149 Hemodynamic Imaging in Brain Metastases: ASL vs. Hypercapnic BOLD study are listed in the Supplementary Materials. The study was performed in accordance with the Declaration of Helsinki20 and the UMCU institutional ethical review (Medisch-Ethische Toetsingscommissie (METC) NedMec) approved the study (METC# 18-747). Written informed consent was obtained from all participants prior to participation. Data acquisition Imaging protocol The participants were scanned on a 3 Tesla MRI scanner (Achieva, Philips Medical Systems, Best, The Netherlands) using a 32 channel receive coil. Whole-brain multislice single shot FE-EPI BOLD images (TR = 1050 ms, TE = 30 ms, flip angle 65°, resolution 2.292 x 2.292 x 2.5 mm3, acquisition matrix 96 x 96 x 51, 1000 dynamics, multi-band factor = 3) were acquired throughout a computer controlled hypercapnic breathing protocol (described below). Perfusion data was acquired using a multiPLD ASL sequence. A whole volume was acquired at each of the 4 post-labeling delays (660, 1325, 1989, 2654ms), using a pCASL Look-Locker multi-slice EPI readout (total scan time = 240 s, labelling train duration = 1650ms, TR = 5s, TE = 12 ms, flip angle 25°, acquired resolution: 3 x 3 x 7 mm3, acquisition matrix: 80 x 80 x 17, 23 averages, SENSE factor = 2, 2 background suppression pulses). A total of 23 label-control pairs were acquired. The first acquired ASL dataset pair here has the labelling, saturation pulses and background suppression turned off and are the M0 images for each post-label delay. An additional dataset pair was acquired for EPI phase (distortion) correction. No breathing challenges were performed during ASL imaging. The ASL was planned using a phase contrast angiography scan, with the labeling plane carefully placed perpendicular to the internal carotid arteries and vertebral arteries. Additionally, a 3D spoiled gradient echo (SPGR) sequence (TR = 8 ms, TE = 3.25 ms, flip angle 10°, isotropic resolution 1 mm, acquisition matrix 240 x 240 x 180), a 3D T2-weighted FLAIR sequence (TR 4800 ms, TE 240 ms, TI = 1650 ms, flip angle 90°, isotropic resolution: 1 mm, acquisition matrix 256 x 256 x 182), as well as a SWI (TR 50 ms, flip angle 17°, inplane slice thickness 2 mm, acquisition matrix 384 x 383 x 63) were acquired as an anatomical reference. A minimum-intensityprojection was constructed from the SWI data, using an in-house developed Matlab script (Matlab R2020a, The MathWorks, Inc., Natick, Massachusetts, United States). Clinical CT and MRI acquisition CT and MRI scans were acquired as part of clinical care as usual 1 to 5 days before receiving radiotherapy. CT scans were acquired on a Brilliance Big bore 22 scanner (Philips Medical Systems, Best, The Netherlands) with a tube potential of 120 kVp, matrix size of 512 × 512 and inplane slice thickness of 1 mm. The participants were 6
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