Technical development and implementation of 3D‐QALAS on a 1.5T MR‐Linac for the brain: A prospective R‐IDEAL stage 0/1 technology development report

BACKGROUND: Quantitative relaxometry of the brain on the integrated MRI / linear accelerator (MR-Linac) at high isotropic reconstructed resolution is currently limited due to prohibitively long scan times and limited field-of-views. PURPOSE: The purpose of this study was to assess the technical feasibility of the 3D-QALAS technique on the 1.5T MR-Linac which can acquire whole-brain 1 mm isotropic quantitative T1, T2, and PD maps along with multiple synthetic images in a 7-minute acquisition time. METHODS: A 1 mm isotropic 3D-QALAS acquisition was scanned in both phantoms and five healthy volunteers on the 1.5T Elekta Unity MR-Linac device with scan times around seven minutes. A test-retest protocol across five independent sessions for the phantom was conducted. The Lin's Concordance Correlation Coefficient (LCCC), repeatability, and reproducibility between measured and reference quantitative T1, T2, and PD values were determined in the phantom. Distortion was also studied. Vendor-provided reconstruction through SyMRI was performed to extract synthetic images and brain volume metric assessments in the healthy volunteers. RESULTS: The slope and LCCC between the measured and phantom reference values was 1.02 (1.00), 1.09 (0.90), and 0.99 (1.00) for T1, T2, and PD, respectively. Median distortion across the phantom remained below 2 mm. The repeatability and reproducibility coefficient-of-variation (CoV) was under 8% for all measured values. The measured brain volumes in the healthy volunteer was within expected age-adjusted reference values. CONCLUSIONS: The technical feasibility of using 3D-QALAS on the integrated 1.5T MR-Linac was confirmed. Applying this technique to the adaptive radiation therapy workflow of the brain will provide new opportunities to integrate quantitative imaging relaxometry biomarkers at 1 mm isotropic reconstructed resolution.