Determining trigger signal for imaging

Abstract

The invention provides a system (100) and a method for determining a trigger signal for imaging with an imaging system (40). The method comprises the steps of: measuring (S10) a patient-specific characteristic of a patient (30), the patient-specific characteristic being measured when the patient (30) is positioned in a first position (51) relative to the imaging system (40); the patient-specific model is generated (S20) based on the patient-specific characteristic, and a trigger waveform of the patient (30) is measured (S30). The trigger waveform is measured when the patient (30) is positioned at a second position (52) relative to the imaging system (40). The second position (52) is different from the first position (51); the trigger signal is determined (S40) based on the patient-specific model and the trigger waveform. The trigger signal is configured to trigger the imaging system (40) to acquire an image of a patient (30) in a time-resolved manner.

Description

Technical Field

[0001] The present invention relates to a system and method for determining a trigger signal for imaging using an imaging system. Background Art

[0002] Electrocardiogram (ECG) and contact photoplethysmography (PPG), such as measured with a finger oximeter, have been used in standard magnetic resonance imaging (MRI) and computed tomography (CT) systems for cardiac triggering or gating of scans.

[0003] MRI usually uses triggering based on ECG measurement to perform scans that need to be synchronized with the heart beat and / or heart phase. The ECG measurement results have a so-called R peak, which indicates the electrical activity that starts the myocardial contraction, that is, the beginning of the systolic period. Based on the R peak, an MRI trigger sequence including a preparation pulse (pre-pulse) and multiple acquisition windows is initiated. Such ECG-based triggering may suffer from disadvantages such as complex electrode placement and multiple wires, which can limit the patient's comfort and disturb the imaging quality due to magnetic field interference.

[0004] Therefore, the feasibility of cardiac-triggered MRI using camera-based PPG of the human face has emerged. Fully automated or semi-automated non-contact camera-PPG solutions can significantly simplify clinical MRI workflows, as no contact with the sensor is required and interference of magnetic fields on the signal can be eliminated.

[0005] However, one possible limitation of camera-PPG based triggering compared to ECG based triggering is that there is a patient specific delay between the R peak commonly used for cardiac triggering and the trigger from the PPG signal. This delay is caused by the pulse transit time from the heart to the surrounding skin site and can lead to inaccurate triggering unless compensated for.

[0006] Publication US2017055934A1 discloses a method and system for determining a trigger signal. The method for determining a trigger signal for an imaging device is based on receiving a film of the surface of a first body part of a patient. In one embodiment, image values ​​in a region in an image in the film are averaged. The noise of the image values ​​in the region is reduced by averaging. On the basis of averaging, the film is transformed into a time signal series. The transformation occurs in such a way that the time series is a measure of the time pattern of blood circulation in the region.

[0007] Camera-based PPG requires a certain amount of processing to achieve robust trigger detection. This can result in costly delays in the workflow before the actual triggered scan can begin.

[0008] Therefore, there is a need to improve the speed and accuracy of triggered imaging. Summary of the invention

[0009] An object of the present invention is to provide a method and system for improving the speed and accuracy of triggered imaging.

[0010] The invention is defined by the independent claim. Advantageous embodiments are defined in the dependent claims.

[0011] According to a first aspect of the present invention, there is provided a method for determining a trigger signal for performing imaging using an imaging system. The method comprises the following steps:

[0012] measuring a patient-specific characteristic of a patient while the patient is positioned at a first position relative to an imaging system;

[0013] generating a patient-specific model based on the patient-specific characteristics;

[0014] Measuring a trigger waveform of the patient. Measuring a trigger waveform when the patient is positioned at a second position relative to the imaging system. The second position is different from the first position;

[0015] A trigger signal is determined based on the patient-specific model and the trigger waveform. The trigger signal is configured to trigger the imaging system to acquire images of the patient in a time-resolved manner.

