Thermal measurements in a soft tissue model during irradiation by a high-power semiconductor diode laser

The temperature of a soft tissue model was measured during laser irradiation. A diode laser with a continuous wave output power of up to 10 W and a wavelength of 990 nm was used to heat and ablate samples of agar gel doped with haemoglobin. The internal temperature of the tissue was measured at depths of 2 - 5 mm below the surface using a thermocouple. The temperature at the surface was measured remotely using an infra-red sensor (over an area 1.4 mm in diameter at the center of the interaction). This method of measurement provides an inexpensive alternative to thermal imaging cameras. Temperature changes in time during the interaction both at the tissue surface and as a function of tissue depth are presented. At the onset of tissue surface rupturing and subsequent tissue ablation the temperature at a depth of 2 mm was found to be 75 ± 6°C. At this time a temperature of 60 ± 2°C, high enough to cause tissue coagulation, had been reached to a depth of 3 mm. After 10 s of continuous tissue ablation, the temperature at a depth of 3.5 mm and beyond had not reached coagulation temperature. The surface temperature rose steadily during irradiation and reached a temperature of 100°C at the time of rupturing of the tissue surface. During the subsequent tissue ablation the measured temperature increased rapidly, reaching a maximum of 250 ± 30°C within a further 10 s.