Monitoring of focused ultrasound-induced blood-brain barrier opening in non-human primates using transcranial cavitation detection in vivo and the primate skull effect

Focused ultrasound (FUS) with microbubbles (MB) is promising for assisting the delivery of drugs across the blood-brain barrier (BBB). To assess the safety and efficacy, the monitoring using passive cavitation detection (PCD) is critical and yet the reliability of transcranial detection in large animals remained questioned. To study the primate skull effect, the PCD through the in-vitro monkey and human skulls and in the in vivo monkeys during the sonication (FUS frequency: 500 kHz) were investigated, with the use of in-house made lipid-shelled, monodisperse MB (median diameter: 4-5 μm) and a flatband hydrophone served as a passive cavitation detector. In the in vitro experiments, the MB were injected to the channel of the phantom under a degassed skull for sonication (peak negative pressure/PNP: 50-450 kPa, pulse length/PL: 0.2 ms, PRF: 10 Hz, duration: 2 s). A diagnostic B-mode imaging system was also used to monitor the cavitation. In the in vivo study, the PCD was realtime monitored during the sonication for PCD calibration (PNP: 50-700 kPa, PL: 0.2 ms and 10 ms, PRF: 2 Hz, duration: 10 s) and BBB opening (PNP: 200-600 kPa, PL: 10 ms, PRF: 2 Hz, duration: 2 min). The stable cavitation dose using harmonics (SCD_h) and ultraharmonics (SCD_u) and the inertial cavitation dose (ICD) were quantified. Results showed that the SCD_h, SCD_u, and ICD were detectable in vitro at 50 kPa and above, and the B-mode imaging showed bubble collapse at 200 kPa and above. The detection thresholds increased with the skulls in place, with the signal reduction of 15.4 dB for the monkey skull and 34.1 dB for the human skull. In the in vivo experiments, the SCD_h and ICD was detectable at and above 100 kPa and 250 kPa, respectively, and the SCD_u was less reliable due to spontaneous occurrence. The BBB was found to be disrupted in 250-600 kPa without edema, hemorrhage, and physiological changes were found. In conclusion- the SCD_h was more detectable and reliable than the SCD_u in assessing stable cavitation in vivo, and the inertial cavitation was detected at 250 kPa and may occur at lower pressures.