Modified thermal clearance technique for determination of blood flow during local hyperthermia

The thermal clearance method utilizes the rate of temperature decay after the applied power is turned off to estimate the local blood flow. A limitation of this method has been its inability to account for the contribution of thermal conduction to the rate of temperature decay. As a result, the blood flow is generally overestimated. A modification of the thermal clearance method is described in this paper which enables the conduction component to be determined. Profiles of the tissue temperature are obtained in three mutually orthogonal directions about the point where thermal clearance is measured. The Laplacian of the temperature is evaluated from these profiles by the method of finite differences. The tissue thermal conductivity is estimated from literature values. The greatest source of error is the uncertainty in the location of the washout point in each catheter. Strict thermometry requirements must be adopted to reduce the localization error to +/- 0.25 cm. The thermometry catheters should be orthogonal to within +/- 10 degrees and all three catheters should be in contact at the washout point. The methodology was tested in a phantom, studied by use of a computer model, and implemented in the clinic. The experimental error in the conduction component is typically 50%. The resulting error in the blood flow depends on the relative rates of energy removal by blood flow and thermal conduction. When perfusion is the dominant mode of energy removal, the resulting uncertainty in the blood flow is typically in the range 20-30%.