Sally Z. Child
Carol H. Raeman
Edwin L. Cartensen
Dept. of Elec. Eng. and the Rochester Ctr. for Biomed. Ultrasound, Univ. of Rochester, Rochester, NY 14627
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 length. Initially, a small (1.8 cm[sup 2]) 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, threshold radiation forces were an order of magnitude greater than for continuous exposure at 200 Hz. 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 the reflecting material over the forearm 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.