Patent Publication Number: US-10310042-B2

Title: Hierrarchical mapping framework for coil compression in magnetic resonance image reconstruction

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application represents the U.S. National Stage of International Application No. PCT/US2015/027297, filed on Apr. 23, 2015 which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/983,690, filed on Apr. 24, 2014, both of which are incorporated herein by reference for all purposes. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under EB012107 and MH093765 awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and method for accelerated MRI and parallel imaging, including simultaneous multislice imaging (“SMS”). 
     MRI scans can often times be lengthy and, thus, not feasible for clinical application. These otherwise lengthy scans can have their scan time significantly reduced by implementing parallel imaging (“PI”) techniques to accelerate the data acquisition process. The data acquisition process is typically accelerated by undersampling k-space, such as by skipping phase-encoding or partition-encoding lines. 
     Typically, PI techniques make use of an array of radio frequency (“RF”) receiver coils. Each coil in the array has a unique spatial sensitivity profile, which can be utilized to supplement the spatial encoding of magnetic resonance signals and to remove aliasing artifacts caused by undersampling k-space. 
     There is a growing trend towards using large coil arrays with PI techniques because larger coil arrays allow greater acceleration of the data acquisition while also increasing the attainable signal-to-noise ratio. These larger coil array are not without their drawbacks, however. For instance, the computational cost of many PI algorithms scales with the square of the number of channels in the coil array, thereby leading to long reconstruction times when larger coil arrays are used. This increase in reconstruction time creates a strong incentive to reduce the effective number of channels used for PI. 
     Coil compression techniques can be used to reduce the computation burden of working with larger coil arrays. In general, both hardware coil compression and software-based coil compression can be used to linearly combine the data acquired on multiple coils into a reduced number of virtual coils. 
     Several techniques for software-based coil compression have been developed so far, with most algorithms being configured for use with two-dimensional Cartesian acquisitions, the geometric-decomposition coil compression (“GCC”) method recently proposed by T. Zhang, et al., in “Coil Compression for Accelerated Imaging with Cartesian Sampling,”  Magn Reson Med,  2013; 69(2):571-582. Other methods can be implemented for a wider range of sampling patterns. For example, global mapping methods, such as SVD compression, are applicable to a wide range of k-space sampling patterns. These global mapping methods suffer from low SNR retention at high coil compression rate, however. 
     In light of the foregoing, there remains a need for a coil compression technique that can be widely applied to different sampling patterns (e.g., both Cartesian and non-Cartesian sampling) without loss of SNR at high coil compression rates. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks by providing a method for reconstructing an image of a subject using a magnetic resonance imaging (“MRI”) system and a hierarchical mapping framework (“HMF”) based coil compression. Data is acquired from a subject using the MRI system, wherein the data is acquired using a plurality of receive channels associated with a radio frequency (“RF”) coil array. The strength of correlation between the plurality of receive channels is determined and used to create subgroups of the receive channels, each subgroup being based on the determined strength of correlation between the plurality of receive channels. At least one virtual channel is produced for each subgroup using a hierarchical semiseparable channel mixing across the subgroups. An image is then reconstructed for each subgroup from the acquired data and using the receive channels and virtual channels in each respective subgroup. An image of the subject is then produced by combining the reconstructed images for each subgroup. 
     The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of using a hierarchical mapping framework (“HMF”) to create four subgroups of receive channels for a coil array that has eight receive channels; 
         FIG. 2  is a flowchart setting forth an example method for reconstructing an image of a subject using an HMF-based coil compression; 
         FIG. 3  is a block diagram of an example of a magnetic resonance imaging (“MRI”) system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Described here are systems and methods for a hierarchical mapping framework (“HMF”) for coil compression that improves upon previously proposed coil compression algorithms. The proposed HMF can be applied to existing coil compression algorithms, such as GCC or SVD coil compression, to reconstruct images from data acquired using a highly-accelerated acquisition, such as a simultaneous multislice (“SMS”) echo-planar imaging acquisition. For instance, using the HMF, the performance of SVD compression can be significantly improved. The application of HMF to SVD compression can therefore extend the benefits seen with GCC for Cartesian acquisitions to non-Cartesian acquisitions, with which GCC is incompatible. 
     The hierarchical mapping framework for coil compression mitigates the loss in signal typically associated with global compression. In HMF, channels are initially organized based on strength of correlation and are then divided into subgroups. Each subgroup has an associated set of virtual channels that will be used to capture contributions from those channels which are excluded from that particular subgroup. This process is schematically illustrated in  FIG. 1 , where a full set  102  of receive channels is divided into four subgroups  104 ,  106 ,  108 ,  110  each having a reduced number of channels. For example, the full set  102  includes eight channels, {C n } for n=1, . . . , 8. Subgroup  104  contains channels {C 1 ,C 2 } and two virtual channels, {H 1,1 ,H 1,2 }. The virtual channels, {H 1,2 ,H 2,2 }, are produced using a coil compression of the channels not included in subgroup  102 . Likewise, subgroup  106  contains channels {C 3 ,C 4 } and two virtual channels {H 2,1 ,H 2,2 }; subgroup  108  contains channels {C 5 ,C 6 } and two virtual channels {H 3,1 ,H 3,2 }; and subgroup  110  contains channels {C 7 ,C 8 } and two virtual channels {H 4,1 ,H 4,2 }. 
     In order to efficiently model virtual channels for all subgroups, a hierarchal relationship is assigned across the subgroups. As one example, a hierarchically semiseparable (“HSS”) channel mixing matrix, such as the following, can be utilized: 
     
