Optimized acquisition of diffusion-weighted measurement data of a test object using a magnetic resonance system

Abstract

By segmenting the acquisition of a first and at least one further echo signal in the readout direction according to the invention, several diffusion-weighted measurement data sets (at least a first and a further measurement data set) are acquired after a common excitation and thus within a repetition time TR. This reduces the overall measurement time required for acquiring measurement data sets with all desired diffusion weights. Furthermore, the influence of movement of the investigated object, for example, the breathing of a patient, can be reduced by acquiring different measurement data sets with varying diffusion weights in close temporal proximity according to the invention.

Description

[0001] The invention relates to an optimized recording of diffusion-weighted measurement data of an examination object using a magnetic resonance system.

[0002] Magnetic resonance imaging (MR) is a well-known technique for generating images of the interior of an object under examination. Simply put, the object under examination is positioned in a magnetic resonance scanner in a comparatively strong, static, homogeneous basic magnetic field, also known as a B0 field, with field strengths ranging from 0.2 Tesla to 7 Tesla and more, so that its nuclear spins are oriented along the basic magnetic field. To trigger nuclear magnetic resonances, which can be measured as signals, high-frequency excitation pulses (RF pulses) are radiated into the object under examination. The triggered nuclear magnetic resonances are measured as so-called k-space data, and based on these, MR images are reconstructed or spectroscopy data is acquired. To spatially encode the measured data, rapidly switched magnetic gradient fields, or gradients for short, are superimposed on the basic magnetic field.A scheme used, which describes a temporal sequence of RF pulses to be delivered and gradients to be switched, is referred to as a pulse sequence (scheme), or simply a sequence. The recorded measurement data are digitized and stored as complex numerical values ​​in a k-space matrix. A corresponding MR image can be reconstructed from the k-space matrix, for example, using a multidimensional Fourier transform.

[0003] In a magnetic resonance system, the measurable volume of a magnetic resonance imaging image is limited in all three spatial directions due to physical and technical conditions, such as limited magnetic field homogeneity and non-linearity of the gradient field. For this reason, an acquisition volume, a so-called field of view or “field of view” (FoV), is restricted to a volume in which the aforementioned physical characteristics lie within a predetermined tolerance range, thus enabling a true-to-original image of the object to be examined using conventional measurement sequences. However, the thus limited field of view or “field of view” is considerably smaller, particularly in the x and y directions, i.e. perpendicular to a longitudinal axis of a tunnel of the magnetic resonance system, than the volume limited by the ring tunnel of the magnetic resonance system. In conventional magnetic resonance systems, the diameter of the ring tunnel, for example, is approximately60cm, whereas the diameter of the commonly used field of view in which the above-mentioned physical characteristics are within the tolerance range is approximately 50cm.

[0004] Typically, a magnetic resonance image consists of a large number of individual partial measurements in which raw data is recorded from different layers of the object under examination in order to subsequently reconstruct volume image data.

[0005] In addition, for many examinations it is also necessary to take several, i.e. a whole series of magnetic resonance images of the object under examination, during which a specific measurement parameter is varied. Based on the measurements, the effect of this measurement parameter on the object under examination is observed in order to later draw diagnostic conclusions. A series is understood to mean at least two, but usually more than two, magnetic resonance images. It is useful to vary a measurement parameter in such a way that the contrast of a specific material type excited during the measurements, for example a tissue type of the object under examination or a chemical substance that is significant for most or certain tissue types, such as water, is influenced as strongly as possible by the variation in the measurement parameter. This ensures that the effect of the measurement parameter on the object under examination is particularly clearly visible.

[0006] A typical example of a series of magnetic resonance images obtained while varying a measurement parameter that strongly influences contrast is so-called diffusion imaging (DWI). Diffusion refers to the Brownian motion of molecules in a medium. In diffusion imaging, multiple images with different diffusion directions and weightings are typically acquired and combined. The strength of the diffusion weighting is usually defined by the so-called "b-value." The diffusion images with different diffusion directions and weightings, or the combined images, can then be used for diagnostic purposes.By appropriately combining the acquired diffusion-weighted images, parameter maps with particularly valuable diagnostic value can be generated, such as maps that represent the apparent diffusion coefficient (ADC) or fractional anisotropy (FA). Diffusion imaging is often based on echo-planar imaging (EPI) due to the short acquisition time of the EPI sequence per image and its robustness to motion.

[0007] In diffusion-weighted imaging, additional gradients reflecting the direction and weight of diffusion are inserted into a pulse sequence to visualize or measure the diffusion properties of the tissue. These gradients result in tissue with rapid diffusion (e.g., cerebrospinal fluid, CSF) experiencing greater signal loss than tissue with slow diffusion (e.g., gray matter in the brain). The resulting diffusion contrast is becoming increasingly important clinically, and applications now extend far beyond the classic early detection of ischemic stroke.

[0008] Thus, MR diffusion imaging provides an important method for both clinical diagnostics and basic medical research to visualize microstructural properties and changes in biological tissue. As the underlying models become increasingly complex, so does the need for acquired data—for example, images with a variety of different diffusion codings (weightings, directions, temporal sequences, etc.)—and thus the need to accelerate measurements in order to capture all desired data in a time frame acceptable, e.g., for clinical procedures.

