DTI has been utilized in imaging nerve injuries and peripheral nerve tumors[1].However, upper extremity(UE) DTI is susceptible to distortion and fat suppression failure, especially in the patient-friendly arms-down position where the area of interest is far from iso-center and has increased B0 inhomogeneity. Spatial mismatch makes it difficult to do proper seeds placement and tracks the nerve in its entirety in fiber tracking software. In this study, we aimed to investigate the utility of a realtime B0 correction(RTB0) method[2], and an image registration-based distortion correction (ADDC) method [3,4] in UE-DTI.

To reduce B0 inhomogeneity & fat suppression failure, we applied the RTB0 technique, which examined the FID for each slice to determine the optimal center frequency and apply it to each slice in realtime (Fig 1). To further reduce misalignment due to tissue susceptibility and eddy-current-induced distortion, the ADDC post-processing technique is used , where a rigid registration of the DTI T2 image to the PD image was performed. To account for susceptibility & eddy current effects, an additional refinement of alignment was performed using 20-parameter cubic-polynomial and 4-parameter linear basis-function respectively. Peripheral nerve DTIs with and without RTB0 were performed on 3 consented healthy volunteers on a GE 3T 60cm bore scanner (MR750) using a 16channel Flexible extremity array in arms down position. Optimized DWI parameters were: FOV:12cm(LR)x9.6cm(AP), Matrix: 80(freq) x 64(phase), TR/TE:5000ms/60.5ms (non-RTB0), 66.5ms(RTB0), single spin echo, BW:250kHz, Slice thickness:3mm, # slices:25/station (i.e. 9cm per station), Adiabatic IR fat suppression , TI=79ms, b-value=0s/mm2(4 NEX), 600s/mm2 (4 NEX), diffusion direction: 15, scan time: 5:25min. Since the most common injury sites of the upper extremity are wrist, elbow and upper arm, we performed DTI at these 3 sites for each volunteer (adjusting the coil placement for each region of interest). Both datasets were then processed through ADDC.

To assess the spatial accuracy, an experience radiologist identified the median, ulnar and radial nerve positions on the proximal, mid, distal locations of each imaging site. The nerve positions were identified on the following datasets: 1. Anatomical PD images, 2. DTI without RTB0, 3. DTI with RTB0, 4. DTI without RTB0 & ADDC, 5. DTI with RTB0 & ADDC. Using the PD image as gold standard, the absolute displacement errors were computed for datasets 2 to 5 in AP and LR direction, and compared using Wilcoxon rank sum test. Fat suppression quality was also noted in the comparison.

Figure 2 shows the center frequency offset calculated from RTB0 at awrist, elbow and arm respectively on a volunteer. For elbow & arm, since they are relatively cylindrical in shape, the CF offset is mainly driven by magnet B0 inhomogenity, which is minimal in the 9cm SI coverage range. While in wrist, where there is shape changes at the wrist joint, there is a higher CF offset due to changes in tissue susceptibility. This large CF offset contributes to fat suppression failure at the distal wrist with conventional DTI in all 3 volunteers. RTB0 eliminates this fat suppression failure since it enables the spectrally selective water excitation pulse to operate at the optimal water peak even at off-isocenter location (Figure 3). Figure 4 shows the absolute displacement error in AP & LR for datasets 2 to 5. We did not observed improvement in spatial accuracy, and upon further inspection, it could be due to the differences in X/Y/Z shim values & CF in DTI vs RTB0 series, which contributed to stretching, skewing and shifting of the image. In future experiement, we should fix the CF & shimming parameters to eliminate this confounding factor. Figure 5 shows a 3D rendering of the peripheral nerve around a nerve sheath tumor at the wrist in a patient. Note that nerve bundles were best tracked in the dataset with both RTB0 & ADDC applied in this case, showing that ADDC & RTB0 could potentially help with visualization of nerve bundles.In this work, we demonstrated fat ghosting reduction using the RTB0 acquisition method. And in our initial investigation, the combination of RTB0 & ADDC method does not provide significant difference or improvement in alignment in this small SI coverage scenario. Future investigation can look at larger SI coverage & comparison with fixed shimming parameters to better assess the spatial alignment differences.