Room: Karl Dean Ballroom B1
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 250CC while rectal air was varied at 30CC, 90CC and 150CC. Dual-echo gradient-recalled 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 150CC rectum, susceptibility-induced distortions were -0.02Â±0.04 (range: -0.26-0.15) and -0.02Â±0.05 (range: -0.44-0.30) for the 1.0T and 0.35T, respectively. At 0.35T, 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 1T, these ranges were 14mm 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 <1mm. Evaluation in higher field strengths is warranted. Future applications of the phantom include benchmarking distortion correction schemes.
Funding Support, Disclosures, and Conflict of Interest: The submitting institution holds research agreements with Philips Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA204189.