Multiparametric MRI Assessment of Focused Ultrasound-Mediated BBB Opening

MRI were acquired to confirm ThUS-mediated BBB opening, track BBB closure, and estimate permeability of the disrupted BBB with K_trans. To confirm BBB opening, mice were intraperitoneally (IP) administered 0.2 mL of a gadolinium-based MR contrast agent (Omniscan, GE Healthcare, Princeton NJ, USA) and anesthetized with isoflurane throughout the scan in a 9.4T MRI system (Ascend, Bruker Medical, Billerca, MA, USA). T₁-weighted 2D FLASH MRI (TR: 230 ms, TE: 3.3 ms, flip angle: 70°, 6 averages, FOV: 25.6 mm x 25.6 mm, matrix size: 256 x 256, slice thickness: 0.4 mm, resolution: 0.1 mm x 0.1 mm, scan time: 5 min) in axial and coronal planes were acquired 30 minutes after BBB disruption for initial confirmation of opening. To track BBB closure, a cohort of mice were administered another bolus injection of contrast agent before acquisition of the same T₁-weighted MRI at 7 hours, 24 hours, 48 hours and 72 hours after BBB opening. To estimate closure on a finer timescale of 2 hours post-ThUS, several mice were given the same 0.2 mL bolus injection of contrast agent 1 hour and 30 minutes after ThUS to avoid confounding contrast enhancement signal with accumulation of remaining contrast agent if also previously injected immediately after ThUS. For estimation of permeability constant K_trans, mice were anesthetized as previously described, catheterized IP, and inserted into the bore of the magnet before dynamic contrast enhanced (DCE) MRI imaging. A modified T₁-weighted FLASH sequence (TR: 40 ms, TE: 1.4 ms, flip angle: 50°, 6 averages, FOV: 25.6 mm x 25.6 mm, matrix size: 160 x 160, slice thickness: 0.6 mm, repetitions: 55, scan time: 35 min) was initialized before injection of 0.3 mL contrast agent through the catheter during the 4^th repetition. The sequence of resulting images was used to estimate K_trans.