[0016] The patient-specific characteristic is a characteristic of a measurement related to a physiological or physical characteristic of the patient, such as, but not limited to, heart rate, respiration, oxygenation, blood pressure, cardiac cycle, temperature, weight, height, body mass ratio, etc. The patient-specific characteristic may be a constant, one or more measurement points, amplitude, frequency, phase, shape, signal-to-noise ratio, etc. of a physiological measurement. This characteristic or combination of characteristics may be used to model patient-specific parameters in the patient-specific model. Similarly, the trigger waveform is a time-resolved measurement of a patient's physiological parameter, such as, but not limited to, heart rate, respiration, oxygenation, cardiac cycle, temperature, etc. The trigger signal directly or indirectly triggers the acquisition of an image of the patient using an imaging system at a certain moment.

[0017] The proposed method may be advantageous for the speed and accuracy of the imaging triggering workflow, since one or more patient-specific characteristics may be measured and the generation of a patient-specific model based on these characteristics may be initiated already before the patient is located at a second position where the triggering waveform is measured. The second position may also be the position of the patient when the imaging system is triggered to acquire images of the patient. In this way, the combination of the patient-specific characteristic information and the measured triggering waveform may form the basis for an accurate imaging triggering without causing unnecessary delays to the workflow.

[0018] In an embodiment of the invention, in the first position the patient anatomy of interest is substantially outside the bore of the imaging system, and in the second position said patient anatomy of interest is substantially inside the bore of the imaging system.

[0019] The patient anatomy of interest is a portion of a patient where a trigger and / or body part to be imaged is determined by the imaging system. By measuring patient-specific characteristics while the patient anatomy of interest is outside of the bore of the imaging system (e.g., during a patient preparation phase), the patient time in the bore can be shortened. In this way, the overall workflow time can be shortened and patient comfort can be improved. The bore of the imaging system can be, for example, the bore of a computed tomography (CT) system or a magnetic resonance imaging (MRI) system or a molecular imaging system such as positron emission tomography (PET) or single photon emission computed tomography (SPECT), or any combination of these systems.

[0020] In an embodiment of the invention, the triggering waveform comprises a waveform from a photoplethysmography (PPG), and the step of measuring the triggering waveform comprises measuring with a sensor, preferably a camera.

[0021] Using, for example, a camera-based PPG to measure trigger waveforms rather than using multiple electrocardiogram (ECG) sensors in contact with the patient can have various advantages, such as automation or simplification of workflow, avoiding magnetic field interference with ECG signals, etc.

[0022] The sensor is preferably, for example, a 2D or 3D camera sensor or the like, configured to non-contactly measure PPG parameters from visible light or infrared light or other wavelengths of light. However, other sensor technologies may also be considered to measure waveforms derived from PPG, such as a transmissive or reflective PPG sensor close to or in contact with the patient, contactless measurement using RADAR or LIDAR, etc.

[0023] In an embodiment of the invention, said patient-specific characteristic comprises a characteristic derived from a PPG, and the step of measuring the patient-specific characteristic comprises measuring with a sensor, preferably a camera.

[0024] Similar to trigger waveforms, measuring patient-specific characteristic parameters using, for example, camera-based PPG (alone or in combination with other measurements) can speed up and simplify imaging and / or patient preparation workflows.

[0025] The sensor used to measure patient-specific characteristics (including characteristics derived from PPG) can be the same sensor as the sensor used to measure the triggering waveform. The sensor can also be a separate sensor of the same, similar, or different type or function as the sensor used to measure the triggering waveform. For example, both sensors can be sensors configured to measure camera-based PPG signals.

[0026] In an embodiment of the invention, the patient-specific model comprises a patient-specific PPG waveform profile.

[0027] Modeling a patient-specific model including PPG waveform profiles (e.g., template morphology and / or amplitude of the waveform) facilitates fast and accurate imaging triggering. This may be particularly important when the trigger waveform includes a waveform derived from PPG.

[0028] In an embodiment of the invention, the patient-specific model includes a patient-specific pulse transit time.

[0029] Similar to patient-specific waveform profiles, information about patient-specific pulse transit times may facilitate fast and accurate imaging triggering.

