MARCH 1981 P H Y S I O T H E R A P Y 13 TEM PER A TU R E CHANGES INDUCED BY CONTINUOUS ULTRASOUND* B. L. MACDONALD, B.Sc. (Physiotherapy), Witwatersrand and S. B. SHIPSTER, B.Sc. (Physiotherapy), Witwatersrand SUMMARY A n investigation was undertaken to measure tempera­ ture changes during therapeutic ultrasound. Various frequencies, intensities and times were used. A s a result o f their work the authors feel that more research should be carried out to determine the point at which high intensities o f ultrasound produce dangerous temperature increases. I n t r o d u c t i o n Controversy exists over the value of ultrasound as a healing modality. We therefore felt that a quantitative evaluation of the temperature changes induced by therapeutic ultrasound was necessary. According to the Grotthus-Draper law, energy must be absorbed before exposure to it can effect a change. Absorption of ultrasonic energy (orderly movement of particles) results in the conversion of th at energy into heat (disorderly movement of particles) (Ter Haar, 1978, Licht, 1958). Absorption by any one tissue depends on that tissue’s absorption coefficient and the frequency of the ultrasound. The resultant hyperther­ mia causes vasodilatation, an increase in blood flow and an increase in metabolic rate. Blood flow increases significantly bu t decreases rapidly after term ination of insonation. The depth at which heating occurs and therefore the temperature increase in the muscle depends mainly on the intensity of ultrasound applied and the soft tissue cover in the area being treated. It has been found that 5 - 10 minutes of insonation at an intensity of 1 W /cm ’ is necessary to heat the muscle adequately. It is also worth noticing that the hyperthermia which results from an increase in temperature in the muscle is not as great as that occurring in the skin with a comparable increase in temperature. B In our study the temperature changes induced were (measured by means of thermocouples inserted into rabbits’ legs. The results obtained show some interesting trends. EXPERIMENTATION An anaesthetized rabbit was insonated to evaluate the temperature changes induced by ultrasound. These tem­ perature changes were measured using copper-constantin thermocouples which were inserted into the rabbit’s thigh at various levels. The thermocouples were placed in the medial subcutaneous tissue, in the muscle, in the femur and in the lateral subcutaneous tissue. One incision was made on the medial aspect of the thigh to allow for the insertion of the thermocouples. The leg was shaved laterally and medially over the middle third of the thigh. A fine covering of hair was left on the lateral side where the ultrasound was applied. * This paper is based on a Dissertation submitted in partial fulfilment of the requirements of the degree of B.Sc. (Physiotherapy), University of the Witwaters­ rand, 1979. Received 22 December 1980. OPSOMMING Navorsing is gedoen om temperatuurveranderings gedurende toepassing van terapeutiese ultraklank te meet. Verskillende frekwensies, intensiteite en toepas- singstye is gebruik. A s gevolg van hul resultate v o e i die skrywers dat meer navorsing gedoen behoort te word om vax te stel waar hoe intensiteite ultraklank gevaar- like temperatuurverhogings produseer. A Sonacel Multiphon M ark II ultrasound machine was used. This unit is capable of producing a maximum power of 15 watts and has a transducer face with an area of 5 cm2. In this study frequencies of 1,5 M Hz and 3 MHz were applied. In order to prevent the core temperature of the rabbit from dropping a homeothermic blanket control was used. A rectal probe monitored the temperature. The blanket automatically switched on when the tem­ perature dropped and switched off when the tem pera­ ture had risen to normal. The ultrasound was applied to the lateral surface of the thigh by/ means of a stroking technique. A section approximately 6 cm long was covered in 10 seconds. At all times the surface of the ultrasound head was held parallel to the thigh. Aquasonic gel was used as a coupling medium. The emf induced in the thermocouples was recorded by a digital voltmeter. These recordings could then be converted to actual temperatures. Frequencies of 3,0 M Hz and 1,5 MHz were applied at intensities of 0,5 W /cm 2, 1,5 W /cm 2 and 2,5 W /cm 2 for durations of 3, 6 and 9 minutes each. Three readings for each thermocouple were recorded on the digital voltmeter and an average of the three was taken. The ultrasound machine was switched off and the rate of cooling was measured after 30, 60, 90, 120, 150, 180, 210, 240 and 270 seconds. Separate control trials were run. The procedure in each case was identical to th a t described above but with the ultrasound machine switched off. During experimentation two im portant observations were made: • Although the volume of coupling medium was kept constant during the application of ultrasound it spread out to form a thin film between the treat­ ment head and the surface being insonated. • The rectal tem perature remained constant. DISCUSSION OF THE RESULTS The graphical representation o f our results showed several interesting features (Figure 1A & IB). The greatest change in tem perature (A T) consistently occurred in the lateral subcutaneous tissue. This is accounted for by the fact that the skin had a higher absorption coefficient and was closest to the ultrasound head. It is important to note th a t the A T in this region was as high as 14°C and in one case the actual tem perature was 49,57°C (Figure 1A). Clearly had the rabbit not been anaesthetized, severe pain would have been felt since temperatures over 45 °C damage living tissue. Although blood flow per unit volume is higher in humans than in rabbits, this high increase in tempe- R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) 14 1 4 ° j 12°. 10°J A T FIG 1A 3 MHz