# Algorithms in Tomography and Related Inverse Problems--A Review
## 7 Optical Coherence Tomography  
Tomographic imaging methods, including X-ray computed tomography [81], magnetic resonance imaging [82], and ultrasound imaging [83], have established extensive utility within the field of medicine. Each of these techniques measures distinct physical properties and offers advantages in terms of resolution and penetration depth for specific medical applications. Optical Coherence Tomography (OCT) enables non-invasive cross-sectional imaging of internal biological tissue structures [158] by assessing the way light reflects within these tissues, as described by [PERSON], [PERSON] et al. [159]. A block diagram of a simplified structure of OCT is provided in Figure 7.  
Both low-coherence light and ultra-short laser pulses have the capacity to assess the internal structures of biological systems. When optical signals pass through or bounce off biological tissues, they contain time-of-flight data, which subsequently provide insight into the spatial details of tissue microstructures. Time-resolved transmission spectroscopy, for instance, has been utilized to gauge the absorption and scattering characteristics within tissues, offering a non-invasive means of diagnosing hemoglobin oxygenation in the brain [84]. In addition, femtosecond laser pulses have enabled optical ranging measurements of microstructures in the eye and skin. Time gating techniques, both coherent and non-coherent, have been employed to selectively capture directly transmitted light, allowing for the acquisition of transmission images in optically opaque tissue. Furthermore, low-coherencereflectometry has proven useful in various applications, including ranging measurements in optical components, surface contour mapping in integrated circuits, and measuring the distance within the retina and other ocular structures. Unlike time-domain techniques, low-coherence reflectometry can be conducted using continuous-wave light, eliminating the necessity for ultra-short pulse laser sources. Recent technological progress in low-coherence reflectometry has made it possible to create compact, modular systems that employ diode light sources and fiber optics, resulting in the achievement of micrometer-level spatial resolutions and heightened detection sensitivities [158; 159]. OCT's ability to provide optical sectioning is comparable to confocal microscopy systems. However, while the longitudinal resolution in confocal microscopy relies on the numerical aperture at hand, OCT's resolution is primarily constrained by the coherence length of the light source. Consequently, OCT can maintain exceptional depth resolution, even when the available aperture is limited. This characteristic is especially advantageous for conducting in vivo assessments of deep tissues, such as in transpupillary imaging of the posterior eye and endoscopic imaging [159]. OCT, being an optical method, offers a versatile range of optical properties that can be harnessed to discern tissue structure and composition. Certain tissues with a defined orientation, such as the elastic lamina of arteries and the retinal nerve fiber layer (RNFL), exhibit birefringence. In OCT, the analysis of reflected light's polarization can be employed to enhance the differentiation of these birefringent tissue structures. Moreover, OCT systems can function across multiple wavelengths to assess spectral properties. This allows for the detection of various characteristics, including chromophore content, hemoglobin oxygenation, hydration levels, or the dimensions of light-scattering structures. Consequently, OCT emerges as a promising technique suitable for both fundamental research and clinical applications. [PERSON], [PERSON] et al. [160] present theoretical models that illustrate the enhanced sensitivity of Swept Source and Fourier Domain that OCT techniques present in comparison to the conventional time-domain approach.