  Photoacoustic computed tomography (PACT), also known as optoacoustic
tomography, is an emerging imaging technique that holds great promise for
biomedical imaging. PACT is a hybrid imaging method that can exploit the strong
endogenous contrast of optical methods along with the high spatial resolution
of ultrasound methods. In its canonical form that is addressed in this article,
PACT seeks to estimate the photoacoustically-induced initial pressure
distribution within the object. Image reconstruction methods are employed to
solve the acoustic inverse problem associated with the image formation process.
When an idealized imaging scenario is considered, analytic solutions to the
PACT inverse problem are available; however, in practice, numerous challenges
exist that are more readily addressed within an optimization-based, or
iterative, image reconstruction framework. In this article, the PACT image
reconstruction problem is reviewed within the context of modern
optimization-based image reconstruction methodologies. Imaging models that
relate the measured photoacoustic wavefields to the sought-after object
function are described in their continuous and discrete forms. The basic
principles of optimization-based image reconstruction from discrete PACT
measurement data are presented, which includes a review of methods for modeling
the PACT measurement system response and other important physical factors.
Non-conventional formulations of the PACT image reconstruction problem, in
which acoustic parameters of the medium are concurrently estimated along with
the PACT image, are also introduced and reviewed.
