**1. Introduction**

Photoacoustic imaging (PAI) is a non-invasive and non-ionized multimodal biomedical imaging method based on transient thermoelastic e ffects 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 [1–7]. 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 e ffectively 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 di ffusion 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 imaging and 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 e ffectively 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 [11–13].

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 di fferent photoacoustic coupling methods, optical transmission models [14], and ultrasonic detection designs [15–20]. However, due to the limited detection angle of ultrasonic transducers, linear array PACT [16–19], 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 [25–28], which results in the loss of deeper tissue structure information, while at the same time, the scattering effect of the light cannot e ffectively illuminate the irregular section and a ffects the imaging e ffect. 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 [29–31]. In addition, in order to improve the resolution of the photoacoustic imaging system, a negative lens can be mounted on the surface of the ultrasonic transducer to increase the receiving angle [32,33]. Due to the large field-of-view of the virtual point ultrasound transducer, some research has applied virtual point ultrasonic transducers to a photoacoustic imaging system [34–36]. Nie LM et al. developed a virtual point ultrasonic transducer with a large receiving angle and applied it to the photoacoustic tomography system. The signal-to-noise ratio of the system is greatly improved compared with the negative-lens transducers [35]. Hao F. Zhang et al. use a synthetic-aperture focusing technique based on the virtual point detector to obtain a high-resolution photoacoustic image of hemoglobin oxygen saturation in living mice [36].

In this paper, a 3D photoacoustic imaging system based on "dual-element" ultrasonic transducer is proposed under the premise of full-view imaging and uniform illumination. The system contains 1-to-8 fan-out fiber bundles and an optical cage design, which delivers approximately uniform laser radiation toward the imaging phantom and provides an exciting source for completing the photoacoustic tomography of each B-scan (B-scan refers to a cross section of photoacoustic tomography in this paper). The rotary motor carries two customized dual-foci virtual point ultrasonic transducers for photoacoustic data acquisition in the process of rotary motion, while the precise vertical stepper motor drives the imaging target to do vertical movement to complete the 3D scanning of it. The dual-foci virtual point ultrasonic transducer can expand the signal acceptance angle in the short focusing direction because of its large field of view, such that a relatively complete photoacoustic signal can be acquired with fewer detection units, which reduces the cost of the photoacoustic imaging system and accelerates the photoacoustic imaging speed. In the long focusing direction, the narrow sound beam and the annular illumination beam are coupled to form a photoacoustic coplanar mode, which improves the signal to noise ratio of the photoacoustic signal. The pattern of 360◦ circular laser radiation provides uniform illumination at all angles around the imaging cross section, which facilitates photoacoustic tomography of imaging phantom with irregular surfaces. Furthermore, the

feasibility and operability of the 3D-PACT system is validated through systematic testing and 3D photoacoustic experiments on a variety of phantoms.
