A method of minimizing thermal trauma during tissue ablation includes the steps of placing an ablation catheter at an ablation site on a first organ in a patients body, providing energy to the ablation catheter to heat first organ tissue at the ablation site, providing microwave radiometry apparatus including a probe containing a microwave antenna and a radiometer responsive to the antenna output for producing a temperature signal corresponding to the thermal radiation picked up by the antenna and positioning the probe in a body passage of a second organ in the patients body having a wall portion adjacent to the ablation site so that the microwave antenna is located at a measurement site opposite the ablation site. Using the radiometry apparatus, the temperature at depth in the second organ tissue at the measurement site is measured to provide a corresponding temperature signal, and the ablation catheter is controlled in response to the temperature signal to maintain the temperature of the second organ tissue below a predetermined value that does not result in thermal trauma to the second organ tissue. Apparatus for carrying out the method is also disclosed.