[0030] In an embodiment of the invention, the patient-specific model comprises a patient-specific reliability factor based on the PPG parameters of the camera.

[0031] The patient-specific reliability factor may provide an indication of the reliability, quality and / or robustness of the camera-based PPG for a particular patient and / or situation. For example, if the reliability is low, additional or alternative measurements of the camera-based PPG may be required to allow for accurate triggering based on the trigger waveform. Obtaining such information at an early stage may save bore time and / or avoid the need to repeat image acquisitions.

[0032] In an embodiment of the present invention, the step of determining a trigger signal comprises calculating a plurality of PPG markers, wherein the PPG markers are derived from the trigger waveform and / or the patient-specific PPG waveform profile, wherein the PPG markers are configured to predict an R-peak.

[0033] PPG markers are specific PPG features of a PPG waveform, such as beat intervals, waveform features, phase, etc. Computing multiple PPG markers may be advantageous because PPG markers may improve the accuracy and / or robustness in predicting R-peaks from, for example, a camera-based PPG.

[0034] In an embodiment of the present invention, the PPG markers described above include a diastolic PPG marker and a rapid flow PPG marker.

[0035] In an embodiment of the present invention, the method further comprises the following steps:

[0036] deriving timing parameters based on trigger waveforms and / or patient-specific models, and

[0037] The patient is scanned triggered by the trigger signal, wherein the scanning is configured to generate imaging scan data.

[0038] In an embodiment of the present invention, the method further comprises an image forming step, wherein the image forming step forms an image of the patient based on processing the imaging scan data and the timing parameters.

[0039] This may be advantageous because using timing parameters from the trigger waveform and / or patient-specific model may help improve the quality and / or speed of image formation.

[0040] According to another aspect of the invention, a computer program element is provided, which, when run by at least one processing unit, is adapted to cause the processing unit to perform for the processing unit any computer executable method steps according to any of the methods described above.

[0041] According to another aspect of the present invention, a computer readable medium having stored thereon a computer program element as described above is provided.

[0042] According to a second aspect of the present invention, there is provided a system for determining a trigger signal for imaging a patient using an imaging system, the system comprising:

[0043] a patient-specific property sensor configured to measure a patient-specific property of a patient when the patient is positioned in a first position relative to the imaging system,

[0044] a trigger waveform sensor configured to measure a trigger waveform of the patient when the patient is positioned at a second position relative to the imaging system, and

[0045] A processing unit is configured to receive the patient-specific characteristic, generate a patient-specific model based on the patient-specific characteristic, receive a trigger waveform, and determine an imaging trigger signal based on the patient-specific model and the trigger waveform.

[0046] In an embodiment of the invention, at least one of the patient-specific characteristic sensor and / or the trigger waveform sensor is a sensor, preferably a camera, and the sensor is configured to measure a sensor-based PPG signal.

[0047] According to another aspect of the invention, an imaging system is provided, comprising the system for determining a trigger signal, wherein the imaging system is preferably a computed tomography system or a magnetic resonance imaging system.

[0048] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 A schematic diagram of a system for determining a trigger signal according to an embodiment of the present invention is illustrated.

[0050] Figure 2 A schematic diagram of measured and processed signals according to an embodiment of the present invention is illustrated.

[0051] Figure 3 Palm and face camera-PPG signals for determining a patient-specific model according to an embodiment of the present invention are shown.

[0052] Figure 4 The determination of a plurality of PPG markers according to an embodiment of the present invention is illustrated.

[0053] Figure 5 A flow chart of a method for determining a trigger signal according to an embodiment of the present invention is shown.

[0054] Figure 6 A flow chart of a method for determining a trigger signal for starting scanning according to an embodiment of the present invention is shown.

[0055] Figure 7 A flowchart representing an image forming method according to an embodiment of the present invention is shown.

