The skullcap's ability to focus ultrasonic beams: computer simulation and experiments

Author(s)

Patricia Cardoso de Andrade

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Faculdade de Engenharia Elétrica e de Computação
• Defense date: 2022-02-21

Examining board members

Eduardo Tavares Costa; Leonardo Abdala Elias; Wagner Coelho de Albuquerque Pereira; Li Min Li; Antonio Adilton Oliveira Carneiro

• Advisor: Eduardo Tavares Costa

Abstract

Focused ultrasound neuromodulation (FUS) has been shown to be a handy neuromodulation tool in treating neuropsychiatric diseases, presenting advantages over other techniques as it is a non-invasive technique with millimeter spatial resolution. In this study, acoustic fields and heat exposure rates were simulated for different ultrasound sources focused on the brain, for the human model, and different protocols used in neuromodulation applications. The success of the ultrasound neuromodulation technique depends on the accuracy of the ultrasonic focal point. This study predicts the penetrability and focus of the ultrasonic pulses allowing to verify the viability and safety of the tFUS technique for neuromodulation. Through an in-silico study, we used the k-Wave toolbox to generate and evaluate 3D acoustic pressure maps generated by one and two single-element FUS transducers. Sound velocity and density of skull, brain tissue and skin (attenuation coefficients of 11, 0.6 e 0.5 dBcm-1MHz-1, respectively) were computed using Hounsfield units from tomographic images. We simulated pulse sequences consisting of sinusoidal bursts with pulse length of 300 µs, 50% or 60% duty cycle and bursts of 0.5s, 1s, 2s, 3s, 4s, in a total of 10 FUS protocols. Safety was evaluated looking at heat deposition throughout the skull and brain tissue. The approach suggested here emphasizes the significance of treatment planning for effective energy supply and upholding healthy brain temperature ranges. The use of two single element transducers allows for transmission and delivery of a higher and more uniform acoustic pressure amplitude in the brain, with an increase of 71% (for the 250 kHz transducer) and 49% (for the 500 kHz transducer) of the maximum amplitude at focus; and targeting deeper structures with the focal point found approximately. enabling a neuromodulation experimental device with a more affordable design than one made up of several transducers. The duration of the sonication and the duty cycle were the two most important variables for the increase in the medium's temperature, and we saw a maximum increase of 0.5º C in the skull as a result. It has been demonstrated that chirp coding of the ultrasonic pulses reduces heat buildup in the focal region and cranium. The feasibility of using planar transducers in applications involving ultrasound neuromodulation was also confirmed by this investigation. If focused through the temporal fossa of the human skull, focused transducers with frequencies higher than those typically used in humans (800 kHz) also have the potential to be used for neuromodulation. This allows a maximum pressure in focus that can cause the activation of channels sensitive to the mechanics of the medium (AU)

• FAPESP's process: 19/14217-8 - Contribution to the study of focused ultrasound transcranial neuromodulation
• Grantee: Patrícia Cardoso de Andrade
• Support Opportunities: Scholarships in Brazil - Doctorate