Stent with Piezoelectric Transducers for High Spatial Resolution Ultrasound Neuromodulation- a Finite Element Analysis

Abstract

D EEP brain stimulation is currently the only technique used in the clinical setting, capable of stimulating the deep brain nuclei. Recently, low intensity focused ultrasound has been shown to reversibly modulate brain activity. At present, ultrasound neuromodulation is performed through the transcranial pathway. This requires the ultrasound waves to pass through the skull, thus attenuating the ultrasound waves. To reduce the effect of attenuation, lower frequencies are used of around 0.5 M H z, however, with decreasing frequency the focal region in which the stimulation occurs also increases. Therefore, a stent with an array of ultrasound transducers operating at high frequencies of around 5 M H z has been proposed to bypass the skull, thus significantly reducing the attenuation. Moreover, such a device would require a minimally invasive procedure removing the need for open brain surgery as currently performed for deep brain stimulation. This research was performed to evaluate the feasibility of using ultrasound arrays within the brain vasculature to stimulate deep brain nuclei using the finite element method.Initially, a literature review was conducted to identify the optimal ultrasound parameters for achieving neuromodulation. For stimulation, a PRF ranging from 500 to 1500 H z and a DC of around 50 to 70% were found to be optimal. Whereas for suppression, a PRF of<100 H z and a DC of around >=10% were found to be ideal. The intensity for both suppression and stimulation was found to greatly depends on the target region and individual as well as anaesthesia for stimulation, nonetheless, for 5 M H z an intensity of around 1 W /cm 2 was required to achieve neural excitation.Next, the brain nuclei were studied and the subthalamic nucleus, ventral intermediate nucleus and globus pallidus internus were identified to be the most commonly targeted nuclei for deep brain stimulation. Brain vasculature was then examined in terms of the blood vessel inner diameter, shape, orientation with respect to the nuclei and the distance to the nuclei. Basal vein of Rosenthal, internal cerebral vein and internal carotid artery were found to be most suitable to target the subthalamic nucleus, ventral intermediate nucleus and globus pallidus internus respectively.Afterwards, two-dimensional simulations were performed using COMSOL Multiphysics to identify the minimum required array width and length for stimulating each nucleus from its respective blood vessel. However, when compared to 3D counterparts it was found that 2D significantly overestimated the size of the focal region.Finally, 3D simulations of 100 x 100 x 262 µm transducers in a 104 (2D 26 x 4) and 105 (Checkered [3 x 2] x 21) arrays within the BVR were analysed targeting the STN. These arrays were found to produce a focal region of full width half intensity within the subthalamic nucleus model boundaries while reaching intensities and pressures of 0.558 W /cm 2 or 0.140 MPa and 0.311 W /cm 2 or 0.101 M P a with a 1 V terminal for 2D and checkered array respectively. Theref...