# Endoscopic Optical Imaging Technologies and Devices for Medical Purposes: State of the Art
## 7 Discussion  
Imaging devices used in medical applications are characterized by their resolution. Non-invasive technologies, such as MRI, CT, and ultrasound, provide in vivo imaging of tissue structures with a large penetration depth, but the resolution is limited. Endoscopic imaging permits imaging body structures with finer details, however, they are invasive in some cases. Fiber optics allow the fabrication of imaging devices that are flexible and can image the target area via hollow cavities. The recent advances in the optical and mechanical fields allow in vivo imaging of tissue surfaces to be performed by using devices having a very fine resolution at the micrometer level. In an endoscopic optical imaging device, the laser light is illuminated on the target surface using an optical fiber or by deviating the light by micro mirrors. The image can be either reconstructed by direct reflectance, or via detection of the fluorescence light using optical fibers, lenses, or CCD/CMOS cameras. In the case of direct reflectance imaging, the morphological information of the tissue structure is obtained, while fluorescence provides information about the inner cell or tissue structures by labelling them using fluorophores. Fluorophore materials are usually added from outside to stain the target sample and emit light at a wavelength higher than that of illumination, increasing the contrast in the images. In biomedical imaging devices, OCT is one of the most used techniques. OCT is a direct reflectance imaging technique where the imaging contrast is provided by the change in the refractive index of the \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Raster** & **Spiral** & **Lissajous** & **Circular** & **Propeller** \\ \hline Scanning pattern & & & & & \\ \hline \multirow{4}{*}{Advantages} & Uniform light intensity & Easy to get, area is swept by changing the driving voltage & Uniform light intensity, most used & Possible to get circular pattern with ID & \\  & Points are scanned at different times & Light intensity is higher in center & High intensity is frequency ratio, quasi-random pattern & Light intensity is higher in center & Easy to generate \\  & motion artifacts & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison table for different scanning patterns. tissue media causing a variation of intensity of the reflected light [7]. Most of the current OCT systems are FD-OCT based, where the information about the depth scan is calculated by using the inverse Fourier transform of the backscattered light spectrum. The fast-axial scanning is obtained by sweeping the light source, which gives high-resolution images due to fast signal acquisition and provides images with a large signal to noise ratio. Given the current state of the art, ophthalmology is the predominating application for OCT due to the low laser output compatible with the ophthalmological structures permitting the imaging without damaging the tissue structures [12; 13; 14]. The transparency and low light scattering of the aqueous humor media allows imaging of the ocular surfaces at a greater depth with resolution independent of the surface depth. In addition to ophthalmology, deep tissue penetration and high resolution morphological and functional imaging make OCT a promising technology to assess the artery lumens in cardiology [10], tissue structures of the biliary tract, and the gastrointestinal (GI) tract, especially for [PERSON]'s esophagus [135]. Furthermore, the skin is a highly light-scattering tissue that contains a large number of inhomogeneities. OCT makes it possible to detect and diagnose various skin diseases and lesions [136]. OCT is also a useful method to monitor lesions in the enamel and dental structures of the teeth [137]. Another promising technology is confocal microscopy, which provides high-resolution imaging of thick specimens by virtually slicing them using optical sectioning [10]. It is possible to image the tissue structures in either confocal or fluorescence mode.