Patent Publication Number: US-10317500-B2

Title: Magnetic resonance imaging with randomly distributed recording of raw data

Description:
RELATED APPLICATION 
     The present application is a continuation of application Ser. No. 14/719,673, filed on May 22, 2015. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention concerns a method for recording magnetic resonance (MR) signals from an examination object in an MR system, wherein at least part of the examination object moves during the data acquisition. The invention also concerns an MR system for implementing such a method. 
     Description of the Prior Art 
     If the examination object moves during recording (acquisition) of the MR signals, this leads to an incorrect registering of the measured MR data and to image artifacts, also called ghosting. The movement of the examination object, e.g. an examination person, usually occurs due to breathing or due to the movement of the heart or due to other undesirable movements such as a tremor in elderly or ill patients. Most movement in an examination object is periodic, wherein each cycle of movement can constitute a problem, particularly in the case of long recording times. One possibility for imaging moving examination objects is known as the single-shot technique, in which the raw data domain (k-space) is filled completely after radiation of an RF pulse and in which recording of the data is quick enough to freeze the movement. A further recording possibility is the segmented recording technique in which the data acquisition is divided into multiple movement phases and MR data are recorded only in comparable movement phases. The first possibility mentioned above limits the measurement time, and the second possibility increases the complexity of the measurement and is not effective since MR signals are recorded only in certain movement phases. 
     There is also the possibility of detecting the movement of the examination object and, as in the segmented recording technique, of using only the movement phases in which a slight movement occurs, but this lengthens the recording time, and this in turn increases the probability of the examination person moving. A method for recording MR signals is therefore needed in which MR data are recorded in all movement phases of a moving examination object. 
     One possibility for correcting movement artifacts is to fill raw data space with raw data lines such that the movement occurs when the peripheral raw data are recorded, i.e. the outer k-space region is filled, with the MR data falsified by movement then being replaced. However, this leads to a reduction in the spatial resolution due to missing portions of raw data. 
     Compressed Sensing Technology also is known, in which severe under-sampling of raw data spaces occurs and additional information e.g. about the recorded MR image, is used. The raw data disrupted by movement are replaced by calculated portions of raw data. Mehdi et al describe in “Compressed Sensing Motion Compensation (CosMo): A Joint Prospective-Retrospective Respiratory Navigator for Coronary MRI”,  Magnetic Resonance in Medicine  66:1674-1681 (2011) the use of a method of this kind in which raw data falsified by movement are replaced with the use of Compressed Sensing Technology. A prerequisite for this is a random distribution of the remaining non-rejected MR raw data in the MR raw data space. 
     WO 2013/140276 A1 describes a reconstruction method in which MR raw data are recorded, wherein the density of the recorded raw data is increased incrementally until the examination person starts to breathe. 
     To summarize, all of the above-described methods have certain drawbacks. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to at least partially avoid these drawbacks and to provide a method in which moving examination objects produce MR images that have few or no movement artifacts. MR data should preferably be recorded during the different movement phases and be used for image reconstruction. 
     According to one aspect of the invention, a method for recording MR signals from an examination object in an MR system is provided in which raw data space is filled with MR signals during recording of the MR signals in raw data lines. In this method, movement information of the examination object is detected during recording of the MR signals and the movement information is grouped into different movement phases of the examination object. A temporally randomly distributed sequence for the recording of the raw data lines, with which at least one predetermined portion of the raw data space is filled with MR signals, is also determined. The MR signals are then recorded in the determined temporally randomly distributed sequence of the raw data lines in the predetermined portion of the raw data space, and each recorded raw data line is allocated to a movement phase of the examination object. 
     The requirements that are necessary for reconstructing portions of the raw data space using Compressed Sensing Technology are better met by the resulting randomly distributed or randomized sequence of the raw data lines in the raw data space, which form part of the same movement phase. There is a statistical randomly distributed data volume in the raw data space as a result of the above-described method. In particular the arrangement of the raw data, in which there was a movement and in which there was no movement, is randomly distributed in the raw data space. The movement is randomly distributed regardless of whether respectively adjacent raw data lines belong to the same or different movement phases. Because in the prior art successive adjacent raw data lines are read, spatially adjacent raw data lines are always affected by the same movement. MR images with slight movement artifacts can be reconstructed with this requirement. 
