Mr scanners

Patients from the CBTN cohort underwent brain MR imaging at 1.5T or 3T Siemens MR scanners (Table S1). Sequences acquired included 2D axial T2-weighted turbo spin-echo (TR/TE, 1000–7300/80–530 ms; 0.5- to 5-mm section thickness), 3D axial or sagittal pre-contrast, and 3D axial gadolinium-based contrast agent–enhanced T1-weighted turbo or fast-field echo. Patients from the BCH cohort underwent brain MR imaging at 1.5T or 3T from various MRI vendors (Table S2). MRIs were performed using the brain tumor protocol of the institution, which included 2D axial T2-weighted fast spin-echo (TR/TE, 2600–12,000/11–120 ms; 2- to 5.5-mm section thickness), 2D axial or sagittal pre-contrast T1-weighted spin- echo, 2D axial T2 FLAIR and 2D axial gadolinium-based contrast agent–enhanced T1-weighted spin-echo sequences (Table S1–S2; Fig. S1–S4). All MR imaging data were extracted from the respective PACS and were de-identified for further analyses. Given that many pLGGs do not enhance with intravenous contrast, are hypointense on T1, and are hyperintense on T2-weighted sequences, we chose to develop a T2-weighted segmentation algorithm with a primary use case of volumetric tumor monitoring.