Evaluation of pseudorandom sonications for reducing cavitation with a clinical neurosurgery HIFU device.

Transcranial high intensity focused ultrasound is used in clinics for treating essential tremor and proposed for many other brain disorders. This promising treatment modality requires high energy resulting eventually in undesired cavitation and potential side effects. The goals of the present work were: 1- to evaluate the potential increase of the cavitation threshold using pseudo-random gated sonications and 2- to assess the heating capabilities with such sonications. The experiments were performed with the transcranial MR-compatible ExAblate Neuro system (InSightec) operating at a frequency of 670 kHz, either in CW or with pseudo-random gated sonications of 50% duty cycle. Cavitation activity with the two types of sonications was compared using chemical dosimetry of hydroxyl radical production at the focus of the transducer, after propagation in water or through a human skull. Heating trials were performed in a hydrogel tissue mimicking material embedded in a human skull, to mimic a clinical situation. The temperature was measured by MR-thermometry when focusing at the geometrical focus, and steering off focus up to 15mm. Compared to CW sonications, the use of gated sonication did not affect the efficiency (60%), nor the steering abilities of the transducer. After propagation through a human skull, gated sonication required higher pressure level (10MPa) to initiate cavitation as compared to CW (5.8 MPa). Moreover, at equivalent acoustic power above the cavitation threshold, the level of cavitation activity initiated with gated sonications was much lower with gated sonication than with continuous sonications, almost half after propagation through water, and one-third after propagation through a skull. This lowered cavitation activity may be attributed to a breaking of the dynamic of the bubbles moving from monochromatic to more broadband sonications, and to the removal of residual cavitation nuclei between pulses with gated sonications. Heating capability was not affected by the gated sonications, and similar temperature increases were reached at focus with both types of sonications when sonicating at equivalent acoustic power, both in water or after propagation through a human skull (+15°C at 325W for 10s). These data, acquired with a clinical system, suggest that gated sonication could be an alternative to continuous sonications when cavitation onset is an issue.