This thesis describes the design and development of an instrument to destroy rectal tumours by freezing them. The work described progresses systematically from the surgical performance requirements to a complete system for operating theatre use. A thermodynamic specification for the freezing tip is derived by using mathematical models of ice-ball growth to relate the performance of the tip to the thermal environment around it. The tip is cooled by boiling liquid nitrogen within it and a test rig is built to measure the heat flux to the tip, and allow the influence of tip variables to be explored. The final tip design incorporates a thin layer of plastic in the wall to encourage nucleate boiling. This tip design meets the specification and is also analysed using finite difference equations to describe the transient heat transfer through the tip wall. The tip tests indicate the mass flow rate of nitrogen needed and a system to transfer the cryogen at this rate from a storage dewar is described. The liquid nitrogen is kept in film boiling (Leidenfrost) flow as this has many advantages for cryoprobe performance. The Leidenfrost flow is maintained by providing a controlled heating of the transfer pipes. A viewer is incorporated in the probe to enable monitoring within the rectum. The viewer forms the interface between the device and the patient and an evolutionary design technique is used. The final viewer design uses gas inflation to expand the rectum. A supporting unit is also designed so that the whole instrument is self-contained to enable safe and effective clinical evaluation.
|Date of Award||1984|