Methods for motion compensation during magnetic resonance imaging

Abstract

Method for motion compensation during magnetic resonance imaging of an object under investigation using a magnetic resonance device, wherein a magnetic resonance sequence is used for the magnetic resonance imaging, comprising the following method steps: - Performing a reference measurement at a first time point during magnetic resonance imaging, where a navigator reference volume is acquired using a simultaneous multi-slice measurement technique with a first acceleration factor and a first number of first slice groups, - Performing a navigator measurement at a second time point during magnetic resonance imaging, wherein a navigator volume is acquired using a simultaneous multi-slice measurement technique with a second acceleration factor and a second number of second slice groups, wherein the second acceleration factor is equal to the first acceleration factor, and wherein the navigator measurement is inserted into the magnetic resonance sequence used for magnetic resonance imaging. - Determining motion information from the navigator reference volume and the navigator volume, where the motion information describes a movement of the object under investigation between the first time point and the second time point, and - Setting recording parameters, which are set after the second time point for magnetic resonance imaging, depending on the motion information.

Description

[0001] The invention relates to a method for motion compensation during magnetic resonance imaging, a magnetic resonance device and a computer program product.

[0002] In a magnetic resonance imaging (MRI) scanner, the body of the subject, especially a patient, is typically exposed to a relatively strong main magnetic field, for example, 1.5, 3, or 7 Tesla, using a main magnet. Additionally, gradient circuits are generated using a gradient coil unit. High-frequency pulses, such as excitation pulses, are then emitted via a radio frequency antenna unit using suitable antenna elements. This causes the nuclear spins of certain atoms, resonantly excited by these radio frequency pulses, to be tilted by a defined angle relative to the magnetic field lines of the main magnetic field. During the relaxation of the nuclear spins, radio frequency signals, known as magnetic resonance signals, are emitted. These signals are received by suitable radio frequency antennas and then further processed.The desired image data can then be reconstructed from the raw data acquired in this way.

[0003] For a specific measurement, a particular magnetic resonance sequence, also called a pulse sequence, must be transmitted. This sequence consists of a series of high-frequency pulses, such as excitation pulses and refocusing pulses, along with correspondingly coordinated gradient circuits transmitted along different gradient axes in various spatial directions. Readout windows are set at the appropriate time intervals to define the periods during which the induced magnetic resonance signals are acquired.

[0004] During magnetic resonance imaging (MRI), the subject may move. For example, the subject may breathe and / or move their heart. Voluntary movements of the subject's limbs may also occur. This movement can lead to undesirable changes in the MRI image data acquired during the MRI scan. For instance, motion artifacts may appear in the acquired MRI data. Furthermore, the movement may reduce the quality of the acquired MRI image data.

[0005] Several methods are known for at least partially compensating for the movement of the object under investigation during magnetic resonance imaging (MRI) data acquisition. One known method is retrospective motion correction, which typically compensates for the movement of the object in the MRI image data after it has been acquired.

[0006] Another well-known method for at least partially compensating for the movement of the object under investigation is prospective motion correction. Prospective motion correction generally involves detecting and correcting any movement of the object during the acquisition of the magnetic resonance imaging (MRI) data. In certain cases, prospective motion correction can be performed in near real-time. This can involve determining motion parameters that characterize any movement of the object after acquiring an initial portion of the MRI data. Acquisition parameters, such as slice selection and / or slice orientation, can then be adjusted based on these motion parameters for acquiring a second portion of the MRI data.In this way, the acquisition of magnetic resonance image data can be adapted to the movement of the object under investigation even during the magnetic resonance imaging procedure.

[0007] A well-known method for prospective motion correction is the use of image-based navigators during magnetic resonance imaging (MRI). These can be particularly advantageous when MRI is performed using a sequence with a long acquisition time and / or extended waiting or dead times during the measurement, during which a navigator volume can be acquired. Acquiring a navigator volume typically involves outputting radio frequency pulses and reading out MRI measurement data in addition to the sequence elements used by the MRI sequence to acquire the diagnostic MRI image data. The navigator volume is typically used solely for prospective motion correction and can be discarded after completion of the MRI sequence.

[0008] The invention is based on the objective of enabling improved prospective compensation of movement of a subject during magnetic resonance imaging. This objective is achieved by the features of the independent claims. Advantageous embodiments are described in the dependent claims.

[0009] The inventive method for motion compensation during magnetic resonance imaging of an object under investigation using a magnetic resonance device comprises the following method steps: - Performing a reference measurement at a first time point during magnetic resonance imaging, where a navigator reference volume is acquired using a simultaneous multislice measurement technique with a first acceleration factor and a first number of first slice groups, - Performing a navigator measurement at a second time point during magnetic resonance imaging, wherein a navigator volume is acquired using a simultaneous multi-slice measurement technique with a second acceleration factor and a second number of second slice groups, wherein the second acceleration factor is equal to the first acceleration factor, - Determining motion information from the navigator reference volume and the navigator volume, where the motion information describes a movement of the object under investigation between the first time point and the second time point, and - Setting recording parameters, which are set after the second time point for magnetic resonance imaging, depending on the motion information.

[0010] The subject of the study can be a patient, a trainee, an animal, or a phantom. During magnetic resonance imaging (MRI), MRI image data is typically acquired, which can be displayed to a user on a screen and / or stored in a database.

