Means of determining the heating effect of a magnetic resonance imaging sequence on a diagnostic modality

Abstract

Method for predicting a heating effect of an imaging sequence of a second imaging modality on a detector of a first modality (12) of a combined imaging device (10) as a function of a reference imaging sequence of the second imaging modality, wherein the first modality (12) is designed to acquire diagnostic data of an object of investigation (15) and the second imaging modality comprises a magnetic resonance imaging device (11) designed to acquire magnetic resonance image data of the object of investigation (15), wherein the first modality (12) comprises a temperature compensation unit (59) designed to compensate for a temperature change of the detector, which comprises the following steps: • Performing (S2) a reference imaging sequence with the second imaging modality, wherein in the reference imaging sequence a gradient magnetic field and / or a high-frequency electromagnetic field is applied to the image acquisition area of ​​the combined imaging device (10) and wherein the reference imaging sequence has several imaging parameters that determine a property of the gradient magnetic field and / or the high-frequency electromagnetic field, • Detection (S3) of at least one temperature-dependent parameter of the detector of the first modality (12), • Determining (S4) the heating effect on the detector of the first modality (12) as a function of several imaging parameters of the reference imaging sequence and the at least one temperature-dependent parameter of the detector, and • Prediction (S7) of the heating effect of the imaging sequence on the detector using a smart algorithm, wherein the smart algorithm is designed to predict the heating effect depending on several imaging parameters of the imaging sequence and data acquired by the reference imaging sequence.

Description

[0001] The invention relates to a method for determining the heating effect of an imaging sequence of a second imaging modality on a detector of a first modality of a combined imaging device as a function of a reference imaging sequence of the second imaging modality. The invention further relates to a method for compensating for the heating effect of an imaging sequence of a second imaging modality on a detector of a first modality of a combined imaging device.Furthermore, the invention relates to a combined imaging device comprising a magnetic resonance imaging device and a first modality, with a detector and a temperature compensation unit designed to compensate for a temperature change of the detector, wherein the combined imaging device is designed to perform a method for determining a heating effect of an imaging sequence of the magnetic resonance imaging device on the detector of the first modality as a function of a reference imaging sequence of the magnetic resonance imaging device.

[0002] In medical diagnostics, particularly diagnostic imaging, examinations are frequently performed using combined imaging devices that offer more than one diagnostic modality, typically two. These examinations involve acquiring diagnostic data from a subject using multiple, especially two, diagnostic modalities. For example, image data can be acquired using imaging modalities such as magnetic resonance imaging (MRI), positron emission tomography (PET), or single-photon emission computed tomography (SPECT). It is particularly useful to combine diagnostic modalities that capture both structural information (e.g., through MRI) and functional information of the subject (e.g., through PET).Accordingly, the assessment of the diagnostic data of the subject under examination can be facilitated, and the accuracy of a diagnosis can be increased. However, for some diagnostic purposes, it can also be useful to combine several diagnostic modalities designed to acquire structural information, or even non-imaging diagnostic modalities.

[0003] However, combining different diagnostic modalities in a single imaging device also has limitations. For example, one limitation, particularly associated with magnetic resonance imaging (MRI) devices, is the heating of electrical conductors and other components due to the application of gradient magnetic fields and high-frequency electromagnetic fields. In an MRI device, gradient magnetic fields are typically induced by passing an electric current through electromagnetic coils with non-zero resistivity, which at least partially enclose an imaging area. Some of the electric current carried through the coils is converted into heat energy, thus heating parts of the MRI device.However, heating can also occur in more distant components of the combined imaging device as a result of eddy currents induced in electrical conductors when a magnetic field is applied. This can cause heating of parts of a detector module, printed circuit boards (PCBs), electromagnetic shields, or electrically conductive housings of other diagnostic modalities of the combined imaging device. Furthermore, high-frequency electromagnetic fields can be absorbed by components of the combined imaging device, contributing to the overall heating effect.

[0004] Gradient magnetic fields and high-frequency electromagnetic fields are typically applied as pulses. The amplitude, duration, rate of rise, and time interval of these pulses can follow a pre-calculated sequence and typically change considerably during a magnetic resonance imaging (MRI) scan or in different types of MRI scans. This can lead to a sudden heating of components of the combined imaging device. A temperature compensation unit, such as a fluid recirculation system, can be used to compensate for the heating effects. However, due to the sudden onset of the heating effect, short-term temperature changes cannot be completely prevented.Furthermore, gradient-induced eddy currents can cause an inhomogeneous temperature distribution because the heating effect changes depending on the geometric orientation of an electrical conductor with respect to the gradient magnetic fields. Consequently, heating effects can impair the quality of diagnostic data acquired by the combined imaging device, especially if a detector module of another diagnostic modality is affected.

[0005] The publication DE 10 2007 044 873 A1 describes a method for stabilizing the amplification of a PET detection system.

[0006] One object of the invention is to provide precise information about the heating effect of a magnetic resonance imaging examination on another diagnostic modality of a combined imaging device. A further object of the invention is to improve the quality of diagnostic data acquired by a combined imaging device.

[0007] This objective is achieved by a method, a combined imaging device, and a computer program product according to the invention. Further advantageous embodiments are specified in the dependent claims.

[0008] According to the inventive method, the heating effect of an imaging sequence of a second imaging modality on a detector of a first modality of a combined imaging device is determined as a function of a reference imaging sequence of the second imaging modality. The first modality is designed to acquire diagnostic data of an object under investigation, and the second imaging modality comprises a magnetic resonance imaging device designed to acquire magnetic resonance image data of the object under investigation. The first modality includes a temperature compensation unit designed to compensate for temperature changes in the detector. The magnetic resonance imaging device may include a magnet arrangement with at least one magnet. Preferably, the magnet arrangement is designed to provide a magnetic field in an image acquisition area of ​​the magnetic resonance imaging device.The magnetic field can be suitable for acquiring magnetic resonance imaging (MRI) data of the object under investigation. It is conceivable that the magnetic field provided by the magnetic array is a homogeneous magnetic field or a gradient magnetic field. It is also conceivable that the magnetic array is designed to provide a high-frequency electromagnetic field in the image acquisition area. According to one embodiment, a homogeneous magnetic field in the image acquisition area can be superimposed by a gradient magnetic field and / or a high-frequency electromagnetic field. The electromagnetic field can be induced by an electric current flowing through coil-like wires of the magnetic array. An image acquisition area can be a designated space in which the object under investigation is positioned to acquire MRI image data of the object.An object of study can be, for example, a patient or a desired area of ​​a patient's body.

[0009] The first modality is designed to acquire diagnostic data of a subject under investigation. Diagnostic data may include structural data, functional data, or other data used to determine a physical and / or medical condition of the subject. Functional data may include data of any kind that provides information about biological activity within the subject. For example, biological activity may include blood flow, tissue movement, and the absorption, distribution, metabolism, and excretion of drugs or other substances in the subject. According to a preferred embodiment, the first modality is a single-photon emission computed tomography (SPECT) device or a positron emission tomography (PET) device.In such devices, radiation from a radiopharmaceutical injected into the subject is detected and used to determine the distribution and / or metabolism of the radiopharmaceutical within the subject. According to another embodiment, the first modality comprises a diagnostic sonography device designed to acquire functional and structural imaging data of the subject. It is also conceivable that the first modality is an electrocardiography device designed to generate an electrocardiogram providing information about the electrical activity of the subject's heart. Naturally, the combined imaging device may include other modalities designed to acquire diagnostic data of the subject.Instead of or in addition to the first modality, the imaging sequence of the magnetic resonance imaging (MRI) device can also induce a heating effect on electronic components of an automated therapeutic device, such as a catheter robot or an automated surgical device located near the MRI device. It is conceivable that this heating effect could lead to deviations in the positioning accuracy of surgical instruments within the automated therapeutic device. Therefore, determining whether the imaging sequence causes a heating effect on an automated therapeutic device could advantageously improve the positioning accuracy of surgical instruments.

[0010] Preferably, the first modality and the second imaging modality are mechanically connected, thereby providing a combined imaging device. For example, the detector of a single-photon emission computed tomography (SPECT) device or a positron emission tomography (PET) device can be integrated into a column of the magnetic resonance imaging (MRI) device. However, the first modality and the second imaging modality can also be separate components or modules that can be assembled to form the combined imaging device for specific diagnostic purposes. It is conceivable that the first modality is supported by or located near the second imaging modality.For example, such specific diagnostic purposes may include recording an electrocardiogram while the coronary vessels of a subject are scanned by the magnetic resonance imaging device.

[0011] The detector of the first modality can be any detector designed to acquire diagnostic data of the subject under investigation. In particular, the detector can be designed to acquire image data of a patient. It is conceivable that the detector is designed to acquire signals from a region of the patient's body, which can be used to derive spatially resolved structural and / or functional information about that region. According to one embodiment, the detector can comprise a photon-sensitive material designed to record incident photons from the subject under investigation. According to another embodiment, the detector can comprise a piezoelectric transducer designed to emit and receive sound waves.

[0012] An imaging sequence performed by the magnetic resonance imaging device can induce eddy currents in electrical conductors of the first modality detector, thereby increasing the temperature of the detector.

