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Low-Field 4D MRI for MRI-Guided Treatment Planning and Dose Delivery

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H Gach

H Gach1*, T Mazur1 , H Wang1 , H Song2 , M Fernandez-Seara3 , V Stenger4 , B Vajko5 , J Dempsey6 , Y Motai7 , S Mutic1 , O Green1 , (1) Washington University in St. Louis, St. Louis, MO,(2) The University of Texas MD Anderson Cancer Center, Houston, TX, (3) University of Navarra Hospital, Pamplona,(4) University of Hawaii at Manoa, Honolulu, HI, (5) ViewRay, Oakwood Village, OH, (6) ViewRay, Mountain View, CA,(7) Virginia Commonwealth University, Richmond, VA


MO-RAM-GePD-JT-3 (Monday, July 31, 2017) 9:30 AM - 10:00 AM Room: Joint Imaging-Therapy ePoster Theater

Purpose: MRI-guided radiotherapy (MRIgRT) systems enable real-time gated dose delivery in the body using 2D cine MRI. Image guided radiotherapy in the body is typically planned using 4D CT imaging to characterize the range of motion for the tumor and organs at risk. Treatment planning is now being informed by 4D MRI acquisitions. 4D MRI provides superior soft tissue contrast without the need for significant ionizing radiation dose versus 4D CT. However, 4D MRI acquisitions can be long and image rebinning is typically required to create 4D movies. In this study, we compare several strategies for acquiring 4D MRIs using a low-field MRIgRT. The goal was to acquire true 4D MRI for MRIgRT treatment planning and dose delivery.

Methods: Healthy volunteers were scanned on a 0.35 T ViewRay MRIgRT. 4D MRI was performed in the MRI-only mode using multislice single-shot 2D acquisitions and single-shot 3D acquisitions. Sequences included 2D and 3D TrueFISP, 2D T2W HASTE, and 3D T1W GRASE. 2D images were retrospectively rebinned using navigator echoes.

Results: TrueFISP image quality was impressive compared to 1.5 T. “True” 4D MRI was achieved using 3D TrueFISP to acquire 10 slices in 0.6 s (3.5 mm isotropic voxels). 3D GRASE images were unsatisfactory due to artifact and signal saturation due to the short TR (< 1 s). 2D HASTE images had good image quality but were also subject to signal saturation particularly in the liver. Duty cycles were higher than 1.5 T due to decreased SAR and the ability to use shorter RF pulses. Higher duty cycles enable higher spatial and temporal resolutions.

Conclusion: Quality 4D MRI over a limited volume was successfully demonstrated at 0.35 T. 3D acquisitions depict motion without the disadvantages of rebinning. Further research is required to accelerate the image acquisition to improve spatial and temporal resolution.

Funding Support, Disclosures, and Conflict of Interest: Drs. Vajko and Dempsey are employees of ViewRay. The research was conducted under a Master Research Agreement between Washington University in St. Louis and ViewRay. Some of the technology resulted from pulse sequences developed under National Institutes of Health National Cancer Institute grant R01 CA159471.

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