# Application of OCT in the Gastrointestinal Tract
## 2 Evolving OCT Technology
Additionally, a MEMS platform has been developed for circumferential scan by [PERSON] et al. using an electrothermal chevron-beam actuator [40]. However, a challenge of MEMS-based scanning endoscopes has been the large probe diameter (4-5.8 mm) [38]. PZT actuators are desirable for their small size and compatibility with an endoscopic accessory channel; however, the length of the tubular PZT actuator is quite long (around 3.5 cm) [37]. Notably, [PERSON] et al. proposed a reverse-mount PZT design with a shorter rigid length that had comparable performance to forward-mount probes, although performed on colonic samples ex vivo [37]. PARS, which rotates a pair of angle-polished gradient index (GRIN) lenses, is advantageous for its miniaturization and variety of scan modes [41], but retaining concentricity and stable separation of the two GRIN lenses is challenging [38]. Most recently, micromotor catheters have been used to achieve high-speed, volumetric en face ultra-high resolution for the in vivo assessment of BE and dysplasia (Figure 2) [42]. Figure 2: Combined en face and volumetric OCT dataset visualization. (**a**) Volumetric OCT dataset with cross-sections (blue), longitudinal (red) and en face (green) views. (**b**) Intrinsic co-registration leads to depth-resolved en face OCT images at various depths, revealing superficial mucosal architecture near the surface and mucosal patterns in greater detail and contrast, as well as microvasculature, in deeper images. Scale bars 1 mm. Figure and caption adapted from [PERSON] et al. 2019. The beam can also be scanned in different patterns such as raster [43], spiral [44] or Lissajou [45]. The use of these scanning patterns depends on the actuators used in the probe design [12]. OCT technology can also be combined with other imaging techniques to give it enhanced molecular sensitivity and imaging depth [38]. However, such endoscopic multi-modal imaging platforms are limited by the small size of the accessory channel of a conventional endoscope [38]. In one paper, such geometric challenges were overcome by putting the imaging and spectroscopy probes together in the same 2 mm diameter silica tube [46]. Notwithstanding the challenges of miniaturizing the imaging catheters, OCT has been combined with a diffuse fluorescence spectroscopy probe to obtain simultaneous 1D laser-induced fluorescence (LIF) and 3D OCT spectra in mice. The LIF spectrum allows chemical specificity to the OCT platform [47]. However, multimodal OCT and fluorescence imaging in the GI tract is challenging, as it requires a high speed, high spatial resolution, uniform scanning system with an FDA approved contrast agent [48]. Nevertheless, one study found that layered architecture and microvasculature could be clearly identified using such a multimodal system in the rat rectum [48]. OCT can also be used simultaneously with ultrasound to result in a greater combined image depth [38]. A study in 2010 used a high-frequency ring transducer to focus the ultrasound (US) beam and OCT light beam toward a common imaging spot for simultaneously co-registered US and OCT images to access tissue beyond the traditional 1-2.5 mm OCT imaging depth [38, 49]. Additionally, trimodal OCT, US and photoacoustic imaging (PAI) endoscopy have been shown to combine the high resolution subsurface imaging capabilities of OCT, deeper penetration by US and the increased optical absorption contrast of PAI for enhanced visualization of neoplasia [50]. Lastly, polarization sensitive (PS) OCT exploits the altered reflected polarization of light from different materials and tissues to quantitate and provide further contrast [51]. This can be used to detect tumors, such as in the colon, where precancerous lesions have been found to be less depolarizing than benign tissues [51, 52]. OCT has been applied to both the upper and lower GI tract either through the accessory channel of an endoscope or independently. While OCT has great potential throughout the entire GI tract, its principal advancements so far have been made in the esophagus.