Best in Physics (Multidisciplinary): Rapid Multicontrast Brain Imaging at 0.35t to Facilitate Tumor Response Assessment and Functional Adaptive Radiation Therapy

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 0.35 T. Low field strength T1 mapping was compared to those references from NMR measurements at 1.5 and 3.0 T. STAGE (two triple-echo gradient echo (GRE) acquisitions) was optimized for 0.35 T. 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). Pretreatment, weekly, and ∼2 months post-treatment STAGE acquisitions were performed for a postsurgical brain cancer case to generate differential quantitative MRI maps and characterize temporal changes Results: In phantom, for T1 values \u3e150 ms, measured T1 errors from 3T-NMR and 1.5T-NMR were 5.0% ± 2.9% and 5.7% ± 2.8%, respectively. B1+ field variations were negligible at 0.35T. Simulations and volunteer experiments yielded final STAGE parameters of FA = 50o/10o, 1 × 1 × 3 mm3 resolution, TR = 40 ms, TE = 5/20/34 ms in 10 min (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 0.35 T. With confirmation in a larger cohort, results may correlate with patient outcomes and facilitate ART