Characterization of Image Quality During MR-Guided Radiotherapy (MRgRT) Localization: Impact of Coil Sensitivity Mapping
R Herrera1, J McCulloch1,2, P Shipman2, A Gutierrez1,2, T Romaguera1,2, D Alvarez1,2, I Aizpurua3, M Chuong1,2, M Mehta1,2, J Bayouth4, K Mittauer1,2*, (1) Miami Cancer Institute, Miami, FL (2) Florida International University, Miami, FL (3) ViewRay Inc, Oakwood, OH (4) University of Wisconsin-Madison, Madison, WI
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PO-GePV-M-226 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Coil sensitivity mapping (CSM) normalizes each coil element and is only performed on first MR acquisition during MRgRT. With introduced IGRT couch shifts, the change in absolute position within Bo field may decrease signal uniformity during subsequent MR imaging. We evaluated uniformity and spatial resolution loss as a function of IGRT couch displacement on a 0.35T MR-Linac.
Methods: A 50x45x43 cm³ TRUFI MR (GRAPPA parallel imaging, 1.5-mm resolution, 172 second) was acquired of a 3D MR-QA phantom (35x27x21 cm³). IGRT-simulated 1D shifts (0.5-5 cm in 0.5 cm increments) and 3D shifts (0.5-3 cm in 0.5x0.5x0.5 cm increments) were applied. Post-IGRT localization, two CSMs were applied at each position: MR with original CSM at isocenter (CSM_isocenter) and MR with manual updated CSM (CSM_localized). Threshold image quality loss (IQL) of percent difference of CSM_isocenter to CSM_localized was taken at ≥30% for uniformity normalized SD (NSD) and ≥3% for spatial resolution as ESF 10%-90% width (mm) and PSF FWHM (mm).
Results: Sixty-four scans (32 CSM_isocenter+32 CSM_localized) were acquired. For 1D lateral, IQL was found only for 2.5 cm for NSD of 50%. For 1D longitudinal, IQL was found for 4 cm and 4.5 cm with 5% and 4% in ESF and PSF, respectively. No uniformity IQL was found for all 1D longitudinal. For 1D vertical, IQL was found for NSD ≥1.5 cm (range 32%-162%) and for ESF was only found for 1 cm at 4%. For 3D shift, IQL was found for NSD ≥1 cm (range 33%-91%), for ESF and PSF at 1, 2, and 3 cm (range 3%-7%).
Conclusion: IGRT 1D shifts ≥ 2.5 (lateral), 4 (longitudinal), or 1 (vertical) cm and 3D shifts ≥ 1x1x1 cm resulted in IQL for uniformity and spatial resolution. It is recommended to perform CSM normalization at new patient position when IGRT shifts exceed above.
Funding Support, Disclosures, and Conflict of Interest: Dr Mittauer reports personal fees from ViewRay Inc, other from MR Guidance LLC, and grants from ViewRay Inc. Dr Chuong reports personal fees and grants from ViewRay Inc. Dr Bayouth reports personal fees from ViewRay Inc and other from MR Guidance LLC.
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IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined (general)
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