[0013] The detector of the first modality may exhibit temperature dependence. As a result of this temperature dependence, the quality of the diagnostic data acquired by the detector may be reduced when the imaging sequence is performed by the magnetic resonance imaging (MRI) device. The imaging sequence may represent any sequence typically used in a diagnostic examination procedure with an MRI device. In particular, the imaging sequence may differ from a reference imaging sequence in at least one imaging parameter.

[0014] The first modality includes a temperature compensation unit designed to compensate for temperature changes in the detector. These temperature changes can represent temporal and / or spatial temperature variations of the detector. In one embodiment, the temperature compensation unit may include a temperature sensor. The temperature sensor may be located near or within the detector and be designed to detect a temperature value of the detector. In another embodiment, the temperature compensation unit may be designed to determine a temperature change of the detector as a function of a temperature-dependent parameter of the first modality.Preferably, the temperature compensation unit is designed to set a compensation parameter depending on the temperature value provided by the temperature sensor and / or the temperature-dependent parameter in order to compensate for unwanted temperature changes in the detector module.

[0015] A compensation parameter can be designed to adjust the detector temperature and / or a temperature-dependent parameter of the detector. For example, the compensation parameter could be a bias applied to the detector. This bias can be set based on the detector temperature and a calibration curve to compensate for temperature changes within the detector. It is also conceivable that the compensation parameter could include the flow rate and / or temperature of a cooling fluid in a cooling system to provide cooling for the detector module. Accordingly, the detector temperature can be directly influenced by the compensation parameter.

[0016] In one step of the method according to the invention, a reference imaging sequence is performed with the second imaging modality, wherein in the reference imaging sequence a gradient magnetic field and / or a high-frequency electromagnetic field is applied to the image acquisition area of ​​the combined imaging device and wherein the reference imaging sequence has several imaging parameters that determine a property of the gradient magnetic field and / or the high-frequency electromagnetic field.

[0017] The gradient magnetic field can be generated by dedicated gradient coils of the magnetic resonance imaging (MRI) device. Using the gradient magnetic field, MRI signals from the object under investigation can be spatially encoded, thus enabling the assignment of received MRI signals to a volume element within the object. According to one embodiment, the gradient magnetic field can comprise a phase-encoding gradient magnetic field, a frequency-encoding gradient magnetic field, and a spatially encoding gradient magnetic field. The MRI device can further include a dedicated coil, for example, a body coil, designed to provide a high-frequency electromagnetic field in the imaging area of ​​the MRI device.The high-frequency electromagnetic field and / or the gradient magnetic field can be generated by passing an electric current through electrically conductive wires or coils of the magnetic array, thereby inducing the magnetic field in the image acquisition area. The electric currents and the resulting magnetic field can depend on several imaging parameters of the reference imaging sequence. For example, the magnetic field within the image acquisition area can change repeatedly within a timeframe of a few microseconds, milliseconds, or seconds when the reference imaging sequence is performed. The reference imaging sequence can be any imaging sequence used in a conventional diagnostic examination of a patient.However, it is also conceivable that the reference imaging sequence has a predefined set of imaging parameters that are selected to induce a specific heating effect in the detector module of the first modality.

[0018] According to one embodiment, the first modality can be a single-photon emission computed tomography device or a positron emission tomography device with a photon-sensitive detector. The photon-sensitive detector can comprise a photon-sensitive material, i.e., a scintillator or a direct-converting material, and a photodetector. The photodetector can comprise an array of photodiodes or silicon photomultiplier tubes (SiPMs), or a charge-coupled device designed to provide an electrical signal in response to a signal supplied by the photon-sensitive material. A direct-converting material can be designed to convert photons into an electrical current. Examples of direct-converting materials include cadmium zinc telluride, cadmium telluride, thallium bromide, and the like.The detector can be integrated into the column of the combined imaging device and may include electrical conductors designed to carry electrical currents from the photodetector to a dedicated evaluation unit. Changes in the magnetic field during the reference imaging sequence can induce eddy currents in the detector's electrical conductors, causing heating. It is also conceivable that the high-frequency electromagnetic field is absorbed in parts of the detector, contributing to the heating effect. However, the reference imaging sequence can also cause heating in electronic conductors of other components located near the column of the combined imaging device.For example, eddy currents can also be induced in the piezoelectric transducer of a diagnostic sonography device or in electronic components of an electrocardiography device when these modalities are combined with the magnetic resonance imaging device for specific diagnostic purposes.

[0019] In a further step of the method according to the invention, at least one temperature-dependent parameter of the detector of the first modality is detected.

[0020] The at least one temperature-dependent parameter can include detector signal noise, detector signal output, detector temperature, the temperature of the cooling fluid in a cooling system, and the like. As described above, the temperature compensation unit and / or the detector can include a temperature sensor designed to detect the temperature value near the detector.

[0021] According to one embodiment, the first modality can be a single-photon emission computed tomography device or a positron emission tomography device with a photon-sensitive detector and a temperature sensor designed to detect a temperature value of the photon-sensitive detector. The temperature sensor can be located close to the photon-sensitive detector to avoid limiting the surface area of ​​the photon-sensitive material. Depending on the relative position between the temperature sensor and the photon-sensitive detector and / or the thermal conductivity of any material connecting the temperature sensor to the photon-sensitive detector, the detected temperature value may not represent the actual temperature level in the photon-sensitive detector.For example, the recorded temperature value may indicate the temperature of the photon-sensitive detector with a delay or even an offset due to the time required for heat conduction and the thermal inertia of the materials involved.

[0022] In another example, the at least one temperature-dependent parameter can be a signal output of the detector, i.e., a photopeak level or a bias current provided by the photon-sensitive detector of a single-photon emission computed tomography (SPET) device or a positron emission tomography (PET) device. The photopeak level can denote an energy level at which the majority of photons are deposited in the photon-sensitive material. The detector's photopeak level can be temperature-dependent. For example, the photopeak level can change depending on the temperature of the photon-sensitive detector, particularly the temperature of the photodetector, even though the rate of incoming photons can remain constant. According to one embodiment, the photodetector can comprise an array of avalanche photodiodes (APDs) with a semiconductor material as the multiplication region.Suitable semiconductors include silicon, germanium, cadmium zinc telluride, mercury cadmium telluride, indium gallium arsenide and the like.

[0023] In another example, at least one temperature-dependent parameter can be the signal noise of the photon-sensitive detector. This signal noise can be caused by temperature-dependent generation-recombination processes within the photodetector. Even in the absence of radiation, these processes can initiate avalanche processes in the photodetector, generating a signal count. The average number of counts per second in the absence of radiation is commonly referred to as the dark count rate. Because the dark count rate changes depending on the temperature of the photodetector, it can be used as a temperature-dependent parameter. Further examples of temperature-dependent parameters include the detector's power consumption and the detector count rate. In particular, the detector's power consumption can include the power consumption of the photodetector.However, power consumption values ​​of other electronic components of the detector can also be used as temperature-dependent parameters. The detector count rate can change when the photopeak level changes, provided the comparator threshold of the detector is kept constant. Accordingly, the detector count rate can also be used as a temperature-dependent parameter. Naturally, the combined imaging device can also include other temperature-dependent variables that can be used as temperature-dependent parameters according to the method of the invention. Preferably, a time interval for acquiring the at least one temperature-dependent parameter of the detector overlaps at least partially with a time interval for executing the reference imaging sequence of the magnetic resonance imaging device.Accordingly, at least one recorded temperature-dependent parameter can indicate the heating effect caused by the reference imaging sequence.

[0024] In a further step of the method according to the invention, the heating effect on the detector of the first modality is determined as a function of the several imaging parameters of the reference imaging sequence and the at least one temperature-dependent parameter of the detector.

[0025] The heating effect can be characterized by the temperature of the detector as well as other components of the combined medical imaging device, i.e., electrical conductors in the magnetic resonance imaging device and / or the first modality. It is conceivable that the heating effect encompasses a temperature value, a temperature distribution, and / or a temporal temperature change in the detector or individual modules of the detector. However, the heating effect can also be characterized by a temperature-dependent property of the detector. For example, the heating effect can be quantified as an offset or a change in a signal output of the detector.

[0026] To determine the heating effect, the temperature value detected by the temperature sensor can be complemented by at least one other temperature-dependent parameter. For example, the temperature value can be corrected or adjusted depending on the photopeak level of the detector. The photopeak level can have a higher spatial resolution and / or a faster response to temperature changes than the temperature value provided by the temperature sensor because the photopeak level can be read directly from multiple photodetectors of the photon-sensitive detector. However, the photopeak level can also be interpreted as a temperature-dependent property of the photon-sensitive detector and converted into a temperature equivalent to complement the temperature value detected by the temperature sensor.

[0027] It is also conceivable to determine the heating effect on the detector without acquiring a temperature value from the temperature sensor. For example, the photopeak level or the dark count rate can be used to determine the temperature distribution in the photon-sensitive detector using a model. Such a model can be designed to determine the heating effect as a function of a physical correlation between a temperature-dependent parameter and the temperature distribution in the detector.

