Technical Note: Validation of an automatic ACR phantom quality assurance tool for an MR-guided radiotherapy system.

Purpose To modify and evaluate an automatic American College of Radiology (ACR) phantom analysis toolbox for ACR quality assurance (QA) on a low-field MR guided radiotherapy system (ViewRay). Methods An open-source toolbox was modified for ACR QA of a 0.35T MRI system (ViewRay MRIdian). A total of 17 ACR datasets were evaluated, including 10 datasets acquired from different systems across the world, and 7 datasets acquired at our center between 2014 and 2020. All required ACR tests, geometric accuracy (GA), high-contrast spatial resolution (HCSR), slice thickness accuracy (ST), slice position accuracy (SP), percent integral uniformity (PIU), percentage signal ghosting (PSG), and low-contrast object detectability (LCOD), were assessed manually and using the toolbox automatically. Measurements between manual and automatic analysis were compared. Precision, recall, and accuracy were calculated, where the manual results were used as the ground truth. Results The software took less than 2 minutes to complete all seven tests, which usually requires 40 minutes or more if analyzed manually. Overall, the automatic measurement was consistent with the manual result. The absolute differences between the two measurements were 0.72±0.66 (mm), 0.01±0.03, 0.50±0.60 (mm), 0.39±0.41 (mm), 1.01±1.00 (%), 0.0016±0.0019, and 2.79±2.29 for the seven tests. The precision of the automatic toolbox was 100% for all tests, indicating that a test would 100% pass a manual analysis if it had passed the automatic analysis. Recall and accuracy were ≥96% for GA, HCSR, SP, PIU, PSG, and LCOD tests, and 91% for the ST test. Conclusion An automatic ACR QA tool was adopted and evaluated for the low-field MR guided radiotherapy (MRgRT) system. Overall, the toolbox provided comparable results as manual analysis, and reduced the processing time from over 40 minutes to less than 2 minutes. This toolbox holds the potential to be widely adopted either as a second check tool or partially replace human measurement for MRgRT programs using the same system.