[0011] In this case, magnetic resonance imaging (MRI) specifically involves the use of a magnetic resonance sequence to acquire the MRI image data. The reference measurement and the navigator measurement are then, in particular, part of the MRI sequence. The first time point at which the reference measurement takes place can represent the start time of the reference measurement. This first time point is located, in particular, at the beginning of the MRI sequence, advantageously before MRI image data are acquired according to the MRI sequence. The second time point at which the navigator measurement takes place can represent the start time of the navigator measurement. This second time point is located, in particular, after the first time point. The second time point can thus, in particular, lie between the acquisition of the MRI image data according to the MRI sequence.

[0012] Naturally, the navigator measurement can involve acquiring multiple navigator volumes at several second time points after the first time point during the playback of the magnetic resonance sequence. In this way, multiple motion data points can be determined at different time points during the magnetic resonance sequence, and the acquisition parameters can be adjusted multiple times based on this motion data. This allows for a particularly advantageous dynamic response to the movement of the subject during magnetic resonance imaging. For example, it is conceivable that a navigator volume is acquired at a defined point during each repetition of the magnetic resonance sequence. For instance, it is conceivable that a navigator volume is acquired between the inversion pulses and the readout blocks of the multiple repetition intervals.In this way, multiple navigator volumes can be acquired at several defined second time points. The wording "one navigator measurement" or "one navigator volume" is intended to explicitly include the acquisition of multiple navigator volumes, possibly during multiple navigator measurements, during the magnetic resonance sequence.

[0013] In contrast, a reference measurement is performed, particularly for magnetic resonance imaging (MRI) sequences, to establish a navigator reference volume at the first time point. In most cases, only one reference measurement is taken to establish exactly one navigator reference volume at the first time point. It is also conceivable that multiple navigator reference volumes are established, for example, at several initial time points. The navigator reference volume can also be calculated as the average of two or more measurements. The navigator reference volume can serve as the basis for prospective motion correction for the entire MRI image or sequence. The first time point can thus be considered the reference point, which can serve as a reference for any subsequently detected motion of the subject.The navigator reference volume can be formed, in particular, from a navigator volume measured first during the magnetic resonance sequence. In certain cases, however, it may also be conceivable that the reference measurement is repeated during the magnetic resonance imaging, that is, another navigator reference volume is acquired at a further first time point during the magnetic resonance sequence.

[0014] According to the proposed procedure, the navigator reference volume is to be acquired during the reference measurement using the simultaneous multislice (SMS) technique. This allows for accelerated acquisition of the navigator reference volume. The simultaneous multislice technique is described, for example, in the following publication: Setsompop et al. “Blipped-controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced g-factor penalty.”, Magn Reson Med. 2012. 67(5):1210-1224. The simultaneous multislice technique involves, in particular, the simultaneous excitation of multiple slices of a measurement volume. The multiple slices that are excited simultaneously are typically grouped into a slice group. The simultaneous multislice technique is typically characterized by an acceleration factor, also called the simultaneous multislice factor (SMS factor).The acceleration factor typically indicates how many layers in a layer group are simultaneously processed by a single radio frequency pulse. The acceleration factor is usually an integer and is at least two for accelerated measurements. With an acceleration factor of one, the measurement is comparable to a conventional, sequential measurement. Typical acceleration factors range up to six and depend on the body region being studied, the coil geometry, and / or the magnetic resonance sequence used. The magnetic resonance signals from the multiple layers, especially when superimposed in a k-space, can then be read out simultaneously. The subsequent reconstruction of the individual layers then involves separating the simultaneously read-out magnetic resonance signals with respect to the multiple layers.To separate the magnetic resonance signals, a known layer separation technique, such as a slice-GRAPPA method in the layer direction, can be used, as described, for example, in the aforementioned publication by Setsompop et al. In this way, several layers can advantageously be acquired simultaneously using the simultaneous multi-layer measurement technique. In particular, interleaved acquisition of the multiple layers is performed to minimize the influence of spatially adjacent layers from temporally successive layer measurements (potential cross-talk).

[0015] In particular, the navigator reference volume comprises several first layers, which are grouped into first layer groups. Specifically, in each first layer group of these multiple first layer groups, a number of first layers within each group are simultaneously excited by a high-frequency pulse. The high-frequency pulse thus acts simultaneously on all first layers of a first layer group. A first layer group within these multiple first layer groups comprises a number of first layers corresponding to the first acceleration factor. Therefore, each high-frequency pulse simultaneously excites a number of first layers corresponding to the first acceleration factor. To excite the navigator reference volume, a number of high-frequency pulses corresponding to the first number of first layer groups is used.Each of the multiple first layer groups comprises, in particular, the same number of first layers. The total number of first layers in the navigator reference volume therefore corresponds, in particular, to the product of the first acceleration factor and the first number of first layer groups.

[0016] Simultaneous acquisition of multiple first slices significantly reduces the time required to acquire the navigator reference volume. This time saving is roughly equivalent to the first acceleration factor. This allows for the elimination of measurement time for the reference measurement. The reduced measurement time can then be invested in improved imaging parameters, such as an increased number of first slices and / or higher resolution within the first slices. Furthermore, the accelerated acquisition of the navigator reference volume advantageously prevents or minimizes any movement of the object under investigation during the reference measurement.In particular, the shorter acquisition time for the reference scan can reduce the probability and / or the proportion of movement of the object under investigation during the reference scan. This is especially advantageous because the navigator reference volume forms the basis for the prospective motion correction of the entire magnetic resonance imaging (MRI) scan. A navigator reference volume distorted by movement of the object under investigation could negatively affect the entire MRI scan, especially the entire prospective motion correction.

