The response of thermometer probes inserted into catheters

The effect of the increased response time of thermometer probes in catheters on measurements of the specific absorption rate (SAR) and blood flow was investigated. An expression was derived for calculation of the response of probes to step changes in temperature based on an empirically determined constant, κ, characteristic of a particular probe and catheter. Values of κ were determined for three commercially available thermometry systems and commonly used catheters. Both SAR and blood flow are determined from the rate of temperature change. The effect of response time on these parameters was studied by use of an algorithm that computes the probe response to continuously changing temperatures. The results show that the rate of temperature change recorded by a probe in a catheter is initially incorrect, but the correct value is attained after a time interval that depends on the value of κ. Data recorded during this interval, as long as 16 seconds for some probes and catheters, must be excluded from SAR and blood flow determinations. Temperatures recorded by probes in catheters while the temperature is changing are incorrect by an amount that depends on the value of κ and the rate of temperature change. This error is usually small but can be as much as 0.50C for a rate of temperature change of 100C/min