Tissue impedance as a function of temperature and time.

Tissue impedance dependence on temperature has been measured for six tissue types. The information was gathered using an automated laboratory under computer control. This information is needed to be able to input these values into a computer model that predicts temperature distribution produced by delivery of radio frequency energy. Due to the thermal dose of time and temperature, tissue properties change and no published data are available that document this. Since there are no theoretical predictions, empirical data were measured to supply this information. Using an aluminum cylindrical cavity of volume 1.69 cm3, muscle, liver, brain, and fat tissue impedance were measured at 500 kHz over a range of temperatures. All tests began at room temperature where baseline measurements were made. The cylinder was then placed in a constant temperature water bath at between 30 and 90 degrees C. The tests were run for a period of 10 to 30 minutes. Temperature homogeneity was carefully studied throughout the volume of the cylinder. It was found that thermal equilibrium occurred within four minutes. Special care was taken with the tissue sample in regard to optimize moisture and freshness, and minimize fat content. Grain orientation was also taken into consideration depending on the test. For all tissue types, resistivity decreased initially as the sample temperature equilibrated with the bath temperature. For temperatures less than 75 degrees C, resistivity values remained approximately constant over time.