[0017] In particular, the navigator volume comprises several second layers, which are grouped into second layer groups. Specifically, in each second layer group of these multiple second layer groups, a number of second layers are simultaneously excited by a high-frequency pulse. The high-frequency pulse thus acts simultaneously on all second layers of a second layer group. Each second layer group of these multiple second layer groups comprises a number of second layers corresponding to the second acceleration factor. Therefore, each high-frequency pulse simultaneously excites a number of second layers corresponding to the second acceleration factor. To excite the navigator volume, a number of high-frequency pulses corresponding to the second number of second layer groups is used.Each second layer group of the multiple second layer groups comprises, in particular, the same number of second layers. The total number of second layers in the navigator volume therefore corresponds, in particular, to the product of the second acceleration factor and the second number of second layer groups. Advantageously, the same calibration scan can be used to reconstruct the navigator reference volume and the navigator volume acquired using the simultaneous multi-layer measurement technique.

[0018] It is particularly advantageous to use the same acceleration factor for both the reference measurement and the navigator measurement. In this way, the number of simultaneously excited first layers in the navigator reference volume is equal to the number of simultaneously excited second layers in the navigator volume. According to one embodiment, the number of layer groups, and thus the number of acquired layers, in the navigator volume is particularly advantageously reduced compared to the navigator reference volume, as described in more detail in the following sections.

[0019] Simultaneous acquisition of multiple second slices significantly reduces the time required to acquire the navigator volume. This time saving is roughly equivalent to the second acceleration factor. This allows for a reduction in measurement time for the navigator measurement itself. The shortened measurement time can then be invested in improved imaging parameters, such as an increased number of second slices and / or higher resolution within the second slices. Consequently, the reduced measurement time enables more accurate detection of the object's movement using the navigator measurement.The reduced measurement time for the navigator measurement is particularly advantageous because the navigator measurement, which is typically inserted into the magnetic resonance imaging (MRI) sequence, has less of an impact on the MRI sequence itself. This is based on the consideration that the navigator measurement is typically inserted during dead times that occur within the MRI sequence used for the actual MRI. Such dead times should typically be at least 150 ms, preferably at least 250 ms, and preferably at least 500 ms.

[0020] The movement of the object under investigation can occur, in particular, between the first and second time points. For example, respiratory and / or cardiac movements of the object may be present. Voluntary movements of the object's limbs may also be present. The movement of the object can be reflected in the image content of the navigator volume compared to the image content of the navigator reference volume. This movement information can be obtained, for example, by recording the navigator volume against the navigator reference volume. Advantageously, this movement information can characterize how the object moved between the first and second time points.

[0021] It is now proposed that the detected motion information be fed back into the magnetic resonance sequence used for MRI, so that the motion of the subject can be at least partially compensated for after the second time point in the MRI scan. For example, the acquisition parameters for the MRI scan can be adjusted after the second time point based on the motion information in such a way that the motion of the subject can be counteracted. In this way, motion correction based on the motion information can be performed, particularly in real time, even during the MRI scan. This approach is also commonly referred to as prospective motion correction.

[0022] To enable particularly rapid determination of motion information from the navigator reference volume and the navigator volume, a model assumption for the motion information is made. This model assumption can include motion parameters that are encompassed by the motion information. A rigid model assumption for the motion information is particularly advantageous, especially one that includes six motion parameters: three translational parameters and three rotational parameters. The use of a rigid model assumption has proven particularly useful in the case of magnetic resonance imaging of the head of the subject. This allows the acquisition parameters to be advantageously set with high temporal resolution. Of course, other model assumptions for the motion information that appear appropriate to those skilled in the art, such as non-rigid model assumptions, can also be made.The use of a non-rigid and / or non-linear motion model is particularly useful for image areas where the model assumption of rigid motion does not apply. In the case of head imaging, this could be, for example, the neck region, the jaw region, and / or the orbital region of the subject.

[0023] Adjusting the acquisition parameters based on motion information can, for example, involve adapting the imaging volume for magnetic resonance imaging (MRI) after the second time point. In the rigid case, for instance, slices of the imaging volume for MRI after the second time point can be adjusted based on the motion parameters, particularly the three translation parameters and three rotation parameters. This allows for direct compensation during the measurement for changes in the position and / or anatomy of the subject caused by movement, such as tilting the subject's head. The imaging volume is adjusted primarily by modifying gradient circuits, which are activated during MRI after the second time point, for example, during excitation radio frequency pulses and / or readout windows.

[0024] The adjustment of acquisition parameters based on motion information can occur with a time delay compared to the acquisition of the navigator volume. For example, at least one repetition time may elapse between the acquisition of a specific navigator volume and the second time point before the acquisition parameters are applied, which are adjusted using the motion information determined from that specific navigator volume. This delay can be due to the computation time required to reconstruct the navigator volume and / or register the navigator volume to the navigator reference volume. However, this delayed motion compensation is typically tolerable, especially if the subject exhibits continuous and / or small movements, such as respiratory movements.

[0025] One embodiment provides that the second number of second layer groups is smaller than the first number of first layer groups.

