The acquisition of ultra-high spatial resolution data with sufficient SNR has always been a challenge in DWI. MUSE not only addresses this issue by enabling sub-millimeter isotropic resolutions with high spatial fidelity, it also provides a framework for incorporating additional tools and sampling techniques such as 3D acquisitions, simultaneous multi-slice imaging and reversed polarity gradients. With these capabilities, MUSE opens new avenues for structural, connectivity and tissue microstructure analyses of the brain and spinal cord.
The ability to achieve high-resolution data with high SNR and high spatial fidelity within a reasonably short amount of time has been a longstanding challenge in diffusion weighted MRI (DWI). To preserve SNR, many DWI protocols acquire data with isotropic voxels on the order of 1.5-2 mm, producing data that is limited in its ability to accurately characterize complex microstructures in organs such as the human brain. Signal decay and spatial distortion artifacts have long been limiting factors in achieving high-resolution DWI with single-shot echo planar imaging (EPI), while shot-to-shot motion artifacts have always hampered the potential SNR and spatial resolution benefits of multi-shot EPI (ms-EPI).
With the advent of MUltiplexed Sensitivity Encoding (MUSE), however, the inter-shot motion induced phase inconsistencies in ms-EPI diffusion data can be accurately accounted for. Included as part of GE Healthcare’s latest SIGNA™Works software releases, MUSE enables the routine acquisition of DWI data at sub millimeter spatial resolutions with sufficient SNR to conduct reliable diffusion tensor imaging (DTI) analyses, opening new avenues for investigations of complex tissue microstructures.
At the Duke University Brain Imaging and Analysis Center, MUSE DWI scans have become a standard protocol in multiple studies exploring structural connectivity in patients with Alzheimer’s disease, autism, epilepsy, cerebral palsy, traumatic brain injury and post-traumatic stress disorder (PTSD). Through MUSE, many of these studies can now routinely acquire data with isotropic spatial resolutions on the order of 0.8-1.0 mm. In Connectome analyses, whole-brain DWI data at this resolution has proven invaluable for both accurately resolving tightly curved intercortical association fibers as well as the delineation of crossing fiber bundles. Furthermore, sub-millimeter DWI has opened the door for more detailed analyses in gray matter, where streamlined fiber tracts and tissue characteristics can be investigated in multiple voxels across the relatively thin cortical layers (Figure 1).
Figure 1.
Whole-brain 4-shot MUSE DWI data acquired at a 0.8 mm isotropic spatial resolution with (A) b0 and (B) b800 s/mm2 (averaged over 18 directions). Imaging with sub-millimeter voxels enables (C) fiber tracking and detailed DTI analysis across multiple cortical layers.
When coupling MUSE with the ultra-high-powered prototype gradient coil‡ in the SIGNA™ Premier 3.0T system at Duke University, the spatial resolution limits of diffusion data can be pushed even further, with whole brain DWI images acquired with 0.7 mm isotropic voxels (Figure 2). Although highly beneficial in brain imaging, MUSE’s ability to account for motion artifacts in sub-millimeter diffusion images is equally significant in body imaging. Figure 3 compares a simple MUSE DWI abdominal scan using 1 mm3 isotropic voxels to that of 0.6 x 0.6 x 2.0 mm, where the anatomical specificity in both the spinal cord and kidneys is immediately apparent in the sub-millimeter scan.
Figure 2.
Whole-brain 0.7 mm MUSE DWI data acquired with (A) b0 and (B) b800 s/mm2 (averaged over 18 directions) in a SIGNA™ Premier 3.0T system with an ultra-high powered 60 cm torque and force balanced gradient coil‡ (peak strength of 115 mT/m) specifically designed for high-resolution DWI.
Figure 3.
MUSE abdominal scan with (A) 1 mm3 and (B) 0.6 x 0.6 x 2 mm voxels offer high fidelity images free of motion artifacts.
