Thermal Characteristics of a Hepatic Cryolesion Formedin Vitroby a 3-mm Implantable Cryoprobe

Abstract

The objective of the investigation was to characterize the hepatic cryolesion formed with an implantable needle (3 × 100 mm) cryoprobe. This was used to produce cryolesions in isolated porcine liver tissue equilibrated to 37°C in a water bath. The shape, size, and temperature zones within the cryolesion and the effect of single versus repeated freeze–thaw cycles on cryolesion size were studied. The final shape of the cryolesion at 15–20 min freezing was cylindrical and its distal hemispherical end extended 8 mm beyond the tip of the cryoprobe. The rate of increase in maximum diameter was logarithmic and decreased from 4.7 mm/min during the first 5 min to 0.4 mm/min during the fourth 5-min period of freezing. By contrast, the rate of increase in volume was linear and ranged from 9.6 to 7.9 ml/min during the corresponding periods. The volume of the hepatic cryolesion after 20 min of continuous freezing was significantly greater than that of the cryolesion formed with 20 min of cumulative freezing interrupted by a 5-min spontaneous thaw. The ultimate temperatures reached and the cooling rates varied in different zones within the cryolesion depending on distances away from and alongside the cryoprobe. Diameter measurements taken in isolation do not reflect the actual growth rate of the cryolesion. Volume measurements define more accurately the amount of tissue frozen and leftin situ.Prolonged freezing beyond 20 min did not increase the diameter of the cryolesion. A single continuous freeze produces a larger cryolesion than two freeze–thaw cycles of the same freezing duration.