Effects of transcranial focused ultrasound on human primary motor cortex using 7T fMRI: a pilot study

Abstract

Background: Transcranial focused ultrasound (tFUS) is a new non-invasive neuromodulation technique that uses mechanical energy to modulate neuronal excitability with high spatial precision. tFUS has been shown to be capable of modulating EEG brain activity in humans that is spatially restricted, and here, we use 7T MRI to extend these findings. We test the effect of tFUS on 7T BOLD fMRI signals from individual finger representations in the human primary motor cortex (M1) and connected cortical motor regions. Participants (N = 5) performed a cued finger tapping task in a 7T MRI scanner with their thumb, index, and middle fingers to produce a BOLD signal for individual M1 finger representations during either tFUS or sham neuromodulation to the thumb representation.

Results: Results demonstrated a statistically significant increase in activation volume of the M1 thumb representation for the tFUS condition as compared to sham. No differences in percent BOLD changes were found. This effect was spatially confined as the index and middle finger M1 finger representations did not show similar significant changes in either percent change or activation volume. No effects were seen during tFUS to M1 in the supplementary motor area or the dorsal premotor cortex.

Conclusions: Single element tFUS can be paired with high field MRI that does not induce significant artifact. tFUS increases activation volumes of the targeted finger representation that is spatially restricted within M1 but does not extend to functionally connected motor regions. Trial registration ClinicalTrials.gov NCT03634631 08/14/18.

Keywords: 7T fMRI; BOLD; Human; Motor cortex; Neuromodulation; Ultrasound.

Fig. 1

a Schematic of the fMRI experimental protocol. Finger movement (thumb, middle, index) was visually cued at 1 Hz across the on blocks. A total of nine 30 s on blocks were collected (3 for each finger) interspersed with 30 s rest blocks. Within each on block transcranial focused ultrasound (tFUS) was delivered every two TRs (2.75 s). b Schematic of the ultrasound pulsing strategy. PRF pulse repetition frequency, Af acoustic frequency

Fig. 2

a Pseudocolor XY plot of ultrasound pressure profile normalized to peak pressure. b XYZ line plots of ultrasound pressure profile normalized to peak pressure. Vertical dashed red lines denote − 3 dB pressure. Note: Red arrow in Z-plot indicates direction of ultrasound from face of transducer (0 mm)

Fig. 3

a 7T anatomical T1 (left, middle) and functional EPI image showing ultrasound transducer. b Overlay of functional MRI thumb activation and acoustic model of the ultrasound beam on subject anatomical T1 scan. Note in right image ultrasound beam is purposefully displaced from the fMRI thumb activation to better show relative size compared to fMRI activation. c Blowup of single subject fMRI BOLD finger representations with overlaid acoustic model that is purposefully displaced to show relative size of ultrasound beam to fMRI activations. For experiments, tFUS would have been placed directly over the thumb activation

Fig. 4

a Individual subject fMRI BOLD thumb activity in primary motor cortex during sham and tFUS neuromodulation. b Group (N = 5) fMRI BOLD M1 volumes for sham and tFUS neuromodulation. *p < 0.05