Focused ultrasound-mediated non-invasive brain stimulation: examination of sonication parameters

Abstract

Background: Transcranial focused ultrasound (FUS) has emerged as a new brain stimulation modality. The range of sonication parameters for successful brain stimulation warrants further investigation.

Objective: The objective of this study was to examine the range of FUS sonication parameters that minimize the acoustic intensity/energy deposition while successfully stimulating the motor brain area in Sprague-Dawley rats.

Methods: We transcranially administered FUS to the somatomotor area of the rat brain and measured the acoustic intensity that caused excitatory effects with respect to different pulsing parameters (tone-burst duration, pulse-repetition frequency, duty cycle, and sonication duration) at 350 and 650 kHz of fundamental frequency.

Results: We observed that motor responses were elicited at minimum threshold acoustic intensities (4.9-5.6 W/cm(2) in spatial-peak pulse-average intensity; 2.5-2.8 W/cm(2) in spatial-peak temporal-average intensity) in a limited range of sonication parameters, i.e. 1-5 ms of tone-burst duration, 50% of duty cycle, and 300 ms of sonication duration, at 350 kHz fundamental frequency. We also found that the pulsed sonication elicited motor responses at lower acoustic intensities than its equivalent continuous sonication.

Conclusion: Our results suggest that the pulsed application of FUS selectively stimulates specific brain areas-of-interest at an acoustic intensity that is compatible with regulatory safety limits on biological tissue, thus allowing for potential applications in neurotherapeutics.

Keywords: Focused ultrasound; Neuromodulation; Neurostimulation; Parameter; Sonication.

Figure 1

(a) Experimental set-up to test excitatory neuromodulation (tail movement) using focused ultrasound in rat: (1) single-element FUS transducer, (2) degassed water bag, (3) rat fixation frame, (4) 3-axes adjustable platform, (5) motion detection sensor, (6) optical-guidance system (dashed lines represent visible lasers); an exemplar record of the induced tail movement by pulsed FUS (350 kHz FF, 50% DC – 250 Hz PRF, 2ms TBD, 300ms SD, 2s ISI) as an inset; we used 3 times the standard deviation (denoted as ‘σ’) of the baseline signal level as the threshold to estimate the response latency from the onset of stimulation, (b) Schematics of sonication control and acquisition system, (c) Definition of sonication parameters modulated by function generators (FGs): tone-burst duration (TBD), pulse-repetition frequency (PRF), duty cycle (DC), sonication duration (SD), inter-stimulus interval (ISI), acoustic intensity (AI), and fundamental frequency (FF).

Figure 2

Acoustic intensity profile of (a) 350 kHz and (b) 650 kHz transducer in longitudinal (left) and transversal (right) plane to the sonication path. The arrows indicate the direction of sonication. The bars indicate the 5 mm scale.

Figure 3

Comparison of threshold acoustic intensities among the three duty cycles across the six tone-burst durations. For the fixed fundamental frequency (350 kHz) and sonication duration (300 ms), the association of the threshold (a) I_sppa and (b) I_spta to the three different duty cycles across various tone-burst durations is shown. The brackets indicate statistically significant differences (one-way ANOVA with Tukey-Kramer post-hoc analysis; p<0.05) at the specified TBD. The error bars indicate ±1 s.e.m.

Figure 4

Comparison of threshold acoustic intensities and energy density among the three sonication durations across the six tone-burst durations. For the fixed duty cycle (50%) and fundamental frequency (350 kHz), the association of the threshold (a) AIs (I_sppa and I_spta) and (b) E_sppa to the three different sonication durations across various tone-burst durations is shown. The brackets indicate statistically significant differences (one-way ANOVA with Tukey-Kramer post-hoc analysis; p<0.05) at the specified TBD. The error bars (I_sppa and E_sppa) indicate ±1 s.e.m.

Figure 5

Comparison of threshold acoustic intensities and energy density between the two fundamental frequencies across the six tone-burst durations. For the fixed duty cycle (50%) and sonication duration (300 ms), the association of threshold AIs (I_sppa and I_spta) and E_sppa to the two different fundamental frequencies at various tone-burst durations is shown. The brackets indicate statistically significant differences (one-tailed t-test; p<0.05) at the specified TBD. The error bars (I_sppa only) indicate ±1 s.e.m. It can be seen that the use of 350 kHz could elicit motor responses at lower threshold acoustic intensities.

Figure 6

Examples of the histological analysis of the sonicated rat brain tissues. (a) Hematoxylin and eosin (H&E) staining confirmed that there was no tissue damage or hemorrhage associated with the sonication in a majority of the animals (n=29 out of 30). (b) Tissue sample from the one animal, whereby several rounded shapes containing hemosiderin were observed. The animal was allowed to survive for 26 days after the sonication before being sacrificed. Several rounded areas containing hemosiderin indicating potential earlier bleeding was observed (shown in arrow). Both sections were sampled from the brain cortex near the mid-line.