High-Resolution Diffusion-Weighted Imaging of Cartilage Using PROPELLER

METHODS: High-resolution in vivo diffusion-weighted imaging of the knee was performed using both PROPELLER and conventional SS-EPI. The sequence parameters were: matrix size= 256 x 256, FOV = 24 cm ( spatial resolution = 0.9 x 0.9 mm), slice thickness = 4 mm, number of slices = 10, TR = 4 s, and b = 600 s/mm 2 . For the PROPELLER scans: echo train length (ETL) = 20, NEX = 1.5 (due to the oversampling of the kspace centre), and echo time (TE) = 120.9 ms. For SS-EPI: TE = 70.2 ms, and NEX = 5. All scanning was performed on a 1.5T Twin Excite MRI (GEMS, Milwaukee, WI) using a transmit/receive extremity coil. A total of 3 healthy subjects were imaged with both protocols. RESULTS: Figures 1a,b are representative diffusion-weighted images of the knee acquired with PROPELLER and SS-EPI respectively. The PROPELLER images provide a significantly sharper depiction of the cartilage and subchondral bone. In contrast, blurring was observed on the SSEPI images, likely caused by T2 and T2 * apodization of the k-space data during the lengthy SS-EPI readout. To evaluate off-resonance sensitivity, the diffusion-weighted images were compared to FGRE images of the same anatomy (Fig. 1c). The overall shape of the anatomy depicted in the PROPELLER images was found to correspond to that in the FGRE images, thus implying minimal geometric distortion. In contrast, the SS-EPI images appear to exhibit significant elongation. This distortion is even more apparent on SS-EPI images acquired with the diffusion-encoding gradients turned off (i.e. b = 0 s/mm 2 ) (Fig. 1d). This severe warping is likely due to B0 inhomogeneity and susceptibility differences over the tissue. DISCUSSION: The results of this study demonstrate that diffusion-weighted PROPELLER is effective at generating high-resolution images of cartilage. Relative to conventional SS-EPI, artifacts due to off-resonance effects were suppressed due to the use of an FSE readout. Image blurring due to T2 and T2 * was reduced due to the use of shorter echo trains. However, a significant drawback of PROPELLER relative to conventional techniques is reduced efficiency. The PROPELLER scans in the present study required 5.8 min, while the SS-EPI studies lasted only 32 s (for a single NEX). Part of the reason for PROPELLER’s reduced efficiency is the oversampling of the central k-space region (the other reason is the FSE readout, vs. a GRE readout in SS-EPI). In the future, greater efficiency may potentially be achieved by optimizing the FSE echo train length and/or by implementing a parallel acquisition strategy. However, even with reduced efficiency, the improvement in image quality provided by PROPELLER may ultimately make it the preferred method for high-resolution diffusion-weighted imaging of cartilage.