[0056] Reference numerals

[0057] 100System for determining a trigger signal

[0058] 10 Patient-specific characteristic sensors

[0059] 20 Trigger Waveform Sensor

[0060] 30 patients

[0061] 40 Imaging System

[0062] 51 First Position

[0063] 52 Second position

[0064] S10 measures patient-specific characteristics

[0065] S20 generates patient-specific models

[0066] S30 measurement trigger waveform

[0067] S35 Export timing parameters

[0068] S40 confirms the trigger signal

[0069] S50 scans the patient

[0070] S60 Image Formation Process DETAILED DESCRIPTION

[0071] Figure 1 A schematic diagram of a system 100 for determining a trigger signal according to an embodiment of the present invention is illustrated in FIG. A patient 30 is positioned on a table of an imaging system 40. In this example, it is proposed to use an exo-bore camera sensor 10 to measure patient-specific characteristics, which can be used to model patient-specific physiology during a preparation phase, and then the patient is brought into the bore for a triggered scan using an ino-bore camera sensor 20.

[0072] Many imaging system environments already provide ceiling mounted cameras, allowing monitoring of the patient during the preparation phase before the table is moved into the chamber. Such camera sensors can therefore be further used for remote PPG analysis etc. The patient specific characteristic sensor 10 and the trigger waveform sensor 20 are connected to a processing unit (not shown) via a wired or wireless connection.

[0073] exist Figure 1 In the example illustrated in FIG. 4 , the two sensors are independent camera sensors positioned at different locations. However, it is also possible that the patient-specific characteristic sensor 10 and the trigger waveform sensor 20 are the same sensor and / or are positioned at the same location. Such as, for example, a camera-based sensor having a field of view both inside and outside the bore of the imaging system 40.

[0074] exist Figure 1 In a, the patient 30 is located in a first position 51 during the preparation phase. In the first position 51, the patient anatomy of interest (e.g., chest or head or other body part) is substantially outside the bore of the imaging system 40. It is worth noting that although Figure 1 a illustrates a patient on a table in conjunction with an imaging system, but it is also contemplated that the first position 51 is further away from the imaging system, such as in another room for preparation.

[0075] exist Figure 1During the stage illustrated in a, when the patient is in the first position 51, patient-specific characteristics are measured using the camera sensor 10. The measurements may be combined with measurements made by additional sensors that are remote or in contact with the patient. Examples of such measured characteristics include, but are not limited to, characteristics derived from PPG, such as pulse waveform, heart rate, respiratory rate, blood pressure, relationship between pulsation characteristics and ECG R peaks, PPG peak detection rate, etc. Based on one or more patient-specific characteristics, a patient-specific model may be created in the processing unit. Non-limiting examples of such patient-specific models that may be created based on patient-specific characteristics measured using the camera sensor 10 include patient-specific PPG waveform profiles, patient-specific characteristic pulse transmission times, or patient-specific reliability factors based on camera-based PPG parameters.

[0076] exist Figure 1 In FIG. 5 , the patient 30 is positioned in a second position 52 during a scanning phase. In the second position 52, the patient anatomy of interest is substantially within the bore of the imaging system 40. In the second position 52, a trigger waveform is measured using the camera sensor 20, in this example an in-bore camera. The trigger waveform may be derived from a camera-based PPG measurement using the camera sensor 20. The processing unit receives the trigger waveform and determines an imaging trigger signal. The trigger signal is configured to trigger the imaging system 40 to acquire images of the patient 30 in a time-resolved manner.

[0077] It can be expected that Figure 1 The system shown in has a number of potential advantages, wherein patient-specific characteristics may be measured during the patient preparation phase and / or at least before the patient 30 is moved further into the bore of the imaging system 40. Examples include;

[0078] By understanding the patient-specific PPG model (e.g., PPG waveform morphology, PPG amplitude, pulse transit time delay, etc.), accurate and fast timing of camera-based MRI and CT acquisition windows, etc., can be simplified.

[0079] Images of MRI cardiac cine are usually annotated with timestamps relative to the R-peak, which is defined as time zero. This is possible using a priori knowledge of the delays.

