MO‐FF‐A3‐01: Advantages of Multi‐Shot, Variable‐Density Spiral, Diffusion‐Weighted MR Imaging for Radiation Treatment Planning

Abstract

Purpose: The prerequisite for intensity‐modulated radiation therapy (IMRT) is accurate target and critical structure definition. Diffusion‐weighted MRI provides information that may improve target delineation and, therefore, radiation treatment outcome. However, geometric distortion artifacts have hampered the use of diffusion‐weighted MRI in radiation treatment planning. We demonstrate here a multi‐shot (i.e. interleaved), variable‐density spiral method that not only diminishes geometric distortion artifacts, but also permits acquisition of higher resolution diffusion‐weighted images for use in radiation treatment planning. Method and Materials: A custom, multi‐shot, spin‐echo, variable‐density spiral sequence with bipolar diffusion gradients was implemented on a 3.0T GE scanner. High‐resolution, multi‐shot, diffusion‐weighted images were acquired on three healthy volunteers (FOV: 240mm2, matrix: 2562, interleaves: 32, TE: 50 msec, TR: 3000 msec, scan time: 3.2 minutes). To demonstrate the inherent self‐navigating capabilities of variable‐density spiral, diffusion‐weighted images were reconstructed without and with self‐navigated phase correction. For comparison to the current state‐of‐the‐art method, diffusion‐weighted images were also collected using a clinical, single‐shot, spin‐echo echo‐planar imaging (EPI) sequence with ramp sampling, partial k‐space, and parallel imaging (FOV: 240mm2, matrix: 1282, TE: 64.7 msec, SENSE, R=2, scan time: ∼1 minute). Results: The inherent self‐navigating capability of variable‐density spiral was effective at diminishing motion‐induced phase errors, encoded by the diffusion gradients, that varied from shot‐to‐shot. The multi‐shot diffusion‐weighted images exhibited good signal‐to‐noise ratio despite the 47% reduction in voxel volume compared to the diffusion‐weighted EPI images. Off‐resonance effects resulted in geometric distortions in ventral and rostral brain regions on diffusion‐weighted images collected with EPI, but did not compromise the geometric integrity of diffusion‐weighted images acquired with multi‐shot spiral. Conclusions: Multi‐shot, variable‐density spiral permits acquisition of high‐resolution, diffusion‐weighted images that do not suffer from geometric distortions that have previously prohibited their use in radiation treatment planning.