# Wavefront Shaping and Its Application to Enhance Photoacoustic Imaging
## 2 Exploiting Scattering of Light
### Wavefront Shaping  
Wavefront shaping is one such technique that scientists have developed. The basic principle is to spatially modulate light incident onto the turbid medium in a way so that scattering-induced phase distortions between the point of incidence and the ROI can be pre-compensated. This technique was first introduced by [PERSON] et al. in 2007 [42]. They used a liquid-crystal-on-silicon (LCoS)-type spatial light modulator (SLM) as the phase/wavefront modulator, with up to 3228 independently controlled pixels to control the phase of every input mode individually. A charge coupled device (CCD) camera was placed to monitor the light intensity at a target position behind a scattering medium. Light intensity reading from the specific CCD pixels was used as the feedback for an optimization algorithm to program the phase patterns on the SLM. In order to maximize the intensity of a chosen area (output mode) recorded by the camera pixels, the phase of each input mode (SLM pixel) was gradually tuned from 0 to 2\(\upmu\)r. The one that yielded highest feedback was assigned as the optimum phase for that specific SLM pixel. Such iterative optimization process was performed for each SLM pixel. After the optimization, a strong optical focus behind an opaque ground glass diffuser is obtained(Figure 1b). In comparison, if wavefront shaping is not employed, a fully-randomized speckle pattern is seen (Figure 1a), which is an indicator of sufficient optical diffusion from the medium. Since the initial implementation, wavefront shaping has seen many exciting advancements and has been explored for wide applications, which has been well reviewed [19; 79; 80; 81] and, thus, will not be reiterated herein. However, it must be pointed out that most efforts in the community have centered on three aspects: to improve the focal intensity enhancement ratio (R), to shorten the optimization process, and to seek appropriate noninvasive guide star to produce the feedback signal. Although the focal enhancement ratio R at the ROI before and after the optimization may vary from case to case (e.g., the working principle of the wavefront modulator), it is overall proportional to \(\alpha\times\)(N/M), where N is the number of independently controlled elements on the SLM, M is the number of output modes (speckle grains) within the ROI, and \(\alpha\) is a constant depending on the type of wavefront modulator (Table 1). As one can imagine, in order to achieve a high R, a large N and/or small M are required: the former is determined by the technical specification of the wavefront modulator, and the latter by the sensitivity zone of the feedback signal with respect to the speckle grain size. That said, a large N usually requires a longer optimization process [43], which is also significantly affected by the refreshing rate of the wavefront modulator, as well as the speed of data transfer and processing between the sensor, the wavefront modulator, and the processor (usually a personal computer [56]). Table 1 lists three types of commercial wavefront modulators, i.e., LCoS-type spatial light modulator (SLM), deformable mirror (DM), and digital micromirror device (DMD), that have been popularly used in wavefront shaping. Each has its own advantages and disadvantages; none is perfect. For example, DMD can respond rapidly, but its modulation relies on the tilting of the micro-mirror arrays, and only binary (i.e., either ON or OFF) intensity modulation is supported by DMD. For the other two types, finer steps of phase modulation are usually supported. On the other hand, LCoS-SLM is slow, and DM is expensive and supports small pixel numbers. Therefore, the choosing of a wavefront modulator is usually a trade-off between the speed, the modulation efficiency, the pixel number, and the cost, among many other requirements from the specific application.