We estimated the peak-to-peak signal to root-mean-square noise ratio (SNR), axial resolution, and transverse resolution as a function of the imaging depth by imaging human hair fibers [Figs. 2] in a 10% porcine gelatin containing 1% Lyposyn II. The reduced scattering coefficient of this phantom8 was , which is close to that of biological tissues. At the depth (the working distance of the transducer), images averaged 5 times achieved a SNR, a axial resolution, and a transverse resolution. The transverse resolution matches approximately the product of the f-number (1.33) and the central acoustic wavelength (, obtained from the PA signals). Therefore, the pixel count in the depth direction was 136, greater than the targeted 100. Compared with the resolution of micro-CT (20 to ) and micro-MRI (100 to ), the axial resolution of the proposed system is similar. However, the imaging depth is less than those of both CT and MRI. PA imaging, however, is sensitive to intrinsic functional optical contrast. Figure 2 demonstrates the possible maximum imaging depth in chicken breast tissue. We imaged two -diam horse hairs located and deep, where the SNRs were measured to be and , respectively, with 30 times averaging. Synthetic-aperture focusing and coherence weighting were applied to improve the transverse resolution as well as the SNR.9 The rubber plate at a depth was also imaged.