       
         
           
             
               
                 
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     Here, the basis parameters {U,V} can be generated using any linear channel compression technique, such as GCC or SVD coil compression, and {D} represents the mixing matrix for each subgroup. The parameters {B,R,W} allow for efficient propagation of the basis parameters {U,V} to produce the virtual channels for each of the subgroups. With comparable compression factors, the HSS mixing matrix, M, allows for the same computational complexity as a global compression operation, but with improved reconstruction. 
     Based on PI reconstruction models such GRAPPA and slice-GRAPPA, the subgroups (and the associated virtual channels) can be used to produce the original PI channels. To further improve efficiency, pseudo-channels can be assigned to each subgroup to capture the dominant features. This is accompanied by solving a non-linear least squares problem to create pseudo-channels whose magnitude is the sum-of-squares combination of the original channels in each subgroup. 
     As an example to demonstrate the efficiency of the HMF method, consider a split slice-GRAPPA (“Sp-SG”) formulation for SMS-EPI reconstruction of 2 mm isotropic voxels with a highly accelerated (e.g., MB=8) simultaneous multi-slice acquisition. In the example, the global SVD-based compression from 64 receive channels to 16 effective channels only retains 87% of the SNR. Using the HMF of the present invention, however, subgroups of size 8 can be formed with 8 virtual channels assigned to each subgroup. Assigning 2 pseudo-channels per group, the HMF method will also have 16 effective channels. This HMF strategy retains 99% of the SNR with the same computational complexity as the global compression. 
     Referring now to  FIG. 2 , a flowchart setting forth the steps of an example method for reconstructing an image from highly-accelerated data using an HMF-based coil compression is illustrated. The method begins by providing data that has been acquired with an MRI system, as indicated at step  202 . In some embodiments, the data has been previously acquired and can thus be provided by retrieving the previously acquired data from storage. In other embodiments, the data can be provided by acquiring it from a subject with an MRI system. Preferably, the provided data is acquired using an accelerated data acquisition scheme. Examples of accelerated acquisitions include undersampling k-space, performing a simultaneous multislice acquisition, or both. 
     To implement the HMF method, the strength of correlation of each coil in the receive coil array is determined, as indicated at step  204 . This strength of correlation is then used to divide the receive channels into a plurality of different subgroups, as indicated at step  206 . After the channels have been assigned to different subgroups, a set of virtual channels is formed for each subgroup, as indicated at step  208 . For instance, a hierarchically semiseparable channel mixing can be implemented to form the virtual channels. Advantageously, the HMF-based coil compression can be interfaced with existing Cartesian and non-Cartesian optimized compression schemes. 
     Optionally, pseudo-channels can be produced for each subgroup, thereby further reducing the number of effective channels, as will be described below in more detail. If pseudo-channels are desired, as determined at decision block  210 , then they are produced as indicated at step  212 . Using the subgroups of reduced channels, one or more images can be reconstructed from the provided data, as indicated at step  214 . Any suitable parallel image reconstruction method can be used. For instance, the HMF-based coil compression described here can be directly used for GRAPPA and slice-GRAPPA based reconstructions. In general, the reconstruction includes reconstructing an image for each subgroup and then combining the subgroups images to produce the final image. 
     PI reconstruction methods such as GRAPPA and Sp-SG are formulated to use all available channels as part of the unaliasing of accelerated data. For example, in the simple eight channel geometry of  FIG. 1 , both methods will employ all of the channels in the full set  102  to unalias any one given channel, C n . Typically, compression methods rely on a global mapping framework (“GMF”) that maps the original channels to a subset of channels to reduce the PI problem size. 