[0028] According to one embodiment, a calibration function and / or a calibration database can be derived to determine the heating effect, thereby associating the multiple imaging parameters of the reference imaging sequence with the at least one temperature-dependent parameter. Accordingly, the temperature development of the detector when an imaging sequence is performed during a diagnostic examination of a patient can be determined by the calibration function and / or the calibration database. According to another embodiment, regression analysis can be used to determine the heating effect of an imaging sequence. The regression analysis can estimate the heating effect as a function of the at least one temperature-dependent parameter and the respective multiple imaging parameters of the reference imaging sequence, as well as multiple imaging parameters of the imaging sequence.However, the heating effect can also be determined based on a database using interpolation or extrapolation methods. The database can be provided by the calibration function or calibration database described above, or obtained via a network connection from other combined imaging devices. According to further conceivable embodiments, the heating effect on the detector can also be determined based on a model or an intelligent algorithm.

[0029] It is conceivable that the steps of the above-described method according to the invention are carried out several times or iteratively. In each iteration of the method according to the invention, the reference imaging sequence can change or remain the same. It is also conceivable that individual steps of the method according to the invention, for example, the acquisition of the at least one temperature-dependent value and / or the determination of the heating effect on the detector, are carried out several times within a time interval of executing a reference imaging sequence.

[0030] The method according to the invention enables the determination of the heating effect as a function of any temperature-dependent parameter. Accordingly, a temperature sensor for detecting a temperature value of the detector can advantageously be omitted, and the manufacturing costs for the combined imaging device can be reduced.

[0031] When determining the heating effect of an imaging sequence of the magnetic resonance imaging device on the first modality detector, the temperature compensation unit of the first modality can advantageously be controlled to compensate for temperature changes in the detector and / or an inaccurate temperature value detected by a temperature sensor located near the first modality detector.

[0032] Accordingly, the detector's signal output can be corrected depending on the specific heating effect, thus increasing the accuracy of the data acquired by the first modality and advantageously improving the quality of the diagnostic data acquired with the combined imaging device.

[0033] According to one embodiment, the method according to the invention includes the step of positioning a radiation source in a relative position to an image acquisition area of ​​the combined imaging device, wherein the radiation source is designed to emit a predefined radiation level in the direction of the detector of the first modality.

[0034] According to this embodiment, the first modality preferably comprises a single-photon emission computed tomography (SPECT) device or a positron emission tomography (PET) device. The detector of the first modality can be configured to receive gamma photons emitted by a subject located in the imaging area of ​​the combined imaging device. In the case of an SPECT device, the radiation source can comprise a gamma-emitting tracer such as technetium-99m, iodine-123, indium-111, and the like, emitting gamma rays with a photon energy in the range of approximately 10 to 500 keV. In the case of a PET device, positron-emitting tracers can be used as the radiation source.The emitted positrons can propagate through the imaging area until they encounter an electron, at which point they are annihilated into a pair of annihilation photons with an energy of approximately 511 keV, traveling in approximately opposite directions. According to one embodiment, the object under investigation may comprise a radiopharmaceutical in the form of a positron-emitting tracer or a gamma-ray-emitting tracer, as well as a phantom containing the radiopharmaceutical. The radiopharmaceutical may be incorporated into compositions commonly used in objects under investigation, such as glucose, water, or ammonia, or into molecules that bind to receptors or other sites of drug action within the object. The phantom may further comprise a tissue or substance to provide suitable electrons for interaction with the emitted positrons.Examples of conceivable positron-emitting tracers are carbon-11, nitrogen-13, oxygen-15, fluorine-18, gallium-68, zirconium-89, rubidium-82, and the like. However, it is also conceivable that a gamma-ray-emitting tracer could be used as the radiation source for the positron emission tomography device. Such a gamma-ray-emitting tracer preferably has a photon energy in the range of approximately 100 to 1500 keV. According to one embodiment, the radiation source can comprise a lutetium oxyorthosilicate (LSO) crystal. The LSO crystal can provide background radiation that can be detected by the detector of the first modality.

[0035] The radiation source is designed to emit a predefined radiation level in the direction of the first modality detector. A predefined radiation level can mean that the radiation source emits an approximately constant number of photons or positrons over a predefined period, for example, a few seconds, a few minutes, or a few hours. Preferably, the radiation source is designed to emit radiation uniformly in all spatial directions. A predefined radiation level can also mean that the energy of emitted gamma photons is well-defined or known.

[0036] When positioning the radiation source relative to the image acquisition area of ​​the combined imaging device, the radiation source can be moved to or near the isocenter of the first modality. For example, the radiation source can be positioned by a patient positioning device designed to transport a patient along an axis of the column of the combined imaging device. However, the radiation source can also be positioned at a distance from the isocenter of the first modality. For example, the distance between the radiation source and the isocenter of the first modality can be a few centimeters, a few tens of centimeters, or a few meters. It is also conceivable that the radiation source is positioned outside the image acquisition area.The relative position between the radiation source and the image acquisition area can be chosen such that radiation emitted by the radiation source can pass through the distance between the detector and the radiation source without interruption.

[0037] According to one embodiment, the radiation source can comprise several separate radiation sources or phantoms distributed around an image acquisition area of ​​the first modality.

[0038] Preferably, a time interval during which radiation is emitted by the radiation source and received by the detector overlaps at least partially with a time interval during which the reference imaging sequence is performed and at least one temperature-dependent parameter is recorded.

[0039] By using a radiation source, a photopeak level can be detected by the detector of the first modality. The temperature of the photodetector and the heating effect of the reference imaging sequence can be determined as a function of the photopeak level. Accordingly, a temperature sensor for detecting the detector's temperature can advantageously be omitted.

[0040] According to one embodiment of the method according to the invention, the multiple imaging parameters of the reference imaging sequence include a strength, a frequency, a rise rate and / or a duty cycle of the gradient magnetic field and / or the high-frequency electromagnetic field.

[0041] For example, the magnetic field strength can be characterized by the magnetic force exerted by the gradient magnetic field and / or the high-frequency electromagnetic field. The magnetic field strengths of the gradient magnetic fields and / or the high-frequency electromagnetic field can be different. The rise rate can be characterized by the ratio between the maximum magnetic field strength and the rise time required to increase the magnetic field to the maximum magnetic field strength. The duty cycle can represent the magnitude or percentage of time during which the magnetic field acts at the maximum magnetic field strength during the reference imaging sequence. In addition to the imaging parameters mentioned above, the reference imaging sequence can, of course, include other imaging parameters commonly associated with an imaging sequence of a magnetic resonance imaging device.

[0042] Preferably, properties of the gradient magnetic fields and / or the high-frequency electromagnetic field can be related to the multiple imaging parameters by performing a reference imaging sequence as described above. The imaging parameters and the corresponding magnetic field properties can be used as a priori knowledge when determining the heating effect of an imaging sequence on the detector of the first modality.

[0043] By using the multiple imaging parameters and the corresponding magnetic field properties of the reference imaging sequence as prior knowledge, temporally and / or spatially limited heating effects on the detector can be correlated with imaging parameters relating to the respective applied gradient magnetic fields and / or the high-frequency electromagnetic field. Accordingly, the contribution of individual imaging parameters to the heating effect can be advantageously identified when determining the heating effect of an imaging sequence on the detector.

[0044] According to one embodiment of the method according to the invention, at least one temperature-dependent parameter of the detector is a temperature value, a photopeak level, or the signal noise of the detector.

[0045] As described above, the temperature sensor can be omitted if the heating effect is determined as a function of the detector's signal output, such as the photopeak level or the dark count rate. Accordingly, the manufacturing costs of the detector can be advantageously reduced. However, by using a temperature value from a temperature sensor located near the detector as a temperature-dependent parameter, an approximate temperature level of the detector can be determined as a reference. This approximate temperature level can advantageously be complemented by other temperature-dependent parameters to determine the heating effect on the detector. In particular, several temperature-dependent parameters can advantageously be used in a complementary manner, thereby increasing the accuracy of the determined heating effect.

[0046] According to one embodiment of the method according to the invention, a first compensation parameter of the temperature compensation unit is kept constant, while at least one temperature-dependent parameter is recorded.

[0047] A first compensation parameter can be used by the temperature compensation unit to compensate for a temperature change of the detector. It is conceivable that the first compensation parameter is a cooling parameter of a cooling system designed to provide cooling to the combined imaging device, in particular to the detector of the first modality of the combined imaging device. According to one embodiment, the detector of the first modality is cooled by a fluid of a cooling system, for example, a water cooling system or an air cooling system. The compensation parameter can, for example, include the flow rate and / or the temperature of the fluid of the cooling system. According to one embodiment, the first modality is a single-photon emission computed tomography device or a positron emission tomography device comprising a photon-sensitive detector with a photodetector.Photodetectors usually require a bias voltage (or supply voltage), which can also be used as a compensation parameter to compensate for temperature changes within the detector module.

[0048] It is conceivable that one or more compensation parameters of the first modality are kept constant, while at least one temperature-dependent parameter is recorded.

[0049] By keeping the compensation parameters of the first modality constant, the heating effect of the imaging sequence on the detector can be advantageously determined independently of the influence of a temperature compensation effect.

[0050] According to one embodiment of the method according to the invention, the first compensation parameter is set to compensate for a temperature change of the detector while the at least one temperature-dependent parameter of the detector is being detected.

