Focusing through skull with time reversal mirrors. Application to hyperthermia

The time reversal process has been applied to aberration correction in transskull focusing. The objective is to focus high-power ultrasound beams through the skull bone for non-invasive local destruction of brain tumours. Although the location of brain tumours may be accurately determined with X-ray or MRI systems, the focusing of an ultrasonic wave remains difficult. Indeed, large differences between acoustic velocities in bone and soft tissues combined with variation of skull thickness result in severe phase aberrations. Additionally, the skull induces severe attenuation of ultrasound and thus, reduces the efficiency of the time reversal approach. An amplitude correction method is proposed to compensate for the attenuation. When the skull is close to the array, this method consists in inverting the amplitude modulation before the time reversal operation. The focal spot is then shifted electronically inside the whole volume of the tumour.