[0026] In this way, the number of acquired second layers of the navigator volume is reduced compared to the number of acquired first layers of the navigator reference volume. Specifically, the number of high-frequency excitation pulses used to excite the navigator volume is smaller than the number of high-frequency excitation pulses used to excite the navigator volume. In this embodiment, the second layer groups represent a subset of the first layer groups. This allows, in particular, the use of different imaging parameters for the reference measurement and the navigator measurement. The number of second layer groups can be dynamically varied for different navigator volumes acquired at different second time points.The variation in the second number of second layer groups can, for example, depend on the available dead times of the magnetic resonance sequence actually used to acquire the magnetic resonance measurement data. Advantageously, the in-plane resolution of the measurement data for the various navigator volumes is kept constant.

[0027] By reducing the number of second layer groups compared to the number of first layer groups, the time required for navigator measurements can be significantly reduced. This advantageously enables even faster detection of the object's movement. Furthermore, reducing the number of second layer groups in the navigator volume allows for high in-plane resolution of the navigator volume measurement data, particularly while maintaining the same acquisition time. An increased in-plane resolution when reducing the number of layers can be more advantageous for determining motion information than an isotropic navigator volume with reduced in-plane resolution.Simultaneously, by reducing the number of second layer groups, the second layer groups can advantageously be selected such that the second layers of the navigator volume are spatially separated as far as possible. In this way, particularly robust detection of the movement of the object under investigation can be achieved using the navigator volume and the navigator reference volume.

[0028] The navigator reference volume can still encompass the complete anatomy of the object under investigation, advantageously at isotropic resolution. In this way, despite a reduced number of slices in the navigator volume, precise registration of the navigator volume to the navigator reference volume for determining motion information can still be advantageously achieved, particularly even when the object is moving. The increased sampling of the object's anatomy in the reference measurement is possible because the reference measurement typically interferes little with the magnetic resonance sequence actually used to acquire the magnetic resonance measurement data. For example, the reference measurement can be performed entirely before the actual acquisition of the magnetic resonance measurement data.

[0029] One embodiment provides that the second number of second layer groups is less than half the number of first layer groups. This can, for example, result in a particularly advantageous acceleration of the navigator measurement.

[0030] One embodiment provides that the second layer groups represent a subset of the first layer groups. In this way, all second layer groups of the navigator volume are already captured in the navigator reference volume. This allows for a particularly advantageous comparison, for example, an advantageous registration, of the navigator volume and the navigator reference volume to determine the motion information.

[0031] One embodiment provides that the second layer groups are selected such that the imaging volume covered by the navigator volume is essentially the same size as the navigator reference volume. Only the resolution of the navigator volume in the layer direction is, in particular, lower than the resolution of the navigator reference volume in the layer direction. Advantageously, one dimension of the navigator volume in the layer direction is similar in size, advantageously at most two layer thicknesses smaller, and most advantageously the same size as a dimension of the navigator volume. Here, the navigator volume can advantageously cover the navigator reference volume to a large extent. In particular, approximately uniform coverage is advantageous.By uniformly covering the navigator reference volume with the navigator volume and / or the corresponding sizes of the navigator volume and the navigator reference volume, the advantage can be achieved that robust detection of motion from the navigator volume and the navigator reference volume is possible. Advantageously, the second layer groups are simultaneously selected such that the multiple second layers in the navigator volume are positioned as far apart as possible, as described in more detail in one of the following sections. In this way, the second layer groups that are excited for the acquisition of the navigator volume can be selected particularly effectively.

[0032] One embodiment provides that the navigator reference volume comprises several first layers, which are grouped into first layer groups, and the navigator volume comprises several second layers, which are grouped into second layer groups. In particular, the grouping of the first layers into first layer groups and the grouping of the second layers into second layer groups are performed by means of the simultaneous and / or coherent excitation of the layers according to the simultaneous multilayer measurement technique. Specifically, those first layers that are excited simultaneously are grouped into a first layer group of the several first layer groups. Furthermore, those second layers that are excited simultaneously are grouped into a second layer group of the several second layer groups.

[0033] One embodiment provides that the second layer groups are selected such that the multiple second layers are positioned as far apart as possible within the navigator volume. The second layers can advantageously be positioned as far apart as possible within the navigator volume. This allows, for example, advantageous uniform coverage of the navigator reference volume by the navigator volume. Advantageously, it can be avoided that certain adjacent second layers of the multiple second layers are positioned closer to each other than other adjacent second layers of the multiple second layers. Positioning the multiple second layers as far apart as possible can lead to the advantage of particularly easy separation of simultaneously excited second layers of the multiple second layers during reconstruction of the navigator volume.Furthermore, increasing the spatial separation of successively excited layer groups advantageously minimizes mutual interference between the layers through cross-talk. The separation of the second layers simultaneously excited using the simultaneous multi-layer measurement technique can thus be advantageously improved and / or simplified.

[0034] One embodiment provides that the layer spacing of the multiple second layers in the navigator volume is greater than the layer spacing of the multiple first layers in the navigator reference volume. This is advantageously the case when the second number of second layer groups is smaller than the first number of first layer groups. The second layers are advantageously positioned as far apart as possible in the navigator volume and are also advantageously positioned such that they sample the navigator reference volume as uniformly as possible.

[0035] One embodiment provides that in the navigator volume there is a first layer spacing between two adjacent second layers of a first layer pair and a second layer spacing between two adjacent second layers of a second layer pair, wherein the difference between the first layer spacing and the second layer spacing is less than or equal to the layer spacing of the multiple first layers in the navigator reference volume. This is one exemplary way in which the second layers can be distributed particularly uniformly in the navigator volume. The difference between various layer spacings between adjacent second layers in the navigator volume is thus advantageously less than or equal to the layer spacing of the multiple first layers in the navigator reference volume.