[0051] It is conceivable that the first compensation parameter is kept constant during a first iteration of a method according to the invention and adjusted during a second iteration of the method according to the invention in order to determine the difference in the heating effect obtained by adjusting the compensation parameter. However, the first compensation parameter can also be adjusted when a single iteration of the method according to the invention is carried out.

[0052] In one example, the bias voltage applied to a photodetector can be adjusted to compensate for an increasing breakdown voltage of the photodetector resulting from the heating effect caused by the reference imaging sequence. In another example, a cooling parameter, such as the flow rate and / or temperature level of the cooling fluid, can be adjusted to compensate for a temperature change of the detector. The heating effect on the detector can be quantified as a function of a required setting of the first compensation parameter. It is also conceivable that multiple compensation parameters—i.e., a first compensation parameter, a second compensation parameter, a third compensation parameter, and so on—can be adjusted while at least one temperature-dependent parameter is being measured.One or more compensation parameters can also be kept constant while other compensation parameters are adjusted to compensate for the heating effect in the detector. The adjustment of the compensation parameter can be based on the temperature reading of a temperature sensor located near the detector. However, as described above, such a temperature reading may exhibit a delay and / or offset compared to the actual temperature level in the detector.

[0053] When setting the first compensation parameter, during which at least one temperature-dependent parameter is acquired, the delay and / or offset of the temperature value acquired by the temperature sensor relative to the actual temperature of the detector can be characterized by correlation with the specific heating effect, for example, by a transfer function. Depending on the delay and / or offset of the temperature sensor, a control strategy for the temperature compensation unit can be implemented to advantageously improve the accuracy of the temperature compensation during the execution of an imaging sequence.

[0054] According to a further embodiment, the method according to the invention comprises the additional steps of detecting a second temperature-dependent parameter of the detector, wherein a second compensation parameter of the temperature compensation unit is defined differently from the first compensation parameter, and determining the heating effect on the detector as a function of the several imaging parameters of the reference imaging sequence, the second compensation parameter and the second temperature-dependent parameter of the detector.

[0055] According to this embodiment, the method according to the invention can be performed several times. In this process, several reference imaging sequences can be performed by the magnetic resonance imaging device, with at least one temperature-dependent parameter being determined each time and the heating effect of the reference imaging sequence on the detector of the first modality being determined. In each iteration of the method according to the invention, a compensation parameter of the temperature compensation unit can be changed, while the several imaging parameters of the reference imaging sequence remain constant. Accordingly, the first compensation parameter can be set to a first value in a first iteration, while the second compensation parameter can be set to a second value different from the first value in a second iteration. Preferably, the compensation parameter is constant during an iteration of the method according to the invention.It is conceivable that a third iteration, a fourth iteration, or further iterations could be performed accordingly. An iteration of the method according to the invention with different compensation parameters can yield different results for the at least one temperature-dependent parameter used to determine the heating effect. Accordingly, the influence of variable compensation parameters on the heating effect can be determined. The heating effect is determined at least as a function of the several imaging parameters of the reference imaging sequence performed in each iteration, as well as the second compensation parameter and the second temperature-dependent parameter of the detector. However, the heating effect can also be determined taking into account the first compensation parameter and the at least one temperature-dependent parameter.For example, the compensation parameter can be a cooling parameter, such as the inlet temperature of the cooling fluid as it enters the detector, the outlet temperature of the cooling fluid as it exits the detector, and / or the flow rate of the cooling fluid. A change in the outlet temperature of the cooling fluid represents a change in the heat capacity of the cooling fluid, which may depend on the heating effect of the reference imaging sequence on the detector. This change in heat capacity can be correlated with the difference between the at least one temperature-dependent parameter and the second temperature-dependent parameter to quantify the heating effect of the reference imaging sequence on the detector.

[0056] According to one embodiment, the temperature compensation unit can also be deactivated in a first iteration and activated in a second iteration of the method according to the invention in order to determine the influence of the compensation parameter and / or the temperature compensation unit on the heating effect.

[0057] When setting the compensation parameter, the influence of the compensation parameter on the heating effect and at least one temperature-dependent value can advantageously be recorded over a large parameter range. For example, the recorded data can be advantageously used to extend the calibration database and / or the calibration function, thereby facilitating the process of determining the heating effect of an imaging sequence on the first modality detector.

[0058] According to a further embodiment, the inventive method further comprises the step of predicting the heating effect of an imaging sequence on the detector using an intelligent algorithm, wherein the intelligent algorithm is designed to predict the heating effect depending on several imaging parameters of the imaging sequence and data acquired by the reference imaging sequence.

[0059] An intelligent algorithm can encompass any type of artificial intelligence application. For example, an intelligent algorithm can include a machine learning algorithm, a neural network, a self-learning algorithm, an expert system, a function optimization, a data mining method, and the like.

[0060] According to one embodiment, a neural network is designed to predict the heating effect of the imaging sequence on the detector as a function of the multiple imaging parameters of the reference imaging sequence, as well as the at least one temperature-dependent parameter and / or the respective heating effect acquired together with the reference imaging sequence. In particular, the neural network can be trained with data acquired by a reference imaging sequence. For example, such data can include the heating effect, the at least one temperature-dependent parameter, the multiple imaging parameters, and / or a compensation parameter acquired together with the reference imaging sequence.

[0061] According to another embodiment, an expert system is used to predict the heating effect of the imaging sequence. The expert system can have access to a database containing data acquired in previously executed reference imaging sequences. The database can be stored in internal memory of the combined imaging device or in external storage, i.e., a server or the cloud. The database can also contain data from previously executed imaging sequences of other combined imaging devices of the same type. It is also conceivable that the expert system includes a model of a relevant component of the combined imaging device and / or a calibration function to predict the heating effect of the imaging sequence on the detector. The expert system can furthermore have access to a sensor (i.e.,a temperature sensor) and / or a signal output from the detector to capture a current value of at least one temperature-dependent parameter.

[0062] An intelligent algorithm can be self-adapting to advantageously compensate for component aging effects and measurement drift, which could otherwise lead to deviations in the determination of heating effect. Furthermore, intelligent algorithms, such as neural networks and deep learning methods, can be easily trained with new calibration data. Consequently, intelligent algorithms can be advantageously transferred to other devices and / or next-generation devices with minimal effort.

[0063] According to a preferred embodiment, the inventive method includes the step of predicting the heating effect of an imaging sequence on the detector using a model-based approach, wherein the model-based approach is designed to predict the heating effect as a function of several imaging parameters of the imaging sequence and data acquired by the reference imaging sequence.

[0064] In a model-based approach, an analytical, empirical, and / or physical model of relevant components of the combined imaging device can be used. Such models can include functions and / or algorithms that establish a relationship between the value of an imaging parameter and a property of the heating effect. For example, an empirical model can correlate the temperature distribution of the detector with the values ​​of individual imaging parameters of a reference imaging sequence using an empirical function. The empirical model can be derived from a calibration database containing data from one or more reference imaging sequences with their respective temperature-dependent parameters and / or heating effects.Accordingly, the heating effect of the imaging sequence can be predicted by the empirical function, even if values ​​of individual imaging parameters are changed with respect to the reference imaging sequence.

[0065] The model can further incorporate an energy equilibrium, assuming that a percentage of the energy from the magnetic field is converted into heat energy, thereby increasing the detector temperature. Accordingly, the heating effect can be determined as a function of the detector's thermal mass and the amount of energy transferred. The percentage of energy transferred and / or the temperature distribution pattern in the detector can be determined based on data derived from one or more reference imaging sequences. For example, data acquired by the reference sequence can include multiple imaging parameters, a temperature-dependent parameter, a specific heating effect, and / or other data associated with the reference imaging sequence.

[0066] The model-based approach can also include a numerical solution to a system of (differential) equations designed to predict the heating effect of the imaging sequence on the detector. To solve the system of (differential) equations, a one- or multi-dimensional model of the detector with discrete grid points can be derived. When predicting the heating effect, the system of (differential) equations can be solved numerically at the discrete grid points as a function of the various imaging parameters of the imaging sequence. It is conceivable that the numerical model uses data from a reference imaging sequence as a boundary condition.

[0067] A model-based approach can advantageously predict the heating effect of an imaging sequence robustly and reproducibly.

[0068] According to one embodiment, the method according to the invention includes the step of correcting a compensation parameter of the temperature compensation unit depending on a correction parameter to reduce the influence of the heating effect on the diagnostic data.

[0069] As described above, the combined imaging device includes a temperature compensation unit designed to compensate for temperature changes in the detector by adjusting one or more compensation parameters. A compensation parameter can be adjusted continuously or at discrete time intervals. The temperature compensation unit can be designed to adjust the compensation parameter based on a temperature value provided by a temperature sensor located near the detector. However, as described above, such a temperature value may have a delay and / or offset relative to the actual temperature of the detector, resulting in inadequate temperature compensation in the detector.