[0009] Methods for accelerating MR acquisition techniques such as parallel imaging (PAT), compressed sensing (CS), deep learning (DL) or simultaneous multi-slice imaging (SMS) are already known, which, for example, reduce the number of measurement data to be acquired and thus the time required for acquiring the measurement data.

[0010] Despite the introduction of such acceleration techniques, the acquisition times of many DWI measurements are still in the range of several minutes, so that any approach to further reduction of measurement time - ideally in combination with at least one of the previously mentioned acceleration techniques - is significant.

[0011] Due to the short acquisition time of per image and its robustness against motion, diffusion imaging is often based on EPI acquisition techniques described below, in particular on RESOLVE techniques extended by a diffusion preparation.

[0012] Such EPI (Echo Planar Imaging) acquisition techniques are among the fastest known MR acquisition techniques. In EPI acquisition techniques, an oscillating, bipolar readout gradient is applied after an RF excitation pulse. With each change in the gradient's polarization direction, this gradient refocuses the transverse magnetization as far as the T2* decay allows, thus generating a gradient echo. In other words, by switching the bipolar readout gradient after an RF excitation pulse within the free induction decay after excitation (FID), or if an additional RF refocusing pulse is applied after the RF excitation pulse, an echo train of rising and falling gradient echoes with alternating signs is generated within the spin echo thus generated. EPI pulse sequences can be used as so-called “single-shot” methods, in which all measurement data are used to generate an image of a subvolume, e.g.a slice of the object under investigation can be recorded after only one RF excitation pulse.

[0013] The displayable spatial resolution that can be achieved using an EPI technique is generally determined by B0 inhomogeneities in the object being examined, e.g., in a patient. These can be caused, for example, by susceptibility jumps at air-tissue or bone-tissue interfaces and lead to local phase accumulations across the readout echo train, which manifests itself as local geometric distortions in the image. To minimize the severity of geometric distortions, phase accumulation must therefore be minimized. This can be achieved, for example, by keeping the temporal length of the readout echo trains short; however, this limits the spatial resolution or requires the use of parallel imaging techniques.

[0014] It is also possible to acquire k-space in segments, i.e., to acquire measurement data in segments. In principle, k-space can be segmented in the phase encoding direction and / or the readout direction.

[0015] The RESOLVE acquisition technique, first described in the article by Porter and Heidemann “High Resolution Diffusion-Weighted Imaging Using Readout-Segmented Echo-Planar Imaging, Parallel Imaging and a Two-Dimensional Navigator-Based Reacquisition”, MRM 62, 2009, pp. 468-475, is a variant of an EPI acquisition technique in which segmentation occurs in the readout direction instead of the phase encoding direction.

[0016] Generally speaking, in a RESOLVE sequence, e.g., after a diffusion preparation, the segment of k-space that is to be filled with measurement data in the following readout phase is defined by a prephasing gradient. During the readout phase, a train of echo signals is acquired as measurement data for the defined segment using a sinusoidal readout gradient. Using a further gradient, which has a polarity opposite to the prephasing gradient and is switched after the readout phase, the system returns to the k-space center in the readout direction before another refocusing pulse is emitted, which leads to the formation of further echo signals that are acquired, e.g., using a navigator readout gradient as navigator data of the k-space center.The navigator data thus acquired for each segment can be used to enable a combination of the various segments without artifacts by performing a non-linear phase correction of possible phase changes between the acquisitions of the measurement data in the individual segments based on the phases of the acquired navigator data, and / or to detect segments whose measurement data are unusable and should be repeated, as described in more detail in the article by Porter and Heidemann mentioned above. Measurement data acquired using RESOLVE can also be acquired accelerated in the phase-encoding direction, e.g., using a parallel acquisition technique. This means that measurement data is not acquired in the phase-encoding direction along all the k-space lines required for a complete scan of k-space according to Nyquist, but rather, depending on the acceleration factor PAT=n, only along every nth k-space line.

[0017] Fig. Figure 2 shows an example of a typical achievable sampling scheme in k-space using a RESOLVE pulse sequence scheme. Here, axis 51 represents the kx direction, and axis 53 represents the ky direction. The kx direction corresponds to the readout direction, and the ky direction to the phase encoding direction.

[0018] A corresponding segment selection gradient in the readout direction can be used to establish a first starting point 54 for scanning a segment 63. The partial k-space line 55 is acquired while a partial gradient with positive polarity of an epi-typical readout gradient is switched. The partial k-space line 56 is acquired while a partial gradient with negative polarity of the epi-typical readout gradient is switched. The shift in the phase-encoding direction between the partial k-space lines 55 and 56 is achieved by switching a gradient blip in the phase-encoding direction.

[0019] The further partial k-space lines 57, 58, 59, 60, 61 and 62 are recorded analogously during switching of further partial gradients of the epi-typical readout gradient with alternating polarity.

[0020] The partial k-space lines 55 to 62 cover the segment 63 of the k-space 52, which is segmented in the readout direction, for example, into the segments 62 to 67.

[0021] If a different segment selection gradient is applied in the readout direction, echo signals from another of the segments 64, 65, 66, or 67 of k-space 52 can be acquired. In the example shown, segment 65 is the central segment in k-space, since it encompasses the k-space center in the readout direction.