[0080] MRI cardiac movies are usually presented for diagnosis and comparison starting at the time of the R-peak. This is possible with a priori knowledge of the delay.

[0081] Using knowledge of the timing relative to the R-peak, functional assessments of cardiac images, such as ejection fraction and wall motion, can be simplified and / or made more accurate.

[0082] Triggering with low latency between the R-peak and the trigger requires a patient-specific PPG template and therefore some initial calibration time to initialize the model. Expensive in-bore time can be saved when the processing unit can do such calibration in advance and not rely on in-bore measurements.

[0083] When the camera-based PPG signal quality can be assessed in advance, patient-specific reliability factors for the camera-based PPG parameters are calculated, which can have a significant positive impact on the workflow time. For example, in the example of triggering an MRI, if the patient is already in the bore and the in-bore camera 20 detects that the patient-specific PPG signal quality is insufficient, the workflow will be significantly extended. In this case, the patient 30 may have to be taken out again, the MR coil removed, the ECG electrodes applied, the MR coil applied again, and then the patient 30 must be put back into the bore.

[0084] exist Figure 1 In the example illustrated in FIG, an exo-camera 10 is positioned above (or to the side of) a patient 30 positioned on a tabletop and measures a physiological signal from the patient's skin (e.g., chest or facial area). The physiological signal may be a PPG signal, but it may also include other physiological signals. In some cases, an ECG sensor is also used. The ECG signal measured using the ECG sensor may be measured, for example, synchronously with the camera-PPG signal.

[0085] Current ECG detection for MRI triggering is typically based on at least four electrodes on the bare chest. Typically at least four electrodes are required for robust R-peak detection in the bore, because the ECG signal here may be corrupted by interference from the MR field. In order to measure the ECG synchronously with PPG outside the bore to measure patient-specific characteristics and initialize the calculation of a patient-specific model, only two ECG electrodes may be applied. The electrodes may be applied to the wrist or hand of the patient 30. Such an ECG setup can greatly simplify the workflow. In addition, in the case where the electrodes are removed before the patient enters the bore, the ECG electrodes, leads, and detection circuits do not have to be compatible with MR because the trigger waveform in the bore is measured using a camera-PPG or the like. It is conceivable that in some cases, the ECG electrodes may be just simple handles or conductive fields on both sides of the patient's support, and the patient can hold them for a few seconds as long as data is needed to calibrate the model.

[0086] Figure 2 An example of a relationship between a patient-specific characteristic measured when the patient 30 is in a first position 51 , a patient-specific model, and a trigger signal measured when the patient 30 is in a second position 52 is illustrated.

[0087] Figure 2a schematically shows patient-specific characteristics measured when the patient 30 is in the first position 51. In this example, the PPG-derived signal is measured using a patient-specific characteristic sensor 10 (e.g., a camera PPG sensor). In addition, an ECG sensor is used to measure an electrocardiogram signal. The synchronized camera-PPG-derived signal and the ECG signal can be used to calculate a patient-specific model, such as Figure 2 b and Figure 2 As shown in c.

[0088] Figure 2 b illustrates the created PPG waveform profile, which can be used as a patient-specific PPG template to achieve fast triggering. The PPG waveform profile is a robust PPG cardiac cycle averaged from multiple PPG cycles, e.g. Figure 2 As the model is based on patient-specific characteristics measured when the patient is in the first position 51 , the model can be initialized before the patient is moved further into the bore of the imaging system 40 .

[0089] Similarly, Figure 2 c shows the R-peak regression model created, which models the relationship between the camera-PPG features and the measured ECG R-peak. The input to the model comes from the patient-specific characteristics measured at the first location 51. The model can relate specific characteristic PPG features (e.g., heartbeat intervals, waveform characteristics, phase) (so-called PPG markers) to the pulse transit time between the R-peak and the systolic trough of the camera-PPG signal. Therefore, such a regression model can be used to predict / regress the R-peak position from the camera-based PPG waveform.

