# Algorithms in Tomography and Related Inverse Problems--A Review
## 7 Optical Coherence Tomography
The alternative method [160] proposed to achieve coherence gating without using a scanning delay line involves collecting the interferometric signal as a function of optical wavenumber by combining sample and reference light at a fixed group delay [161]. Two distinct techniques have been developed based on this spectral discrimination (SD) approach. The first is Fourier domain OCT [161; 162; 163; 164; 165; 166], which utilizes a broadband light source and employs spectral discrimination with a dispersive spectrometer in the detector arm. The second technique is swept  
Figure 7: OCT simplified block diagram. The output from the super luminescent diode is coupled into a single-mode fiber and divided at a 50/50 coupler. The resulting optical signals are directed into both the sample and the reference arm. The reflections are combined at the sample coupler and subsequently detected by a photodiode. The detector output is then demodulated to generate the envelope of the interferometric signal, which is subsequently digitized and stored on a computer. This process involves a series of longitudinal scans, with the lateral beam position being translated after each longitudinal scan. source OCT [161; 165; 166; 167; 168], which encodes time with wavenumber by rapidly tuning a narrowband light source across a broad optical bandwidth. Dental OCT applications in oral tissue imaging, caries, periodontal disease, and oral cancers have been discussed by [PERSON], [PERSON] et al. [169]. The article also compares OCT with other oral diagnostic methods. Dental OCT enables the qualitative and quantitative assessment of morphological changes in dental hard and soft tissues in vivo. Early detection and treatment can enhance both tooth and patient survival rates. Another advantage of dental OCT is its three-dimensional imaging capability, facilitating more precise and rapid identification of issues in soft and hard tissues. In the early days of OCT, the primary focus was on investigating dental soft and hard tissue morphology. This was partly due to limitations in the size of the OCT systems and the technology for manufacturing light sources [170; 171; 172]. However, in recent years, with the advancement of components and technology, this powerful tool has found new applications in advanced diagnostic challenges. Dental OCT is useful for visualizing tissues like the gingival, periodontal structures, and mucosa. With longer center wavelengths, OCT can also be applied to imaging bone-related conditions. Looking ahead, the development of an OCT system with a handheld optical probe and a more streamlined setup holds promise for telemedicine integration, where it could be utilized with Picture Archiving and Communication Systems (PACS). This advancement could prove invaluable for home nursing care plans in our aging society. The work by [PERSON], [PERSON] et al. [173] introduces distortion-free OCT volumetric imaging via a handheld probe equipped with a dual-axis micro-electro-mechanical system (MEMS). In the context of this imaging probe, where optics are positioned between the 2D MEMS scanner and the sample, the work discusses the implementation of pre-shaped open-loop input signals containing customized nonlinear elements on a dedicated control board. Unlike the common use of sinusoidal signals for MEMS scanning, this approach enables real-time distortion-free imaging without the need for post-processing. The MEMS mirror has been successfully integrated into a compact and lightweight handheld probe, achieving a significant 12-fold reduction in volume and a 17-fold reduction in weight compared to a previous dual-mirror galvanometer-based scanner. Experimental results demonstrate distortion-free imaging without post-processing using a Gabor-domain optical coherence microscope (GD-OCM) with exceptional 2 \(\upmu\)m axial and lateral resolutions, covering a \(1\times 1\) mm\({}^{2}\) field of view. This work presents utilizing a MEMS-based scanning device for the generation of distortion-free images in conjunction with a GD-OCM.