Optical Coherence Tomography (called OCT) is an optical radar that detects the reflection of light from optical discontinuities. The TOP figure shows the x,z image of a stack of glass cover slips interleaved with layers of water. At each x position, a z-scan is made, which is displayed in the BOTTOM figure.
The detection method is interferometric, involving interference of light reflected from the sample with a reference beam reflected from a movable mirror. As the reference mirror is moved, the depth of the local region of reflectance within the sample is scanned down through the sample. During scanning, the pathlength of the sample arm and the reference arm varies, creating a sinusoidal pattern of interference at each water/glass interface.
Later, just like an AM radio, digital filtering selects for this spatial frequency of interference (the carrier frequency) and acquires the envelope of such oscillations, which rejects unwanted noise at lower and higher spatial frequencies. The envelope of the sinusoidal oscillations is the reflectivity of the tissue as a function of depth.