Optical Coherence Tomography Fundamentals

Optical coherence tomography is an interferometric imaging technique that uses broadband light to achieve micrometer-scale axial resolution in biological tissue. The key insight is that low-coherence interferometry provides depth sectioning without confocal gating — the coherence length of the source directly determines the axial point spread function.

Axial Resolution

The axial resolution is determined by the coherence length of the source:

where is the center wavelength and is the full-width half-maximum bandwidth. This is fundamentally different from confocal microscopy, where axial resolution depends on the numerical aperture.

Fourier-Domain Detection

Modern OCT systems use Fourier-domain detection — either spectral-domain (spectrometer-based) or swept-source (tunable laser). The key advantage is the sensitivity gain: Fourier-domain OCT achieves 20–30 dB higher sensitivity than time-domain OCT because all depth points are measured simultaneously. This connects to the Fellgett Advantage in Spectroscopy principle from Fourier transform spectroscopy.

Spectroscopic Extensions

By analyzing the wavelength-dependent signal, we can extract tissue optical properties — see Inverse Problems in Optical Imaging for the mathematical framework. This is the basis of spectroscopic OCT (S-OCT), which adds quantitative biochemical contrast to the structural images.

Polarization-Sensitive OCT

Adding polarization diversity detection enables measurement of tissue birefringence, which is related to collagen fiber organization. The Jones matrix formalism provides the mathematical framework — see Jones Calculus for Fiber Optics for details.

The combination of spectroscopic and polarization-sensitive measurements provides three independent contrast mechanisms (scattering, absorption, birefringence) from a single imaging modality.