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An Evaluation of Patient Independent Image Distortion in a 0.35T MRI-Guided Radiotherapy System

J Ginn

J Ginn1*, N Agazaryan1 , M Cao1 , U Baharom2 , D Low1 , Y Yang1 , Y Gao1 , P Hu1 , P Lee1 , J Lamb1 , (1) UCLA, Los Angeles, CA (2) Integrated Medical Technologies, Troy, NY


TU-C1-GePD-JT-1 (Tuesday, August 1, 2017) 9:30 AM - 10:00 AM Room: Joint Imaging-Therapy ePoster Theater

Purpose: Image distortion in MRI-guided radiotherapy has the potential to adversely affect both tumor targeting and dose calculation accuracy. The authors report a comprehensive evaluation of patient-independent distortion arising in a 0.35T MR guided radiotherapy unit using two commercially available phantoms.

Methods: Each phantom contains a grid of regularly spaced markers that were localized through template matching using custom MATLAB software. Spatial distortion was quantified by comparing measured sphere locations to expected positions defined by phantom specifications and computed tomography measurements. Corrections were applied to compensate for imperfect phantom alignment. All image acquisition was performed using a balanced steady state free precession sequence with various acquisition parameters. Image distortion was evaluated in 3D for two 3D imaging protocols used to align patients and perform dose calculations for adaptive radiotherapy, and in 2D for three real-time imaging protocols commonly used for tumor tracking during radiotherapy.

Results: Average 3D distortions within 50 mm, 100 mm and 175 mm of isocenter were 0.28 mm, 0.37 mm and 0.49 mm respectively for one of the 3D imaging protocols. The corresponding maximum 3D distortions in these regions were 0.76 mm, 1.15 mm and 1.88 mm. The average 2D distortions within 50 mm, 100 mm and 175 mm of isocenter using one of the real time imaging protocols were 0.52 mm, 0.51 mm and 0.79 mm. The maximum 2D distortions in these regions were 1.10 mm, 1.67 mm, and 4.15 mm.

Conclusion: Our evaluation showed that distortion within 50 mm of isocenter is small relative to the 5 mm planning target volume (PTV) margins typically used in our clinic for MRI guided stereotactic radiotherapy. However, when targets are located further than 50 mm from isocenter, or PTV margins are less than or equal to 3 mm, image distortion must be taken into consideration.

Funding Support, Disclosures, and Conflict of Interest: Some authors have received consulting fees, speaking fees and research funding from ViewRay.

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