### Key Devices in PAI  
The most important (and costly) component in a PAI device is the photoacoustic emission source, usually in the form of a pulsed laser. Important laser parameters in PAI applications are pulse energy, wavelength, pulse duration, pulse repetition rate and pulse stability. Different PAI methods require different laser energies. For example, OR-PAM, AR-PAM and PAT require laser devices with an output energy range of a few tens of micro-joules, a couple of hundred micro-joules and mill joules, respectively, in visible and near-infrared (NIR) wavelengths. OR-PAM requires a pulse duration shorter than 10 ns to satisfy the stress limitation condition of maximum PA generation in the biological cell scale. Using a single detector entails a raster scan of targeted samples, before image reconstruction. For real-time imaging, lasers with a high pulse repetition rate from kHz to MHz are required, depending on the imaging zone and spatial resolution. Although the effect of the pulse-to-pulse energy stability of PAI can be calibrated by a photodetector, an energy stability above 2-3% is recommended for a quantitative real-time measurement of chromophores in a tissue without fluence calibration [26, 27]. Another key component in PAI is the ultrasonic (US) detector that is used to receive the PA signal. These detectors can be divided into two categories: piezoelectric transducers and optical sensors. Usually, piezoelectric transducers have a higher sensitivity and are more convenient for an imaging setup, while optical sensors are not disturbed by electromagnetic interference and have a wider response bandwidth, but lower sensitivity. Piezoelectric transducers are the most common type of US detector used in PAI, due to their high sensitivity, simple signal detection and sufficient bandwidth. Indeed, the bandwidth and central frequency of the piezoelectric transducer determine the axial resolution of PAM. OR-PAM typically employs a 50-75 MHz central frequency transducer [8, 16], whereas AR-PAM has a lower frequency, such as 20-50 MHz [8, 11, 28]. For PAMac and PAT, the center frequency of the transducer or array should be as low as 1-10 MHz [8] for detecting lower frequency PA signals from deep tissues with less US attenuation. In addition, PAI requires a scanning system, which can involve a single element, a line or a ring-array, with the system type determining the imaging speed and quality. PAM tends to use single element detectors with a focused or converged light beam in forward-view detection to achieve high lateral resolution. As the PA signal naturally provides one-dimensional (depth) information (signal arrival time multiplied by acoustic speed in tissue), 2D raster-scanning will produce a 3D PA image. Common scanning devices include mechanical scanners (stepping motor or DC motor), voice-coil scanners, optical scanners and MEMS-mirror scanners. A mechanical scanner has a low B-scan rate, but a large scanning range, while an optical scanner's properties are the opposite. A voice-coil scanner represents a negotiation between the scanning rate and range. A B-scan rate of  
Figure 1: Schemes of PAI techniques for tissue imaging, classified according to spatial resolution and imaging depth: (**a**) OR-PAM, (**b**) AR-PAM and PAMac, (**c,d**) PACT. 40 Hz/mm and a scanning range of up to \(\sim\)10 mm [29] allows for the real-time imaging of small animals. However, a voice-coil scanner has a relatively low loading ability, and fast scanning requires the detection head to be light in weight, limiting its design and performance. Recently, a scanner based on a water-immersible microelectromechanical system (MEMS) has been developed for the simultaneous scanning of light beams and PA waves. MEMS can maintain optical and acoustic confocal alignment with fast scanning and, at a 400 Hz B-scan rate over a 3 mm range, as it has a large scanning range [30].