[0070] According to one embodiment, the correction parameter can represent a temperature value or temperature distribution of the detector that is determined or predicted as a function of the heating effect, as described above. Accordingly, in contrast to a temperature sensor located near the detector, the correction parameter can provide a close representation of the actual temperature or temperature distribution within the detector. In one example, the correction parameter can correspond to a heating effect that is determined and / or predicted as a function of at least one temperature-dependent parameter and several imaging parameters of the reference imaging sequence.In another example, the correction parameter can correspond to a variable designed to compensate for a delay and / or offset of the temperature sensor when correlated with the temperature sensor reading or the compensation parameter. As described above, the correction parameter can also include a transfer function that characterizes the difference between the temperature reading detected by the temperature sensor and the temperature of the detector determined by the heating effect.

[0071] The correction parameter can be transmitted to the temperature compensation unit either instead of or in addition to the temperature value provided by the temperature sensor. The temperature compensation unit can be designed to correlate the temperature value detected by the temperature sensor with the correction parameter. It is also conceivable that the temperature compensation unit includes a logic unit designed to use the correction parameter instead of the temperature value provided by the temperature sensor when a corresponding correction parameter signal is present.

[0072] According to another embodiment, the correction parameter is used to modify a signal from the temperature sensor before the signal is transmitted to the temperature compensation unit. The temperature sensor signal can be an analog or a digital signal. In another example, the correction parameter can be used to modify a compensation parameter at the output of the temperature compensation unit. For example, the compensation parameter can be output as an analog or digital signal. This analog or digital signal can be modified, for example, by an analog circuit, a processing circuit, a logic unit, or the like, to take into account the temperature value or temperature distribution determined as a function of the heating effect.

[0073] A correction parameter can be used to increase the accuracy of the temperature compensation unit by correcting a delayed and / or inaccurate temperature value provided by a temperature sensor near the detector. This can reduce unwanted temperature changes in the detector module and advantageously improve the quality of diagnostic data acquired by the detector.

[0074] According to the inventive method, the heating effect of an imaging sequence of a second imaging modality on a detector of a first modality of a combined imaging device is compensated, wherein the first modality is designed to acquire diagnostic data of an object under investigation, and the second imaging modality comprises a magnetic resonance imaging device designed to acquire magnetic resonance imaging data of the object under investigation, wherein the first modality comprises a temperature compensation unit designed to compensate for a temperature change of the detector.

[0075] The first modality, the second modality and the temperature compensation unit of the combined imaging device can correspond to any of the embodiments described above.

[0076] In one step of the method according to the invention, the heating effect of an imaging sequence of the second imaging modality on the detector of the first modality is determined using a model-based approach. As described above, in a model-based approach, an analytical model, a physical model, and / or a numerical model of one or more components of the combined imaging device can be used to determine the heating effect. In contrast to performing a reference imaging sequence, however, the heating effect can be determined primarily as a function of physical correlations and / or physical interactions between the first modality and the magnetic resonance imaging device, as well as data relating to geometric dimensions and / or structural properties of components of the combined imaging device and imaging parameters of the imaging sequence.It is conceivable that the heating effect determined by the model-based approach is an averaged quantity, thus providing the average heating rate of the detector and / or the amount of heat energy absorbed by the detector during the imaging sequence. However, it is also conceivable that the heating effect comprises a time-resolved history of the heating rate and / or the heat energy absorbed by the detector. For example, the heating effect can be determined when imaging parameters of an imaging sequence are specified, such as before an imaging examination of a patient is performed. However, it is also conceivable that the heating effect is determined in discrete time steps while the imaging sequence is being executed.Accordingly, temperature-dependent parameters can be captured using the combined imaging device to refine and / or update the heating effect via the model-based approach while the imaging sequence is running.

[0077] In a further step of the method according to the invention, a compensation parameter of the temperature compensation unit is set as a function of the heating effect. It is conceivable that the temperature compensation unit has an input interface designed to receive the heating effect determined by the model-based approach. The heating effect can, for example, be transmitted as an analog or digital signal via a suitable signal connection. The temperature compensation unit can also be designed to determine the compensation parameter as a function of a temperature value from a temperature sensor located near the detector of the first modality and of the heating effect.According to a preferred embodiment, the temperature compensation unit includes a control unit designed to adjust one or more compensation parameters in discrete time steps depending on the heating effect while the imaging sequence is being executed. The temperature compensation unit may, for example, include a control loop designed to adjust a compensation parameter depending on the heating effect.

[0078] The combined imaging device according to the invention comprises a magnetic resonance imaging device and a first modality comprising a detector and a temperature compensation unit designed to compensate for temperature changes in the detector. The combined imaging device is designed to acquire diagnostic data and magnetic resonance image data of an object of investigation arranged in an image acquisition area of ​​the combined imaging device and to perform a method according to one of the embodiments described above.

[0079] The combined imaging device may comprise one of the aforementioned first modalities. According to a preferred embodiment, the first modality is a single-photon emission computed tomography device or a positron emission tomography device. For acquiring, processing, and / or storing data, for example, the at least one temperature-dependent parameter, the multiple imaging parameters, and / or the heating effect, the combined imaging device may comprise components such as a control unit, a processing unit, a memory, an internal and / or an external storage unit, and a suitable interface designed to send and receive data and / or convert data into a desired data format. The processing unit may comprise a logic unit, a control device, a microcontroller, a CPU, a GPU, a DSP, an FPGA, and the like.The memory and internal storage can include RAM, ROM, PROM, EPROM, EEPROM, flash memory, HDD, SSD, and the like. The storage unit can also include an external database, i.e., a database located on an external server or in the cloud and connected to the combined imaging device via a network connection. Data can be transported between components using appropriate signal connections via analog and / or digital signals.The combined imaging device may have a single control unit designed to control the first modality and the second imaging modality, or multiple control units designed to control specific components of the combined imaging device, such as the magnetic resonance imaging device, the first modality, the temperature compensation unit, a patient positioning device for positioning the subject in relation to the combined imaging device, etc.

[0080] The first modality and the second imaging modality can be carried by and / or mechanically attached to the combined imaging device. It is also conceivable that the first modality includes a portable device, such as an ultrasound or electrocardiography device, which can be positioned on or near the second imaging modality to acquire both diagnostic and magnetic resonance imaging data from a subject.

[0081] According to one embodiment of the combined imaging device according to the invention, the temperature compensation unit is designed to set a compensation parameter depending on a heating effect and / or a correction parameter.

[0082] According to a preferred embodiment, the temperature compensation unit can include a high-voltage digital-to-analog converter configured to supply a bias voltage to a photodetector of a single-photon emission computed tomography (SPECT) device or a positron emission tomography (PET) device. The bias voltage can be set as a function of a signal from a logic unit. The logic unit can be configured to receive a current temperature value detected by a temperature sensor, the correction parameter determined as a function of the heating effect, and / or the heating effect itself, as described above in an embodiment of the method according to the invention. The logic unit can further be configured to send a signal to the high-voltage digital-to-analog converter that determines the desired bias voltage as a function of the temperature value and the correction parameter.

[0083] According to another embodiment, the temperature compensation unit is designed to output the compensation parameter to a cooling system. The cooling system can be designed to adjust the flow rate or temperature of the cooling fluid to compensate for the heating effect on the detector of the first modality.

[0084] By providing a temperature compensation unit designed to adjust a compensation parameter depending on a heating effect and / or a correction parameter, temperature changes resulting from an imaging sequence of the magnetic resonance imaging device can be avoided. Accordingly, the quality of diagnostic data acquired by the detector of the first modality can be advantageously improved.

[0085] The computer program product according to the invention can be loaded into a memory of a programmable processing unit of a combined imaging device and comprises program code means for carrying out a method according to the invention when the computer program product is executed in the processing unit of the combined imaging device.

[0086] This allows the method according to the invention to be executed quickly, robustly, and repeatably. The computer program product is designed such that it can execute the process steps according to the invention via the processing unit. The processing unit must in any case have the necessary prerequisites in the form of a suitable main memory, a suitable graphics card, or a suitable logic unit so that the respective process steps can be executed effectively.

[0087] The computer program product is stored, for example, on a computer-readable medium or on a network, server, or cloud, from which it can be loaded into the processor of a local processing unit. The local processing unit can be directly connected to the combined imaging device or designed as part of the combined imaging device. Furthermore, control information of the computer program product can be stored on an electronically readable medium. The control information on the electronically readable medium can be designed such that, when the medium is used, it executes a method according to the invention in a processing unit of the combined imaging device. Examples of an electronically readable medium are a DVD, a magnetic tape, or a USB flash drive on which electronically readable control information, in particular software, is stored.If this control information is read from the medium and stored in a control and / or processing unit of a combined imaging device, all embodiments of the above-described method according to the invention can be carried out.

[0088] Further advantages and details of the present invention can be seen from the embodiments described below and the drawings. The figures show: Fig. 1 a schematic representation of a combined imaging device according to the invention, Fig. 2 an exemplary temperature distribution in a detector module, Fig. 3 a schematic temperature development in a detector module during an imaging sequence, Fig. 4 a schematic representation of a temperature compensation unit according to the invention, Fig. 5 a flowchart of a method according to the invention and Fig. 6 a flowchart of a method according to the invention.

