Transcranial focused ultrasound with multielement arrays: Decreasing the number of transducers without compromising the focusing quality

Abstract

Transcranial ultrasonic brain therapy at frequencies higher than 500 kHz requires adaptive focusing to compensate for the aberrations induced by the skull bone. This can be achieved by using multi-element arrays driven by a dedicated electronics. A growing number of elements was used to improve the focusing: 64 elements in 2000 [1], 300 in 2003 [2], 1024 in 2013 [3], and more to come. We will present some of the salient results obtained pre-clinically and clinically with such multi-element transcranial devices. Nevertheless, we will show that comparable transcranial focusing can be achieved with a novel approach in rupture with the current trend. It consists in a single-element transducer covered with a 3D silicone acoustic lens of variable and controlled thickness. Similar lenses have been introduced in the past to perform multiple focusing patterns in homogenous media [4] but recent 3d printing capabilities make 3D lenses a feasible option for transcranial adaptive focusing. We will show that the use of a lenses increases the acoustic intensity by 97 ± 56% on average, on three human skulls. [1] G. Clement et al., “A hemisphere array for non-invasive ultrasound brain therapy and surgery,” Phys. Med. Biol. 2000. [2] M. Pernot et al., “High power transcranial beam steering for ultrasonic brain therapy,” Phys. Med. Biol. 2003. [3] N. Lipsman et al., “MR-guided focused ultrasound thalamotomy for essential tremor: a proof-of-concept study,” The Lancet Neurology 2013. [4] T. Fjield et al., “Low-profile lenses for ultrasound surgery,” Phys. Med. Biol. 1999.