[0022] In the example shown, trajectories 68 and 69 (in segments 63 and 64) are shown with a certain distance in the readout direction. This is purely for clarity. In fact, echo signals can be acquired in such a way that no gaps arise in the acquired k-space in the readout direction, so that segmented but overall complete k-space lines can be sampled in the readout direction. The partial k-space lines contained in the exemplary marked fields 70 and 85 then complement each other to form a complete k-space line.

[0023] A disadvantage of the RESOLVE sequence is the long acquisition time required overall, as multiple segments must be acquired for a complete k-space image. Acquiring each segment requires a repetition time (TR). Furthermore, each echo train, and thus each repetition time (TR), is extended by the acquisition of the image navigator.

[0024] To shorten the acquisition time of a measurement data set that is complete according to Nyquist, under certain conditions certain measurement data from the complete set can be omitted and supplemented later. Acquiring an incomplete measurement data set requires less time than recording a complete measurement data set. One such method is a partial Fourier technique (PF). With PF techniques, it is not usually the entire k-space that is sampled, i.e., recorded or measured, but only a part of k-space specified by a PF factor and further determined by symmetry considerations of k-space. The symmetry of k-space is used in PF techniques to supplement or fill in the unmeasured part of k-space using various reconstruction methods.In a method known as "zero-filling," unacquired regions of k-space are filled with zeros or zero values. This is a very simple method that requires little computational power.

[0025] An alternative method for supplementing non-acquired measurement data in PF procedures uses a so-called POCS algorithm (Projection Onto Convex Sets). This algorithm estimates missing, i.e., unmeasured, parts of k-space in a measurement data set in an iterative process, ensuring data consistency with the actually measured parts of k-space in the measurement data set, i.e., the actually measured k-space values. For this purpose, reference is made, for example, to the publication "Implementation and Assessment of Diffusion-Weighted Partial Fourier Readout-Segmented Echo-Planar Imaging" by Robert Frost et al. in Magnetic Resonance in Medicine 68:441-451 (2012). This methodology can lead to improved sharpness or spatial resolution, but is not always reliably applicable, for example, depending on certain phase variations in the underlying measurement data set.

[0026] Especially with segmented acquisition techniques, such as the above-mentioned RESOLVE technique, which is susceptible to phase changes, phase inconsistencies between the individual segments contained in the measurement data sets in conjunction with PF techniques can lead to artifacts, in particular so-called ringing artifacts and / or grid-like mesh artifacts.

[0027] In the article by Dan et al., "Single-Shot Multi-b-value (SSMb) Diffusion-weighted MRI Using Spin Echo and Stimulated Echoes with Variable Flip Angles," Proc. Intl. Soc. Mag. Reson. Med. 30; p. 1865, 2022, a conventional echo-planar imaging technique is presented. By shifting magnetization to the longitudinal axis and re-exciting this magnetization, it is possible to obtain multiple diffusion weights per slice after excitation. The achievable resolution is still limited by inhomogeneities in the object under investigation.

[0028] An SMS technique can also be used to reduce the repetition time (TR) by simultaneously acquiring multiple slices. However, to achieve effective diffusion contrast, the minimum TR is limited to approximately 3 seconds, so a reduction in the repetition time (TR) is often only possible to a limited extent, thus limiting the benefits achievable through the use of an SMS technique.

[0029] The invention is based on the object of enabling the recording of diffusion-weighted measurement data with good quality and the shortest possible total measurement time.

[0030] The object is achieved by a method for the optimized recording of diffusion-weighted measurement data of an examination object by means of a magnetic resonance system according to claim 1, a magnetic resonance system according to claim 12, a computer program according to claim 13, and an electronically readable data carrier according to claim 14.

[0031] A method according to the invention for the optimized acquisition of diffusion-weighted measurement data of an examination object using a magnetic resonance system comprises the steps: a) Imparting a first RF pulse that tilts a longitudinal magnetization into the transverse plane, b) switching a diffusion dephasing gradient of a desired diffusion strength in a desired diffusion direction after the first RF pulse, c) radiating a second RF pulse which shifts part of the magnetization in the transverse plane back into the longitudinal axis and leaves another part of the magnetization in the transverse plane in this plane, d) switching a first diffusion rephasing gradient in the desired diffusion direction, which rephases the magnetization remaining in the transverse plane, so that a first echo signal is formed after the first diffusion rephasing gradient, e) switching a segment selection gradient in the readout direction to select one of at least three segments, f) recording the first echo signal in the segment selected by the segment selection gradient as measurement data according to an echo-planar recording technique and capturing this as measurement data of a first measurement data set, g) after recording the first echo signal, radiating a third RF pulse which tilts at least part of the magnetization shifted back into the longitudinal axis by the second RF pulse into the transverse plane, h) switching a further diffusion rephasing gradient in the desired diffusion direction, which rephases the magnetization tilted into the transverse plane by the third RF pulse, so that a further echo signal is formed after the further diffusion rephasing gradient, i) switching a further segment selection gradient in the readout direction to select one of at least three segments, j) Recording the further echo signal in the segment selected by the further segment selection gradient as measurement data according to an echo-planar recording technique and capturing this as measurement data of a further measurement data set, k) Storing and / or further processing the first measurement data set recorded and any further measurement data sets recorded.

