# Photoacoustic Imaging for Management of Breast Cancer: A Literature Review and Future Perspectives
## 1 Introduction
Another advantage with PA imaging is that since the receiving technology is the same for both PA and US imaging, the already commercially available US imaging system can be modified to produce a dual mode PA-US imaging system. This dual mode system can be used to acquire co-registered PA and US images of the breast, which can be potentially applied for early detection of breast cancer. Using the morphological cues provided by the US images, it is possible to localize a malignant lesion in the early stage which has already been detected by the PA image. Photoacoustic (PA) imaging systems are broadly classified into two categories, namely PA tomography and PA microscopy [34]. In PA tomography, a large-diameter pulsed laser beam is used to irradiate the tissue surface, and different US transducer arrays (planar, linear, spherical, hemispherical, circular, cylindrical) are used to scan the tissue/organ along different trajectories to collect the PA signals generated by the tissue. Grayscale 2D PA images (B scan, C scan) are formed using specific reconstruction algorithms with the acquired PA data. A schematic of the PA tomography (PAT) system for breasts cancer detection is shown in Figure 1, which illustrates the process of PA signal generation from the breast and the process of generating grayscale PA images from the acquired PA signals. Cylindrical, circular, spherical, and hemispherical US arrays require access to major portions around the region to be imaged, so, barring a handful of cases, like small animal imaging or breast  
Figure 1: Schematic of the PAT system for breast cancer detection. cancer imaging, these arrays are not suitable for in vivo imaging. On the other hand, planar and linear US arrays can be used widely for in vivo imaging as they can work with restricted access. In the case of in vivo breast imaging, it is not possible to use spherical US arrays, as they require access to the complete region around the organ to be imaged. In such cases, where there is restricted access, planar [35; 36], and linear [37; 38] US arrays can be used. However, hemispherical [39; 40] US arrays were used by some researchers for breast imaging. The reconstruction algorithms are primarily different variants of back projection algorithms which are used to recover the spatially-varying PA signals generated at each location inside the tissue. In PA microscopy, the tissue/organ is illuminated using a tightly focused beam, and the PA signal is detected generally using focused single element US transducer. The tissue/organ is scanned along certain trajectories by laser beam along with the US transducer and the PA signals acquired by the US transducer are used directly to form the 2D PA images [41]. Using contrast agents, both PA imaging contrast and imaging depth can be significantly improved. In cancer detection, there are two strategies used to navigate contrast agents to the target, a passive and an active targeting strategy. In passive targeting, the physical properties of the tumor, such as enhanced permeation and retention (EPR) effect, are utilized. This facility is more prominent in xenograft tumor models than in human patients [42; 43; 44; 45]. In the active targeting strategy, molecules which have the ability to selectively bind to a particular target are used. These binding molecules or targeting moieties are conjugated with the contrast agents before administration to the organism [46]. Researchers performed animal studies using dyes, such as Indocyanine green (ICG), Methylene Blue, Evans blue, and different nanomaterials for breast cancer detection, as well as sentinel lymph node (SLN) detection. This review paper attempts to present an in-depth analysis of the research performed so far towards the development of PA imaging technology for breast cancer detection. In some earlier research articles [25; 47], comparative analysis of the system parameters of different PA imaging system was presented.