# Endoscopic Optical Imaging Technologies and Devices for Medical Purposes: State of the Art
## 3 Scanning Directions  
Considering the basis of the scanning mode and the light illumination direction, optical scanner probes can be divided into two groups: side-view imaging and forward-view imaging. Side imaging probes generate circumferential images by scanning the target area from the side of the probe. By moving  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multirow{2}{*}{**OCT**} & \multirow{2}{*}{**CM**} & \multicolumn{3}{c}{**Nonlinear Microscopy**} & \multicolumn{3}{c}{**Photoacoustic Imaging**} \\ \cline{5-8}  & & & **Multiphoton** & **Harmonic** & **Raman** & **Geratering** & **OR-PAM** & **AR-PAM** & **PAT** \\ \hline Lateral & \multirow{2}{*}{2-25 μm [2,10]} & \multirow{2}{*}{1 μm [10,18]} & \multirow{2}{*}{40.2 μm [47]} & \multirow{2}{*}{4.9 μm [48]} & \multirow{2}{*}{300 μm [49]} & \multirow{2}{*}{-0.5-10 μm} & \multirow{2}{*}{45 μm [41,45]} & \multirow{2}{*}{70-720 μm} \\ resolution & & & & & & [44,45] & & \\ Axial & & & & & & & [44,45] & & \\ resolution & 1-15 μm [2,10] & 7 μm [18] & \multirow{2}{*}{0.6 μm [47]} & \multirow{2}{*}{3.1 μm [48]} & \multirow{2}{*}{1.6 μm [49]} & \multirow{2}{*}{10 μm [44]} & \multirow{2}{*}{15 μm [41,45]} & \multirow{2}{*}{25-600 μm} \\ Penetration & 1-3 mm & & & & & & [41,45] & \\ depth & [2,3,10] & \multirow{2}{*}{-400 μm [18]} & \multirow{2}{*}{\(>\)200 μm [47]} & \multirow{2}{*}{100-300 μm [50]} & \multirow{2}{*}{-1.1 mm [45]} & \multirow{2}{*}{-3-5 mm [45]} & \multirow{2}{*}{70 mm [45]} \\ FOV & 2-5 mm [2] & & & & & & \\  & & & & & & & \\ Orientation & Cross-section & & & & & & \\  & [10] & & & & & & \\  & & & & & & & \\ wavelength & Near IR & & & & & & & Near IR or IR \\  & Low coherence & Continuous & & & & & \\  & [10] & & & & & & \\  & & & & & & & \\  & & & & & & & \\ Frame rate & \textgreater{}60 Hz [2] & \multirow{2}{*}{\(>\)15 Hz [2]} & \multirow{2}{*}{\(>\)5 Hz [2]} & \multirow{2}{*}{\(\sim\)10 Hz (depends on scanning area) [2]} \\  & & & High contrast & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\ \multirow{2}{*}{Advantages} & High & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\ \multirow{2}{*}{Disadvantage} & Expensive & Two or more & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\ \end{tabular}
\end{table}
Table 1: Performance of different imaging applications.  
Figure 10: Photoacoustic image of a mouse ear: (**a**) maximum amplitude image of 1 mm \(\times\) 1 mm; (**b**) image of a small fragment of image a (taken with the permission of [44]).  
the probe linearly along the axis while scanning, the beam will generate a cylindrical 2-D image area [53]. In the case of distal scanning, such devices can be as small as 1.65 mm [54]. On the other hand, forward imaging probes provide the target tissue surface image from the front. By scanning the light beam, 2-D images are generated. By moving the probe along the axis, it is possible to get 3-D images. Such devices are characterized by a high scan speed and can be used as a guide for medical devices used during surgery and other endoscopic procedures. However, these devices have a much larger size as their miniaturization is limited by the scanning elements. The smallest forward view imaging probe with the distal scanning method has a diameter of 2.4 mm [55]. It is possible to get more compact-sized probes using the proximal scanning. The diameter of the probe can be reduced up to 250 \(\upmu\)m [56]. Further details about these devices are described below in the paper.