  MRI offers outstanding soft tissue contrast that may reduce uncertainties in
target and organ-at-risk delineation and enable online adaptive image-guided
treatment. Spatial distortions resulting from non-linearities in the gradient
fields and non-uniformity in the main magnetic field must be accounted for
across the imaging field-of-view to prevent systematic errors during treatment
delivery. This work presents a modular phantom and software application to
characterize geometric distortion (GD) within the large field-of-view MRI
images required for radiation therapy simulation. The modular phantom is
assembled from a series of rectangular foam blocks containing high-contrast
fiducial markers in a known configuration. The modular phantom design
facilitates transportation of the phantom between different MR scanners and
MR-guided linear accelerators and allows the phantom to be adapted to fit
different sized bores or coils. The phantom was evaluated using a 1.5T
MR-guided linear accelerator (MR-Linac) and 1.5T and 3.0T diagnostic scanners.
Performance was assessed by varying acquisition parameters to induce image
distortions in a known manner. Imaging was performed using T1 and T2 weighted
pulse sequences with 2D and 3D distortion correction algorithms and the
receiver bandwidth (BW) varied as 250-815 Hz/pixel. Phantom set-up
reproducibility was evaluated across independent set-ups. The software was
validated by comparison with a non-modular phantom. Average geometric
distortion was 0.94+/-0.58 mm for the MR-Linac, 0.90+/-0.53 mm for the 1.5 T
scanner, and 1.15+/-0.62 mm for the 3.0T scanner, for a 400 mm diameter
volume-of-interest. GD increased, as expected, with decreasing BW, and with the
2D versus 3D correction algorithm. Differences in GD attributed to phantom
set-up were 0.13 mm or less. Differences in GD for the two software
applications were less than 0.07 mm.
