Purpose: MR-only treatment planning and MR-IGRT require high fidelity anatomical patient models for precise radiation therapy. Computer simulations have shown that local susceptibility changes at tissue/air interfaces increase with increased field strength and size of the inhomogeneity. However, it has been challenging to validate these effects. This work evaluates the potential of using a novel pelvic phantom with variable bladder and air-filled rectal balloon inserts to benchmark susceptibility-related distortions. Methods: A novel pelvic end-to-end (PETE) MR-compatible phantom with variable anatomies was imaged on a 1.0 T MR-Simulator and 0.35 T MR-Linac. The phantom consists of anthropomorphic pelvic bones and two fillable silicone balloons designed to mimic the bladder and rectum. For all experiments, the fluid-filled bladder volume was kept constant at 250 CC while rectal air was varied at 30 CC, 90 CC and 150 CC. Dual-echo gradientrecalled echo images were acquired on both systems. Maps of the phase differences were reconstructed after complex division of the complex data from the two echoes. Distortion maps were then generated using magnet-specific image acquisition parameters. Results: Distortion maps showed an increase in distortion magnitude and distortion ranges in surrounding tissues with increased rectal air. In a 2 cm ring around the 150 CC rectum, susceptibilityinduced distortions were -0.02 ± 0.04 (range: -0.26-0.15) and -0.02 ± 0.05 (range: -0.44-0.30) for the 1.0 T and 0.35 T, respectively. At 0.35 T, peak distortions at the rectal boundary decayed radially to half its value within 16 mm and 14 mm for the 150 CC and 90 CC rectal gas volumes, respectively. At 1 T, these ranges were 14 mm and 12 mm, respectively. Conclusion: Our phantom mimicking varied rectal conditions allowed quantification of local susceptibility distortions. While distortions increased with increased air volume, the distortion measured for low fields was \u3c1mm. Evaluation in higher field strengths is warranted. Future applications of the phantom include benchmarking distortion correction schemes
