  Photoacoustic computed tomography (PACT) is an emerging imaging modality that
exploits optical contrast and ultrasonic detection principles to form images of
the photoacoustically induced initial pressure distribution within tissue. The
PACT reconstruction problem corresponds to an inverse source problem in which
the initial pressure distribution is recovered from measurements of the
radiated wavefield.
  A major challenge in transcranial PACT brain imaging is compensation for
aberrations in the measured data due to the presence of the skull. Ultrasonic
waves undergo absorption, scattering and longitudinal-to-shear wave mode
conversion as they propagate through the skull. To properly account for these
effects, a wave-equation-based inversion method should be employed that can
model the heterogeneous elastic properties of the skull. In this work, a
forward model based on a finite-difference time-domain discretization of the
three-dimensional elastic wave equation is established and a procedure for
computing the corresponding adjoint of the forward operator is presented.
Massively parallel implementations of these operators employing multiple
graphics processing units (GPUs) are also developed. The developed numerical
framework is validated and investigated in computer-simulation and experimental
phantom studies whose designs are motivated by transcranial PACT applications.