[0032] By recording a first and at least one further echo signal segmented in the readout direction according to the invention after a common exciting first RF pulse, several diffusion-weighted measurement data sets (at least one first and one further measurement data set) are recorded after a common excitation and thus within a repetition time TR. This makes it possible to reduce the overall measurement time required for recording measurement data sets with all desired diffusion weightings. Furthermore, the influence of a movement of the examined object, for example, the breathing of a patient under examination, can be reduced by the temporally closely related recordings of different measurement data sets with different diffusion weightings according to the invention.

[0033] A magnetic resonance system according to the invention comprises a magnet unit, a gradient unit, a radio frequency unit and a control device designed to carry out a method according to the invention, having a diffusion control unit.

[0034] A computer program according to the invention implements a method according to the invention on a control device when it is executed on the control device. For example, the computer program comprises instructions that, when the program is executed by a control device, e.g., a control device of a magnetic resonance imaging system, cause this control device to execute a method according to the invention. The control device can be configured in the form of a computer.

[0035] The computer program can also be in the form of a computer program product which can be loaded directly into a memory of a control device, with program code means for carrying out a method according to the invention when the computer program product is executed in a computing unit of the computing system.

[0036] A computer-readable storage medium according to the invention comprises instructions which, when executed by a control device, e.g. a control device of a magnetic resonance system, cause the control device to carry out a method according to the invention.

[0037] The computer-readable storage medium can be designed as an electronically readable data carrier which comprises electronically readable control information stored thereon, which control information comprises at least one computer program according to the invention and is designed such that it carries out a method according to the invention when the data carrier is used in a control device of a magnetic resonance system.

[0038] The advantages and details given with regard to the method also apply analogously to the magnetic resonance system, the computer program product and the electronically readable data carrier.

[0039] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings. The examples listed do not represent a limitation of the invention. They show: Fig. 1 a schematic flow diagram of a method according to the invention, Fig. 2 an example of a typical k-space sampling scheme using a RESOLVE pulse sequence scheme, Fig. 3 a schematically illustrated part of a pulse sequence scheme for the acquisition of measurement data according to the invention, Fig. 4 a schematic representation of a magnetic resonance system according to the invention.

[0040] Fig. 1 is a schematic flow diagram of a method according to the invention for the optimized acquisition of diffusion-weighted measurement data of an examination object by means of a magnetic resonance system.

[0041] A first RF pulse RF1, which tilts a longitudinal magnetization into the transverse plane, is radiated (block 101).

[0042] After the first RF pulse has been radiated, a diffusion dephasing gradient D1 of a desired diffusion strength in a desired diffusion direction is switched (block 102).

[0043] A second RF pulse is applied, shifting a portion of the magnetization in the transverse plane caused by the first RF pulse back to the longitudinal axis, while leaving another portion of the magnetization in the transverse plane in this plane (block 103). Such a second RF pulse is known, for example, from the above-mentioned article by Dan et al.

[0044] A first diffusion rephasing gradient D2 is switched in the desired diffusion direction, which rephases the magnetization remaining in the transverse plane so that a first echo signal is formed after the first diffusion rephasing gradient (block 104).

[0045] By means of the switched diffusion dephasing gradient D1 and the switched first diffusion rephasing gradient D2, which are both switched before the formation of the first echo signal and have an influence on the magnetization from which the first echo signal is formed, the first echo signal can be diffusion-prepared according to a first diffusion value which is based on the previously switched diffusion dephasing gradient D1 and the first diffusion rephasing gradient D2 and the first time period DT1 elapsed between the switched diffusion dephasing gradient D1 and the first diffusion rephasing gradient D2, so that the acquired measurement data are diffusion-weighted according to the first diffusion value.

[0046] The first diffusion value (also called b-value) can be calculated according to b = γ 2 G 2 δ 2(DT1-δ / 3), where γ is the gyromagnetic ratio, G is the gradient amplitude of the diffusion dephasing gradient D1 and the first diffusion rephasing gradient D2, δ is the gradient duration of the diffusion dephasing gradient D1 and the first diffusion rephasing gradient D2, and DT1 is the time duration between the pair of diffusion dephasing gradient D1 and first diffusion rephasing gradient D2.

[0047] A segment selection gradient Se1 is switched in the readout direction to select one of at least three segments in which a k-space is to be filled with measurement data (block 105).

[0048] The formed first echo signal is recorded as measurement data in the segment selected by the segment selection gradient Se1 according to an echo-planar recording technique and the measurement data is acquired as measurement data of a first measurement data set MDS1 (block 106).

[0049] After recording the first echo signal, a third RF pulse RF3 is radiated, which tilts at least part of the magnetization shifted back into the longitudinal axis by the second RF pulse RF2 into the transverse plane (block 107).

[0050] A further diffusion rephasing gradient D3 is switched in the desired diffusion direction, which rephases the magnetization tilted into the transverse plane by the third RF pulse RF3, so that after the further diffusion rephasing gradient D3 a further echo signal is formed (block 108).

