# Wavefront Shaping and Its Application to Enhance Photoacoustic Imaging
## 4 Enhanced Photoacoustic Imaging with PAWS  
Using photoacoustic signal as an internal guide star for wavefront shaping, acoustic-, or optical-diffraction-limited optical focusing can be formed within scattering media. After PAWS, the in situ optical fluence can be greatly increased, and so can be the strength of PA signals. Therefore, PAWS has the potential to enhance the SNR, sensitivity, as well as the imaging resolution and penetration depth of current photoacoustic imaging implementations. The first demonstration came from [PERSON] et al. [28] and [PERSON] et al. [26], where optical wavefront shaping was combined with genetic algorithm to improve the imaging quality of photoacoustic sensing. The authors [28] placed an absorbing object (two polypropylene 90 \(\upmu\)m-inner-diameter tubes filled with India ink) \(\sim\)8 mm behind a glass diffuser as guidestar for feedback--the distance was calibrated to match the size of photoacoustic focal region with that of a speckle grain. Linear PAWS was firstly used for optimizing the input wavefront to enhance the photoacoustic signal by localizing the optical fluence enhancement. It turned out that a peak-to-peak amplitude enhancement of 10 upon the optimized photoacoustic signal was observed in their study. They, thereby, scanned the absorber with an auto-translation stage in two dimensions around the optimized focus, forming the shown two-dimensional imaging. As for the third dimension, the temporal profile of the ultrasound signal was used to provide the information on that dimension and, hence, the reconstruction of the three-dimensional photoacoustic imaging of the object accordingly. As shown in Figure 6, the PA signal was enhanced by one order of magnitude with PAWS, and the three-dimensional photoacoustic imaging with PAWS shows a much higher SNR and much better well-defined tube structure than that without PAWS. Figure 5: **(a)** The phase of each SLM input mode (in Hadamard basis [84]) is scanned from 0 to 2\(\pi\) in 16 steps; the corresponding photoacoustic trace is measured for each phase step; **(b)** the peak-to-peak amplitude of each PA signal window closely follows a cosine modulation as a function of the phase of an input mode; the photoacoustic transmission-matrix elements are retrieved directly from these cosine modulation phases \(\theta ij\) and amplitudes \(\beta_{ij}\) for all input-output mode pairs \(j\), \(i\). The matrix is converted to the SLM-pixel input basis by a basis transformation (Figure from [22]). The same group also furthered the research to obtain super-resolution photoacoustic imaging [26]. Due to the spatially non-uniformity of acoustic detection (a Gaussian spatial distribution caused by the acoustic focus), optical speckles with higher contribution weighting to the photoacoustic response center more with respect to the center of the acoustic focus. Therefore, photoacoustic signal contributions produced by a single optical speckle near the acoustic focus center are preferentially detected and optimized. As a result, the local intensity of the optical speckle was significantly enhanced (optical enhancement of 24--three times that of the photoacoustic signal amplitude enhancement ratio), whose size is smaller than the acoustic focal size. In the reported experiment, the optical speckle size in the acoustic focus region is 13 \(\upmu\)m while the acoustic focus size is 50 \(\upmu\)m, which makes it possible to imaging beyond the acoustic resolution. By scanning the sample (without moving the optimized optical focus), an image of a black alpaca hair (25 \(\upmu\)m diameter) placed behind a scattering medium with a FWHM of 30 \(\upmu\)m was demonstrated in Figure 7a,b. Using the same procedures, they imaged a sweat bee wing, obtaining fine structures and details of the sample (Figure 7g), which is fairly consistent with the scattering-free optical image (Figure 7h) captured by a camera.