Wideband transskull refocusing of ultrasound beams using dual-mode ultrasound arrays: Ex vivo results

Abstract

Transcranial focused ultrasound (tFUS) is capable of providing subtherapeutic and therapeutic ablative treatments for a variety of brain disorders. A major challenge towards widespread use of tFUS-based therapies stems from the complexity of the skull that could result in severe loss of focusing gain. Using extensive hydrophone scan measurements in plain water as well as transskull, we have documented a range of tFUS beam distortions for a variety of target points and access angles. In this paper, we present quantitative measurements of tFUS distortions due to skull aberrations at different operating frequencies. In addition, refocusing results for a variety of target points at different frequencies within the transducer bandwidth are presented in terms of improvement in focusing gain. Dual-mode ultrasound array (DMUA) prototype (64 elements, concave with 40-mm radius of curvature) was used. Skull samples were extracted from animal subjects that have undergone tFUS treatments using the DMUA prototype were utilized. Experiments were performed at a set of 31 discrete frequencies in the range 2.0 MHz–5.0 MHz. A needle hydrophone was used to measure the pressure waveforms at the target locations. The element transmission efficiency varied as a function of frequency in a nonmonotonic manner with a range of 5–15 dB variation for the different target points. The array focusing gain also varied nonmonotonically suggesting the need for broadband refocusing.Transcranial focused ultrasound (tFUS) is capable of providing subtherapeutic and therapeutic ablative treatments for a variety of brain disorders. A major challenge towards widespread use of tFUS-based therapies stems from the complexity of the skull that could result in severe loss of focusing gain. Using extensive hydrophone scan measurements in plain water as well as transskull, we have documented a range of tFUS beam distortions for a variety of target points and access angles. In this paper, we present quantitative measurements of tFUS distortions due to skull aberrations at different operating frequencies. In addition, refocusing results for a variety of target points at different frequencies within the transducer bandwidth are presented in terms of improvement in focusing gain. Dual-mode ultrasound array (DMUA) prototype (64 elements, concave with 40-mm radius of curvature) was used. Skull samples were extracted from animal subjects that have undergone tFUS treatments using the DMUA prototype were...