[0089] Fig. Figure 1 is a schematic representation of a combined imaging device 10 designed to perform a method according to the invention. According to the illustrated embodiment, the combined imaging device 10 comprises a magnetic resonance imaging device 11 and a positron emission tomography device 12.

[0090] The magnetic resonance imaging device 11 comprises a magnet assembly 13 and an image acquisition area 14 for accommodating an examination subject 15, in particular a patient 15. The image acquisition area 14 is enclosed circumferentially by the magnet assembly 13. The patient 15 can be inserted into the image acquisition area 14 by means of a patient positioning device 16, which is movably arranged within the image acquisition area 14. The magnet assembly 13 can be supported by a cylindrical column (not shown) that encloses the image acquisition area 14 and has at least one opening to allow the patient to enter the image acquisition area 14 via the patient positioning device 16.

[0091] The magnet assembly 13 includes a main magnet 17 designed to generate a strong and constant main magnetic field 18 in the image acquisition area 14. The magnet assembly 13 also includes a gradient coil unit 19 for generating gradient magnetic fields used for spatial coding during a diagnostic examination. Additionally, the magnet assembly 13 includes a high-frequency coil 20 integrated into the magnet assembly 13, designed to provide a high-frequency electromagnetic field in the image acquisition area 14. The high-frequency coil 20 may also be designed to receive magnetic resonance signals.

[0092] The combined imaging device 10 includes a magnetic resonance control unit 21 (MR control unit 21) for controlling the main magnet 17 of the gradient coil unit 19 and the high-frequency coil 20. The MR control unit 21 centrally controls the magnetic resonance imaging device 11, i.e., for executing an imaging frequency in the form of a gradient echo sequence or a turbo spin echo sequence. The MR control unit 21 may further include a gradient control unit (not shown in detail) and a high-frequency antenna control unit (not shown in detail). Furthermore, the MR control unit 21 may include a magnetic resonance evaluation unit for evaluating magnetic resonance imaging data.

[0093] The magnetic resonance imaging device 11 can have a surface coil 30 designed to receive magnetic resonance signals from the patient 15. The surface coil 30 can be applied to a body region of the patient 15 to be examined by the magnetic resonance imaging device 11. According to the illustrated embodiment, the surface coil 30 is designed as a body antenna unit. The surface coil 30 can also be designed to accommodate other body regions of the patient 15, for example, the knee and / or the back of the patient 15. It is also conceivable that more than one surface coil 30 is arranged on the patient 15.

[0094] The depicted magnetic resonance imaging device 11 may, of course, include other components that magnetic resonance imaging devices 11 usually have. The general operating mode of a magnetic resonance imaging device 11 is well known to those skilled in the art, so a detailed description of the general components is not considered necessary.

[0095] The positron emission tomography device 12 has a photon-sensitive detector comprising several detector modules 22 arranged in a ring and circumferentially enclosing the image acquisition area 14. Each of the detector modules 22 has several detector elements (not shown in detail) in the form of a scintillator array with scintillation crystals (for example, LSO crystals) and a photodetector with a photodiode array (for example, an avalanche photodiode array) arranged behind the scintillator array within the detector modules 22.

[0096] Photon pairs resulting from the annihilation of a positron with an electron are detected by the detector modules 22. The two photons each have an energy of approximately 511 keV and are typically emitted along paths with an angle of approximately 180°. Positrons can be emitted by a radiopharmaceutical (positron-emitting tracer), which is usually administered to the patient 15 by injection.

[0097] Each of the detector modules 22 additionally includes detector electronics comprising an electrical amplifier circuit and other electronic components (not shown). For controlling the detector electronics and the detector modules 22, the combined imaging device 10, in particular the positron emission tomography device 12, includes a positron emission tomography control unit 23 (PET control unit 23). The PET control unit 23 is designed to control the positron emission tomography device 12. Furthermore, the PET control unit 23 can include an evaluation unit for assessing functional data acquired by the positron emission tomography device 12. The illustrated positron emission tomography device 12 can, of course, include other components typically found in positron emission tomography devices 12.The general operating mode of a positron emission tomography device 12 is well known to those skilled in the art, so that a detailed description of the general components is considered unnecessary.

[0098] The combined imaging device 10 may include a central processing unit 24 designed to coordinate the acquisition and / or evaluation of magnetic resonance imaging (MRI) and positron emission tomography (PET) image data. The central processing unit 24 may also include a central system control unit (not shown) designed to control the MR control unit 21 and / or the PET control unit 23. The central processing unit 24 may also include the MR control unit 21 and / or the PET control unit 23. The central processing unit 24, as well as the MR control unit 21 and / or the PET control unit 23, may be connected to an internal storage unit 27. The internal storage unit 27 may store data about previously executed reference imaging sequences of the magnetic resonance imaging device 11.This data can include, for example, several imaging parameters of a reference imaging sequence, one or more temperature-dependent parameters, and the heating effect of the reference imaging sequence on a detector module 22 of the positron emission tomography device 12. The central processing unit 24 can also be connected to a cloud storage device 28 via a network connection. The cloud storage device 28 can contain the respective data or other combined imaging devices 10 of the same type.

[0099] Control information in the form of imaging parameters, as well as the reconstructed image data, can be displayed on a display unit 25. The display unit 25 can have at least one screen designed to show control information and / or image data from the combined imaging device 10 to an operator. The combined imaging device 10 also has an input unit 26 designed to receive information and / or parameters entered by the operator during a measurement procedure.

[0100] The PET control unit 23 can further comprise a temperature compensation unit 59 designed to adjust a bias voltage supplied to the detector module 22. The temperature compensation unit 59 can be connected to a temperature sensor 56 (not shown) located near the photodiode array of the detector module 22 and designed to detect a temperature value of the detector module 22. The temperature compensation unit 59 is operated by means of Fig. 4 described in further detail.

[0101] Fig. Figure 2 shows an exemplary temperature distribution in a detector module 22 of a positron emission tomography (PET) device 12. The temperature distribution can represent a heating effect determined by a model-based approach, i.e., a numerical simulation designed to predict the heating effect of an imaging sequence from a magnetic resonance imaging (MRI) device 11 on the detector module 22 of the positron emission tomography device 12. In the example shown, black areas of the detector module 22 can denote a higher temperature than light areas. The heating effect predicted by the model indicates that the temperature is inhomogeneously distributed across the detector module 22. It is conceivable that the end of the detector module 22 oriented in the Z-direction is positioned closer to a gradient magnetic field provided by the gradient unit 19 of the MRI device 11.To compensate for localized hotspots with elevated temperatures, a bias voltage of affected sections of the detector module 22 can be set by the temperature compensation unit 59.

[0102] Fig. Figure 3 shows a schematic temperature development in the detector module 22 of a positron emission tomography device 12 during an imaging sequence of the magnetic resonance imaging device 11. The diagram shows the development of the temperature T of an arbitrarily chosen volume element (D) of the detector module 22 and the temperature value (S) recorded by the temperature sensor 56 over time t. The combined imaging device 10 may be in a standby operating mode before an imaging sequence of the magnetic resonance imaging device 11 is initialized at time A. In standby mode, the temperature of the volume element (D) and the temperature value (S) can increase in the same way, for example, as a result of the heating of electrical components in standby mode.When the imaging sequence is initialized at time A, the temperature of the volume element of the detector module 22 immediately increases due to eddy currents induced in electrical conductors within the detector module 22. In contrast, the temperature value (S) detected by the temperature sensor 56 cannot immediately reflect the actual temperature in the volume element (D) of the detector module 22 because it is separated from the detector module 22 by a thermal bridge 55 (see ). Fig. 4) Depending on the thermal conductivity properties of the detector module 22, the thermal bridge 55, and the temperature sensor 56, the heating effect of the imaging sequence on the detector module 22 can be registered at time B when the temperature of the volume element (D) of the detector module 22 has already risen further. Accordingly, the temperature sensor 56 cannot provide an accurate indication of the temperature of the detector module 22. To compensate for this deficiency, the combined imaging device 10 can include a temperature compensation unit 59 with access to a specific and / or predicted heating effect and / or to a correction parameter provided by a method according to the invention.

[0103] Fig. Figure 4 shows a schematic representation of a temperature compensation unit 59 of a positron emission tomography device 12. The temperature compensation unit 59 can comprise a current monitoring device 51 designed to monitor a bias current of the photodiode array, a high-voltage digital-to-analog converter 52 designed to supply a bias voltage to the photodiode array, and a logic unit 53 designed to control the high-voltage digital-to-analog converter 52. The logic unit 53 can, for example, comprise a logic circuit, a control device, a microcontroller, a processor, a memory, and an input and output interface for receiving and processing signals from the current monitoring device 51 and from the temperature sensor 56 located near the detector module 22.The signals from the temperature sensor 56 and the current monitoring device 51 can be analog or digital signals. In particular, the logic unit 51 can be configured to receive a predicted and / or determined heating effect and / or a correction parameter via a dedicated signal port 60. The predicted and / or determined heating effect and / or the correction parameter can be provided by the PET control unit 23 or the central processing unit 24, which can be configured to execute a method according to the invention. Depending on the predicted and / or determined heating effect and / or the correction parameter, as well as the temperature value detected by the temperature sensor 56, the logic unit 53 controls the high-voltage digital-to-analog converter 52 to output a bias voltage to the photodiode array 54 of the detector module 22.The output from the logic unit 53 and / or the output from the high-voltage analog-to-digital converter can be considered as a compensation parameter designed to compensate for a temperature change in the photodiode array 54 of the detector module 22. It is conceivable that one or more elements of the temperature compensation unit, for example the logic unit 53, the current monitoring device 51, and / or the high-voltage digital-to-analog converter 52, are integrated into a combined analog circuit.