     The HMF-based coil compression can be used to create a hierarchical grouping of the channels with smaller associated distinct PI systems to solve. For example, as illustrated in  FIG. 1 , an eight channel geometry can partition the original channels into four subgroups of channels that are strongly correlated to one another. In this example, two virtual coils are generated for each subgroup from the other six original channels (e.g., using SVD or GCC). A PI problem is solved for each subgroup in order to estimate unaliased images for the two original channels assigned to that subgroup. To improve efficiency, a pseudo-channel can also be solved for, where the pseudo-channel has a magnitude that is the sum-of-squares combination of the two original channels in the subgroup. 
     The additional flexibility provided by HMF enables alternative coil compression techniques to be used for many acquisition types. In the case of Cartesian sampling, HMF can be used to bring the level of performance for SVD compression sustainably closer to that observed with the Cartesian-optimized GCC compression. Because HMF exploits the coil array topology through correlation, the benefits should extend to irregular sampling patterns and coil geometries. 
     Referring particularly now to  FIG. 3 , an example of a magnetic resonance imaging (“MRI”) system  300  is illustrated. The MRI system  300  includes an operator workstation  302 , which will typically include a display  304 ; one or more input devices  306 , such as a keyboard and mouse; and a processor  308 . The processor  308  may include a commercially available programmable machine running a commercially available operating system. The operator workstation  302  provides the operator interface that enables scan prescriptions to be entered into the MRI system  300 . In general, the operator workstation  302  may be coupled to four servers: a pulse sequence server  310 ; a data acquisition server  312 ; a data processing server  314 ; and a data store server  316 . The operator workstation  302  and each server  310 ,  312 ,  314 , and  316  are connected to communicate with each other. For example, the servers  310 ,  312 ,  314 , and  316  may be connected via a communication system  340 , which may include any suitable network connection, whether wired, wireless, or a combination of both. As an example, the communication system  340  may include both proprietary or dedicated networks, as well as open networks, such as the internet. 
     The pulse sequence server  310  functions in response to instructions downloaded from the operator workstation  302  to operate a gradient system  318  and a radiofrequency (“RF”) system  320 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  318 , which excites gradient coils in an assembly  322  to produce the magnetic field gradients G x , G y , and G z  used for position encoding magnetic resonance signals. The gradient coil assembly  322  forms part of a magnet assembly  324  that includes a polarizing magnet  326  and a whole-body RF coil  328 . 
     RF waveforms are applied by the RF system  320  to the RF coil  328 , or a separate local coil (not shown in  FIG. 3 ), in order to perform the prescribed magnetic resonance pulse sequence. Responsive magnetic resonance signals detected by the RF coil  328 , or a separate local coil (not shown in  FIG. 3 ), are received by the RF system  320 , where they are amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server  310 . The RF system  320  includes an RF transmitter for producing a wide variety of RF pulses used in MRI pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server  310  to produce RF pulses of the desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to the whole-body RF coil  328  or to one or more local coils or coil arrays (not shown in  FIG. 3 ). 
     The RF system  320  also includes one or more RF receiver channels. Each RF receiver channel includes an RF preamplifier that amplifies the magnetic resonance signal received by the coil  328  to which it is connected, and a detector that detects and digitizes the I and Q quadrature components of the received magnetic resonance signal. The magnitude of the received magnetic resonance signal may, therefore, be determined at any sampled point by the square root of the sum of the squares of the I and Q components:
 
