Apparatus for detecting vulnerable plaques embedded in the wall of a patient's blood vessel includes an intravascular catheter containing a microwave antenna, an extra-corporeal radiometer having a signal input, a reference input and an output, a cable for electrically connecting the antenna to the signal input, and a device for applying an indication of the patient's normal tissue temperature to the reference input so that when the catheter is moved along the vessel, the locations of the vulnerable plaques are reflected in a signal from the output as thermal anomalies due to the higher emissivity of the vulnerable plaques as compared to the normal tissue. A second embodiment of the apparatus has two coaxial antennas in the catheter serving two radiometers. One measures the temperature at locations in the vessel wall, the other measures the temperature at the surface. By subtracting the two signals, the locations of vulnerable plaque may be visualized. The apparatus employs a special diplexer for separating the signals from the two antennas and a method of detecting and possibly destroying the plaques is also disclosed.