Over the past few decades, endoscopes have been used to view the interior of cavities in the human body or the surfaces of internal human organs noninvasively for diagnosis or treatment. However, white light endoscopy and magnifying endoscopy widely used in clinical practice have poor resolution and contrast and require pathological biopsy examination to confirm the diagnosis. In recent years, narrow-spectrum technology has used blue light via optical or digital filtering to irradiate tissues and enhance the microstructural and microvascular morphology of the mucosal surface, improving the imaging contrast. However, it still exhibits poor resolution. White light and narrow-spectrum endoscopy cannot achieve cellular-level resolution; therefore, a purely optical biopsy cannot be performed, significantly reducing diagnosis accuracy. Confocal endoscopy has emerged owing to its submicron resolution and optical sectioning capability. Cell morphology observed using confocal endoscopy is highly consistent with the biopsy pathology. Since its introduction in 2004, confocal laser endomicroscopy (CLE) has become a vital technique in gastrointestinal endoscopic imaging. Confocal laser endoscopy enables endoscopists to perform cellular imaging and tissue structure assessments at the focal plane during endoscopic testing. Thus, real-time in vivo histological information can be obtained, enabling "optical biopsy."

Confocal microscopy was first developed in 1957 by Minsky, who used pinholes on the illumination and detection sides in the same conjugate image plane to achieve "confocal." In 1967, Egger and Petrǎn successfully used confocal microscopy for label-free imaging of neural tissues. The key to confocal microscopy imaging technology is that the "double focus" of the two pinholes can shield all signals from the nonfocal plane, and the photomultiplier tube behind the detection pinhole can detect only the signal from the focal plane to achieve optical sectioning. Depending on the source of the image contrast, laser scanning confocal microscopy can be performed in the fluorescence or reflectance mode. Fluorescence confocal microscopy requires fluorescent contrast agents to generate contrast, yielding spatial and functional information about endogenous autofluorescence and exogenously labeled molecules and structures. Reflection confocal microscopy relies on differences in the refractive indices of cellular structures to generate natural contrast.

Based on the scanning method, confocal endoscopy in confocal endoscopic imaging technology is divided into endoscopy-integrated and probe-based confocal endoscopy. As shown in Figure 2, the endoscopy-integrated confocal endoscope adopts the distal scanning mode [Fig. 2(b)], whereas the probe-based confocal endoscope adopts the proximal scanning mode [Fig. 2(a)]. The eCLE uses a point-scanning method to drive a single optical fiber to scan through a scanning device, achieving high-resolution confocal endoscopic imaging. Because the eCLE adopts a distal scanning method and the mechanical scanning device is included in the imaging probe, it is necessary to miniaturize the mechanical scanning device. However, the miniaturization of this device required for confocal endoscopy is technically challenging and expensive. Therefore, eCLE is limited to clinical applications because of the limited size of the mechanical scanning device. The pCLE probe does not contain a scanning device, and the scanning device does not have size limitation. However, its resolution is limited by the distance between the cores, and the imaging quality is affected by the honeycomb structure of the fiber bundle.

As both eCLE and pCLE are based on traditional confocal microscopy imaging techniques, they use a single excitation wavelength. However, the traditional confocal endoscope requires mechanical scanning to complete three-dimensional imaging, and the imaging speed is low. Therefore, traditional confocal endoscopic microscopy-imaging solutions cannot achieve rapid three-dimensional deep tissue imaging or real-time optical diagnosis in clinical practice. Spectral-encoded confocal microscopy (SECM) is a reflection confocal microscopy technique. It can be used to determine the spatial position of a sample by measuring the spectrum of light reflected from the sample. It can significantly increase the confocal imaging speed, enabling large-area imaging within a short time. The high imaging rate of SECM can potentially increase the confocal field of view, but the imaging depth of the focus is still limited to 200 μm. At more significant imaging depths, the effective resolution of SECM is significantly reduced owing to light scattering and optical aberrations.

In recent years, chromatic confocal technology has been used to achieve high-resolution, fast, and multi-depth imaging. Chromatic confocal endoscopy solves the problem of insufficient imaging depth in traditional confocal endoscopy, and shows significant potential in gastric cancer diagnosis. However, it cannot guarantee large chromatic and small spherical aberration simultaneously in miniaturization, owing to the limitations of lens-manufacturing technology, resulting in limited axial resolution.

Confocal endoscopes enable in vivo and real-time imaging of different tissues, cells, molecules, and even bacteria owing to the higher magnification and resolution of confocal endoscopes than those of conventional endoscopes. In particular, confocal endoscopic imaging technology has promising applications in diagnosing diseases in the human body, such as the gastrointestinal tract, skin, cervix, and eye.

In this paper, confocal endoscopic imaging technology is briefly described. A comparative introduction is presented for fluorescence confocal imaging, reflectance confocal imaging, and probe-based and endoscope-integrated confocal endoscopic imaging. Furthermore, the application of confocal endomicroscopy in biomedical science is discussed.