[0051] By means of the switched diffusion dephasing gradient D1 and the switched further diffusion rephasing gradient D3, which are both switched before the formation of the further echo signal and have an influence on the magnetization from which the further echo signal is formed, the further echo signal can be diffusion-prepared according to a further diffusion value which is based on the previously switched diffusion dephasing gradient D1 and the further diffusion rephasing gradient D3 and the further time period DTw elapsed between the switched diffusion dephasing gradient D1 and the further diffusion rephasing gradient D3, so that the acquired measurement data are diffusion-weighted according to the further diffusion value.

[0052] The further diffusion value (b-value) can be calculated again, for example, according to b = γ 2 G 2 δ 2(DTw-δ / 3), where γ is the gyromagnetic ratio, G is the gradient amplitude of the diffusion dephasing gradient D1 and the further diffusion rephasing gradient D3, δ is the gradient duration of the diffusion dephasing gradient D1 and the further diffusion rephasing gradient D3, and DTw is the time duration between the pair of diffusion dephasing gradient D1 and further diffusion rephasing gradient D3.

[0053] A further segment selection gradient Sew is switched in the readout direction to select one of the at least three segments (block 109).

[0054] The further echo signal formed is recorded as measurement data in the segment selected by the further segment selection gradient Sew according to an echo-planar recording technique and the measurement data is acquired as measurement data of a further measurement data set MDSw (block 110).

[0055] Recorded first measurement data sets MDS1 and recorded further measurement data sets MDSw can be saved and / or further processed.

[0056] For illustration purposes, a schematically illustrated part of a pulse sequence scheme for the inventive acquisition of measurement data in Fig. 3 shown.

[0057] The top line RF shows the possible RF pulses RF1, RF2, RF3, and RF4 to be switched, their temporal profile and in relation to the gradients described herein to be switched in the slice selection direction GS, the readout direction GR, and the phase encoding direction GP. For example, for slice selection, gradients can be switched in a known manner in the slice selection direction GS simultaneously with the RF pulses RF1, RF2, RF3, and / or RF4 to be radiated, which can limit the effect of the respective RF pulses to a desired slice.

[0058] For recording a first formed echo signal according to an echo-planar recording technique, for example, a bipolar readout gradient R1 in the readout direction GR with associated gradient blips to be switched in its course in the phase encoding direction GP can be used to acquire measurement data of a first measurement data set MDS1.

[0059] Analogously, for recording a further formed echo signal according to an echo-planar recording technique, a bipolar readout gradient Rw in the readout direction GR with associated gradient blips to be switched in its course in the phase encoding direction GP can be used to acquire measurement data of a first measurement data set MDSw.

[0060] The first and second RF pulses RF1 and RF2 to be radiated as described above, together with the diffusion dephasing gradient D1, which is shown here as being switched in all coding directions GS, GR, GD, but can be switched in any diffusion direction, can be referred to as a preparation module Prep.

[0061] Switching a first diffusion rephasing gradient D2 and the subsequent recording according to blocks 105 and 106 can be referred to as a module EZ1 of a first echo signal and switching a further diffusion rephasing gradient D3 and the subsequent recording according to blocks 109 and 110 can be referred to as a module EZw of a further echo signal.

[0062] Blocks 107 to 110 can be repeated at least once in direct succession, with a third RF pulse RF3 leaving a portion of the magnetization shifted into the longitudinal axis by the second RF pulse RF2 in the longitudinal axis at least during a first execution of block 107, so that a third RF pulse RF3 tilts at least a portion of the magnetization shifted into the longitudinal axis by the second RF pulse RF2 and left in this axis by a preceding third RF pulse RF3 into the transverse plane during a subsequent execution of block 107. For this purpose, a query 121 can be provided which checks, for example, whether blocks 107 to 110 should be executed multiple times after a first RF excitation pulse RF1, and, if so, ensures that all desired repetitions are carried out.

[0063] In the example of Fig. 3, for example, after recording the measurement data of another measurement data set MDSw, a third RF pulse RF3 would be transmitted, and another module of another echo signal EZw would be carried out. Depending on the desired maximum diffusion value (b-value) or the desired maximum repetition time TR, the number of sampling points can be increased, for example, for calculating ADC maps and / or SNR efficiency.

[0064] For further echo signals and corresponding further measurement data sets MDSw, the relationship to a further diffusion value (b-value) described above applies, whereby the time duration DTw between the pair of diffusion dephasing gradients D1 and (repeated) further diffusion rephasing gradients is extended accordingly, whereby larger b-values ​​are achieved by later repetitions of blocks 107 to 110 after a common first RF pulse RF1.

[0065] After recording at least one further echo signal as measurement data of a further measurement data set MDSw, a center selection gradient Se3 can be switched in the readout direction (block 111), which selects a central segment of the at least three segments in the readout direction, and then a fourth RF pulse RF4 can be radiated (block 112), which refocuses the further echo signal into a third echo signal, which can be recorded in the central segment selected by the center selection gradient according to an echo-planar recording technique and acquired as navigator data NAVw for the immediately previously recorded measurement data of the further measurement data set MDSw of the further echo signal (block 113). For example, an R-refocusing pulse can be used as such a fourth RF pulse RF4.

