Spatial distortion results in image deformation that can degrade accurate targeting and dose calculations in MRI-guided
adaptive radiotherapy. The authors present a comprehensive assessment of a 0.35 T MRI-guided radiotherapy system’s spatial
distortion using two commercially-available phantoms with regularly spaced markers.
Images of the spatial integrity phantoms were acquired using five clinical protocols on the MRI-guided radiotherapy machine
with the radiotherapy gantry positioned at various angles. Software was developed to identify and localize all phantom markers
using a template matching approach. Rotational and translational corrections were implemented to account for imperfect phantom
alignment. Measurements were made to assess uncertainties arising from susceptibility artifacts, image noise, and phantom
construction accuracy.
For a clinical 3D imaging protocol with a 1.5 mm reconstructed slice thickness, 100% of spheres within a 50 mm radius of
isocenter had a 3D deviation of 1 mm or less. Of the spheres within 100 mm of isocenter, 99.9% had a 3D deviation less than 1 mm.
94.8% and 100% of the spheres within 175 mm were found to be within 1 mm and 2 mm of the expected positions in 3D respectively.
Maximum 3D distortions within 50 mm, 100 mm and 175 mm of isocenter were 0.76 mm, 1.15 mm and 1.88 mm respectively. Distortions
present in images acquired using the real-time imaging sequence were less than 1 mm for 98.1% and 95.0% of the cylinders within 50
mm and 100 mm of isocenter. The corresponding maximum distortion in these regions was 1.10 mm and 1.67 mm.
These results may be used to inform appropriate planning target volume (PTV) margins for 0.35 T MRI-guided radiotherapy.
Observed levels of spatial distortion should be explicitly considered when using PTV margins of 3 mm or less or in the case of
targets displaced from isocenter by more than 50 mm.