     Recorded raw data lines are preferably identified in which the examination object is in a movement rest phase in which there are the most raw data lines per movement phase. A reference image can be calculated from the movement rest phase with the use of these identified raw data lines. A further MR image is also calculated in at least one other movement phase. The further MR image is then calculated using the raw data lines which were recorded in the at least one other movement phase and the reference image, with the reference image being used by way of example in the PICCS method (Prior Image Constraint Compressed Sensing) to determine the missing raw data lines in the other movement phase with the use of Compressed Sensing Technology. Since the examination object remains longest in the movement phase in which the most raw data lines were recorded, this phase is also called a movement rest phase. 
     The further MR image can be determined, for example, with the use of an iterative reconstruction method in this connection, in which a cost function is minimized. 
     The temporally randomly distributed recording of the raw data lines can occur in a region of the raw data space outside of a predetermined central region of the raw data space. The central region of the raw data space can be recorded, for example, in the movement rest phase, it being possible to record the raw data lines outside of this predetermined central region during all movement phases. 
     If the raw data space is a two-dimensional Cartesian raw data space that is filled with MR signals entered into different raw data lines a phase encoding direction, it is temporally randomly chosen when a raw data line is filled with MR signals in the phase encoding direction. The raw data lines in which a pronounced movement occurs are therefore randomly distributed spatially in the phase encoding direction. The raw data space can also be a three-dimensional raw data space, with a temporally randomly distributed sequence of the recording of the raw data lines being determined in a first direction of the three-dimensional raw data space and in a second direction perpendicular thereto a temporally randomly distributed sequence of the recording of the raw data lines being determined. If the raw data lines run parallel to the third spatial direction, then it is chosen in a randomly distributed manner when which raw data lines in the two other spatial directions perpendicular thereto are recorded. 
     Radial recording of the raw data space is also possible with radial raw data lines that run through the center of the raw data space. The read sequence of the radial raw data lines can also be chosen so as to be temporally randomly distributed. This means that the individual raw data lines or spokes have an angle, relative to an arbitrarily chosen reference data line, with the sequence of angles in the sequence of measured raw data lines being temporally randomly distributed. This means that raw data lines that are adjacent are not recorded, and instead raw data lines are recorded that are temporally randomly distributed one after the other over the circle. 
     The invention also concerns a magnetic resonance apparatus for implementing the method described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates an MR system with which inventive MR images of moving examination objects are generated, in which the movement artifacts are minimized. 
         FIG. 2  schematically shows the movement sequence of an examination object and the associated filling of the raw data space with raw data lines according to the prior art, as well as according to the invention. 
         FIG. 3  schematically shows aspects of the invention with the application of the radial recording methods. 
         FIG. 4  schematically shows aspects of the invention in the recording of MR signals in a three-dimensional Cartesian raw data space. 
         FIG. 5  schematically shows the calculation of MR images in movement phases in which there is movement, taking into account prior knowledge of an MR image which was recorded during a rest phase. 