[0066] For this purpose, a query 121 can additionally or alternatively check whether blocks 111 to 113 are to be carried out and whether navigator data NAVw are to be recorded for a further measurement data set MDSw, and, if so, initiate the execution of blocks 111 to 113. Whether navigator data and, if so, for which further measurement data sets MDSw are to be recorded can, for example, be determined in advance by a user input or can be carried out automatically for all or all Nth (N is a natural number) or all Kth (K is a natural number) repetitions of blocks 107 to 110 to be carried out directly one after the other.

[0067] A recording of navigator data NAVw according to a block 113 can be referred to as a module EZn of a third echo signal to be recorded as navigator data.

[0068] Thus, after each or selected acquisitions of further echo signals as measurement data of a further measurement data set MDSw, i.e., after each or selected executions of blocks 107 to 110, a center selection gradient Se3 can be switched, a fourth RF pulse RF4 can be radiated, and a modulus EZn of a third echo signal to be acquired as navigator data can be carried out. This can be particularly advantageous if several further measurement data sets MDSw with different diffusion values ​​or if a lowest desired diffusion value is significantly above 50, because navigator data NAVw acquired in this way directly after measurement data to be corrected from further measurement data sets MDSw can further improve a phase correction between segments of the respective further measurement data sets MDSw.

[0069] On the basis of acquired navigator data NAVw, a phase correction of acquired measurement data MDSw can be carried out in a manner known per se (see, for example, the introduction to the description) (block 114) in order to obtain further phase-corrected measurement data sets MDSw'.

[0070] Alternatively or additionally, navigator data NAVw can be used to identify corrupted measurement data from further measurement data sets MDSw (block 114). By recording third echo signals and capturing them as navigator data NAVw directly after measurement data from further measurement data sets MDSw that are to be checked for corruption, the detection of corrupted measurement data and thus of newly recorded segments can be improved. If, for example, only one set of navigator data NAVw belonging to a specific segment of another measurement data set MDSw out of several sets of navigator data NAVw acquired after a common first RF pulse RF1 identifies the measurement data of this specific segment of the further measurement data set MDSw as corrupted (but not all further measurement data acquired, for example, in other segments), measurement data only need to be recorded again for this specific segment of the further measurement data set MDSw.However, if in another case all navigator data NAVw acquired for further measurement data sets MDSw after a common first RF pulse RF1 identify all measurement data of the further measurement data sets MDSw as corrupted, it can be assumed that a signal responsible for the corruption was already detected during the first diffusion coding by the diffusion dephasing gradient D1 and all echo signals acquired after this diffusion dephasing gradient D1 should be re-acquired.

[0071] A query 122 can be used to check whether corrupted measurement data has been identified in further measurement data sets MDSw and, if so, to initiate a new recording of further corresponding echo signals.

[0072] In this case, further echo signals from measurement data identified as corrupted from further measurement data sets can be recorded again, in particular in a renewed execution of blocks 101 to 110, and again stored and / or further processed.

[0073] Blocks 101 to 110 (and storage and / or further processing of the first and further measurement data sets MDS2 and MDSw acquired thereby) can be repeated with different segment selection gradients Se1, Sew until measurement data from the first measurement data set MDS1 and all further measurement data sets MDSw have been acquired in all desired segments. For this purpose, a query 123 can be provided which, for example, checks whether measurement data from the first measurement data set MDS1 and all further measurement data sets MDSw have been acquired in all desired segments and, if not, ensures that blocks 101 to 110 are repeated until measurement data from the first and further measurement data sets MDS1, MDSw have been acquired in all segments.

[0074] The first and further segment selection gradients Se1 and Sew can select different segments of the at least three segments in at least one implementation of blocks 101 to 110. This is particularly useful if, for example, corrupted measurement data has been identified for certain segments in further measurement data sets and measurement data is to be acquired again only in these specific segments. By varying the recorded segments in this way between the respective modules of first or further echo signals EZ1, EZw, the re-acquisition of the segments identified as defective using previously only corrupted measurement data can be designed particularly efficiently, for example in a re-acquisition section at the end of the imaging sequence in which measurement data identified as corrupted is to be acquired again.

[0075] Blocks 101 to 110 (and storage and / or further processing of the first and further measurement data sets MDS2 and MDSw acquired thereby) can be repeated with different diffusion dephasing gradients D1 and first and further diffusion rephasing gradients D2, D3 of different diffusion strengths and different desired diffusion directions until measurement data have been acquired in a first or further measurement data set MDS1, MDSw for all desired diffusion strengths and diffusion directions. For this purpose, a query 123 can additionally or alternatively check whether measurement data have been recorded in a first or further measurement data set MDS1, MDSw for all desired diffusion strengths and diffusion directions and, if not, can cause blocks 101 to 110 to be repeated until measurement data have been recorded in a first or further measurement data set MDS1, MDSw for all desired diffusion strengths and diffusion directions.

[0076] In this way, for example, further diffusion values ​​can be generated, according to which the first and further measurement data sets MDS1, MDSw are diffusion-weighted. Further diffusion values ​​can again be calculated, for example, according to b = γ 2 G 2 δ 2 (DT1-δ / 3) or according to b = γ 2 G 2 δ 2 (DTw-δ / 3), where γ is the gyromagnetic ratio, G is the gradient amplitude of the respective gradients (see above), δ is the gradient duration of the respective gradients (see above) and DT1 or DTw is the time duration between the pair of diffusion dephasing gradients D1 and first diffusion rephasing gradients D1 or between the pair of diffusion dephasing gradients D1 and further diffusion rephasing gradients D3.

