Purpose: Longitudinal on-board diffusion MRI (DWI) using MR-guided radiotherapy (MRgRT) system has the potential to monitor localized treatment response throughout treatment course. Clinically, DWI is acquired together with an anatomical MRI with a large field-of-view (FOV), while the DWI focuses primarily on the tumor within a smaller FOV. Accordingly, reduced FOV (rFOV) DWI has been introduced to remove artifacts from motion, flow and other signal interferences from the surrounding regions. In this study, a rFOV DW-single-shot echo planar imaging (DW-ssEPI) using saturation bands was implemented on an MRgRT system and its performance was evaluated with phantom studies.
Methods: Full FOV DW-ssEPI (fFOV-EPI) and rFOV DW-ssEPI (rFOV-EPI) were evaluated on a 0.35T MRgRT system using a diffusion phantom and a spatial integrity phantom. Both scans used the same resolution (2.7x2.7x6mm³), TE (120ms), and TR (3200ms). The FOV of fFOV-EPI and rFOV-EPI was 350x350mm² and 225x225mm², respectively. Saturation bands were placed immediately outside of the rFOV in the phase encoding direction to minimize aliasing artifacts. Signal-to-noise ratio (SNR) compensation was introduced for the rFOV-EPI to equalize SNR between the two scans. As a result, fFOV-EPI and rFOV-EPI used 6 and 9 averages for a scan time of 1:46 and 2:34min, respectively. Apparent diffusion coefficient (ADC) measurements were evaluated relative to the reference values by the manufacturer. Spatial integrity was evaluated using an open-source toolbox to quantify spatial distortion.
Results: Both fFOV-EPI and rFOV-EPI provided accurate ADC quantification on the diffusion phantom (p<0.05). rFOV-EPI had similarly low levels of distortion as fFOV-EPI at 50, 75, and 100mm radius of the spatial integrity phantom.
Conclusion: rFOV-EPI is a feasible method to generate rFOV DWI and is comparable to conventional fFOV-EPI on phantom scans. Future work will focus on patients and volunteers to test the efficacy of rFOV-EPI for image quality and ADC quantification improvements.
Funding Support, Disclosures, and Conflict of Interest: Dr. Yang has received honoraria and consulting fees from ViewRay. Dr. Hu has received consulting fees and research funding from ViewRay.