Estimation of temperature artifact from a short interruption in ultrasonic power

An error in temperature measurement, commonly referred to as a temperature artifact, frequently occurs during ultrasound hyperthermia as a result of viscous and absorption heating of the thermometer probe. At the present time there is no convenient method for correcting the clinical data for this error, which can be 0.5 degrees C, or greater. A technique is described by which the artifact can be estimated from a 10-s interruption in ultrasonic power. This technique is based on the observation that thermal decay recorded by a probe can be represented by the summation of two exponential decay rates, one of which represents the decay of tissue temperature and other decay of the artifact. Temperature drop during the first 10 s arises primarily from decay of the artifact because its time constant is approximately 7 s whereas that of the tissue generally ranges from 100 to 1000 s. A relationship between artifact and temperature drop derived from clinical data shows that the artifact is directly proportional to temperature drop. This relationship can be used to estimate the artifact during therapy if power is interrupted for 10 s. Because the interruption in power is brief, it is feasible to sample the artifact periodically during therapy and to make an on-line correction for the temperature artifact.