Focusing ultrasound through the skull for neuromodulation

Abstract

Focused ultrasound for neuromodulation is emerging as a non-invasive brain stimulation method, whereby low-intensity pulsed ultrasound is focused through the skull to locations within the brain. The ultrasound results in excitation of the targeted brain region, and stimulation in the motor and visual centers has already been reported. One barrier is that the strongly heterogeneous skull bone distorts, aberrates, and attenuates the ultrasound beam leading to disruption and shifting of the focus. While transducer arrays can be used to correct for these aberrations, this equipment is expensive and complex. Here, numerical modeling is used to determine the optimal placement of a single element focused transducer to achieve the required focusing. Numerical simulations, using a point source at target locations in the visual cortex, are employed to determine the phase and amplitude on a spherical surface placed outside the head. The optimal placement of the transducer is determined by minimizing the weighted phase error over the transduce surface. Appropriate focusing is then confirmed by simulating the pressure field in the brain tissue for the optimal transducer location. Both elastic and fluid-type models of the skull are considered to assess the impact of shear waves on the targeting.