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Immobilization and Geometric Distortion Study for Magnetic Resonance Imaging Guided Stereotactic Radiosurgery On the ViewRay MR-Linac

E Omari*, G Hugo , O Green , S Mutic , H Gach , N Knutson , F Reynoso , Washington University in St. Louis, St Louis, MO


(Tuesday, 7/31/2018) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 5

Purpose: To investigate the feasibility of stereotactic radiosurgery (SRS) on a Linac with onboard MRI capabilities. The reproducibility of a frameless MRI-compatible positioning and immobilization standalone system was evaluated, and system specific geometric distortion uncertainties were quantified.

Methods: A head mask was used to immobilize an SRS head phantom with an MRI-compatible U-Frame modified to fit the MRI RF coils. A repeatability study was conducted over five days using rigid registrations of subsequent CT scans. To evaluate geometric distortion uncertainties, an anthropomorphic MRI distortion SRS phantom was used. The phantom was immobilized and simulated using a CT scanner and then scanned in the ViewRay MRIdian system using a clinical 3D-TrueFISP sequence (TE/TR: 2/4 ms, 1.5 mm isotropic resolution, 550 Hz/pixel). Several features in the central and peripheral parts of the phantom were selected. Distances between features were measured by five independent observers on both the MRI and CT images.

Results: The Dice and Jaccard similarity coefficients were evaluated over repeated CT scans on the external contour to evaluate immobilization and reproducibility. The average Dice and Jaccard coefficients were 0.994±0.003 and 0.988±0.006, respectively, prior to image registration. Maximum rigid errors for the (x,y,z) directions were (0.60, 0.07, and 0.35 cm) and 0.09°, 0°, and 0.22° for rotations in the axial, sagittal, and coronal planes. The geometric distortion analysis showed a maximum observed distortion of 0.8 mm around the periphery of the phantom and 0.6 mm in central regions. Analysis of variance was calculated over all 21 observations per observer and no statistical significance was observed between the CT and MRI (α=0.05, mean p=0.6).

Conclusion: A frameless MRI-compatible immobilization system was modified and evaluated for SRS to be performed on the ViewRay MR-Linac. System specific geometric distortions were quantified. These are important steps for establishing an SRS service on the MR-Linac.

Funding Support, Disclosures, and Conflict of Interest: Disclosures for Geoffrey Hugo: Research support from Varian, NIH Disclosures for H. Michael Gach: Owns common stock for ViewRay


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