Room: Exhibit Hall
Purpose: Magnetic Resonance Imaging (MRI) scanners are widely used for 3D gel dosimeters readout. However, limited access to MRI scanners is a challenge in MRI based gel dosimetry. Recent commercialization of MRI guided radiation therapy machines such as ViewRay provides potential opportunity for on-board gel dosimetry using its integrated MRI scanner. The objective of this study was to investigate the feasibility of gel dosimetry using ViewRayâ€™s onboard 0.35 T MRI scanner.
Methods: A BANG polymer gel dosimeter was irradiated by three beams of 3 cm Ã— 3 cm field size with 432 MU (left and right) and 720 MU (center). The readout was performed with both Philips 1.5 T and ViewRayâ€™s 0.35 T on-board MRI scanners. Two Hahn spin echo images were taken with repetition time 8000 ms and two different echo times (TEs) of TE1 = 20 ms and TE2 = 100 ms. The number of signal averages (NSA) was set to 1 on the Philips scanner and 16 on the ViewRay scanner. The in-plane spatial resolution was set at 1.5Ã—1.5 mmÂ² and the slice thickness was 5 mm. For the images obtained from the ViewRay scanner the 1.5Ã—1.5 mmÂ² pixel size was downsampled to 3 Ã— 3 mmÂ² for averaging. Another approach was to denoise the images. Dose versus R2 curve was plotted and the dose uncertainty was derived.
Results: When the dose was increased from 2 Gy to 7.5 Gy the dose uncertainty decreased from 12% to 3.5% using the Philips scanner and from 13% to 4.2% using the ViewRay scanner with 3Ã—3 mmÂ² pixel size. Similar results were obtained with 1.5Ã—1.5 mmÂ² pixel size when the images were denoised.
Conclusion: Comparable dose uncertainty was obtained using the ViewRayâ€™s onboard 0.35 T and Philips 1.5 T MRI scanners when using a spin echo pulse sequence with NSA=16.