A new cryosurgical device for controlled freezing.

A new cryosurgical device utilizing liquid nitrogen, which is a modification of an existing commercial system, was developed. In the new computer-controlled cryodevice the temperature of the cryoprobe is controlled by means of an electrical heating element. The desired temperature-forcing function is calculated to ensure a specified constant cooling rate at the freezing front. The new device facilitates real-time data processing, and, in particular, simulation of the heat transfer processes. A series of tests was performed to study the characteristics of the cryodevice and to validate the underlying assumptions. These tests were performed using organic tissue, i.e., potatoes, as an in vivo simulating medium of biological tissue. The differences between experimental data and computed results were found to be within +/-0.5 degrees C, which falls within the uncertainty range of the experimental temperature measurements. A typical control error of the new device is within +/-0.3 degrees C, prior to the formation of the freezing front, and +/-0.6 degrees C thereafter, which is of the same order of magnitude as the uncertainty range of the temperature measurements. The new device is capable of producing maximal cooling rates of 50 degrees C/min down to temperatures of -165 degrees C and a maximal heating rate of 300 degrees C/min. The maximal cooling power of the cryoprobe, due to LN2 boiling, is 80 W; the maximal electrical heating power of the cryoprobe is 160 W. Precooling of the device requires about 30 min, and it can be operated continuously for about 3 h. Initial results of experimental in vivo cryosurgery performed on rabbit hindlimbs, including histological observations and thermal analysis, are presented in the second part of this study.