Comparison of thermal effects induced by transcranial focused ultrasound and transcranial pulse stimulation

Abstract The objective of the study was to investigate thermal effects induced through two low-intensity focused ultrasound (LIFU)–based brain stimulation techniques—transcranial focused ultrasound stimulation (tFUS) and transcranial pulse stimulation (TPS). Long trains of acoustic pulses are delivered using tFUS, whereas, the newly introduced TPS delivers ultrashort (∼3 μs) pulses repeated at 4 Hz. It is presumed that TPS by virtue of delivering ultrashort ultrasound pulses reduces the risk for brain heating, but this has not been quantified in the literature. While overall temperature increase depends on the exact combination of stimulation parameters, simulations for tFUS have predicted ∼3°C and ∼<0.2 °C rises in a monkey skull and brain respectively. To our knowledge, the maximum rise ever reported is ∼3°C in rat brain but via a combination of longer duration and higher Ispta than typically used in tFUS. Following our previous work developing an image-derived model for TPS, we extended the models to assess transient heating in TPS as compared to tFUS. A finite difference time domain approach was used to solve the Westervelt-Lighthill equation for the pressure distribution in the head. Average intensity was calculated from the pressure distribution, which was then coupled to Pennes bioheat equation as a heat source. Initial results using certain assumptions indicate brain temperature rises in TPS to be <0.01°C. This indicates temperature rise at least an order of magnitude lower than some common tFUS parameters. While the absolute heating induced by either modality can be modulated by adjusting Ispta (through transducer intensity, duration, or duty cycle), typical tFUS and TPS protocols produce inherently different proportions of temperature versus pressure changes. In addition to risk, knowing the relative differences in temperature versus pressure generated could lend to a deeper understanding of the thermal versus mechanical mechanisms underlying the two techniques. Research Category and Technology and Methods Translational Research: 19. Modeling and computational methods Keywords: Ultrasound, tFUS, TPS, Thermal