# Endoscopic Optical Imaging Technologies and Devices for Medical Purposes: State of the Art
## 7 Discussion
Raster and Lissajous patterns provide uniform light intensity within the light scanning area. The spiral pattern is easy to obtain but has more light intensity in the center compared to the edges. Each pattern has its own advantages and disadvantages. Table 5 summarizes the pros and cons of the different scanning patterns in detail. A descriptive summary of some recently developed optical imaging devices for medical purposes using a variety of different imaging techniques, configurations, actuating methods, and scanning patterns is shown in Table 6. The scanning catheter developed by [PERSON] et al. is a side-viewing OCT endoscope, where the light is transported to the distal end using an SMF, and the forward mirror used to deflect the light is vibrated at resonance using angular vertical electrostatic comb drives and generates a raster scanning pattern [96]. [PERSON] et al. developed a forward-viewing spectral domain OCT endomicroscope. In this case, an SMF is excited at resonance using piezoelectric tube actuators and describes a Lissajous scanning pattern [63]. [PERSON] et al. reported a forward-viewing two-photon fluorescence microscope. A double clad fiber (DCF) is used to deliver light to the target sample and collect the fluorescence signal. The spiral pattern is generated by the fiber tip using a piezoelectric actuator exciting the tip at resonance [108]. Recently, [PERSON] et al. developed a 2.4-mm confocal endomicroscope using electrostatic actuators for laser light scanning. Such a probe with a 1.5-\(\upmu\)m lateral and 12-\(\upmu\)m axial resolution is compatible with the finer operating channel of current endoscopes [24]. \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline  & \begin{tabular}{c} **Working** \\ **Principle** \\ \end{tabular} & **Frequency** & **FOV** & **Drive Voltage** & **Scanner Dimensions** & \begin{tabular}{c} **Scanning** \\ **Pattern** \\ \end{tabular} & **References** \\ \hline OCT using & & & & & & & & \\ rotational & Micromotor & 1 kHz & - & \(>\)1 V & 2.4 mm (in diameter) & Radial & [54] \\ MEMS fiber & & & & & & & \\ scanner for & Electrothermal & 239 Hz (x-axis) & 378 \(\upmu\)m \(\times\) & \begin{tabular}{c} 16 Vpp \\ (duty cycle \\ 13\%) \\ \end{tabular} & \begin{tabular}{c} Diameter (1.65 mm) \\ Rigid length (28 mm) \\ \end{tabular} & Lissajous & [69] \\ confocal & actuation & 207 Hz (y-axis) & 439 \(\upmu\)m & \begin{tabular}{c} 13\%) \\ \end{tabular} & \begin{tabular}{c} Rigid length (28 mm) \\ \end{tabular} & \begin{tabular}{c} Scanning \\ \end{tabular} & [63] \\ microscopy & & & & & & \\ Fiber scanner & & & & & & \\ for forward & Piezoelectric & 86 Hz (x-axis) 97 & 732 \(\upmu\)m \(\times\) & \begin{tabular}{c} 40 Vac \\ \end{tabular} & \
\begin{tabular}{c} Diameter (3.2 mm) \\ Rigid length (50 mm) \\ \end{tabular} &
\begin{tabular}{c} Lissajous \\ scanning \\ \end{tabular} & [63] \\ viewing & tube & & & & & \\ endoscope & & & & & & \\ \hline OCT based on & & 1.25 Hz (fast & & 0–4 V ramp & \\
2D MEMS & Electrothermal & scan actuator & 2.3 mm x & (fast scan) & Diameter (5.8 mm) & Lissajous & [140] \\ mirror & & & 0.0125 Hz & 2.3 mm & (0.5-3.5 V ramp & Rigid length (12 mm) & scanning & \\  & & & & (slow axis) & & \\ Scanning fiber & Piezoelectric & 5 kHz & 200 \(\upmu\)m & \(<\)20 Vac & \
\begin{tabular}{c} Diameter \\ (1.2 mm-1.7 mm) \\ \end{tabular} & Spiral scan & \\ endoscope & tube & & & & & \\ Multi-Photon & Piezoelectric & 35 Hz & 900 \(\upmu\)m & 40 Vac &
\begin{tabular}{c} Diameter (5 mm) Rigid \\ length (4 cm) \\ \end{tabular} & Circular & [141] \\ Endoscope & tube & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of some advanced optical scanning devices. In summary, the choice of an optical scanner over another type often depends on the target tissue and the constraints it poses. For example, some tissue surfaces, such as the eye, cannot tolerate shining a laser light on it for a long period of time due to the energy of the laser beam causing photoablation of the tissue.