Tactile perception of ultrasound

TLDR

The study used acoustic radiation force to determine tactile perception thresholds in the finger and upper forearm across frequencies and pulse durations. The authors delivered a 2.2‑MHz unfocused ultrasound source modulated into square waves (50–1000 Hz) with a 1.8‑cm² reflecting disk to maximize radiation force and measured thresholds on the finger and forearm. Finger sensitivity peaked at 200 Hz with a ~0.4 mN threshold, single‑pulse thresholds were ten times higher than continuous 200 Hz exposure, and the presence of a reflecting disk confirmed that tactile sensation results from radiation‑force‑induced momentum transfer.

Abstract

In this investigation, acoustic radiation force was used as a stimulus to determine the threshold for tactile perception in the human finger and upper forearm as a function of frequency and pulse duration. Initially, a small (1.8-cm2) acoustically reflecting disk was affixed to the anatomical exposure site to maximize the delivered radiation force. Exposures were performed using a 2.2-MHz unfocused source modulated to produce square waves at 50, 100, 200, 500, and 1000 Hz. For the finger, maximum tactile sensitivity occurred at 200 Hz with a threshold radiation force of ∼0.4 mN. For single pulses of 1 to 100 ms at 2.2 MHz, the threshold forces were an order of magnitude greater than for continuous exposure modulated at 200 Hz. Thresholds for pulse durations of 0.1 ms were somewhat greater than for pulses longer than 1 ms. Subsequently, thresholds of tactile perception were determined for direct exposure of the upper forearm (avoiding bone) to single pulses of 2.2-MHz ultrasound. Comparison of perception thresholds with and without a reflecting material over the tissue were consistent with the hypothesis that the tactile sensation experienced when tissue is exposed to ultrasound is its response to the radiation force associated with the transfer of momentum from the sound field to the tissue medium.