# 3D Photoacoustic Tomography System Based on Full-View Illumination and Ultrasound Detection
## 1 Introduction  
Photoacoustic imaging (PAI) is a non-invasive and non-ionized multimodal biomedical imaging method based on transient thermoelastic effects of the biological tissue. The principle is that the energy of pulsed laser light deposited in biological tissue during the process of laser absorption is converted into an ultrasonic signal which is called the photoacoustic signal through instantaneous thermoelastic expansion, and the ultrasonic transducer can receive the signal that carries information about the properties of laser absorption in biological tissue [1; 2; 3; 4; 5; 6; 7]. Through the corresponding signal processing and image reconstruction algorithms, a photoacoustic image reflecting the internal structure and function of the tissue can be obtained. Photoacoustic imaging effectively overcomes the limitations of existing pure optical imaging and pure ultrasound imaging, the contrast is based on the absorption of laser light during the photoacoustic excitation period, while the resolution is derived from the ultrasonic detection during the photoacoustic emission period. Photoacoustic imaging breaks through the limit depth of diffusion of high-resolution optical imaging (about 1 mm), and uses laser-generated ultrasonic waves as a carrier to obtain optical absorption information of tissue [8; 9]. It is mainly suitable for the tissues whose acoustic properties are uniform but optical properties are not. Therefore, photoacoustic imaging combines the advantages of high contrast characteristics of optical imagingand high penetration characteristics of ultrasonic imaging, and can provide photoacoustic images with high optical contrast and high acoustic resolution at a deeper imaging depth [10]. As a new generation of biomedical imaging technology, photoacoustic imaging can effectively realize the structure and functional imaging of biological tissue, which provides an important method for studying the morphological structure, physiological characteristics, pathological characteristics, and metabolic functions of biological tissues [11; 12; 13]. Scientific research on 3D photoacoustic imaging of small animals has become increasingly popular in recent years. There have been many 3D photoacoustic tomography (PACT) systems using different photoacoustic coupling methods, optical transmission models [14], and ultrasonic detection designs [15; 16; 17; 18; 19; 20]. However, due to the limited detection angle of ultrasonic transducers, linear array PACT [16; 17; 18; 19], spherical array PACT [20; 21], arc-shaped array PACT [22], and the half-ring multispectral optoacoustic tomography (MSOT) [23] generally need to obtain complete photoacoustic signal via spatial multiplexing. In photoacoustic tomography, uniform illumination helps to obtain more comprehensive light absorption information of the imaging cross section [24]. The traditional circular scanning mainly irradiates the laser beam from the top to the bottom of the object [25; 26; 27; 28], which results in the loss of deeper tissue structure information, while at the same time, the scattering effect of the light cannot effectively illuminate the irregular section and affects the imaging effect. From the perspective of the cost, the array ultrasonic transducer with high performance makes it not only expensive, but also a huge challenge for transducer manufacturing and data acquisition. Once formed, its scanning radius cannot be adjusted according to the size of the imaging targets, which limits the scope and flexibility of its application. There are also some photoacoustic tomography systems that acquire photoacoustic signals point by point through a single-element transducer carried by a rotary motor, whose advantages are their low cost and easy operability. The ultrasonic transducer performs a rotary motion at a predefined speed and completes the acquisition of the photoacoustic signal of the imaging section during the continuous motion process, the continuous scan of data acquisition is faster than a stop-and-go scan [29; 30; 31].