Technical Note: Quantification of Blood Spinal Cord Barrier Permeability after Application of Magnetic Resonance Guided Focused Ultrasound in Spinal Cord Injury.

PURPOSE To demonstrate that magnetic resonance guided focused ultrasound (MRgFUS) facilitates blood-spinal cord barrier (BSCB) permeability and develop observer-independent MRI quantification of BSCB permeability after MRgFUS for spinal cord injury (SCI). METHODS Non-injured Sprague-Dawley rats (n=3) underwent MRgFUS and were administered Evans blue post-MRgFUS to confirm BSCB opening. Absorbance was measured by spectrophotometry and correlated to its corresponding image intensity. Rats (n=21) underwent T8-T10 laminectomy and extradural compression of the spinal cord (23g weighted aneurysm-type clip, 1 min). The intervention group (n=11) were placed on a preclinical MRgFUS system, administered microbubbles (Optison, 0.2 mL/kg), and received 3 MRgFUS sonications (25 ms bursts, 1 Hz pulses for 3 min, 3 acoustic W, approximately 1.0-2.1 MPa peak pressure as measured via hydrophone as described in [Payne et al., 2017]) [1]. The sham group (n=10) received equivalent procedures with no sonications. T1w MRI were obtained both pre- and post-MRgFUS BSCB opening. Spinal cords were segmented manually or semi-automatically and a Pearson correlation with P-value ≤ 0.001 was used to correlate the two segmentation methods. MRgFUS sonication and control regions intensity values were evaluated with a paired t-test with a P-value ≤ 0.01. RESULTS Semi-automatic segmentation reduced computational time by 95% and was correlated to manual segmentation (Pearson = 0.92, p<0.001, n=71 regions). In the non-injured rat group, Evans blue absorbance correlated to image intensity in the MRgFUS and control regions (Pearson = 0.82, p=.02, n=6). In rats that underwent the SCI procedure, an increase in signal intensity in the MRgFUS targeted region relative to control was seen in all SCI rats (10.65±12.4%, range: 0.96-43.9%, n=11, p=.002). SCI sham MRgFUS revealed no change (0.63 ± 0.52%, 95% CI 0.32-0.95, n=10). This result was significant between both groups (p=.003). CONCLUSION The implemented semi-automatic segmentation procedure improved data analysis efficiency. Quantitative methods using contrast-enhanced MRI with histological validation are sensitive for detection of blood-spinal cord barrier opening induced by magnetic resonance-guided focused ultrasound.