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Three-Dimensional Polymer Gel Dosimetry Using An Onboard 0.35 T Magnetic Resonance Imaging Scanner

B Maraghechi , H Gach , D Yang , H Li*, Washington University School of Medicine, St Louis, MO


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

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 ViewRay’s onboard 0.35 T MRI scanner.

Methods: A BANG polymer gel dosimeter was irradiated by three beams of 3 cm × 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 ViewRay’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 1.5×1.5 mm² and the slice thickness was 5 mm. For the images obtained from the ViewRay scanner the 1.5×1.5 mm² pixel size was downsampled to 3 × 3 mm² 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 3×3 mm² pixel size. Similar results were obtained with 1.5×1.5 mm² pixel size when the images were denoised.

Conclusion: Comparable dose uncertainty was obtained using the ViewRay’s onboard 0.35 T and Philips 1.5 T MRI scanners when using a spin echo pulse sequence with NSA=16.


Gel Dosimeter, MRI, Quality Assurance


TH- Radiation dose measurement devices: 3D solid gel/plastic

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