The response of thermometer probes inserted into catheters.
The time constants of three commercially available thermometer probes were measured when inserted into catheters commonly used in hyperthermia. The catheter typically increases the time constant by a factor of 3. Following the transition from a constant to continuously changing temperature, the rate of change of temperature indicated by such a probe is initially incorrect, but it attains the true rate of change of temperature after an elapsed time that depends on the rate of change of temperature and the time constant of the probe. This elapsed time varies from 3.5 to 8 time constants, as the rate of change of temperature increases from 0.05 to 10 degrees C/min. The specific absorption rate determined from a least-squares fit to temperatures recorded during the first 30 s following such a transition, are typically in error by 1%, 3%, and 6% when thermometer probes having time constants of 1, 2, and 3 s are used. These errors can be reduced to less than 0.1% by excluding the data recorded during an initial period of the first 3 time constants following the transition. Errors of similar magnitude result when blood flow is determined from the first 30 s of thermal clearance. These errors are similarly reduced to typically less than 1% by excluding the data recorded during the first 3 time constants from the determination.
D000818:Animals; D002404:Catheterization / Q000295:instrumentation*; D006801:Humans; D006979:Hyperthermia, Induced; D008872:Microwaves / Q000627:therapeutic use; D009369:Neoplasms / Q000098:blood supply / Q000628:therapy*; D012039:Regional Blood Flow; D013821:Thermometers; D013997:Time Factors