[0036] One embodiment provides that the acquisition parameters are set such that the acquisition of magnetic resonance measurement data during magnetic resonance imaging after the second time point is performed in such a way that the movement of the object under investigation, as described in the motion information, is compensated for as much as possible between the first and second time points. In this way, the movement of the object under investigation can advantageously be compensated for directly during the acquisition of the magnetic resonance measurement data. A loss of signal information due to the movement of the object under investigation can thus be advantageously reduced even during the acquisition process.

[0037] One embodiment provides that the navigator reference volume and / or the navigator volume is smaller than the scan volume for acquiring diagnostic magnetic resonance (MRI) measurement data during MRI imaging. The scan volume, also called the acquisition volume (field of view, FOV), is, in particular, a volume that is represented in the acquired MRI measurement data. The scan volume is typically defined by a user, for example, on an overview image (localizer). Advantageously, the navigator volume is positioned within the scan volume such that a model assumption, such as the assumption of rigid movement of the subject, is particularly well fulfilled. In the case of head imaging, for example, it may be advantageous to position the navigator volume in a body region of a, in particular, posterior, skull base of the subject.

[0038] The magnetic resonance device according to the invention comprises a reference measurement unit, a navigator measurement unit and a computing unit with a detection unit and a setting unit, wherein the magnetic resonance device is designed to perform a method according to one of the preceding claims.

[0039] The magnetic resonance imaging (MRI) device is configured to perform a motion compensation procedure during MRI imaging of a subject. The reference measurement unit is configured to perform a reference measurement at a first time point during MRI imaging, acquiring a navigator reference volume using a simultaneous multi-slice measurement technique with a first acceleration factor and a first number of first slice groups. The navigator measurement unit is configured to perform a navigator measurement at a second time point during MRI imaging, acquiring a navigator volume using a simultaneous multi-slice measurement technique with a second acceleration factor and a second number of second slice groups, where the second acceleration factor is equal to the first acceleration factor.The acquisition unit is designed to determine motion information from the navigator reference volume and the navigator volume, where the motion information describes a movement of the object under investigation between the first and second time points. The setting unit is designed to adjust acquisition parameters, which are set for magnetic resonance imaging after the second time point, depending on the motion information.

[0040] The computer program product according to the invention can be directly loaded into the memory of a programmable processing unit of a magnetic resonance imaging (MRI) device and includes program code means for executing a method according to the invention when the computer program product is executed in the processing unit of the MRI device. The computer program product particularly comprises a computer program. This allows the method according to the invention to be executed quickly, identically, and robustly. The computer program product is configured so that it can execute the method steps according to the invention by means of the processing unit. The processing unit must have the necessary prerequisites, such as sufficient main memory, a suitable graphics card, or a suitable logic unit, so that the respective method steps can be executed efficiently.The computer program product is, for example, stored on a computer-readable medium or on a network or server, from where it can be loaded into the processor of a local computing unit, which may be directly connected to the magnetic resonance device or be designed as part of the magnetic resonance device. Furthermore, control information of the computer program product can be stored on an electronically readable data carrier. The control information of the electronically readable data carrier can be designed such that, when the data carrier is used in a computing unit of the magnetic resonance device, it executes a method according to the invention. Examples of electronically readable data carriers are a DVD, a magnetic tape, or a USB flash drive on which electronically readable control information, in particular software (see above), is stored.If this control information (software) is read from the data carrier and stored in a control and / or processing unit of the magnetic resonance device, all embodiments of the methods described above can be carried out according to the invention. Thus, the invention can also be based on the aforementioned computer-readable medium and / or the aforementioned electronically readable data carrier.

[0041] The advantages of the magnetic resonance device and the computer program product according to the invention essentially correspond to the advantages of the method according to the invention, which have been described in detail above. Features, advantages, or alternative embodiments mentioned herein can also be applied to the other claimed items and vice versa. In other words, the claims can also be further developed with features described or claimed in connection with a method. The corresponding functional features of the method are implemented by corresponding physical modules, in particular hardware modules.

[0042] The invention will now be described and explained in more detail with reference to the exemplary embodiments shown in the figures.

[0043] They show: Fig. 1 a magnetic resonance device according to the invention in a schematic representation, Fig. 2 a flowchart of an embodiment of a method according to the invention and Fig. 3 an exemplary illustration of the procedure according to the invention Fig. 2.

[0044] Fig. Figure 1 schematically illustrates a magnetic resonance imaging (MRI) device 11 according to the invention. The MRI device 11 comprises a detector unit formed by a magnetic unit 13, with a main magnet 17 for generating a strong and, in particular, constant main magnetic field 18. Furthermore, the MRI device 11 has a cylindrical patient reception area 14 for receiving a test subject 15, in this case a patient, wherein the patient reception area 14 is cylindrically enclosed in a circumferential direction by the magnetic unit 13. The patient 15 can be moved into the patient reception area 14 by means of a patient positioning device 16 of the MRI device 11. For this purpose, the patient positioning device 16 has a table that is movably arranged within the MRI device 11. The magnetic unit 13 is shielded from the outside by means of a housing 31 of the MRI device.

