Transcranial focused ultrasound selectively increases perfusion and modulates functional connectivity of deep brain regions in humans

Abstract

Background: Low intensity, transcranial focused ultrasound (tFUS) is a re-emerging brain stimulation technique with the unique capability of reaching deep brain structures non-invasively.

Objective/hypothesis: We sought to demonstrate that tFUS can selectively and accurately target and modulate deep brain structures in humans important for emotional functioning as well as learning and memory. We hypothesized that tFUS would result in significant longitudinal changes in perfusion in the targeted brain region as well as selective modulation of BOLD activity and BOLD-based functional connectivity of the target region.

Methods: In this study, we collected MRI before, simultaneously during, and after tFUS of two deep brain structures on different days in sixteen healthy adults each serving as their own control. Using longitudinal arterial spin labeling (ASL) MRI and simultaneous blood oxygen level dependent (BOLD) functional MRI, we found changes in cerebral perfusion, regional brain activity and functional connectivity specific to the targeted regions of the amygdala and entorhinal cortex (ErC).

Results: tFUS selectively increased perfusion in the targeted brain region and not in the contralateral homolog or either bilateral control region. Additionally, tFUS directly affected BOLD activity in a target specific fashion without engaging auditory cortex in any analysis. Finally, tFUS resulted in selective modulation of the targeted functional network connectivity.

Conclusion: We demonstrate that tFUS can selectively modulate perfusion, neural activity and connectivity in deep brain structures and connected networks. Lack of auditory cortex findings suggests that the mechanism of tFUS action is not due to auditory or acoustic startle response but rather a direct neuromodulatory process. Our findings suggest that tFUS has the potential for future application as a novel therapy in a wide range of neurological and psychiatric disorders associated with subcortical pathology.

Keywords: amygdala; brain perfusion; entorhina cortex; functional connectivity; transcranial focused ultrasound.

FIGURE 1

Study design. A visual representation of the randomized, double-blind, within-subject crossover study design. Participants completed two study visits, separated by a 14-day between-session window. During each session, participants underwent a baseline MRI assessment of regional blood flow using Arterial Spin Labeling (ASL) MRI. Thereafter, participants received tFUS in the MRI with simultaneously collected blood oxygenation level-dependent (BOLD) MRI. After tFUS was administered, ASL was collected again to compare to baseline. The brain region targeted during each study session was randomized and counterbalanced across participants such that 47% received amygdala tFUS during the first study session and 53% received ErC tFUS during their first session. Examples of amygdala and ErC tFUS targeting are provided, as well as a chart detailing sample demographics.

FIGURE 2

Group perfusion findings. (Left Column) Analysis of ASL MRI demonstrated that tFUS was associated with significant increase in perfusion to the targeted region and not the control region (i.e., when targeting ErC, increased perfusion to ErC and not amygdala, and vice versa). Increased perfusion was also seen in functionally connected regions. For ErC: anterior cingulate cortex, medial prefrontal cortex and basal ganglia regions including anterior thalamus. For amygdala: medial prefrontal cortex and ventral forebrain. (Right Column) Bar graph illustrating the mean, normalized percent perfusion change associated with tFUS in the four regions of interest: right amygdala, left amygdala, right entorhinal cortex, left entorhinal cortex. When sonicating the left ErC, increased perfusion was found in the sonicated left ErC and not the right ErC or bilateral amygdala. Similarly, when sonicating the right amygdala increased perfusion was found in the sonicated right amygdala and not the left amygdala or bilateral ErC.

FIGURE 3

Group BOLD findings. (Left Column) Analysis of tFUS on BOLD showed significantly increased activity in small areas of the temporo-occipito-parietal junction, occipital cortex and right cerebellum. Additionally, left ErC tFUS was associated with significantly reduced BOLD in anterior frontal, anterior temporal, and posterior parietal cortices. tFUS targeting the right amygdala resulted in significantly decreased BOLD activity in the posterior cingulate, pre-sensorimotor cortex, dorsolateral prefrontal cortex and pons. (Right Column) When sonicating the ErC, group PPI analysis revealed tFUS-related increased connectivity between the left ErC and left dorsolateral prefrontal cortex. Group PPI analysis of data collected when sonicating the amygdala revealed tFUS-related decreased FC between the right amygdala and posterior cingulate, anterior cingulate, medial prefrontal and posterior parietal regions. PPI control analyses confirmed that these findings were specific to the target region.

FIGURE 4

tFUS paradigm. (A) Illustration of the present study’s sonication block design, wherein the transducer alternated between 30-s blocks of active stimulation and no stimulation. This cycle occurred ten times, totaling five minutes of non-consecutive tFUS. (B) Visualization of the shape, orientation and intensity distribution of the tFUS beam. When measured in a water tank using a hydrophone, the tFUS beam appears ellipsoid in shape with a central focus of higher intensity and a surround of lower intensity. The longitudinal dimension is approximately 2 cm in length while the cross section, which also evidences a higher intensity centroid, is approximately 0.5 cm in diameter.

FIGURE 5

tFUS targeting. Visualization of transducer placement on 3D model (1^st column) when targeting the right amygdala (Top Row) and left entorhinal cortex (Bottom Row). Examples of MRI-console guided targeting using transducer fiducial markers are provided in coronal view (2^nd column) and axial view (3^rd column).