Low-Distortion and High-SNR Diffusion-Weighted MRI on an MR-Linac with Spiral Readouts
Abstract
Purpose
Conventional diffusion-weighted MRI (DWI) uses single-shot echo planar imaging (ssEPI), which is highly susceptible to geometric distortion, limiting its use for radiotherapy guidance on MR-LINAC systems. Turbo-spin-echo (TSE) and multi-shot EPI (msEPI) DWI reduce distortion but also reduce SNR efficiency. This work is the first to implement Spiral DWI for the MR-LINAC to achieve low distortion while improving SNR.
Methods
Multi-shot Spiral-DWI sequence was implemented on a 1.5 T MR-LINAC (Elekta Unity). DWI scans were acquired at 2×2×5mm³ (b=0/500s/mm²) using ssEPI, multi-shot Spiral (5/10-shots), 10-shot msEPI, and DW-TSE. All sequences were tested on quantitative diffusion and geometric distortion phantom, with a 3D-T2-weighted TSE as geometric reference. ADC and SNR were quantified in the diffusion phantom and geometric distortion was quantified in the grid phantom by grid-point displacements relative to 3D-T2-TSE. In-vivo Spiral and ssEPI were acquired in the prostate and abdomen (with respiratory gating) of a healthy volunteer. Spiral data were reconstructed using both the scanner default (gridding) and a custom shot-locally-low-rank (shot-LLR) reconstruction algorithm to mitigate inter-shot phase inconsistencies due to motion.
Results
Spiral DWI measured similar phantom ADC to ssEPI with lower bias (-1.75±7.00%), than msEPI (7.93±7.54%) and TSE (-13.70±33.37%). Spiral achieved higher SNR (804.80 for b=0) than msEPI (226.34), TSE (58.82), and ssEPI (756.11). Distortion was significantly reduced with Spiral (0.32±0.28 mm) compared with ssEPI (2.41±2.05mm]), p<0.05. Distortions were larger in msEPI vs spiral (0.50±0.44mm, p<0.05) and not significantly different between spiral and DW-TSE (0.23±0.21mm). With scanner reconstructions (DICOM), Spiral overestimated ADC compared with ssEPI due to inter-shot motion, but shot-LLR visually improved reconstruction and reported similar ADC (unit: x10-3mm2/s) in prostate (ADCssEPI=1.4±0.4 vs. ADCSpiralDICOM=1.9±0.5, ADCSpiralLLR=1.5±0.5) and pancreas (ADCssEPI=3.2±0.8 vs. ADCSpiralDICOM=5.1±0.8, ADCSpiralLLR=3.0±0.9).
Conclusion
Multi-shot Spiral DWI enabled low-distortion, high-SNR diffusion imaging on MR-LINAC. This may improve DWI-guided online adaptive radiotherapy planning and online treatment response monitoring.