TU‐AB‐BRA‐05: Optimization of a Novel Large Field of View Distortion Phantom for MR‐Only Treatment Planning

Abstract

Purpose:MR‐only treatment planning requires images of high geometric fidelity, particularly for large fields of view (FOV). However, the availability of large FOV distortion phantoms with accompanying analysis software is currently limited to in‐house solutions. This work sought to optimize the design of a modular distortion phantom that can accommodate several bore configurations and implement distortion corrections.Methods:1.0T MR and CT images were acquired of twelve urethane foam samples of various densities and strengths. Samples were precision machined to accommodate 6 mm diameter paintballs used as landmarks and signal‐to‐noise ratio was calculated. Final material candidates were selected by balancing strength, machinability, weight, and cost. Bore sizes and minimum aperture width resulting from couch position were tabulated from the literature for 13 systems across 4 vendors. Bore geometry and couch position were simulated in MATLAB to generate machine‐specific models to optimize the phantom build. Previously developed software for distortion characterization (control point detection, polynomial fitting, and correction map generation) was modified for several magnet geometries and integrated into Slicer, a widely available image analysis toolkit.Results:All foam samples provided sufficient image contrast with paintball landmarks. Urethane foam with a compressive strength of ∼1000psi and density of 20lb/ft3 was chosen for its accurate machinability and relatively light weight. A modular phantom design was optimized to accommodate all magnet geometries with the following parameters: 15 foam plates, 55×55×37.5cm, 5,082 landmarks, and 66 lbs. To accommodate wide bore magnets (>70 cm), an extended build spanned 55×55×50cm over 20 plates with 7,497 landmarks and weighing 97lbs. Distortion characterization software was implemented as an external module into Slicer's plugin framework.Conclusion:We optimized the design and implementation of a modular, extendable distortion phantom to support an MR‐only workflow and MR‐IGRT. The phantom will be useful for future multi‐institution collaborations and cross‐validation studies.The submitting institution has research agreements with Philips Healthcare. Research sponsored by a Henry Ford Health System Internal Mentored Grant. Technical support and initial distortion phantom design was provided by GE Healthcare.