# Endoscopic Optical Imaging Technologies and Devices for Medical Purposes: State of the Art
## 1 Introduction  
The use of optical devices in medical applications has increased in the last few decades. The main purpose of optical imaging techniques is the direct localization of lesions and malignancies in the organs to inform and assist with surgical procedures. The imaging of internal organs can either be structure-based or surface-based. The former among these is a non-invasive technique used to scan a larger area and quickly localize the abnormality using an X-ray, ultrasound, computed tomography, or magnetic resonance imaging technique. These techniques are characterized by high penetration depths; but, the spatial resolution is limited (~50 \(\upmu\)m to 2 mm), which hides the finer structure details. Thus, to accurately detect a malignant surface, follow-up imaging using surface-based imaging is performed. Even though these imaging modalities like endoscopy are invasive in some cases, they provide live and high-resolution imaging (in the micrometer range). In surface-based imaging, the lesion area can be directly monitored by inserting an endoscopic device to observe the desired area. The size of the endoscopic probe determines the accessibility to the body area of interest. Traditional endoscopic devices have a rigid structure where the rigidity and difficulty of maneuvering it inside the body limits the accessible area. Miniaturization of micro-electro-mechanical systems (MEMSs) in conjunction with the evolution of optics permits the fabrication of small-sized and flexible endoscopes. Thus, the evolution in these fields allows medical users to image smaller areas of the body that were inaccessible in the past. The early detection of lesions, tumors, etc. permits more accurate diagnoses, which can enhance life expectancy in the case of cancerous tumors. The diagnostic procedure based on endoscopic imaging is a multi- step process where the lesion is first detected using an imaging device, characterized using advanced techniques, and confirmed using histological procedures [1]. Tissue imaging can be either performed using a backscattered or fluorescence light configuration [2]. In the former case, the image is provided by the light diffusely reflected (or backscattered) from the superficial layers of the tissue and contrast is provided by the differences in the scattered light from different tissue structures. In the latter case, the incoming light from the laser is absorbed by fluorophores in the tissue and re-emitted at a longer wavelength. In this case, the image contrast is provided by the fluorescence signatures of the different structures. The image contrast in fluorescence imaging can be reduced by background fluorescence. To increase the contrast, the tissue is often stained using a fluorophore material. A large number of optical devices have been researched and developed for imaging purposes. Among these, optical coherence tomography (OCT), confocal microscopy (CM), and photoacoustic (PA) imaging are the predominant ones. The performance of an endoscopic device depends on the choice of the actuation method and the optical components used in its fabrication. Early flexible endoscopes were developed using coherent optical fiber bundles (CFBs) relying on multiple optical fibers to deliver the light to the imaging target surface, which subsequently receive the reflected light using a charge-coupled device (CCD), video chips, or optical fibers. The resolution of these devices is limited by the discrete fiber pixilation, which avoids the miniaturization of such devices below 3 mm [3,4]. Most of the current endoscopes use a complementary metal oxide semiconductor (CMOS) chip at the tip of the endoscopes due to its low fabrication cost, ability to withstand in magnetic fields, and the possibility of miniaturization [5]. The limited resolution in such cases can be improved using laser light scanning at the proximal end of the fiber. In this case, the resolution of the image is obtained in a temporal basis instead of a spatial one as a single pixel is detected at a time.