[0090] Figure 2 d illustrates the trigger waveform measured when the patient 30 moves further into the bore of the imaging system 40. In this example, the trigger waveform is derived solely from the camera-based PPG measurements when the patient is in the second position 52. Thus, an ECG internal to the imaging system is not required to measure the trigger waveform.

[0091] As in Figure 2 As shown in e, the PPG waveform profile created previously ( Figure 2 b) to quickly identify the PPG period in the trigger waveform, which can be used for trigger detection. Since the construction of the model is initialized early in the workflow, fast triggering can be enabled.

[0092] Similarly, as in Figure 2 f shows the pre-built R peak regression model ( Figure 2 c) Can be used to achieve accurate triggering by predicting the ECG R-peak from the camera-based trigger waveform.

[0093] A trigger signal is generated based on the identified cardiac cycle and R-peak, thereby triggering the imaging system to acquire images of the patient.

[0094] exist Figure 2 In the example illustrated in , the patient-specific characteristics used to model the patient-specific pulse transit time are measured during the patient preparation phase by a camera-based PPG and ECG combination. Alternatively, the pulse transit time and related parameters can also be modeled by PPG measurements (e.g., multi-point PPG imaging) without ECG.

[0095] Figure 3 The diagram shows multi-point PPG imaging, where PPG signals are collected simultaneously from the patient's face and palm using a ceiling camera, which can be a sensor of patient-specific characteristics 10. Information about the simultaneous signals between the face and palm can be used to estimate alternative transit times. Such alternative transit times can then be used to improve the accuracy of the PPG signal R peak regression.

[0096] Additional parameters such as patient height, age, body mass ratio, etc. can be used to further improve the model.

[0097] In order to improve the robustness of detecting PPG markers from the trigger waveform, multiple PPG markers can be generated in conjunction with the patient's specific PPG waveform profile. In this way, one marker can predict the next marker and thereby improve the robustness of the model.

[0098] In addition to creating a single patient-specific PPG waveform profile, multiple profiles associated with a specific cardiac cycle duration or a range of cardiac cycle durations can be created. The creation of different profiles based on a range of cycle durations is possible due to the fact that the temporal curve of the cardiac beat and related physiological quantities (such as arterial blood flow velocity and PPG signal) do not simply scale linearly if the heart rate changes. Instead, some parts of the cardiac cycle are prolonged in time (usually the resting period of diastole), while some parts remain more or less unchanged (such as the systole).

[0099] In each waveform profile, one or more PPG markers can be defined. Of particular importance are the zero crossings in the signal. Negative-going zero crossings are associated with the highest blood volume changes at the observation point, e.g. the forehead when using camera-PPG in response to the contraction of the heart. Positive-going zero crossings are associated with diastole and usually precede the ECG R peak. Multiple markers can be generated from a single waveform profile, making it possible to track the current phase of the PPG signal in the cardiac cycle.

[0100] The detection of PPG markers can be performed, for example, as follows. The waveform profile can be shifted so that it starts and stops at a zero crossing. In this way, two waveform profiles are created, each with a complete cardiac waveform: one starting and ending at a positive zero crossing, and the other starting and ending at a negative zero crossing.

[0101] Both waveforms are shown in Figure 4 a, where the PPG waveform profile is stretched or time-warped to cover 33 samples. The purpose is to check whether the latest received sample of the incoming PPG waveform corresponds to the last sample in these prototypes. If so, a corresponding marker is generated.

[0102] A patient-specific model can be used to decompose the patient-specific PPG signal. The model describes the waveform as consisting of three parts: a PPG waveform profile, a low-frequency (LF) signal component, and a noise component. The LF signal component can be modeled as a low-order polynomial. More formally, let the discrete-time PPG signal segment be denoted as s(n), n = [1,…,N], the prototype be denoted as p(n), and the LF signal component be denoted as b

[0103] (n), the signal is described as:

[0104] s=αp+b+e,

[0105] where e(n) is an error signal describing noise (e.g. measurement noise) and interference, and α is a scaling factor. The signal component b captures signal drifts in the PPG and PPG variations that are related to breathing (and therefore have a frequency lower than the heart rate). There are many known ways to achieve such a decomposition, for example, a least squares fitting procedure can be used to obtain the decomposition, where windows can be used to emphasize the importance of certain parts of the segment in the fit.

