# Wavefront Shaping and Its Application to Enhance Photoacoustic Imaging
## 4 Enhanced Photoacoustic Imaging with PAWS
Furthermore, the authors built a point-by-point image, by scanning the transducer to improve the imaging resolution: an optical focus was optimized and moved from one absorber to another to obtain the spatial responses of photoacoustics. The results (Figure 7c,d) indicate that this method can clearly distinguish individual chromium bars (55 \(\pm\) 3 \(\upmu\)m wide) after the optimization of PAWS. In the latter case that transducer is used to scan, the ultrasound transducer had a frequency of 50 MHz and an acoustic focal diameter of 61 \(\upmu\)m, which apparently wider than the resolution of the PAWS-enabled photoacoustic imaging. Super-resolution in PAWS can also be obtained by using spectral filter detection [45], where a broadband transducer was used for sensing photoacoustic signals, which were high-pass filtered and used as feedback PAWS optimization. This study unveils non-uniform PAWS focusing capability regardless of different photoacoustic spectra and focusing enhancement ratio. Although not directly shown, this method could be helpful in enhancing the sensitivity, as well as the resolution of photoacoustic imaging. Figure 6: Three-dimensional photoacoustic imaging of two capillary samples: (**a–c**) represent the imaging optimized by PAWS; (**d–f**) represent the imaging without PAWS by displaying a flat phase pattern on SLM; (**a,d**) three-dimensional scan with a 2% intensity threshold regarding the maximum intensity; (**b,e**) two-dimensional scan; and (**c,f**) 1D scan of the capillary tubes at \(x\) = 0.02 mm (Figure from [28]). The aforementioned enhanced PA imaging cases were all implemented based on iterative PAWS optimization. [PERSON] et al. [53] recently showed that similar PA imaging enhancement could be obtained by measuring the photoacoustic transmission matrix of the complex medium, following a protocol from [22] by applying cosine modulations to the photoacoustic signals. In their approach (Figure 8), a black leaf skeleton was placed behind a ground glass diffuser as an optical absorber, in order to generate photoacoustic signals used as feedback. The PA signals were detected by a linear ultrasound array (15 MHz central frequency). The hypothesis is that all optical information had been well included in the transmission matrix, so that the photoacoustic image could be reconstructed without any additional measurements. By analyzing the modulation, the authors successfully reconstructed detailed infrastructure of the leaf skeleton (Figure 8e), which is consistent and comparable with the skeleton information provided by a photograph of the same leaf captured by a CCD camera in a scattering-free setting (Figure 8b). Compared with the standard photoacoustic images by various speckle illuminations (Figure 8b,c), the enhancement in imaging quality is quite obvious. Figure 7: Super-resolution photoacoustic imaging. Scale bar: 100 μm. (**a**) Photoacoustic image of a black alpaca hair (25 μm diameter) and (**b**) PA signal intensity profile along the dashed line in (**a**), showing a diameter of 30 μm defined by the full width of half maximum (FWHM). (**c**) Direct microscopic image of multiple hairs. (**d**) Experimental one-dimensional (1D) scan images with direct optical microscopic, and with uniform, random, and optimized phase compensations, repsectively. (**e**–**g**) Sweat bee wing images with uniform, random speckle, as well as optimized wavefront illuminations, respectively. (**h**) Direction microscopic image of the bee wing serves as the gold standard for comparison. (Figure from [26]).