[0077] After recording a first echo signal, at least one crusher gradient C can be switched in at least one coding direction, which dephases any remaining signal of the first echo signal before a third RF pulse RF3 is radiated.

[0078] An increase in the number of modules of further echo signals and / or third echo signals carried out after a first RF pulse RF1 and thus an extension of the repetition time TR can be particularly useful in combination with SMS acquisition techniques, since an achievable reduction in the repetition time TR is often already limited by the required image contrast and thus can possibly only be optimally utilized by an increase in the number of modules of further echo signals and / or third echo signals carried out after a first RF pulse RF1.

[0079] The proposed method offers the advantage of reducing measurement time for echo-planar acquisition techniques that are segmented in the readout direction, such as RESOLVE sequences. By acquiring MDS1 and MDSw measurement data sets with different diffusion weightings in close temporal proximity, otherwise harmful influences caused by movement, including respiratory movement, can be reduced.

[0080] Fig. 4 schematically illustrates a magnetic resonance system 1 according to the invention. This comprises a magnet unit 3 for generating the basic magnetic field, a gradient unit 5 for generating the gradient fields, a radio-frequency unit 7 for irradiating and receiving radio-frequency signals, and a control device 9 designed to carry out a method according to the invention.

[0081] In the Fig.4, these subunits of the magnetic resonance system 1 are shown only schematically. In particular, the radio-frequency unit 7 can consist of several subunits, for example, several coils such as the schematically shown coils 7.1 and 7.2, or more coils, which can be configured either only for transmitting radio-frequency signals or only for receiving the triggered radio-frequency signals, or for both.

[0082] To examine an object U, for example a patient or a phantom, it can be placed on a couch L in the magnetic resonance system 1 in its measurement volume. The slice or slab S i represents an exemplary target volume of the object under investigation from which echo signals are to be recorded and recorded as measurement data.

[0083] The control device 9 serves to control the magnetic resonance system 1 and can, in particular, control the gradient unit 5 by means of a gradient controller 5' and the radio-frequency unit 7 by means of a radio-frequency transmit / receive controller 7'. The radio-frequency unit 7 can comprise several channels on which signals can be transmitted or received.

[0084] The radio-frequency unit 7, together with its radio-frequency transmit / receive control 7', is responsible for generating and transmitting (transmitting) a radio-frequency alternating field for manipulating the spins in a region to be manipulated (for example, in slices S to be measured) of the object U under investigation. The center frequency of the radio-frequency alternating field, also referred to as the B1 field, is generally set as close as possible to the resonance frequency of the spins to be manipulated. Deviations from the center frequency to the resonance frequency are referred to as off-resonance. To generate the B1 field, controlled currents are applied to the RF coils in the radio-frequency unit 7 by means of the radio-frequency transmit / receive control 7'.

[0085] Furthermore, the control device 9 comprises a diffusion control unit 15, with which RF pulses and gradients according to the invention for the diffusion preparation of measurement data can be determined, which can be implemented by the gradient controller 5' and the radio-frequency transmit / receive controller 7'. The control device 9 is designed overall to carry out a method according to the invention.

[0086] A computing unit 13 included in the control device 9 is configured to perform all the computational operations required for the necessary measurements and determinations. Intermediate results and results required for this purpose or determined in this process can be stored in a memory unit S of the control device 9. The units shown are not necessarily to be understood as physically separate units, but merely represent a subdivision into meaningful units, which, however, can also be realized, for example, in fewer or even in only a single physical unit.

[0087] Via an input / output device I / O of the magnetic resonance system 1, control commands can be sent to the magnetic resonance system, e.g. by a user, and / or results of the control device 9, such as image data, can be displayed.

[0088] A method described herein may also be in the form of a computer program comprising instructions that execute the described method on a control device 9. Likewise, a computer-readable storage medium may be present comprising instructions that, when executed by a control device 9 of a magnetic resonance system 1, cause the system 1 to execute the described method.

[0089] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Zitierte Nicht-Patentliteratur

[0000] High Resolution Diffusion-Weighted Imaging Using Readout-Segmented Echo-Planar Imaging, Parallel Imaging and a Two-Dimensional Navigator-Based Reacquisition“, MRM 62, 2009, S.468-475

[0015] Implementation and Assessment of Diffusion-Weighted Partial Fourier Readout-Segmented Echo-Planar Imaging“ von Robert Frost et.al. in Magnetic Resonance in Medicine 68:441-451 (2012

[0025]

