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MRI Isocenter Consistency at Various Gantry Angles Using MODUS MRID3D Phantom

D Sievert1, B Lewis1, J Shin1,2, E Barberi3, B Quinn3, J Kim2, T Kim1, (1) Washington University in St. Louis, St. Louis, MO, (2) Yonsei University College of Medicine, Seoul, ,KR, (3) Modusqa, London, ON, CA

Presentations

TH-IePD-TRACK 2-5 (Thursday, 7/29/2021) 3:00 PM - 3:30 PM [Eastern Time (GMT-4)]

Purpose: MRI isocenter consistency at all gantry angles is critical to ensuring accurate treatment delivery in MRgRT. Previous reports from our group showed an imaging isocenter shift of <1mm utilizing a rectangular phantom at multiple imaging locations with ViewRay’s analysis software. In this study, we verified MRI isocenter fidelity using an independent 3rd party 3D geometric phantom and corresponding software.

Methods: This experiment used a QUASAR™ MRID³ᴰ phantom on a 0.35T ViewRay MRIdian MR-linac alongside the QUASAR™ MRID³ᴰ geometric distortion analysis system. Images were acquired at twelve gantry angles from 0–330° at 30° increments. Acquisitions utilized a 3D TRUFI sequence with TR: 3ms, TE: 1ms, flip angle: 60°, FoV: 450x450x360mm³, imaging matrix: 300x300x240. Images were acquired with the onboard Siemens distortion correction function turned on and off to measure the ability of the system to correct geometric distortion. All images were imported into the distortion analysis software to quantify MRI isocenter shift and geometric distortion of the phantom per gantry angle. The baseline for all measurements is G300 for Gantry home position in RT.

Results: The mean and standard deviation deformation vectors for Distortion On were 0.28±0.37mm, 0.27±0.32mm, and 0.28±0.33mm in the AP, RL, and SI directions respectively, and 2.91±4.44mm, 3.15±4.75mm, and 1.82±2.49mm for Distortion Off. The mean isocenter shift was 0.62mm with a range of 0.21mm to 0.96mm for Distortion On, and 0.65mm ranged from 0.24mm to 0.93mm for Distortion Off.

Conclusion: The magnitude of MRI isocenter shift from 3D MRID³ᴰ phantom showed similar trends as previous work with a 2D slab phantom for all gantry angles. This study showed that the gantry position can impact the image isocenter yet can be corrected by applying a deformation vector field at all measured gantry angles using the QUASAR™ MRID³ᴰ phantom and software.

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