Assessment of echography as a monitoring technique for cryosurgery

One of the principal difficulties encountered in cryosurgery is to know the extent and depth of the frozen tissue boundary during the freezing process and the extent of the cryonecrosis in the postfreezing period. These problems have to be solved for cryosurgery to be a controlled therapeutic method and to avoid injuries to tissues adjacent to the tumor. The aim of this study was to evaluate echographic possibilities in monitoring the freezing process during cryotherapy in dermatology. In this paper, we describe the first experimental results of ultrasound monitoring of the freezing process with a 15 MHz transducer on pig skin in vitro. We show that it is possible to easily detect the depth boundary of the frozen region. Conventional linear filtering of the echographic signals can be very useful for SNR enhancement in the detection of a target signal drowned in speckle noise. Measurement of the kinetics of the freeze-thaw cycle (thickness of the frozen region versus time) is also measurable, which is of major importance as it allows the exact adjustment of the freezing time as a function the extent of tumor depth. Cryotherapy monitoring by ultrasound, added to the new diagnostic possibilities of high frequency ultrasound transducers, makes it an interesting controlled therapeutic method in such clinical fields as dermatology.