[0106] The strength of the decomposed signal may be analyzed, for example in terms of energy. If the energy associated with the PPG component p is significantly greater than the energy associated with e, then a good fit is achieved. Further evidence in favor of a marker being generated is that the coefficient α is positive. Further evidence is that when considering the next segment consisting of a simple shifted input signal, the coefficient α becomes lower, or that the balance between the energy in (αp) and e becomes less favorable. By these or other means, a decision may be made that a marker has occurred.

[0107] Since the length of the current cardiac cycle is unknown, the system will run with different lengths. Therefore, the detection has an extra dimension, that along the scale (cardiac cycle length). The various clues that a marker is present must also be observed through this extra dimension. The hard decision can be made based on thresholding, a more statistical approach can be used, or a neural network can be trained to create the final decision mechanism.

[0108] Figure 4 b shows an example of multiple PPG markers generated to improve R-peak prediction. More importantly, the prediction can be improved by selecting at least one marker that is located before but close to the R-peak and at least one second marker that indicates that it has occurred, for example, by observing the blood volume changes caused by the induced cardiac contraction. In this case, the PPG markers are generated during diastole and at or near the moment when the blood volume change is the largest (i.e., the flush phase). Diastolic PPG markers ( Figure 4 The asterisk in b) usually appears before the R peak, while the PPG marker in the rapid period ( Figure 4 The circle in b) usually appears after the R peak. Therefore, the combination of the two can improve the timely prediction ability of the R peak.

[0109] In the above description, several embodiments of the present invention have been described in detail. Figure 5 Also illustrated is one aspect of the present invention, showing a flow chart representing a method for determining a trigger signal. The method comprises the following steps:

[0110] measuring S10 a patient-specific characteristic of a patient 30, said patient-specific characteristic being measured when said patient is positioned in a first position relative to an imaging system;

[0111] generating S20 the patient-specific model based on the patient-specific characteristics,

[0112] Measuring S30 the trigger waveform of the patient. When the patient is positioned at a second position relative to the imaging system, measuring the trigger waveform. The second position is different from the first position;

[0113] The trigger signal is determined S40 based on the patient-specific model and the trigger waveform. The trigger signal is configured to trigger the imaging system to acquire images of the patient in a time-resolved manner.

[0114] Similarly, Figure 6 A flow chart of a method for determining a trigger signal for starting a scan according to an embodiment of the present invention is shown. In this case, the method includes Figure 5 In addition to the method shown, the following steps are also included: deriving S35 timing parameters from a trigger waveform and / or a patient-specific model, and triggering S50 scanning the patient by the trigger signal, wherein the scanning S50 is configured to generate imaging scan data.

[0115] In addition to enabling more robust triggering from the measured trigger waveform, the patient-specific model (e.g. including PPG markers) may contain timing parameters that can be further exploited in the imaging workflow. In the image formation process S60, where the scan data are combined into images or video sequences, these timing parameters can be used as additional information to control the image formation process. For example, by weighting or as compensation factors for data acquired at different data acquisition windows.

[0116] For example, in MRI, knowledge of temporal parameters, such as the distance between pre-pulse generation and data acquisition time, can be used to improve image formation. Alternatively, the trigger mechanism can be configured to maintain a stricter requirement for a constant pre-pulse to acquisition window distance. In this case, information about the occurrence of each MR sequence event, such as the motion state of the myocardium, can be used in the image formation process.

[0117] Figure 7 An embodiment of the present invention is illustrated, wherein Figure 6 The illustrated method further comprises an image forming step S60, wherein the image forming step S60 forms an image of the patient based on processing the imaging scan data and the timing parameters.

