Transcranial focusing and HIFU beam localization with dual-mode ultrasound arrays

Focused ultrasound (FUS) has been proposed for a variety of transcranial applications, including neuromodulation, tumor ablation, blood brain barrier opening. A flurry of activity in recent years have generated encouraging results demonstrating its feasibility in these and other applications. To date, monitoring of FUS beams have been primarily accomplished using MR guidance, where both MR thermography and elastography have been used. With the use of dual-mode ultrasound array (DMUA) systems with real-time imaging capability, it is possible to localize sub-therapeutic and therapeutic transcranial FUS (tFUS) beams using ultrasound. We have designed and implemented a 64-channel DMUA system with real-time GPU-based beamformer capable of supporting single transmit focus (STF) imaging at frame rates in excess of 500 fps. A 3.5-MHz, concave (40-mm roc), 32-element DMUA transducer was used to produce sub therapeutic tFUS while simultaneously imaging a decapitated head of a Copenhagen rat. A 150-μm needle thermocouple was placed at the center of the brain to measure the tFUS induced temperature. DMUA imaging was used to localize the T/C needle and place the tFUS beam near the T/C junction. A number of sub-therapeutic tFUS beams with varying power were then generated while T/C measurements were recorded synchronously with the STF frames. Speckle tracking of echo data from the tFUS beam location within the brain was used to produce transcranial images of temperature change. We have also characterized the distortion to the array psf due to the skull using images of a 50-μm wire at the center of the brain. The results show that, while the DMUA focus is degraded due to the propagation through the skull, it still produces localized heating effects within sub-millimeter volume. In addition, DMUA transcranial echo data from brain tissue allow for reliable estimation of temperature change through speckle tracking.