[0045] The magnet unit 13 further comprises a gradient coil unit 19 for generating magnetic field gradients, which are used for spatial encoding during imaging. The gradient coil unit 19 is controlled by a gradient control unit 28. The magnet unit 13 also includes a high-frequency antenna unit 20, which in the illustrated case is configured as a body coil permanently integrated into the magnetic resonance device 10, and a high-frequency antenna control unit 29 for exciting a polarization that arises in the main magnetic field 18 generated by the main magnet 17. The high-frequency antenna unit 20 is controlled by the high-frequency antenna control unit 29 and transmits high-frequency magnetic resonance sequences into an examination space, which is essentially formed by the patient acquisition area 14.The high-frequency antenna unit 20 is further equipped to receive magnetic resonance signals, especially from patient 15.

[0046] The magnetic resonance imaging (MRI) device 11 includes a processing unit 24 for controlling the main magnet 17, the gradient control unit 28, and the high-frequency antenna control unit 29. The processing unit 24 centrally controls the MRI device 11, for example, by performing a predetermined imaging gradient echo sequence. Control information, such as imaging parameters, as well as reconstructed MRI images, can be made available to a user on a display unit 25 of the MRI device 11. The MRI device 11 also includes an input unit 26, which allows a user to enter information and / or parameters during a measurement procedure. The processing unit 24 can comprise the gradient control unit 28 and / or the high-frequency antenna control unit 29 and / or the display unit 25 and / or the input unit 26.

[0047] In the case shown, the computing unit 24 comprises an investigation unit 33 and an adjustment unit 34.

[0048] The magnetic resonance imaging (MRI) device 11 further comprises a reference measurement unit 32 and a navigator measurement unit 30. In this case, the reference measurement unit 32 and the navigator measurement unit 30 are formed by an image data acquisition unit. The image data acquisition unit is formed by the magnet unit 13 together with the high-frequency antenna control unit 29 and the gradient control unit 28. The MRI device 11, together with the reference measurement unit 32, the navigator measurement unit 30, and the processing unit 24, is thus designed to carry out a method according to the invention for motion compensation during magnetic resonance imaging.

[0049] The depicted magnetic resonance device 11 can, of course, include further components that magnetic resonance devices 11 typically possess. Furthermore, the general operating principle of a magnetic resonance device 11 is known to those skilled in the art, so a detailed description of the further components is omitted.

[0050] Fig. Figure 2 shows a flowchart of a first embodiment of a method according to the invention for motion compensation during magnetic resonance imaging of an object 15 using a magnetic resonance device 11.

[0051] In a first process step 40, a reference measurement is performed at a first time point during the magnetic resonance imaging using the reference measurement unit 32, wherein a navigator reference volume REF is acquired using a simultaneous multi-layer measurement technique with a first acceleration factor and a first number of first layer groups A, B, C, D, E.

[0052] This process step 40 is illustrated in Fig. 3 in section 50. Here, the situation at the first time point is schematically illustrated using an example. Diagnostic magnetic resonance measurement data are to be acquired from a head region of the subject 15 using a magnetic resonance sequence that has a first examination volume FOV1. The first examination volume FOV1 encompasses the entire head of the subject 15, which is to be imaged.

[0053] The navigator reference volume REF is acquired at the first time point, specifically before or at the beginning of the magnetic resonance sequence. The navigator reference volume REF is located in the Fig. The case shown in 3 is smaller than the first examination volume FOV1. The navigator reference volume REF is in the Fig. In the case shown in Figure 3, the object under investigation is positioned at an advantageous location within the object 15, namely in an upper head region of the object 15. Here, in particular, an assumption of rigid movement of the object 15 may be justified.

[0054] In the case shown, the navigator reference volume REF comprises fifteen first layers A1, B1, C1, D1, E1, A2, B2, C2, D2, E2, A3, B3, C3, D3, E3 (hereinafter referred to as A1, B1, ..., E3), which are grouped into five first layer groups A, B, C, D, E. Each first layer group A of the five first layer groups A, B, C, D, E comprises three first layers A1, A2, A3 of the fifteen first layers A1, B1, ..., E3. Similarly, each of the five first layer groups A, B, C, D, E comprises three first layers A1, B1, ..., E3. The first layers A1, B1, ..., E3 belonging to each first layer group A, B, C, D, E are measured simultaneously during the measurement of the navigator reference volume REF and are excited by means of an excitation pulse. This is how it is done in Fig. In the case shown, the navigator reference volume REF is acquired using a simultaneous multi-layer measurement technique with a first acceleration factor of three and an initial number of five first layer groups A, B, C, D, E. The number of first layers A1, B1, ..., E3 of the navigator reference volume REF results from the product of the first acceleration factor and the initial number of first layer groups A, B, C, D, E and is therefore fifteen in the example shown.

[0055] The acquisition sequence of the first layers A1, B1, ..., E3 of the navigator reference volume REF can be interleaved. Advantageously, layer groups A, B, C, D, E of the navigator reference volume REF are never acquired directly one after the other if they contain directly adjacent first layers A1, B1, ..., E3. For example, a possible interleaved acquisition sequence of the navigator reference volume REF would be to first acquire the first layers A1, A2, A3 of the first layer group A simultaneously, then the first layers C1, C2, C3 of the third layer group C, then the first layers E1, E2, E3 of the fifth layer group E, then the first layers B1, B2, B3 of the second layer group B, and then the first layers D1, D2, D3 of the fourth layer group D. In this way, interactions (cross talks) between the first layers A1, B1, ..., E3 during acquisition can be particularly advantageously reduced and / or avoided.