         FIG. 6  is a flowchart of basic steps for calculating an MR image according to one aspect of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically shows a magnetic resonance apparatus, with which inventive MR images of an examination object can be generated, which object executes a movement, preferably a cyclical movement. The magnetic resonance apparatus has a scanner with magnet  10  that generates a polarization field BO, with an examination person  12 , as the examination object, arranged on a bed  11  that is moved into the center of the magnet  10  in order to record spatially encoded magnetic resonance signals of the examination object from at least a portion of the examination object. By radiating radio-frequency pulse sequences and switching magnetic field gradients nuclear spins with the magnetization produced by the polarization field BO are deflected from the state of equilibrium and the resulting signal echoes are detected as MR signals by receiving coils (not shown). The magnetic resonance apparatus also has a central control computer  13  that is configured to operate the MR scanner. The central control computer  13  has a gradient controller  14  for controlling and switching magnetic field gradients that are generated by one or more gradient coils  15 . A radio-frequency (RF) controller  16  is provided for controlling and radiating the RF pulses for producing the magnetization with the use of an MR coil  17 . The coil can be a transmitting and receiving coil and can be designed as a body coil or as a local coil or a combination of the two. After selection of an imaging sequence, an image sequence controller  18  controls the sequence of the magnetic field gradients and the RF pulses and therefore also controls the RF controller  16  and the gradient controller  14 . The MR signals detected by the coil are converted in an arithmetic processor  19  into an MR image and can be displayed on a display unit  20 . Inputs for the MR system can be made via an input interface  21 . Different imaging sequences and sequences of imaging sequences are used to acquire MR data from specific regions of the body can be stored in and accessed from a memory unit  22 . 
     Program codes or programs may also be stored in the memory unit  22 , which, when run in the control computer  13 , are required for operation of the MR system. Those skilled in the art know how MR signals can be produced by a sequence of magnetic field gradients and RF pulses, so this need not be explained in more detail herein. A movement detector  23  detects movement of at least a portion of the examination person and groups the movement into different movement phases. It is also possible for the grouping into different movement phases to occur in the arithmetic processor  19 , which analyses the data of the detector  23 . The movement detector can be, for example, a breathing belt, an ECG or a camera that detects the movement of the object and possibly also analyzes the movement. 
     It will be explained in more detail below how the imaging sequence controller  18  inventively controls the temporal sequence of the recorded MR signals such that a randomized distribution of the raw data that belong to the same movement phase is possible in the raw data space (k-space). 
     The raw data space is filled with MR signals such that a random acquisition sequence of the raw data lines is implemented. As an example, a randomization can occur over time if the instant at which a raw data line is read is randomly chosen. There is therefore also a spatial random distribution of the raw data lines in which there is a movement position or amplitude. The raw data lines, which belong to one movement phase, are also randomly distributed spatially in the raw data space. As will be explained in detail below, the inventive recording method can be used in the two-dimensional or three-dimensional acquisition of raw data. 
       FIG. 2  shows how a random or statistical distribution of the recorded MR signals relative to a movement phase is achieved in a two-dimensional Cartesian recording of the raw data space. The examination object chosen in the examination person, such as by way of example part of the abdomen, can execute a movement, as is shown by course over time  25  in the upper section of  FIG. 2 . This may be the respiratory movement, for example, wherein in a first section  25   a  the examined region of the body was almost completely at rest, such as because the examination person held his or her breath. During the time frames  25   b ,  25   c  and  25   d  there was a movement, here a respiratory movement, of the examined region. The left side of  FIG. 2  shows how in a two-dimensional Cartesian case the raw data space is filled with raw data according to the prior art. A central region, as an example, of the raw data space  30  can be recorded during the time frame  25   a , wherein the k-space or raw data lines outside of the central region  30  are recorded consecutively. The outer raw data regions are then recorded in such a way that the raw data region  31  is recorded during the time frame  25   b , the raw data region  32  during the time frame  25   c , and the raw data region  33  during the time frame  25   d . This means, as shown on the left of  FIG. 2 , that the raw data lines, which are affected by the movement, are located in a respectively continuous k-space. If there is then an attempt to calculate the raw data regions  31 ,  32  and  33 , which constitutes gaps, for example using Compressed Sensing Technology, i.e. to fill the gaps, blurry images result since the necessary random distribution of missing raw data in the raw data space is absent. 