[0104] As in Fig. As shown in Figure 4, the photodiode array 54 can be connected to the temperature sensor 56 via a thermal bridge 55, which is designed to provide heat conduction from the photodiode array 54 to the temperature sensor 56. However, the temperature compensation unit 59 can also be designed to output a compensation parameter to a cooling system (not shown), which is designed to adjust the temperature and / or flow rate of a cooling fluid to compensate for the heating effect of an imaging sequence on the detector module 22 of the positron emission tomography device 12.

[0105] Fig. Figure 5 shows a flowchart of a method according to the invention for determining the heating effect of an imaging sequence of a magnetic resonance imaging device 11 on a detector of a positron emission tomography device 12 of a combined imaging device 10 as a function of a reference imaging sequence of the magnetic resonance imaging device 11, wherein the positron emission tomography device 12 is designed to acquire functional data of a patient 15, and the magnetic resonance imaging device 11 is designed to acquire magnetic resonance image data of the patient 15, wherein the positron emission tomography device 12 has a temperature compensation unit 59 which is designed to compensate for a temperature change of the detector.

[0106] In an optional step S1, a radiation source is positioned relative to the image acquisition area 14 of the combined imaging device 10, the radiation source being designed to emit a predefined radiation level in the direction of the detector of the positron emission tomography device 12. To position the radiation source relative to the image acquisition area 14 of the combined imaging device 10, the radiation source can be mounted on a patient positioning device 16 and moved manually or automatically into the image acquisition area 14. When positioned in the image acquisition area 14, the radiation source is circumferentially enclosed by the detector modules 22 of the positron emission tomography device 12. As described above, the radiation source can comprise an aqueous solution containing a radiopharmaceutical in the form of a positron emission tomography tracer.The radiation source may also include a phantom made of tissue or fabric, which can provide a container for the radiation source and also serves to interact with the emitted positrons.

[0107] In step S2, a reference imaging sequence is performed using the magnetic resonance imaging device 11, wherein a gradient magnetic field and / or a high-frequency electromagnetic field is applied to the image acquisition area 14 of the combined imaging device 10 during the reference imaging sequence. The reference imaging sequence can be characterized by several imaging parameters that determine a property of the gradient magnetic field and / or the high-frequency electromagnetic field. For example, such a property could be a strength, a frequency, a rate of rise, and / or a duty cycle of the respective magnetic field applied to the image acquisition area 14. However, other properties of the magnetic fields commonly used in magnetic resonance imaging devices are also conceivable.

[0108] The reference imaging sequence can be any imaging sequence used in a conventional diagnostic examination of a patient 15. According to a preferred embodiment, the reference imaging sequence has a predetermined set of imaging parameters selected to induce a specific heating effect in the detector module 22 of the positron emission tomography device 12. The multiple imaging parameters of the reference imaging sequence can be entered or selected by an operator of the combined imaging device 10 via the input unit 26. For example, the reference imaging sequence can be stored in a memory unit connected to the central processing unit 24 of the combined imaging device 10 and presented to the operator for selection via the display unit 25.The central processing unit 24 and / or the MR control unit 21 are designed to control the gradient coil unit 19 and / or the high-frequency coil 20 to provide a gradient magnetic field and / or a high-frequency electromagnetic field according to the reference imaging sequence within the image acquisition area 14 of the magnetic resonance imaging device 11. The execution of the reference imaging sequence can cause the gradient magnetic fields and / or the high-frequency electromagnetic field to change within short time intervals, thereby inducing eddy currents in electrical conductors of the detector modules 22 of the positron emission tomography device 12, resulting in a heating effect.

[0109] In step S3, at least one temperature-dependent parameter of the positron emission tomography device 12 is acquired. For example, a signal noise (e.g., a dark count rate), a photopeak level, and / or a temperature value of the detector module 22 are acquired during and / or after the reference imaging sequence has been completed. In another example, the inlet temperature and / or the outlet temperature of a fluid in the cooling system are acquired as a temperature-dependent parameter. Preferably, the at least one temperature-dependent parameter is acquired by a suitable sensor. However, the temperature-dependent parameter can also be a quantity that can be determined or calculated as a function of one or more suitable sensors of the type mentioned above.

[0110] According to one embodiment, the bias voltage provided by the temperature compensation unit 59 (the first compensation parameter) is kept constant while the photopeak level (at least one temperature-dependent parameter) is measured. By keeping the bias voltage of the photodiode array 54 constant, the influence of the bias voltage on the photopeak level and / or the dark count rate can be prevented. Accordingly, the heating effect on the detector can be derived from the photopeak level and / or the dark count rate as a relative quantity.

[0111] According to another embodiment, the bias voltage provided by the temperature compensation unit 59 is adjusted to compensate for the temperature variation of the detector while the photopeak level is being acquired. For example, the bias voltage of the photodiode array 54 can be adjusted depending on the temperature value provided by the temperature sensor 56 to compensate for an increasing breakdown voltage of the photodiode array 54 resulting from the heating effect caused by the reference imaging sequence. As in Fig. As shown in Figure 3, the temperature value detected by the temperature sensor 56 can provide a delayed indication of the actual temperature in the detector. Accordingly, determining the heating effect as a function of a photopeak level detected while the bias voltage is being set can provide a quantification of the delay or error associated with the temperature sensor 56. This quantification can be used to adapt a control strategy of the temperature compensation unit 59 to compensate for the heating effect when relying on the temperature value of the temperature sensor 56.

[0112] In step S4, the heating effect on the detector of the positron emission tomography device 12 is determined as a function of the several imaging parameters of the reference imaging sequence and the at least one temperature-dependent parameter of the detector. In one example, the heating effect on the detector can be determined by complementing the temperature value detected by the temperature sensor 56 with one or more temperature-dependent parameters such as the photopeak level and / or the dark count rate of the detector. In another example, the heating effect on the detector can be determined by a physical model that correlates the photopeak level and / or the dark count rate with the temperature distribution in the detector.In further examples, the heating effect on the detector can be determined depending on a calibration function and / or a calibration database, an interpolation or extrapolation method, and / or an intelligent algorithm. It is conceivable that one or more of the aforementioned approaches could be used to determine the heating effect of the reference imaging sequence on the detector of the positron emission tomography device 12.

[0113] In an optional step S5, a second temperature-dependent parameter of the detector is acquired, wherein a second compensation parameter of the temperature compensation unit is set differently from the first compensation parameter. Preferably, the second temperature-dependent parameter corresponds to the at least one temperature-dependent parameter acquired in step S3, for example, the photopeak level of the detector. According to one embodiment, the first and second compensation parameters are represented by the flow rate of a fluid in a cooling system. Accordingly, in step S5, the second flow rate (the second compensation parameter) is set differently from the first flow rate (the first compensation parameter) in step S3 in order to call up a different value for the second photopeak level (second temperature-dependent value).

[0114] In an optional step S6, the heating effect on the detector is determined as a function of the several imaging parameters of the reference imaging sequence, the second compensation parameter, and the second temperature-dependent parameter of the detector. According to one embodiment, the heating effect on the detector can be determined as a function of the second flow rate and the second photopeak level, as described above. However, the heating effect can also be determined as a function of the difference between the first and second photopeak levels, as well as the first and second flow rates. The difference between the first and second photopeak levels can quantify the temperature difference of the detector.This quantification of the temperature difference can be correlated with the thermal energy absorbed by the cooling fluid, which is obtained, for example, from the difference between the first and second flow rates and the difference between the first and second outlet temperatures of the cooling fluid as it exits the detector module 22. The absorbed thermal energy can be correlated with the change in at least one temperature-dependent parameter to determine the heating effect of the reference imaging sequence on the detector.

[0115] In an optional step S7, the heating effect of an imaging sequence on the detector is predicted using an intelligent algorithm. The intelligent algorithm is designed to predict the heating effect as a function of several imaging parameters of the imaging sequence and data acquired by the reference imaging sequence. According to one embodiment, the intelligent algorithm includes an expert system designed to predict the heating effect of the imaging sequence as a function of data obtained from a previously executed reference imaging sequence. The expert system can have access to an internal storage unit 27 and / or an external storage unit 28, wherein data relating to such reference imaging sequences are stored in a database.Preferably, the database contains data from previously executed imaging sequences and / or reference imaging sequences, for example, from the same combined imaging device 10 or from other combined imaging devices 10 of the same type. According to one embodiment, the expert system is designed to interpolate or extrapolate the heating effect as a function of a heating effect and several imaging parameters of a previously executed imaging sequence. However, the expert system can also include models of relevant components of the combined imaging device 10. These models can be designed to correlate individual imaging parameters of a reference imaging sequence with a corresponding heating effect.Furthermore, the models can be used to complement the heating effect obtained by interpolation or extrapolation, or to account for dependencies between the imaging parameters and the heating effect that cannot be represented by a regression analysis of available data.