[0056] In a further process step 41, a navigator measurement is performed at a second time point during the magnetic resonance imaging using the navigator measurement unit 30, wherein a navigator volume VOL is acquired using a simultaneous multi-layer measurement technique with a second acceleration factor and a second number of second layer groups A', C', wherein the second acceleration factor is equal to the first acceleration factor.

[0057] This process step 41 is illustrated in Fig. 3 in section 51. Here, the situation at the second time point is schematically illustrated using an example. Diagnostic magnetic resonance (MRI) measurement data from a head region of the subject 15 are to be acquired using a MRI sequence that has a first examination volume FOV1. However, between the first and second time points, i.e., between the situation according to section 50 and the situation according to section 51, the subject 15 has moved or tilted its head. Thus, the first examination volume FOV1 is no longer optimally formed to acquire MRI measurement data of the tilted head.

[0058] At the second time point, specifically during the magnetic resonance sequence, the navigator volume VOL is acquired. The navigator volume VOL is positioned and / or oriented similarly to the navigator reference volume REF.

[0059] In the case shown, the navigator reference volume REF comprises six second layers A1', C1', A2', C2', A3', C3' (hereinafter referred to as A1', C1', ..., C3'), which are grouped into two second layer groups A', C'. One second layer group A' of the two second layer groups A', C' comprises three second layers A1', A2', A3'. Another second layer group C' of the two second layer groups A', C' comprises the three other second layers C1', C2', C3'. The second layers A1', C1', ..., C3' belonging to each second layer group A', C' are measured simultaneously during the measurement of the navigator volume VOL and are excited for this purpose by means of an excitation pulse. In this way, in the Fig. In the case shown, the navigator volume VOL is measured using a simultaneous multi-layer measurement technique with a second acceleration factor of three and a second number of second layer groups A', C' of two. The number of second layers A1', C1', ..., C3' of the navigator volume VOL is the product of the second acceleration factor and the second number of second layer groups A', C' and is therefore six in the example shown.

[0060] Compared to the acquisition of the navigator reference volume REF, the acceleration factor used for the acquisition of the navigator volume VOL remained the same. Only the number of layer groups decreased for the acquisition of the navigator volume VOL compared to the acquisition of the navigator reference volume REF. Thus, in the case shown, the second number of second layer groups A', C' is smaller than the first number of first layer groups A, B, C, D, E. In fact, the second number of second layer groups A', C' is less than half the first number of first layer groups A, B, C, D, E.

[0061] Furthermore, the second layer groups A', C' represent a subset of the first layer groups A, B, C, D, E. The second layers A1', C1', A2', C2', A3', C3' are therefore oriented and arranged in the same way as the corresponding first layers A1, C1, A2, C2, A3, C3. Consequently, no changes were made to the layer parameters of the second layers A1', C1', A2', C2', A3', C3' of the navigator volume VOL compared to the corresponding first layers A1, C1, A2, C2, A3, C3 of the navigator reference volume REF. Only the number of acquired second layer groups A', C' or second layers A1', C1', ..., C3' has decreased compared to the number of acquired first layer groups A, B, C, D, E or first layers A1, B1, ..., C3.

[0062] Furthermore, in Fig. In the case shown in Figure 3, the second layer groups A', C' are chosen such that the imaging volume covered by the navigator volume VOL is essentially the same size as the navigator reference volume REF.

[0063] In particular, the second layer groups A', C' were chosen such that the multiple second layers A1', C1', ..., C3' are positioned as far apart as possible in the navigator volume VOL. Thus, the layer spacing of the multiple second layers A1', C1', ..., C3' in the navigator volume VOL is greater than the layer spacing of the multiple first layers A1, B1, ..., C3 in the navigator reference volume REF.

[0064] Simultaneously, in the navigator volume VOL, there exists a first distance between two adjacent second layers A1', C1', ..., C3' of a first layer pair and a second distance between two adjacent second layers A1', C1', ..., C3' of a second layer pair, where the difference between the first distance and the second distance is less than or equal to a layer spacing of the multiple first layers A1, B1, ..., C3 in the navigator reference volume REF. For example, if we consider the second layers A1' and C1' as the first layer pair and the second layers C2' and A3' as the second layer pair, then the first distance between the second layers A1', C1' of the first layer pair is two, in units of the layer spacing of the first layers A1, B1, ..., C3, and the second distance between the second layers C2', A3' of the second layer pair is three, in these units.The difference between the first distance and the second distance is one in this unit and therefore equal to the layer distance of the first layers A1, B1, ..., C3.

[0065] In a further process step 42, motion information is determined from the navigator reference volume REF and the navigator volume VOL using the detection unit 33, wherein the motion information describes a movement of the object under investigation 15 between the first time point and the second time point. The motion information can be obtained, for example, by registering the captured navigator volume VOL onto the navigator reference volume REF. Of course, other possibilities for determining the motion information from the navigator volume VOL and the navigator reference volume REF that would appear sensible to a person skilled in the art are also conceivable.

[0066] In the Fig. In the case shown in Figure 3, a head movement of the subject 15 occurred between the first time point shown in Section 50 and the second time point shown in Section 51. For example, the subject 15 tilted its head, so that the first examination volume 15 no longer optimally represents the head of the subject 15 at the second time point. The motion information determined from the navigator reference volume REF and the navigator volume VOL can describe this tilting of the head, for example, by means of determined rotation and / or translation parameters.

