Standardized in Vitro Measurement of Heat Flux and Diameter of Frozen Region for Cryosurgical Probes

Abstract

Cryosurgery has become an economic and easy to use technique for the surgical therapy of tumours in many fields of medicine. The volume of tissue that can be frozen by locally removing heat from the body with a thin cryosurgical probe is limited. To exactly predict the extent of the frozen region and for an efficient control of the therapy it is necessary to know the power and capacities of cryoprobes.

We present devices for the measurement of the transient ice ball diameter and the heat flux that is removed during the in vitro operation of a certain cryoprobe under various controlled thermal conditions. A segmented metal volume of nearly spherical inner shape is filled with a mixture of water and gelatine to avoid convection within the volume. The device is placed into a temperature controlled water bath to keep the temperature of the outer metallic surface constant. The transient temperature field of the inner water volume is recorded with an array of thermocouples located radially at various distances from the probes tip. The position of the advancing ice front is recorded continuously by a one-dimensional ultrasound signal that is evaluated by a computer. The heat flux that can be removed from the tissue during the steady state operation of a cryoprobe is measured quantitatively with a compensation heating device in an insulated stirred volume of water of computer-controlled temperature. Standardized conditions enable us to compare the power and therefore the cooling effects of cryosurgical probes. We suggest to use similar devices to quantify future improvements in cryosurgical technique more precisely.