Dynamic Low-Frequency Electrical Impedance of Biological Materials Subject to Freezing and Its Implementation in Cryosurgical Monitoring

A precise monitoring of the extent of freezing during a cryosurgical process has been an important problem in health-care clinics. Among various existing techniques, the dynamic electrical impedance utilizing the impedance jump to detect the ice moving front, is a suitable way because the impedance of frozen tissue is much higher (3 to 4 orders of magnitude or even larger) than that of unfrozen tissue. Based on two experimental setups, the dynamic low-frequency impedance (DLFI) and impedance changing rate (ICR) for selected biological materials (fresh pork and rabbit tissues) subject to freezing were systematically measured. Their transient behaviors were investigated, and implementations in a practical cryosurgery to detect ice front propagation were analyzed. Furthermore, in vitro experiments were performed on distilled water and phantom gel. The experimental results, obtained with a two-electrode measuring technique, are as follows: (1) The impedance of all samples has a rapid response to the external freezing. (2) The impedance will not jump into an insulation region when the cooling temperature is not low enough. (3) As an alternative to DLFI, ICR imaging can also give important information for the phase-change process, which may lead to an efficient method to detect the ice-ball growth. (4) There is an evident variation in DLFI for different biological tissues when subjected to the same cooling temperatures; this value also differs for the same tissue under different cooling conditions.