     The right lower region of  FIG. 2  shows the recording of MR data according to one embodiment of the invention. A central raw data region  35 , for example, can again be recorded during a time frame in which it is known that the movement of the examination region is minimal, or in which as many raw data lines as possible belong to one movement phase. In a region of the raw data space outside of the predetermined central region  35  the phase encoding values ky are then temporally randomly, chosen so that it is randomly chosen when each k-space line in the raw data regions  36  and  37  is read. In the raw data image shown on the right in  FIG. 2  the regions are again shown, analogously to the left raw data image, as regions without raw data lines in which a movement has taken place, such as a movement during the time frames  25   b - d . As may be seen in the right part of  FIG. 2 , here the regions with missing raw data lines are not distributed so as to be continuous as on the left in  FIG. 2 , but indiscriminately over ky. In the right part of  FIG. 2  the raw data lines corrupted by movement are randomly distributed over the outer raw data regions  36  and  37 . This spatial random distribution of the raw data is particularly suitable for Compressed Sensing Reconstruction Technology and to a large extent minimizes the movement artifacts that consequently arise. If the central raw data region  35  is read (filled) in a movement rest phase in which a large number of raw data lines can be recorded, then this allows a good contrast, by way of example in the case of T2-weighted multi-spin echo sequence or spin echo epo sequences or perfusion studies. 
       FIG. 3  shows the inventive method in the case of a two-dimensional radial recording technique. The raw data lines designed here as spokes are not measured in a sequential sequence in such a way that an incremental change in the angle leads to adjacent spokes always being successively read over the whole circle. In the case of the spokes shown in  FIG. 3  the sequence over time of the individual spokes is instead randomly chosen. This leads to a pattern in which the raw data lines (shown in broken lines), in which the examination object is moving, are randomly distributed over the disk. If there is then a time frame with movement in the movement phase, such as by way of example one of the phases  25   b - d  of  FIG. 2 , this means that the raw data lines affected by movement errors have a random distribution in k-space. If the raw data lines, which are affected by movement, are replaced with the use of Compressed Sensing Technology then this leads to an MR image with much fewer artifacts or no artifacts compared to the case where segments of a circle, such as the segment  31  or  32  with the associated raw data lines have to be reconstructed completely using Compressed Sensing Technology if the spokes in the circle are read consecutively, as is conventional in the prior art. 
     An embodiment is shown in connection with  FIG. 4  in which a three-dimensional image is acquired with individual raw data lines which run into the drawing plane, the read direction is along kx, so that a cylindrical raw data space results with the axis in the kx direction. In this embodiment the central raw data region is again filled with MR signals during the rest phase  25   a  shown in  FIG. 2 . The raw data region outside of the center with the reference numeral  41  is then read such that the individual dots shown in  FIG. 4 , which constitute a raw data line in the kx direction, are read in a temporally randomly distributed manner. The choice is made in a temporally randomly distributed manner in the kz direction and in the ky direction perpendicular thereto. This means that it is chosen in a temporally randomly distributed manner when the raw data line shown as a dot is read with a specific ky and a specific kz value. This in turn leads to a spatially random distribution of the raw data lines in the three-dimensional space in which the recorded MR signals are adversely affected by movement. If the three-dimensional raw data space had been read using a method of the prior art, segments such as the segment  43  by way of example would be filled as a whole with raw data lines which are affected by the movement. This would mean that all raw data lines in segment  43  would have to be replaced, and this constitutes a continuous block of raw data lines. The reconstruction of continuous blocks of this kind, such as block  43 , can be prevented according to the invention, however, since the raw data lines to be reconstructed are randomly distributed over the raw data space. 
     In the previously described method the raw data lines, in which a movement occurred which is greater than a limit value, are discarded and can be calculated by way of example with the aid of Compressed Sensing Technology. In the exemplary embodiment shown below in  FIG. 5  all raw data or k-space lines can be used for image reconstruction, even the raw data lines which were recorded during movement of the examination object. As shown in  FIG. 5 , different movement phases  51 ,  52  or  53  are identified in this embodiment. The movement phase  53  is, as an example, the movement phase that is to be categorized as the movement rest phase, in which as good as no movement occurs in the examined region in which the most raw data lines can be read. There was a movement in the movement phase  52 , as well as in the movement phase  53 . If what is involved is an MR image of examination objects that are affected by the respiratory movement, the three movement phases can be by way of example the inhalation phase, the exhalation phase and an intermediate phase. With examinations in the region of the heart, phases such as the systole, the diastole and the interim contraction can be distinguished as different movement phases. In this embodiment all raw data lines of the raw data space are again recorded and allocated to a movement phase. The raw data lines can then be detected in which there was no movement, i.e. the movement phase in which the most raw data lines are present. 