[0067] In a further process step 43, acquisition parameters are set after the second time point for magnetic resonance imaging, depending on the motion information, using the setting unit 34. The acquisition parameters can be set in such a way that the acquisition of magnetic resonance measurement data during magnetic resonance imaging after the second time point is such that the movement of the object under investigation 15 described in the motion information between the first time point and the second time point is compensated as much as possible.

[0068] In the Fig. In the case shown in section 52, which represents the period after the second time point, a second examination volume FOV2 is shown, adapted based on the motion information. This second volume is tilted relative to the first examination volume FOV1 according to the movement of the head of the subject 15. The second examination volume FOV2 can thus optimally image the head region of the subject 15.

[0069] The magnetic resonance measurement data are primarily diagnostic magnetic resonance measurement data. These diagnostic magnetic resonance measurement data can be acquired using a magnetic resonance sequence typically employed for magnetic resonance imaging. Magnetic resonance image data can be reconstructed from the magnetic resonance measurement data and made available, i.e., displayed to a user on the display unit 25 and / or stored in a database. The magnetic resonance measurement data can be acquired from an examination volume FOV1, FOV2, where advantageously the navigator reference volume REF and / or the navigator volume VOL is smaller than the examination volume FOV1, FOV2.

[0070] The in Fig. The process steps of the method according to the invention, as illustrated in point 2, are executed by the computing unit. For this purpose, the computing unit comprises the necessary software and / or computer programs, which are stored in a memory unit of the computing unit. The software and / or computer programs include program elements designed to execute the method according to the invention when the computer program and / or the software is executed in the computing unit by means of a processor unit of the computing unit.

[0071] It should be noted that the in Fig.Figure 3 illustrates the procedure according to the invention only as an example. Naturally, a different acceleration factor, a different number of layer groups or layers, and different positioning or orientation of layers can be used. Naturally, a different body region of the object 15 can also be examined. Naturally, a different movement of the object 15, for example, a breathing movement, can also be compensated for.

[0072] Although the invention has been illustrated and described in detail by the preferred embodiments, the invention is nevertheless not limited by the disclosed examples and other variations can be derived from them by the person skilled in the art without leaving the scope of protection of the invention.

Claims

[1] Method for motion compensation during magnetic resonance imaging of an object under investigation using a magnetic resonance device, wherein a magnetic resonance sequence is used for the magnetic resonance imaging, comprising the following method steps: - Performing a reference measurement at a first time point during magnetic resonance imaging, where a navigator reference volume is acquired using a simultaneous multi-slice measurement technique with a first acceleration factor and a first number of first slice groups, - Performing a navigator measurement at a second time point during magnetic resonance imaging, wherein a navigator volume is acquired using a simultaneous multi-slice measurement technique with a second acceleration factor and a second number of second slice groups, wherein the second acceleration factor is equal to the first acceleration factor, and wherein the navigator measurement is inserted into the magnetic resonance sequence used for magnetic resonance imaging. - Determining motion information from the navigator reference volume and the navigator volume, where the motion information describes a movement of the object under investigation between the first time point and the second time point, and - Setting recording parameters, which are set after the second time point for magnetic resonance imaging, depending on the motion information. [2] Method according to claim 1, wherein the second number of second layer groups is smaller than the first number of first layer groups. [3] Method according to claim 2, wherein the second number of second layer groups is less than half the number of first layer groups. [4] Method according to any of the preceding claims, wherein the second layer groups are a subset of the first layer groups. [5] Method according to one of the preceding claims, wherein the second layer groups are selected such that the imaging volume covered by the navigator volume is substantially the same size as the navigator reference volume. [6] Method according to any of the preceding claims, wherein the navigator reference volume comprises several first layers which are grouped into first layer groups, and the navigator volume comprises several second layers which are grouped into second layer groups. [7] Method according to claim 6, wherein the second layer groups are selected such that the multiple second layers are positioned as far apart as possible in the navigator volume. [8] Method according to one of claims 6-7, wherein a layer spacing of the multiple second layers in the navigator volume is greater than a layer spacing of the multiple first layers in the navigator reference volume. [9] Method according to one of claims 6-8, wherein in the navigator volume there is a first layer spacing between two adjacent second layers of a first layer pair and a second layer spacing between two adjacent second layers of a second layer pair, wherein a difference between the first layer spacing and the second layer spacing is less than or equal to a layer spacing of the several first layers in the navigator reference volume. [10] Method according to one of the preceding claims, wherein the setting of the recording parameters is carried out such that the acquisition of magnetic resonance measurement data during magnetic resonance imaging after the second time point is carried out in such a way that the movement of the object of investigation described in the motion information between the first time point and the second time point is compensated as far as possible. [11] Method according to any of the preceding claims, wherein the navigator reference volume and / or the navigator volume is smaller than an examination volume for acquiring diagnostic magnetic resonance measurement data during magnetic resonance imaging. [12] Magnetic resonance device comprising a reference measurement unit, a navigator measurement unit and a computing unit with a detection unit and a setting unit, wherein the magnetic resonance device is designed to perform a method according to one of the preceding claims. [13] Computer program product which can be directly loaded into a memory of a programmable computing unit of a magnetic resonance device, comprising program code means for executing a method according to any one of claims 1-11 when the computer program product is executed in the computing unit of the magnetic resonance device.

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