Claims

[1] Method for the optimized recording of diffusion-weighted measurement data of an examination object (U) by means of a magnetic resonance system (1) comprising the steps: a) Radiation of a first RF pulse (RF1) which tilts a longitudinal magnetization into the transverse plane, b) switching a diffusion dephasing gradient (D1) of a desired diffusion strength in a desired diffusion direction after the first RF pulse (RF1), c) radiating a second RF pulse (RF2) which shifts part of the magnetization in the transverse plane back into the longitudinal axis and leaves another part of the magnetization in the transverse plane in this plane, d) switching a first diffusion rephasing gradient (D2) in the desired diffusion direction, which rephases the magnetization remaining in the transverse plane, so that a first echo signal is formed after the first diffusion rephasing gradient, e) switching a segment selection gradient (Se1) in the readout direction to select one of at least three segments, f) Recording the first echo signal in the segment selected by the segment selection gradient as measurement data according to an echo-planar recording technique and capturing this as measurement data of a first measurement data set (MDS1), g) after recording the first echo signal, radiating a third RF pulse (RF3) which tilts at least part of the magnetization shifted back into the longitudinal axis by the second RF pulse into the transverse plane, h) switching a further diffusion rephasing gradient (D3) in the desired diffusion direction, which rephases the magnetization tilted into the transverse plane by the third RF pulse (RF3), so that a further echo signal is formed after the further diffusion rephasing gradient (D3), i) switching a further segment selection gradient (Sew) in the readout direction to select one of the at least three segments, j) Recording the further echo signal in the segment selected by the further segment selection gradient (Sew) as measurement data according to an echo-planar recording technique and capturing this as measurement data of a further measurement data set (MDSw), k) Storing and / or further processing the recorded first measurement data set (MDS1) and recorded further measurement data sets (MDSw). [2] Method according to claim 1, wherein a first echo signal is diffusion-prepared in the desired diffusion direction with a first diffusion value according to the previously switched diffusion dephasing gradient (D1) and the first diffusion rephasing gradient (D2) and the first time period (DT1) elapsed between the switched diffusion dephasing gradient (D1) and the first diffusion rephasing gradient (D2), and the measurement data of the first measurement data set (MDS1) are diffusion-weighted according to the first diffusion value, and wherein a further echo signal is diffusion-prepared in the desired diffusion direction with a further diffusion value according to the previously switched diffusion dephasing gradient (D1) and the further diffusion rephasing gradient (D3) and the first time period (DT1) elapsed between the switched diffusion dephasing gradient (D1) and the further Diffusio-rephasing gradient (D3) elapsed further time period (DTw) is diffusion-prepared,and the measurement data of further measurement data sets are diffusion-weighted according to the respective further diffusion value. [3] Method according to one of the preceding claims, wherein steps g) to j) are repeated at least once directly one after the other, wherein a third RF pulse (RF3) at least during a first execution of step g) leaves a part of the magnetization shifted into the longitudinal axis by the second RF pulse (RF2) in this axis, so that a third RF pulse (RF3) during a subsequent execution of step g) tilts at least a part of the magnetization shifted into the longitudinal axis by the second RF pulse (RF2) and left in this axis by a preceding third RF pulse (RF3) into the transverse plane. [4] Method according to one of the preceding claims, wherein after recording at least one further echo signal, a center selection gradient (Se3) is switched in the readout direction, which selects a central segment of the at least three segments in the readout direction, and then a fourth RF pulse (RF4) is radiated, which refocuses the further echo signal to a third echo signal, which is recorded in the central segment selected by the center selection gradient (Se3) according to an echo-planar recording technique and is acquired as navigator data (NAVw) for the further measurement data set (MDSw) of the further echo signal. [5] Method according to claim 4, wherein a phase correction of acquired measurement data is carried out on the basis of acquired navigator data (NAVw). [6] Method according to one of claims 4 or 5, wherein navigator data (NAVw) are used to identify corrupted measurement data of further measurement data sets (MDSw). [7] Method according to claim 6, wherein further echo signals from measurement data of further measurement data sets (MDSw) identified as corrupted are recorded again, in particular in a renewed execution of steps a) to k). [8] Method according to one of the preceding claims, wherein steps a) to k) are repeated with different segment selection gradients (Se1, Sew) until measurement data of the first measurement data set (MDS1) and all desired further measurement data sets (MSDw) have been recorded in all desired segments. [9] Method according to one of the preceding claims, wherein the first and the further segment selection gradient (Se1, Sew) select different segments of the at least three segments in at least one implementation of steps a) to k). [10] Method according to one of the preceding claims, wherein steps a) to k) are repeated with different diffusion dephasing gradients (D1) and first and further diffusion rephasing gradients (D2, D3) of different diffusion strengths and different desired diffusion directions until measurement data have been recorded in a first or further measurement data set (MDS1, MDSw) for all desired diffusion strengths and diffusion directions. [11] Method according to one of the preceding claims, wherein after the recording of a first echo signal at least one crusher gradient (C) is switched in at least one coding direction, which dephases any remaining signal of the first echo signal. [12] Magnetic resonance system (1) comprising a magnet unit (3), a gradient unit (5), a radio-frequency unit (7) and a control device (9) with a radio-frequency transmission / reception control (7') and with a diffusion control unit (15), wherein the control device (9) is designed to carry out a method according to one of claims 1 to 11 on the magnetic resonance system (1). [13] Computer program comprising instructions which, when the program is executed by a control device (9) of a magnetic resonance system (1), cause the system (1) to carry out the method according to one of claims 1 to 11. [14] Computer-readable storage medium, comprising instructions which, when executed by a control device (9) of a magnetic resonance system (1), cause the system (1) to carry out the method according to one of claims 1 to 11.

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