### PAI Technique and Its Categories  
PAI is based on the photoacoustic (PA) effect, first reported by [PERSON] in 1880. The PA effect is produced by the absorption of modulated light (or electromagnetic radiation) in a material. The absorbed optical energy is converted into heat by non-radiative relaxation, resulting in a pressure variation. This fast pressure variation is released through acoustic waves, which propagate outside the material. By applying acoustic (usually ultrasonic) detectors to receive acoustic waves and scanning the PA generation zone, it is possible to obtain an optical absorbing image of the material through a specific image reconstruction algorithm. This technique is known as PA (or optoacoustic) imaging and has found wide application in biotissue studies and in vivo small animal studies [8; 9]. Examples include visualizing blood vessels [10; 11], imaging tissue tumours [12; 13], determining blood hemoglobin (HbR) [14] and deoxyhemoglobin (HbO) [15] levels as well as oxygen saturation [16; 17] and oxygen metabolism [15; 18]. It is also used in measuring blood flow [15; 19], counting melanomas and cancerous cells in blood [20; 21] and sensing fatty deposits in blood vessels [22; 23]. Together with exogenous contrast agents [24], such as bio-compatible nanoparticles, chemical dyes and report genes, researchers have greatly expanded the PAI application range, while improving imaging depth and quality in animal and human tissues alike.  
Depending on their imaging resolution and depth, PAI methods are approximately divided into PA microscopy (PAM), PA macroscopy (PAMac) and PA computed tomography (PAT or PACT). PAM can be further classified as optical-resolution microscopy (OR-PAM) and acoustic-resolution microscopy (AR-PAM), while PAT can be classified as single, linear-array, ring-array, planar-array, cylindrical-array and spherical-array PAT, based on the detector number and shape [25]. All of these imaging techniques have their own properties and application areas. OR-PAM utilizes ballistic and quasi-ballistic photons to generate images and provides the highest resolution, to the micrometer or even sub-micrometer range. As shown in Figure 1a, the exciting light and detected ultrasound are co-focused. However, OR-PAM's imaging depth in brain tissue is less than 1 mm. AR-PAM and PAMac, on the other hand, use diffusive light to generate images with a lateral resolution from a few tens of micrometers to sub-millimeters, depending on the transducer frequency and focus. The imaging depth of AR-PAM and PAMac in brain tissue can be up to 3-5 mm and 10-20 mm, respectively, at red and near-infrared wavelengths. Due to its limited imaging depth, PAM is mainly suited to small animal imaging, for which it has achieved great success in brain cortex studies. Though similar to PAM, PAMac has a greater imaging depth, achieved by sacrificing lateral resolution to some degree, which is a result of using transducers with a lower frequency and a smaller acoustic focus (Figure 1b). Differing from both PAM and PAMac, in which the detector receives PA waves along or nearly along its acoustic axis, PACT usually receives PA waves from the imaging plane perpendicular to the direction of excitation light. As a result, it has the capacity to image tissues up to a depth of several centimeters with sub-millimeter resolution. Hence, while PAT is capable of imaging the entire body or head of a small animal, it can also produce images of the outer layers of the human brain, which is covered by a scalp and skull with a thickness of up to 10 mm. Although single-detector PAT has a simpler setup, array-detector PAT offers faster data acquisition without scanning or with a shorter scanning period, making it more suitable for real-time imaging (Figure 1c,d).