Room: AAPM ePoster Library
Purpose: To evaluate the percent image uniformity (PIU) and spatial integrity of cine images used for MRgRT target tracking during MLC and gantry motion. The moving component results were compared to the static conditions currently used during MRgRT treatment.
Methods: The phantoms (24 cm spherical NEMA phantom for PIU, uniform linearity phantom for spatial integrity) were imaged at isocenter in cine mode under the following conditions: (1) static MLC and gantry, (2) moving MLC and static gantry (3) static MLC and moving gantry. PIU was calculated according to the ACR MRI Quality Control Manual. Spatial integrity was evaluated by measuring the deviation of 16 measured marker positions (10 cm or 15.225 cm from isocenter) from their true positions.
Results: The variation of cine image PIU from static linac conditions was within 1% throughout MLC motion and within 3% throughout gantry rotation. The mean deviation of the measured marker positions in the cine images was 0.45 mm (max 0.72 mm) at 10 cm from isocenter and 0.60 mm (max 0.88 mm) at 15.225 cm from isocenter during MLC motion. During gantry rotation, the mean deviation of the measured marker positions in the cine images was 0.77 mm (max 0.92 mm) at 10 cm from isocenter and 0.95 mm (max 1.60 mm) at 15.225 cm from isocenter.
Conclusion: Low-resolution cine images are not held to the same standards as 3D images, but quality must still be maintained during MRgRT treatment. This work demonstrates the potential for VMAT or sliding window delivery during MRgRT. During linac component motion, PIU did not vary greatly, and spatial distortion was less than 1 mm for all measurement points 10 cm from isocenter. Future work should evaluate spatial integrity using more measurement locations and should investigate the effects imaging artifacts have on MRgRT target tracking.