This is a hybrid optoacoustic/ultrasound imaging system for imaging mice. Tissues undergo thermoelastic expansion after they are illuminated by a pulsed laser, which generates ultrasound waves that are detected by a 270° array of ultrasound detectors. Co-registered tomographic ultrasound and photoacoustic images are produced. The whole mouse body can be scanned by moving the mouse through the detector ring. Contrast can be endogenous (deoxy hemoglobin vs oxy hemoglobin) or from exogenous agents such as indocyanine green or nanoparticles. There is also a hand-held unit for imaging of rats and larger subjects.