A design method for mixed gas Joule–Thomson refrigeration cryosurgical probes

A cryosurgical probe is a medical instrument that is designed to kill cancerous tissue by exposure to cryogenic temperatures. The most important figure of merit for a cryosurgical probe of fixed size is the dimension of the cryolesion that can be generated within a specified time during a surgical procedure. The optimization of the cryoprobe, through variation of the geometry, operating conditions, or working fluid composition, should be aimed directly at this clinical goal rather than at maximizing the refrigeration capacity, COP, or minimizing the tip temperature. This unique optimization criterion has created the need for a design method that considers both the probe design and the heat transfer details external to the probe (e.g. the blood perfusion and metabolic heat rate in the patient). This paper outlines a design method that will allow manufacturers of cryosurgical probes to identify an optimum combination of refrigerant mixture, cryoprobe geometry, and other cycle operating conditions. Specifically, a numerical model of the development of a cryolesion has been developed and verified; this model relates the size of the cryolesion, refrigeration required, and tip temperature. A detailed model of the cryoprobe refrigeration cycle relates the refrigeration to the tip temperature that can be achieved. The integration of these models allows the designer to select the optimal cryoprobe based on cryolesion size. This paper focuses on optimizing the mixed gas composition; however, the design technique is generic and can be used to select other system parameters.