[0116] In an optional step S8, the heating effect of an imaging sequence on the detector is predicted using a model-based approach. This model-based approach is designed to predict the heating effect as a function of several imaging parameters of the imaging sequence and data acquired by the reference imaging sequence. The model-based approach can employ an analytical, empirical, and / or physical model of relevant components of the combined imaging device 10. For example, an empirical model is derived that correlates the heating effect on the detector with a predetermined set of imaging parameters from a previously executed reference imaging sequence via an empirical function.The empirical function can then be used to predict the heating effect of the imaging sequence with a different set of imaging parameters than the previously executed reference imaging sequence. Preferably, the empirical model is derived from a calibration database containing data from one or more reference imaging sequences with their respective temperature-dependent parameters and / or heating effects. As described above, data relating to one or more reference imaging sequences can be stored in an internal storage unit 27 and / or an external storage unit 28, and can be accessed by the central processing unit 24 and / or the PET control unit 23 of the combined imaging device 10.

[0117] In step S9, a compensation parameter of the combined imaging system is corrected as a function of a correction parameter to reduce the influence of the heating effect on the diagnostic data. According to one embodiment, the correction parameter represents a temperature distribution of the detector module 22, which can be determined or predicted by the heating effect, as described above. According to another embodiment, the correction parameter corresponds to a variable designed to compensate for a delay and / or offset of the temperature sensor positioned near the detector when correlated with the temperature value of the temperature sensor or the compensation parameter.

[0118] The correction parameter can be used to modify an analog or digital signal from the temperature sensor 56 and / or the bias output by the high-voltage analog-to-digital converter 52. In particular, the correction parameter can be used to compensate for a delay and / or offset of the temperature sensor 56 positioned near the detector module 22.

[0119] For example, the correction parameter can be transmitted via signal port 60 to the logic unit 53 of the temperature compensation unit 59. Depending on the correction parameter and / or the temperature value detected by the temperature sensor 56, the logic unit 53 controls the high-voltage digital-to-analog converter 52 to output a bias voltage to the photodiode array 54 of the detector module 22 in order to compensate for a temperature change of the detector module 22.

[0120] Fig. Figure 6 shows a schematic flow diagram of a method according to the invention for compensating the heating effect of an imaging sequence of the magnetic resonance imaging device 11 on the detector of the positron emission tomography device 12.

[0121] In step S1a, the heating effect of an imaging sequence from the magnetic resonance imaging device 11 on the detector of the positron emission tomography device 12 is determined using a model-based approach. As described above, a model-based approach can use an analytical model, a physical model, and / or a numerical model of one or more components of the combined imaging device 10 to determine the heating effect. It is conceivable that the model-based approach could be implemented on the central processing unit 24 such that the central processing unit 24 is designed to determine the heating effect.The central processing unit 24 can access a current set of imaging parameters, which are entered, for example, by an operator via the input unit 26, and / or data relating to geometric dimensions and / or structural properties of components of the combined imaging device 10, which are stored in the internal storage unit 27 or the external storage unit 28. Preferably, when determining the heating effect, the temporal profile of the heating rate and / or the heating energy absorbed by the detector of the first modality is determined. According to one embodiment, the heating effect is determined in discrete time steps while the imaging sequence is being executed. It is conceivable that temperature-dependent parameters are acquired at the same time steps or at similarly spaced time intervals in order to refine and / or update the determination of the heating effect by the model-based approach.However, the heating effect can also be determined depending on a specific set of imaging parameters before an imaging sequence is performed that has the specific set of imaging parameters.

[0122] In step S2a, a compensation parameter of the temperature compensation unit is set depending on the heating effect. The heating effect can be provided to the logic unit 53 of the temperature compensation unit 59 via signal port 60 (see Fig.4) According to a preferred embodiment, the logic unit 53 of the temperature compensation unit 59 can further be configured to adjust the compensation parameter in discrete time steps as a function of a temperature value provided by the temperature sensor 56 and the heating effect while the imaging sequence is being executed. In particular, the temperature compensation unit 59 can have a control loop configured to adjust a compensation parameter as a function of the heating effect.

[0123] It should be noted that the embodiments described above are to be regarded as examples. Individual embodiments can be extended by features of other embodiments. In particular, the sequence of steps of the methods according to the invention is to be understood as exemplary. The individual steps can be carried out in a different order and / or partially or completely overlap in time.

Claims

[1] Method for predicting a heating effect of an imaging sequence of a second imaging modality on a detector of a first modality (12) of a combined imaging device (10) as a function of a reference imaging sequence of the second imaging modality, wherein the first modality (12) is designed to acquire diagnostic data of an object of investigation (15) and the second imaging modality comprises a magnetic resonance imaging device (11) designed to acquire magnetic resonance image data of the object of investigation (15), wherein the first modality (12) comprises a temperature compensation unit (59) designed to compensate for a temperature change of the detector, comprising the following steps: • Performing (S2) a reference imaging sequence with the second imaging modality, wherein in the reference imaging sequence a gradient magnetic field and / or a high-frequency electromagnetic field is applied to the image acquisition area of ​​the combined imaging device (10) and wherein the reference imaging sequence has several imaging parameters that determine a property of the gradient magnetic field and / or the high-frequency electromagnetic field, • Detection (S3) of at least one temperature-dependent parameter of the detector of the first modality (12), • Determining (S4) the heating effect on the detector of the first modality (12) as a function of several imaging parameters of the reference imaging sequence and the at least one temperature-dependent parameter of the detector, and • Prediction (S7) of the heating effect of the imaging sequence on the detector using a smart algorithm, wherein the smart algorithm is designed to predict the heating effect depending on several imaging parameters of the imaging sequence and data acquired by the reference imaging sequence. [2] The method of claim 1, which further comprises the following step: • Positioning (S1) a radiation source in a relative position to the image acquisition area of ​​the combined imaging device (10), wherein the radiation source is designed to emit a predefined radiation level in the direction of the detector of the first modality (12). [3] Method according to one of claims 1 or 2, wherein the multiple imaging parameters of the reference imaging sequence comprise a strength, a frequency, a rise rate and / or a duty cycle of the gradient magnetic field and / or the high-frequency electromagnetic field. [4] Method according to any of the preceding claims, wherein the at least one temperature-dependent parameter of the detector is a temperature value, a photopeak level, a current consumption, a bias current or a signal noise of the detector. [5] Method according to one of the preceding claims, wherein the first compensation parameter of the temperature compensation unit (59) is kept constant while the at least one temperature-dependent parameter is recorded. [6] Method according to any one of claims 1 to 4, wherein the first compensation parameter is set to compensate for a temperature change of the detector while the at least one temperature-dependent parameter of the detector is being detected. [7] The method of claim 5, which further comprises the following steps: • Acquiring (S5) a second temperature-dependent parameter of the detector, wherein a second compensation parameter of the temperature compensation unit (59) is set differently from the first compensation parameter, • Determine (S6) the heating effect on the detector as a function of the several imaging parameters of the reference imaging sequence, the second compensation parameter and the second temperature-dependent parameter of the detector. [8] Method according to any one of claims 1 to 7, which further comprises the following step: • Predictions (S8) of the heating effect of an imaging sequence on the detector using a model-based approach, wherein the model-based approach is designed to predict the heating effect as a function of several imaging parameters of the imaging sequence and data acquired by the reference imaging sequence. [9] Method according to any one of claims 1 to 8, which further comprises the following step: • Correcting (S9) a compensation parameter of the temperature compensation unit (59) depending on a correction parameter to reduce the influence of the heating effect on the diagnostic data. [10] Method for compensating a heating effect of an imaging sequence of a second imaging modality on a detector of a first modality (12) of a combined imaging device (10), wherein the first modality (12) is designed to acquire diagnostic data of an object of investigation (15) and the second imaging modality comprises a magnetic resonance imaging device (11) designed to acquire magnetic resonance image data of the object of investigation (15), wherein the first modality (12) comprises a temperature compensation unit (59) designed to compensate for a temperature change of the detector, comprising the following steps: • Determine (S1a) the heating effect of the imaging sequence of the second imaging modality on the detector of the first modality (12) using a model-based approach and • Setting (S2a) a compensation parameter of the temperature compensation unit (59) depending on the heating effect. [11] Combined imaging device (10) comprising a magnetic resonance imaging device (11) and a first modality (12) with a detector and a temperature compensation unit (59) designed to compensate for a temperature change of the detector, wherein the combined imaging device (10) is designed to acquire diagnostic data and magnetic resonance image data of an object of investigation (15) arranged in an image acquisition area of ​​the combined imaging device (10), and wherein the combined imaging device (10) is designed to perform a method according to any of the preceding claims. [12] Combined imaging device (10) according to claim 11, wherein the temperature compensation unit (59) is designed to adjust a compensation parameter depending on a heating effect and / or a correction parameter. [13] Computer program product that can be loaded into a memory of a programmable processing unit (24) of a combined imaging device (10), comprising program code means for executing a method according to any one of claims 1 to 10 when the computer program product is executed in the processing unit (24) of the combined imaging device (10).

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