MRI Scanning Protocol

All MRI experiments were conducted on a 3T Siemens MAGNETOM Skyra scanner (Siemens Healthcare GmbH, Erlangen, Germany) with a 32-channel head coil. The imaging protocol included a T1-weighted anatomical 3D MPRAGE sequence (with inversion time (TI) = 950 ms, repetition time (TR) = 1760 ms, echo-time (TE) = 3.1 ms, resolution = 1 mm isotropic, scan time = 5:08 min), a time-of-flight (TOF) angiography sequence and a PCASL sequence. The angiograms acquired with the TOF sequence, depicting the carotid and vertebral arteries were used to position the PCASL labeling plane. The position was selected above the carotid bifurcation and below the V3 segment. The plane was oriented as perpendicular as possible to the carotid and vertebral arteries. The PCASL sequence combined PCASL (labeling time = 1.2 s, post-labeling delay = 1.4 s) with background suppression and single-shot 3D RARE stack-of-spirals readout with through-plane acceleration (1D-GRAPPA), as previously described (Vidorreta et al., 2017). The imaging parameters were as follows: TE = 10.33 ms, TR = 3 s, resolution = 3.75 mm isotropic, in-plane FOV = 240 mm × 240 mm, excitation flip angle (FA) = 90°, refocusing FA = 180°, in-plane reconstructed matrix = 64 × 64, 26 slices acquired with 15% oversampling, slice Partial Fourier = 5/8 and R = 2 acceleration in the HF phase-encoding direction (i.e., slice-encoding direction) to achieve a single-shot acquisition (readout length <300 ms). The in-plane spiral readout consisted of two spiral arms acquired in interleaved fashion along the RARE train, with maximum gradient amplitude = 36 mT/m, slew rate = 120 mT/m/ms, and receiver bandwidth = 400 Hz/px (dwell time = 2.5 us). Background suppression consisted of two adiabatic inversion pulses played during PLD time, with timings optimized to null the static tissue signal to 10% of its equilibrium value at the time of readout. A proton density image was also acquired at the start of the ASL scan without background suppression or labeling pulses, for CBF quantification.