The present technique utilizes microwave radiometry to detect the presence of vulnerable plaques engrained in the wall of a blood vessel. In accordance with the technique, an intravascular catheter containing at least one microwave antenna is moved along the suspect vessel. The antenna, in combination with an external microwave detection and display unit, is able to detect and display thermal anomalies due to the difference in the thermal emissivity (brightness) of vulnerable plaques as compared to normal tissue even though the two may have a common temperature.