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:

$\delta z = \frac{2 \ln 2}{\pi} \cdot \frac{\lambda_0^2}{\Delta\lambda}$

where $\lambda_0$ is the center wavelength and $\Delta\lambda$ 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. 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.

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