One of the main difficulty encountered in cryosurgery is to know the extent and depth of the frozen tissue boundary during the freezing process. It has been previously demonstrated that the echo of the advancing freezing front could be detected by means of ultrasound on in vitro pig skin. The ultrasonic measurement consists in noting the total transit time for an ultrasonic pulse travelling through the frozen skin and reflected by the ice boundary. In this paper we describe the experimental results obtained with a prototype that combines the cryoprobe and the ultrasonic transducer. A small transducer of 5 mm diameter with a center frequency of 20 MHz was inserted into a hole that was drilled in the center of the cryoprobe. This study permits to design the optimal characteristics of the transducer operating in this range of low temperatures (impedance matching, backing, center frequency, broadband). Our prototype enables in vivo cryosurgery real time monitoring.
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