Apparatus for detecting thermal radiation emanating from two different intracranial depths in a patient includes a transducer having a surface for placement against the patient's cranium. The transducer includes a first antenna with an aperture adjacent that surface the antenna being tuned to a first frequency and producing a first signal indicative of thermal radiation received within an antenna pattern extending a first distance from the surface and a second antenna having a second aperture adjacent to that surface within the first aperture. The second antenna is tuned to a second frequency and produces a second signal indicative of radiation received within an antenna pattern extending a selected second distance from the working surface, the second distance being shorter than the first distance. The two signals are coupled to a receiver/control unit which thereupon monitors and displays the temperature at the two intracranial depths.