Determination of the temperature artifact during ultrasound hyperthermia

Temperature artifacts produced by very small uncoated thermocouples during ultrasonic heating are evaluated by backward extrapolation of the linear portion of the temperature rise curve or by backward extrapolation of the exponential portion of the temperature decay curve. The accuracy of these techniques for larger clinically used thermocouples is investigated by use of a two-dimensional model of the bioheat equation which simulates the transfer of heat radially from a probe 1 mm in diameter. The accuracy of these techniques is found to depend upon the perfusion rate. In the absence of perfusion, both extrapolation techniques underestimate the artifact by nearly 40%. Extrapolation of the temperature rise curve is very sensitive to the perfusion rate and this technique results in errors exceeding 100% when the perfusion rate is high. Extrapolation of the temperature decay curve produces more consistent results. Over a blood flow range of 0-100 ml/100 g per min, the artifact is underestimated by an amount that varies from approximately 40% to 30% respectively. Thus, the artifact can be determined to within 5% by this technique by increasing the extrapolated value by 35%.