The integration of a MR scanner with a linear accelerator (MR-Linac) may improve image guidance in radiation therapy (RT). Spatial fidelity is an important consideration in RT and should be evaluated for MR-Linac imaging. This work presents a phantom and software developed in-house to measure geometric distortion across a large MR imaging field-of-view. A comparative assessment of geometric distortion within a 1.5T MR scanner integrated with a 7 MV linear accelerator is performed versus several commercial scanners with field strengths ranging from 1.5T-3.0T. The geometric distortion within the MR-Linac was comparable or less than that measured with the diagnostic scanners.