Beyond the increase in achievable spatial resolutions, one of the most appealing aspects of the MUSE framework is the foundation it provides for incorporating additional imaging tools and techniques. For example, the traditional 2D MUSE model has been expanded to acquire and reconstruct 3D ms-EPI data, where an additional SNR advantage is gained by Fourier encoding thick slabs across the imaging volume to produce images with increased anatomical detail (Figure 4).‡
Figure 4.
1 mm3 MUSE DWI images with b1000 s/mm2 using (A) 2D and (B) 3D encoding.‡
Although the shortened readout and echo spacing of ms-EPI does reduce distortion artifacts in DWI, a recent adaptation of MUSE (marked by GE as PROGRES) to incorporate reversed polarity gradients (RPG) between shots has shown great promise in eliminating both residual static field inhomogeneities as well as dynamic gradient-induced eddy currents. While traditional RPG distortion corrections require additional imaging volumes with opposing phase encoding directions (PE), RPG-MUSE simply reverses PE in the odd/even shots of a typical ms-EPI scan. This approach requires no additional volumes or increase in scan duration, as it allows for field inhomogeneities to be estimated directly from the ms-EPI data and incorporated into the reconstruction. As shown in Figure 5, the resulting RPG-MUSE images exhibit tremendous anatomical specificity when compared with a traditional correction of data with a single PE.
Figure 5.
Average DWI (b800 s/mm2) and fractional anisotropy maps from distortion corrected data acquired with (A) forward phase encoding MUSE, (B) reversed phase encoding MUSE and (C) RPG-MUSE.
With its small cross-sectional area, another region that has benefited greatly from the high resolution afforded by MUSE is in the spinal cord. Unlike traditional ms-EPI, where each shot is separated by the duration of a predefined TR, spinal cord imaging often employs cardiac gating (CG) in an effort to mitigate cerebrospinal fluid pulsation artifacts. Although CG both increases and varies the time between shots, MUSE remains fully capable of accounting for intershot motion artifacts. As illustrated in Figure 6, the sub-millimeter spatial resolution and SNR achieved by MUSE facilitates the delineation of complex microstructures in the brain stem, such as the bifurcation of corticospinal tracts and the decussation of medullary pyramids.
Figure 6.
Using MUSE DWI data with a 0.8 mm isotropic spatial resolution, (A) fibers passing through the brain stem can be shown to bifurcate into (B) lateral and (C) anterior corticospinal tracts.
Despite its remarkable advantages, the one remaining drawback of ms-EPI is the prolonged acquisition time. To that end, the MUSE framework has since been expanded to incorporate simultaneous multi-slice imaging, or HyperBand. Through HyperBand, a whole brain 1 mm3 MUSE scan with 4 shots and 25 diffusion volumes can be reduced from approximately 20 minutes to around 7 minutes, while preserving both the increased SNR and spatial fidelity of ms EPI (Figure 7). Alternatively, MUSE with HyperBand can also be used to extend the coverage achieved within a given scan time. This is of particular use in applications such as spinal cord imaging, as it would enable fiber bundles to be tracked at sub-millimeter isotropic spatial resolutions from their origins within the brain as they extend down into the spinal cord.
Figure 7.
Whole-brain (A) b0 and (B) b800 s/mm2 (averaged over 18 directions), and (C) fractional anisotropy maps derived from MUSE with HyperBand data acquired with 4 shots and 2 bands at a 1 mm3 spatial resolution.
Although MUSE has only been available on GE scanners for a short time, its impact in the field of DWI has been great. The increased spatial resolution, fidelity and SNR achieved through MUSE has already broadened the scope of many structural and connectivity studies. As its utility continues to grow with the inclusion of tools such as HyperBand, it will soon become a staple in many DWI protocols.
‡ Technology in development that represents ongoing research and development efforts. These technologies are not products and may never become products. Not for sale. Not cleared or approved by the U.S. FDA or any other global regulator for commercial availability.
