Purpose: Magnetic resonance-guided radiation therapy (MRgRT) has shown great promise for localization and real-time tumor monitoring. However, quantitative imaging has been limited at low field strengths. This work benchmarks functional T1-mapping in phantom and implements Strategically Acquired Gradient Echo (STAGE) MRI to facilitate functional and anatomical adaptive radiation therapy (ART) in the brain.
Methods: A benchmarking ISMRM/NIST phantom consisting of spheres with variable NiCl2 concentrations was scanned using variable flip angles (2-40 degrees) at 0.35T. Low field strength T1 mapping was compared to those references from NMR measurements at 1.5T and 3.0T. STAGE (two triple-echo gradient echo (GRE) acquisitions) was optimized for 0.35T. Simulations were performed to choose two flip angles to optimize signal-to-noise ratio (SNR) and T1 mapping precisions. Tradeoffs between SNR, scan time, and spatial resolution for whole-brain coverage were evaluated in volunteers. Data were inputted into the STAGE pipeline to yield 4 qualitative images (T1-weighted, enhanced T1-weighted, proton-density (PD) weighted, and FLAIR), and 3 quantitative datasets (T1, PD, and R2* mappings). Pre-treatment, weekly, and ~2 months post-treatment STAGE acquisitions were performed for a post-surgical brain cancer case to generate differential quantitative MRI maps and characterize temporal changes
Results: In phantom, for T1 values >150 ms, measured T1 errors from 3T-NMR and 1.5T-NMR were 5.0%Â±2.9% and 5.7%Â±2.8%, respectively. B1+ field variations were negligible at 0.35T. Simulations and volunteer experiments yielded final STAGE parameters of FA=50o/10o, 1x1x3 mm3 resolution, TR=40ms, TE=5/20/34ms in 10 minutes (64 slices). In patient, differential maps for R2* and T1 maps were sensitive to local tumor changes and spatially correlated to 3T follow up MRI datasets.
Conclusion: We are the first to highlight that quantitative T1 mapping is meaningful and STAGE is promising for functional brain cancer imaging at 0.35T. With confirmation in a larger cohort, results may correlate with patient outcomes and facilitate ART.
Funding Support, Disclosures, and Conflict of Interest: The submitting institution holds research agreements with Philips Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA204189