# Application of OCT in the Gastrointestinal Tract
## 2 Evolving OCT Technology  
Briefly, the working principle of OCT is analogous to that of an ultrasound, with analysis of reflected light rather than sound waves. An optical ranging technique called low coherence interferometry [13; 14] uses a Michelson interferometer to measure the time delay and subsequent signal intensity of light reflected from a tissue sample. Since its first publication in the early 1990s [15], endoscopic OCT techniques [16] and applications have become major areas of research [17; 18; 19; 20]. OCT can be broadly divided into two types: time-domain OCT (TD-OCT) and Fourier-domain OCT (FD-OCT). They differ in that TD-OCT considers each scan individually with a moving mirror (Figure 1a) [21; 22; 23], while FD-OCT applies a Fourier transform on the detected spectrum (Figure 1b) [24; 25; 26]. FD-OCT's introduction in the early 2000s generated a great increase in imaging speed and sensitivity of OCT [23; 24; 25; 26]. FD-OCT can be further divided into subcategories, spectral-domain OCT (SD-OCT) and swept-source OCT (SS-OCT) [21]. SD-OCT uses a spectrometer with a broadband light source and typically operates at shorter wavelengths [27; 28]. SS-OCT measures interference spectra using a wavelength-swept light source and a photodetector, operating at longer wavelengths and with a high imaging frequency, sometimes reaching the MHz region [29; 30; 31]. Due to its improved tissue depth penetration at longer wavelengths, SS-OCT is the technology of choice for endoscopic applications [32; 33]. Different imaging probe designs allow different perspectives for acquisition of OCT images. Side-viewing probes are the most common in endoscopic OCT applications due to their large luminal surface area coverage [12]. Different forms of side-viewing probe delivery include the small diameter flexible sheath [16], large diameter inflatable balloon [34] and rigid housing [35]. An introductory video  
Figure 1: (**a**) Time-domain optical coherence tomography (TD-OCT) uses low coherence interferometry. The light source is split and the backscattered light is captured by the detector. The depth information is provided by the scanned reference path. Adapted from [PERSON] et al. 2014. (**b**). Swept-source optical coherence tomography (SS-OCT) allows 10–200-fold faster imaging compared to TD-OCT. (A) Interferometer with swept laser source and beam splitter and path difference AL. (B) Time delay of light from sample (dotted) and reference light (dashed). (C) Interference signal proportional to delay. (D) Fourier transform of the interference signal measures AL. Figure and caption adapted from [PERSON] et al. 2014.  
demonstrating imaging in the gastrointestinal tract using a probe-based OCT is available [36]. Small diameter flexible sheath, also called \"optical biopsy,\" is excellent for obtaining high-speed visualization, but is less resolved than forward viewing probes [16]. The balloon catheter is excellent for tubular structures such as the esophagus, but is not optimized for the bends and folds of the rest of the GI tract. Rigid housing, such as in a capsule, is advantageous for a number of reasons that will be discussed in a subsequent section. Forward viewing (en face) probes have smaller FOVs but provide a more intuitive viewing scheme akin to magnified endoscopy and endomicroscopy, and are generally integrated with other modes of microscopy and applied to tasks requiring high magnification. Forward-imaging probe designs include microelectromechanical system (MEMS) scanners, piezoelectric transducer (PZT) actuators, and the paired-angle-rotation scanner (PARS) [37; 38]. While MEMS scanners have been shown to combat distortion, they often use a nonlinear scan pattern which necessitates oversampling; however, a promising study by [PERSON] et al. reported the implementation of pre-shaped open-loop input signals with non-linear parts to achieve live, distortion-free imaging without post-processing with a Gabor-domain optical coherence microscope [39].