[0118] It should be noted that the embodiments mentioned above are examples rather than limitations of the invention, and that a person skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference numerals placed in brackets shall not be construed as limiting the claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in the claims. The word "one" or "an" before an element does not exclude the presence of a plurality of such elements. The present invention may be implemented by means of hardware comprising several different elements, and by means of a suitably programmed processor. In a device-type claim enumerating several devices, several of these devices may be implemented by the same item of hardware. The measures recited in mutually different dependent claims may be advantageously used in combination.

Claims

1. A method for determining a trigger signal for imaging using an imaging system (40), the method comprising the following steps: measuring (S10) a patient-specific characteristic of a patient (30), wherein the patient-specific characteristic is measured when the patient (30) is positioned in a first position (51) relative to the imaging system (40); generating (S20) a patient-specific model based on the patient-specific characteristics, measuring (S30) a triggering waveform of the patient (30), wherein the triggering waveform is measured when the patient is positioned at a second position (52) relative to the imaging system (40), wherein the second position (52) is different from the first position (51), and The trigger signal is determined (S40) based on the patient-specific model and the trigger waveform, wherein the trigger signal is configured to trigger the imaging system (40) to acquire images of the patient (30) in a time-resolved manner.

2. The method according to claim 1, wherein: In the first position (51), the patient anatomy of interest is positioned substantially outside the bore of the imaging system (40), and wherein, in the second position (52), the patient anatomy of interest is positioned substantially inside the bore of the imaging system (40).

3. The method according to claim 1 or 2, wherein: The triggering waveform comprises a waveform derived from photoplethysmography (PPG), and wherein the step of measuring the triggering waveform comprises measuring with a sensor, preferably a camera.

4. A method according to any one of the preceding claims, wherein: The patient-specific characteristic comprises a characteristic derived from a PPG, and wherein the step of measuring the patient-specific characteristic comprises measuring with a sensor, preferably a camera.

5. A method according to any one of the preceding claims, wherein: The patient-specific model includes a patient-specific PPG waveform profile.

6. A method according to any one of the preceding claims, wherein: The patient-specific model includes patient-specific pulse transit times.

7. A method according to any one of the preceding claims, wherein: The patient-specific model includes patient-specific reliability factors based on camera-based PPG parameters.

8. The method according to claim 5, wherein: The step of determining the trigger signal comprises calculating a plurality of PPG markers, wherein the PPG markers are derived from the trigger waveform and / or the patient-specific PPG waveform profile, and wherein the PPG markers are configured to predict an R-peak.

9. The method according to claim 8, wherein: The PPG markers include a diastolic PPG marker and a rapid flow PPG marker.

10. The method according to any one of the preceding claims, further comprising the steps of: deriving (S35) a time parameter based on the trigger waveform and / or the patient-specific model, and scanning the patient (S50), which is triggered by the trigger signal, wherein the scanning is configured to generate imaging scan data, and Image formation (S60), wherein an image of the patient (30) is formed based on processing the imaging scan data and the timing parameters.

11. A computer program element adapted, when run by at least one processing unit, to cause the processing unit to perform any computer executable method steps according to any of the preceding claims for the processing unit.

12. A computer-readable medium having a program element according to claim 11 stored thereon.

13. A system (100) for determining a trigger signal for imaging a patient (30) using an imaging system (40), the system (100) comprising: a patient-specific property sensor (10) configured to measure a patient-specific property of the patient (30) when the patient (30) is positioned in a first position (51) relative to the imaging system (40), a trigger waveform sensor (20) configured to measure a trigger waveform of the patient (30) when the patient (30) is positioned at a second position (52) relative to the imaging system (40), and A processing unit is configured to receive the patient-specific characteristic, generate a patient-specific model based on the patient-specific characteristic, receive the trigger waveform, and determine an imaging trigger signal based on the patient-specific model and the trigger waveform.

14. An imaging system (40), comprising the system (100) for determining a trigger signal according to claim 13, wherein: The imaging system (40) is preferably a computed tomography system or a magnetic resonance imaging system.

Images