Ultrasonic modeling and hydrophone measurements of dual divergent transducers for wearable therapeutic ultrasound device

Intramuscular Heating Characteristics of Multihour Low-Intensity Therapeutic Ultrasound

CONTEXT

The heating characteristics of a stationary device delivering sustained acoustic medicine with low-intensity therapeutic ultrasound (LITUS) are unknown.

OBJECTIVE

To measure intramuscular (IM) heating produced by a LITUS device developed for long-duration treatment of musculoskeletal injuries.

DESIGN

Controlled laboratory study.

SETTING

University research laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 26 healthy volunteers (16 men, 10 women; age = 23.0 ± 2.1 years, height = 1.74 ± 0.09 m, mass = 73.48 ± 14.65 kg).

INTERVENTION(S)

Participants were assigned randomly to receive active (n = 20) or placebo (n = 6) LITUS at a frequency of 3 MHz and an energy intensity of 0.132 W/cm(2) continuously for 3 hours with a single transducer or dual transducers on the triceps surae muscle. We measured IM temperature using thermocouples inserted at 1.5- and 3-cm depths into muscle. Temperatures were recorded throughout treatment and 30 minutes posttreatment.

MAIN OUTCOME MEASURE(S)

We used 2-sample t tests to determine the heating curve of the LITUS treatment and differences in final temperatures between depth and number of transducers.

RESULTS

A mild IM temperature increase of 1 °C was reached 10 ± 5 minutes into the treatment, and a more vigorous temperature increase of 4 °C was reached 80 ± 10 minutes into the treatment. The maximal steady-state IM temperatures produced during the final 60 minutes of treatment at the 1.5-cm depth were 4.42 °C ± 0.08 °C and 3.92 °C ± 0.06 °C using 1 and 2 transducers, respectively. At the 3.0-cm depth, the maximal steady-state IM temperatures during the final 60 minutes of treatment were 3.05 °C ± 0.09 °C and 3.17 °C ± 0.05 °C using 1 and 2 transducers, respectively. We observed a difference between the temperatures measured at each depth (t78 = -2.45, P = .02), but the number of transducers used to generate heating was not different (t78 = 1.79, P = .08).

CONCLUSIONS

The LITUS device elicited tissue heating equivalent to traditional ultrasound but could be sustained for multiple hours. It is a safe and effective alternative tool for delivering therapeutic ultrasound and exploring dosimetry for desired physiologic responses.