Figure 1 shows the schematics of the system. The system integrates a tunable optical parametric oscillator (OPO) laser (Vibrant B, Opotek Inc, Carlsbad, California) pumped by the second harmonic output of an Nd:YAG pulsed laser (Brilliant B, Quantel, Bozeman, Montana) as the illumination source. The laser system is tuned to 740 nm which gives the maximum output energy of 80 mJ per pulse. The laser pulses, with a repetition rate of 10 Hz and pulse width of 5.5 ns, are coupled into an optical fiber bundle (CeramOptec Industries Inc, East Longmeadow, Massachusetts) which consists of 18 fibers fused together at the input end for best coupling of light energy. As shown in Fig. 2, the fibers, at the output end of the bundle, are arranged into four linear arrays which bilaterally illuminate the finger horizontally rested on the imaging platform [marked with the blue lines in Fig. 2(a)]. Considering the coupling efficiency of 30 to 40 percent of the fiber bundle (approximately 30 mJ per pulse delivered to the finger) as well as the beam spreading before light reaching the target finger, the light energy density, on the skin surface, is approximately which is significantly lower than the ANSI safety limit. A commercial US unit (Z.ONE, ZONARE Inc.) is used to acquire the PA signals. A linear array (L10-5, ZONARE Inc.), with a working band of 5 to 10 MHz [delineated with red curves in Fig. 2(a)], is positioned perpendicular to the imaging platform and coupled to the finger surface with a block of agarose gel, as delineated with the blue lines in Fig. 2(b). The lateral resolution, of the PAT system, in the focal plane is approximately 300 μm at a distance of 20 mm from the probe surface. For each finger joint, both the volar side of the coronal [as shown in Fig. 2(a)] and the sagittal [as shown in Fig. 2(b)] middle planes are scanned. The scanning plane, for PA imaging of a human finger joint, corresponds to that of standard ultrasound evaluation in clinical practice.