This thesis describes the design and development of a hybrid photoacoustic-ultrasound transmission tomographic imager optimised for small animal imaging (mice). The system allows imaging of ultrasound transmission properties (speed of sound (SOS) and acoustic attenuation (AA) ) simultaneously with photoacoustic imaging. The method is based on passive elements of ultrasound. Here we define the 'passive' element as a strong absorber of light with a small geometrical cross section, which functions as an ultrasound generator by the photoacoustic effect when it is placed in the path of the laser pulse. The situation is in contrast to conventional electrical activation of piezoelectric (active) elements for producing ultrasound and thus we refer our method as a Passive Element enRiched Computed Tomography (PER-PACT).