 M =√{square root over ( I   2   +Q   2 )}  (2);
 
     and the phase of the received magnetic resonance signal may also be determined according to the following relationship: 
     
       
         
           
             
               
                 
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     The pulse sequence server  310  also optionally receives patient data from a physiological acquisition controller  330 . By way of example, the physiological acquisition controller  330  may receive signals from a number of different sensors connected to the patient, such as electrocardiograph (“ECG”) signals from electrodes, or respiratory signals from a respiratory bellows or other respiratory monitoring device. Such signals are typically used by the pulse sequence server  310  to synchronize, or “gate,” the performance of the scan with the subject&#39;s heart beat or respiration. 
     The pulse sequence server  310  also connects to a scan room interface circuit  332  that receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  332  that a patient positioning system  334  receives commands to move the patient to desired positions during the scan. 
     The digitized magnetic resonance signal samples produced by the RF system  320  are received by the data acquisition server  312 . The data acquisition server  312  operates in response to instructions downloaded from the operator workstation  302  to receive the real-time magnetic resonance data and provide buffer storage, such that no data is lost by data overrun. In some scans, the data acquisition server  312  does little more than pass the acquired magnetic resonance data to the data processor server  314 . However, in scans that require information derived from acquired magnetic resonance data to control the further performance of the scan, the data acquisition server  312  is programmed to produce such information and convey it to the pulse sequence server  310 . For example, during prescans, magnetic resonance data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  310 . As another example, navigator signals may be acquired and used to adjust the operating parameters of the RF system  320  or the gradient system  318 , or to control the view order in which k-space is sampled. In still another example, the data acquisition server  312  may also be employed to process magnetic resonance signals used to detect the arrival of a contrast agent in a magnetic resonance angiography (“MRA”) scan. By way of example, the data acquisition server  312  acquires magnetic resonance data and processes it in real-time to produce information that is used to control the scan. 
     The data processing server  314  receives magnetic resonance data from the data acquisition server  312  and processes it in accordance with instructions downloaded from the operator workstation  302 . Such processing may, for example, include one or more of the following: reconstructing two-dimensional or three-dimensional images by performing a Fourier transformation of raw k-space data; performing other image reconstruction algorithms, such as iterative or backprojection reconstruction algorithms; applying filters to raw k-space data or to reconstructed images; generating functional magnetic resonance images; calculating motion or flow images; and so on. 
     Images reconstructed by the data processing server  314  are conveyed back to the operator workstation  302  where they are stored. Real-time images are stored in a data base memory cache (not shown in  FIG. 3 ), from which they may be output to operator display  312  or a display  336  that is located near the magnet assembly  324  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  338 . When such images have been reconstructed and transferred to storage, the data processing server  314  notifies the data store server  316  on the operator workstation  302 . The operator workstation  302  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
     The MRI system  300  may also include one or more networked workstations  342 . By way of example, a networked workstation  342  may include a display  344 ; one or more input devices  346 , such as a keyboard and mouse; and a processor  348 . The networked workstation  342  may be located within the same facility as the operator workstation  302 , or in a different facility, such as a different healthcare institution or clinic. 
     The networked workstation  342 , whether within the same facility or in a different facility as the operator workstation  302 , may gain remote access to the data processing server  314  or data store server  316  via the communication system  340 . Accordingly, multiple networked workstations  342  may have access to the data processing server  314  and the data store server  316 . In this manner, magnetic resonance data, reconstructed images, or other data may exchanged between the data processing server  314  or the data store server  316  and the networked workstations  342 , such that the data or images may be remotely processed by a networked workstation  342 . This data may be exchanged in any suitable format, such as in accordance with the transmission control protocol (“TCP”), the internet protocol (“IP”), or other known or suitable protocols. 
     The present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.