  Existing approaches to image reconstruction in photoacoustic computed
tomography (PACT) with acoustically heterogeneous media are limited to weakly
varying media, are computationally burdensome, and/or cannot effectively
mitigate the effects of measurement data incompleteness and noise. In this
work, we develop and investigate a discrete imaging model for PACT that is
based on the exact photoacoustic (PA) wave equation and facilitates the
circumvention of these limitations. A key contribution of the work is the
establishment of a procedure to implement a matched forward and backprojection
operator pair associated with the discrete imaging model, which permits
application of a wide-range of modern image reconstruction algorithms that can
mitigate the effects of data incompleteness and noise. The forward and
backprojection operators are based on the k-space pseudospectral method for
computing numerical solutions to the PA wave equation in the time domain. The
developed reconstruction methodology is investigated by use of both
computer-simulated and experimental PACT measurement data.
