DIFFUSION IMAGING USING MIND SAP-EPI

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: Even with modern MRI scanners, EPI distortions remain problematic, especially at higher image resolutions or in DWI. Recently, we introduced a redesigned propeller EPI, known as SAP-EPI (that has higher pseudo receiver bandwidth than previous propeller EPI methods. For high resolution 256¿256 SAP-EPI images, obtained by using e.g. 32 (freq.) ¿ 256 (phase) blade resolutions, the sensitivity to off-resonances is reduced by about a factor of 3. In this abstract we combine the new SAP-EPI sequence with diffusion gradients and two additional means of reducing the distortions. These are a) the use of multi-shot with GRAPPA and b) distortion correction using the reversed gradient polarity method (RGPM) (2,3), both of which are applied on the blades prior to regridding and Fourier transformation to the image domain. By this method an undistorted image is achieved by minimizing the difference between two oppositely distorted magnitude images. For EPI imaging, the distortion direction is altered by changing the sign of the phase blip gradients. Recently, Andersson et al. (2) and Skare et al. (3) have developed RGPM further by the use of basis functions (cosine set or cubic 3D B-splines) and smooth and continuous 3D fields as well as 3D motion correction. RGPM may be introduced into SAP imaging by using a pair of half-Fourier blades, oriented 180¿ apart. Materials and methods: Multi-shot DW-EPI with GRAPPA allows reduced distortions while not introducing ghosting due to the random phase between the shots (induced by the diffusion gradients). This is accomplished by estimating GRAPPA weights on the b=0 image and using these weights to reconstruct each of the shots in the DWI¿s. Each image is then combined with magnitude averaging (4). Unlike multi-shot DW-EPI with navigators, reconstructing each shot separately is guaranteed to be successful w.r.t. ghosting. In this work we have combined SAP-EPI with up to 4-shot acquisitions. GRAPPA weights are estimated on the T2w and applied on each shot in each DW blade to create 4 DW blades. For a target resolution of 256¿256 and blade resolution of 32¿256, we are able to reduce distortions by ~12x compared to conventional 256¿256 ssEPI. Further improvement is possible by correcting the residual distortions using a 2D variant of RGPM using cubic B-splines as basis set to build the distortion field (3). A pair of propeller blades rotated 180 degrees apart is acquired. As these are acquired in opposite directions in k-space, the effect of the phase accrual due to ?B0 is reversed in the reconstructed image for these blade images. This is illustrated in Fig. 2. The second blade image is rotated to match the first, followed by the 2D distortion and motion correction, which results in two corrected blades and a ?B0 field. As the homodyne reconstruction clears the phase of the blade images, full-size echo centered blades in k-space result. The processing steps in Fig. 2 are repeated for all other blade pairs. The corrected k-space data from all blades are regridded and 2D Fourier transformed to produce the final image. Acquisition was performed on a 1.5T GE Excite system (GEHC, Waukesha, WI) with 50mT/m gradients on a volunteer using an 8 channel phased-array head coil (MRI Devices, Milwaukee, WI). Eight axial 4 mm slices were acquired with a 24¿24 FOV and 32¿256 (freq.¿phase) blade resolution, reconstructed to 256¿256. Results: T2w and DW images from 1-shot EPI, 1-shot SAP-EPI, 4-shot GRAPPA SAP-EPI with distortion correction are presented. For geometric reference, a fat-sat FSE is included. When comparing these images one should disregard SNR as scan times are not identical, and focus on the distortion and blurring reductions. One can appreciate the improvement with SAP-EPI vs. single-shot EPI. Structures in near the pile-up regions are recovered in SAP-EPI, yet some distortion artifacts remain. When using the current method of a 4-shot SAP-EPI with RGPM, these artifacts disappear, making it similar to the FSE image. References: 1. Wang FN, Huang TY, Lin FH, Chuang TC, Chen NK, Chung HW, Chen CY, Kwong KK. Magn Reson Med 2005;54(5):1232-1240. 2. Andersson JL, Skare S, Ashburner J. Neuroimage 2003;20(2):870-888. 3. Skare S, Andersson JL. Magn Reson Med 2005;54(1):169-181. 4. Skare S, Newbould R, Clayton D, Bammer R. 2005; Lake Louise. p 16. Acknowledgements: This work was supported in part by the NIH (1R01EB002771), the Center of Advanced MR Technology at Stanford (P41RR09784), Lucas Foundation, and Oak Foundation.