A NEW CRYOSURGICAL DEVICE FOR CONTROLLED FREEZING .1. SETUP AND VALIDATION TESTS

A new cryosurgical device utilizing liquid nitrogen, which is a modifi cation of an existing commercial system, was developed. In the new com puter-controlled cryodevice the temperature of the cryoprobe is contro lled by means of an electrical heating element. The desired temperatur e-forcing function is calculated to ensure a specified constant coolin g rate at the freezing front. The new device facilitates real-time dat a processing, and, in particular, simulation of the heat transfer proc esses. A series of tests was performed to study the characteristics of the cryodevice and to validate the underlying assumptions. These test s were performed using organic tissue, i.e., potatoes, as an in vivo s imulating medium of biological tissue. The differences between experim ental data and computed results were found to be within +/-0.5 degrees C, which falls within the uncertainty range of the experimental tempe rature measurements, A typical control error of the new device is with in +/-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 devi ce is capable of producing maximal cooling rates of 50 degrees C/min d own to temperatures of -165 degrees C and a maximal heating rate of 30 0 degrees C/min. The maximal cooling power of the cryoprobe, due to LN (2) boiling, is 80 W; the maximal electrical heating power of the cryo probe is 160 W. Precooling of the device requires about 30 min, and it can be operated continuously for about 3 h. Initial results of experi mental in vivo cryosurgery performed on rabbit hindlimbs, including hi stological observations and thermal analysis, are presented in the sec ond part of this study. (C) 1996 Academic Press, Inc.