Coupling Agents and their thermal properties
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Grey, K: “The thermal effect of aqueous gel and mineral oil in therapeutic ultrasound”. 1998.
Patients report that gel feels cold when applied to the skin and during treatment, while this is not the case with oil.
No data is available about the thermal effects of gel or oil.
The choice of coupling medium influences the temperature distribution in the exposed tissues.
This investigation compares the change in surface temperature during ultrasound treatment with gel versus mineral oil. The intensity was 1W/cm² continuous ultrasound at 1 MHz. The local exposure time was 2min. With a treated area of 30cm² and a 5cm² transducer (Effective Radiation Area), the total treatment time was 12min. The anterior mid-thigh was treated in 6 subjects. There were 5 treated measuring points and 3 control points. Temperatures were measured before the treatment. When the temperature reached steady state, the treatment was started. After the treatment the measurements were resumed.
The results showed a decrease in temperature of 2.1°C for the gel treatments and a temperature rise of 2.2°C for the treatments with oil.
The significant (p<0.00005) difference of 4.3°C found between ultrasound treatments with gel and mineral oil is due to evaporation from the gel. The selection of gel as a coupling medium imposes a decrease in the surface temperature, that may interfere with the expected therapeutic effect of the ultrasound treatment.
From the dosage given in this trial the generated heat can be calculated in the skin surface to 3.4°C, in subcutis (fat) 1.5 °C and in the underlying muscle 4,0°C, if the thickness of the skin is 0,3 cm, subcutis = 0,7 cm and muscle = 3cm. The bone peaks at 9°C. These calculated temperatures are without regard to perfusion or heat conduction or other temperature regulators of the body.
Reference for temperature model:
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