We present systematic characterization of a photoacoustic imaging system optimized for rapid, high-resolution tomographic imaging of small animals. The system is based on a 128-element ultrasonic transducer array with a 5-MHz center frequency and 80% bandwidth shaped to a quarter circle of radius. A 16-channel data-acquisition module and dedicated channel detection electronics enable capture of a 90-deg field-of-view image in less than and a complete 360-deg scan using sample rotation within . Measurements on cylindrical phantom targets demonstrate a resolution of better than and high-sensitivity detection of blood tubing to depths greater than in a turbid medium with reduced scattering coefficient . The system is used to systematically investigate the effects of target size, orientation, and geometry on tomographic imaging. As a demonstration of these effects and the system imaging capabilities, we present tomographic photoacoustic images of the brain vasculature of an ex vivo mouse with varying measurement aperture. For the first time, according to our knowledge, resolution of sub- vessels with an overlying turbid medium of greater than depth is demonstrated using only intrinsic biological contrast.