Reaching the optimal focusing and steering capabilities of transcranial HIFU arrays based on time reversal of acoustically induced cavitation bubble signature

Brain treatment with High Intensity Focused Ultrasound (HIFU) can be achieved by multichannel arrays through the skull using time-reversal focusing. Such a method requires a reference signal either sent by a real source embedded in brain tissues or computed from a virtual source, using the acoustic properties of the skull deduced from CT images. This non-invasive computational method allows precise focusing, but is time consuming and suffers from unavoidable modeling and repositioning errors which reduce the accessible acoustic pressure at the focus in comparison with real experimental time-reversal using an implanted hydrophone. Ex vivo simulations with a half skull immersed in a water tank allow us to reach at low amplitude levels a pressure ratio of 83% of the reference pressure (real time-reversal) at 1 MHz. Using this method to transcranially focus a pulse signal in an agar gel (model for in vivo bubble formation), we induced a cavitation bubble that generated an ultrasonic wave received by the array. Selecting the 1 MHz component, the signal was time reversed and re-emitted, allowing 97%plusmn1.1% of pressure ratio to be restored. To target points in the vicinity of the geometrical focus, conventional electronic steering from the reference signal has been achieved. Skull aberrations severely degrade the accessible pressure while moving away from the focus ( ~90% at 11 mm in the focal plane). Nevertheless, inducing cavitation bubbles close to the limit of the primary accessible zone allowed us to acquire multiple references signal to increase the electronic steering area by 50%.