Enhanced Detection of Bubble Emissions Through the Intact Spine for Monitoring Ultrasound-Mediated Blood-Spinal Cord Barrier Opening

Objective: We previously developed short burst, phase keying (SBPK) focused ultrasound (FUS) to mitigate standing waves in the human vertebral canal. Here, we show microbubble emissions from these pulses can be detected through the human vertebral arch and that these pulses are effective for blood-spinal cord barrier (BSCB) opening. Methods: At f(0) = 514 kHz, circulating microbubbles were sonicated through ex vivo human vertebrae (60 kPa-1 MPa) using a dual-aperture approach and SBPK exposures engineered to incorporate pulse inversion (PI). Signals from a 250 kHz receiver were analyzed using PI, short-time Fourier analysis and the maximum projection over the pulse train. In rats (n = 14), SBPK FUS+microbubbles was applied to 3 locations/spinal cord at fixed pressures (similar to 0.20-0.47 MPa). MRI and histology were used to assess opening and tissue damage. Results: In human vertebrae between 0.2-0.4 MPa, PI amplified the microbubble/baseline ratio at f(0)/2 and 2f(0) by 202 +/- 40% (132-291%). This was maximal at 0.4 MPa, coinciding with the onset of broadband emissions. In vivo, opening was achieved at 40/42 locations, with mean MRI enhancement of 46 +/- 32%(16%-178%). Using PI, f(0)/2 was detected at 14/40 opening locations. At the highest pressures (f(0)/2 present) histology showed widespread bleeding throughout the focal region. At the lowest pressures, opening was achieved without bleeding. Conclusion: This study confirmed that PI can increase sensitivity to transvertebral detection of microbubble signals. Preliminary in vivo investigations show that SBPK FUS can increase BSCB permeability without tissue damage. Significance: SBPK is a clinically relevant pulse scheme and, in combination with PI, provides a means of mediating and monitoring BSCB opening noninvasively.