Effective Ultrasonic Stimulation in Human Peripheral Nervous System

Abstract

Objective: Low-intensity ultrasound can stimulate excitable cells in a noninvasive and targeted manner, but which parameters are effective has remained elusive. This question has been difficult to answer because differences in transducers and parameters-frequency in particular-lead to profound differences in the stimulated tissue volumes. The objective of this study is to control for these differences and evaluate which ultrasound parameters are effective in stimulating excitable cells.

Methods: Here, we stimulated the human peripheral nervous system using a single transducer operating in a range of frequencies, and matched the stimulated volumes with an acoustic aperture.

Results: We found that low frequencies (300 kHz) are substantially more effective in generating tactile and nociceptive responses in humans compared to high frequencies (900 kHz). The strong effect of ultrasound frequency was observed for all pressures tested, for continuous and pulsed stimuli, and for tactile and nociceptive responses.

Conclusion: This prominent effect may be explained by a mechanical force associated with ultrasound. The effect is not due to heating, which would be weaker at the low frequency.

Significance: This controlled study reveals that ultrasonic stimulation of excitable cells is stronger at lower frequencies, which guides the choice of transducer hardware for effective ultrasonic stimulation of the peripheral nervous system in humans.

Fig. 1.. Ultrasound stimulation apparatus and stimuli.

(a) Setup. A focused ultrasound transducer delivered a 300 kHz or 900 kHz stimulus into a subject’s index finger through water. The ultrasound beam profile (dashed lines) was controlled through an aperture 4 mm in diameter. The water was either one-time degassed (18 subjects) or continuously degassed (8 subjects), and was kept at about 30°C. (b) Peak-normalized ultrasound pressure field for the two frequencies. The aperture size and pressure levels were set such that both frequencies produced a comparable focal volume (see Methods). The pressure profile was averaged over the x and y dimensions. The dotted lines show the 0.707 (0.5) pressure (intensity) levels to characterize the fields using full-width-at-half-maximum values. (c) Stimuli. Each subject experienced 10 repetitions of 12 distinct stimuli. The stimuli, 200 ms in duration, were selected randomly and delivered each 8–12 seconds. We tested 2 frequencies, 3 pressure levels, and continuous and pulsed (100 Hz, 20% duty) stimuli.

Fig. 2.. Low frequencies stimulate excitable cells more effectively than high frequencies.

Mean±s.e.m. response frequency for all 300 kHz stimuli (blue) and all 900 kHz stimuli (green). The data contain responses from all 26 subjects. Significance was assessed using paired t-test.

Fig. 3.. Low frequencies stimulate excitable cells more effectively than high frequencies, in all tested conditions.

Mean±s.e.m. response frequency for the specific stimuli at the 300 kHz carrier frequency (blue) and 900 kHz (green). Response frequency is presented as a function of the ultrasound pressure amplitude (top row), stimulus kind (continuous or pulsed; middle row), and the type of subjects’ response (tactile (tap or vibration) or nociceptive; bottom row). The significance of these effects is assessed using an omnibus linear model (Table I).

Fig. 4.. Individual responses to the individual stimuli.

Mean±s.e.m. response frequency separately for each of the 12 stimuli, and separately for the individual response kinds (see legend). The abscissa provides peak pressure (I_SPTA) values. The lines represent second-order polynomial fits.

Fig. 5.. Pulsed (continuous) stimuli more effectively elicit tactile (nociceptive) responses.

Mean±s.e.m. response frequency for vibrotactile (left) and nociceptive (right) responses, separately for pulsed and continuous stimuli. Significance is assessed using paired t-tests.

Fig. 6.. Detailed characteristics of the ultrasonic fields.

The pressure fields measured with hydrophone for the two distinct ultrasound frequencies. The fields were centered on the aperture. The values were normalized by their maximum (see color scale). The arrows represent gradients.