Author: Krystal M. Kirby, Aliasghar Rohani, Christopher W. Schneider, Sotirios Stathakis 👨🔬
Affiliation: Mary Bird Perkins Cancer Center, Louisiana State University, Baton Rouge, Louisiana 🌍
Purpose: Accurate treatment delivery in MR-guided radiotherapy (MRgRT) systems requires consistent imaging quality assurance (QA) to assess MR image quality. However, standard QA phantoms have limited field-of-view (FOV) coverage, which may not represent the larger anatomical regions, such as the torso. This study provides a comprehensive assessment of a novel large FOV phantom for routine imaging QA on a 1.5T MRgRT system.
Methods: The QUASAR™ MRgRT Insight Phantom (Modus Medical Devices Inc.) and ACR MR phantoms were scanned on a 1.5T MRgRT system (Unity, Elekta) and a 1.5T MR simulator (Magnetom Sola, Siemens Healthineers) with T1- and T2-weighted acquisitions. Evaluations included geometric distortion, slice position and thickness accuracy, spatial resolution, and image intensity uniformity, performed in the vendor-provided software.
Results: In slice thickness analysis, maximum errors on the Unity were 16.3% (T1) and 12.36% (T2) for the Insight, compared to 4.2% (T1) and 9.2% (T2) for the ACR. On the Sola, errors were 13.55% (T1) and 9.95% (T2) for the Insight, and 5% (T1) and 6.2% (T2) for the ACR phantom. Image intensity uniformity exceeded 92.96% across all scans, with the lowest value recorded for the Insight on Unity with T2. Spatial resolution, evaluated using the Modulation Transfer Function (MTF), revealed a failure in one large FOV region for the Sola. Mean geometric distortion for the Insight was 0.428±0.001mm, 0.575±0.003mm, and 0.964±0.001mm on the Sola, and 0.275±0.001mm, 0.343±0.002mm, and 0.631±0.011mm on the Unity for FOVs of 21cm, 31cm, and 41cm, respectively. Maximum geometric distortion remained below the 3.015mm across all FOVs, agreeing with ACR phantom scans.
Conclusion: The results highlight the significance of large FOV phantoms for routine QA. The Insight phantom enables measurements at the periphery of a large FOV, providing a more comprehensive assessment of MR image quality compared to the ACR phantom.