# Endoscopic Optical Imaging Technologies and Devices for Medical Purposes: State of the Art
## 2 Medical Imaging Applications
### Photoacoustic Imaging  
Photoacoustic (PA) imaging is a hybrid technology that uses light, which generates sound waves to image a tissue surface. In this technique, a tissue surface to be imaged absorbs the short pulsed high-intensity laser light, causing the heating of the local surface, which then emits a high-frequency acoustic wave due to the pressure rise. The emitted ultrasonic signal is very poor as compared to signals from other technologies, and thus requires the use of an acoustic transducer without blocking the optical signal. The PA microscopy provides an absorption contrast with a resolution at a subcellular level and has a capability to image anatomical, functional, and molecular structures at the same time [37; 38]. A schematic of a photoacoustic system is reported in Figure 9[39].  
Figure 8: Images of brain metastasis of squamous cell carcinoma: (**A**) Hematoxylin and eosin-stained sample; (**B**) TPEF image; (**C**) Coherent anti-Stokes Raman scattering (CARS) image; (**D**) combining CARS and TPEF (used with the permission of [33]).  
PA microscopy can be classified into optical resolution PA microscopy (OR-PAM) and acoustic resolution PA microscopy (AR-PAM) on the basis of the resolution technique used for imaging. In an OR-PAM device, the optical beam is focused inside the acoustic focus point. On the other hand, in an AR-PAM device, laser light is focused on a tissue surface through an optical system at a tight acoustic focus point. The lateral resolution of the PA microscopic device depends on the size of the focal spot. Since the wavelength of the optical wave is smaller than that of an acoustical wave, a small spot size can be obtained in the former case. Consequently, an OR-PAM system provides a finer resolution (ranges from submicrometer to submillimeter) compared to that of an AR-PAM (\(\sim\)45 \(\upmu\)m). Conversely, a greater illumination area in AR-PAM allows higher numbers of photons to penetrate deep into the tissue, giving a higher penetration depth [37; 38; 40].  
Another imaging technique based on the use of optical and acoustical wave interaction is PA computed tomography (PACT) [39]. It is based on a similar working principle as PA microscopy, with the difference in the fact that a reconstruction-based image is formed instead of a focused-based one.  
Two-dimensional or three-dimensional tissue imaging is possible by scanning the optical and acoustical foci along a tissue surface using appropriate scanning methods. The penetration depth of imaging in this case ranges from a few micrometers to a few millimeters.  
PA-based microscope catheters are largely used to image the bladder [41] and neurovascular systems [42], characterize microvascular parameters [39], and detect breast cancer [43]. A large number of devices based on PA microscopy are described in [2]. OR-PAM devices can achieve lateral and axial resolution as small as \(\sim\)0.5 and 10 \(\upmu\)m, respectively [44]. [PERSON] et al. imaged a large FOV image of the mouse ear using this technique. They are able to image the ear capillaries with finer resolution even at a depth of 150 \(\upmu\)m as can be seen in Figure 10[44]. It is possible to get an even finer resolution image using a high-frequency signal, but the higher attenuation of the acoustical wave limits the penetration depth at this point. Thus, there is a trade-off between the resolution and imaging depth, with the penetration depth:resolution ratio being about 200 [45]. The penetration depth also depends on the PA modality used for imaging and on the nature of the transducer used for signal detection [46]. Using NIR light, the penetration depth can reach up to 7 cm [45].  
Figure 9: Schematic diagram of a photoacoustic microscope (taken with the permission of [39]).  
It is possible to use more than one technology to image the surface under consideration as every modality shows different aspects of the surface. The performance of different imaging techniques is compared in Table 1.