     In the illustrated case the movement information has been divided into three different movement phases  51 - 53 . Of course more or less than three movement phases can be distinguished. 
     An MR image can be determined with the use of Compressed Sensing Technology using the raw data lines that were recorded during the movement phase  53 . This normally delivers good results for undersampling factors to a factor of 4. If Compressed Sensing Technology is to be used in order by way of example to reconstruct MR image in the movement phases  52  or  51  in which there are undersampling factors, such as greater than 6, the MR image calculated in the movement phase  53  is used as a reference image. This reference image is taken into account in the calculation of MR images in the movement phases  52  or  51  in which the raw data lines, which are each located outside of phases  52  or  51 , are reconstructed. By use of the reference image of the movement phase  53 , the fact is used in this connection that during a movement a large number of commonalities conventionally occur between an MR image in the movement phase  51  and the MR images in the movement phase  52  or  53 , e.g. the contours of organs, the position of organ in the vicinity, etc. If the difference between these images is then produced in the movement phases  51 ,  52  or  53 , an MR image with very little spatial information is obtained, so that reference is also made to a “sparse image”. As a result of this requirement and the further requirement that there is a spatially randomly distributed undersampling of the raw data space, MR images can also be calculated using Compressed Sensing Technology for the movement phases  51  and  52 . One solution in this connection is the use of Prior Image Constraint Compressed Sensing (PICCS) Technology with iterative image reconstruction. Technology of this kind operates up to undersampling factors of 20. In this method a cost function, which has a constancy term DF, is minimized, wherein the difference between the images and what is known as the sparsity transformation of the image to be calculated is minimized according to the following equation: 
     
       
         
           
             
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     where DF(Ax,y)=(y−Ax) T (y−Ax), ∥ψ(x)∥ 1 =Sparsity Enforcing Term, M0, M1 are different movement phases of the examination object, x is the image to be reconstructed, y the measured raw data lines, x M1  the image to be reconstructed in movement phase M1, β a regularization term, A the transfer function of the MR system, and x M0  is the reference image in the movement rest phase. The Sparsity Enforcing Term indicates how thinly occupied the image or difference image is. 
       FIG. 6  summarizes the steps with which MR images can be created in different movement phases with high undersampling factors, with the artifacts caused by the undersampling being nevertheless minimal. 
     In a step  61  the movement of the examination object or of the part of the examination object is detected from which an MR image is to be created. The detection of the movement by the movement detector  23  of  FIG. 1  can occur by way of example through a breathing belt, ECG triggering or any other method that enables the movement of the examination object to be identified. As explained in connection with  FIG. 5 , the movement is also divided into different movement phases. In a step  62  the raw data lines are recorded in a temporally randomly distributed manner, as has been explained inter alia in connection with  FIGS. 2-4 . In a step  63  the recorded raw data line in the raw data space is stored with the associated movement information, so that it is known for each raw data line in which movement phase it was recorded. In a step  64  a check is made as to whether the data acquisition has finished, i.e. whether all desired raw data lines have been recorded. This can be the entire raw data space or just a portion thereof. 
     The reference image is created in a step  65 . As has been explained in connection with  FIG. 5 , the reference image is the image in which the examination object was at rest and in which a large number of raw data lines were recorded. This reference image, shown schematically in  FIG. 6  as reference image  67 , is then taken into account as image information to determine MR images in the other movement phases (step  65 ), for example using the PICCS technology mentioned above. It is thereby possible to determine MR images in different movement phases, wherein the artifacts in the case of undersampled MR images in particular are minimized. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.