Optoacoustic (OA) imaging allows optical absorption contrast to be visualised using thermoelastically generated ultrasound. To date, optoacoustic theory has been applied to homogeneously absorbing tissue models that may describe, for example, large vessels filled with blood, where the whole target will act as a coherent source of sound. Here we describe a new model in which the optical absorbers are distributed inhomogeneously, as appropriate to describe microvasculature, or perhaps the distribution of molecularly targeted OA contrast agents inside a tumour. The degree of coherence over the resulting distributed acoustic source is influenced by parameters that describe the scale of the inhomogeneity, such as the sizes of the absorbers and the distances between them. To investigate the influence of these parameters on OA image appearance, phantoms with homogeneously and imhomogeneously absorbing regions were built and imaged. Simulations of the same situation were conducted using a time domain acoustic propagation method. Both simulations and experiments showed that introducing inhomogeneity of absorption produces more complete images of macroscopic targets than are obtained with a homogeneous absorption. Image improvement and target detectability were found to reach a maximum at an intermediate value of the length-scale of the inhomogeneity that was similar to the axial resolution of the acoustic receiver employed. As the scale of inhomogeneity became finer than this the target's detectability and appearance began to revert to that for homogeneous absorption. Further understanding of this topic is believed to be important for optimising the design of clinical optoacoustic imaging systems.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.