  Photoacoustic computed tomography (PACT), also known as optoacoustic
tomography, is an emerging imaging modality that has great potential for a wide
range of biomedical imaging applications. In this Note, we derive a hybrid
reconstruction formula that is mathematically exact and operates on a data
function that is expressed in the temporal frequency and spatial domains. This
formula explicitly reveals new insights into how the spatial frequency
components of the sought-after object function are determined by the temporal
frequency components of the data function measured with a circular or spherical
measurement geometry in two- and three-dimensional implementations of PACT,
respectively. The structure of the reconstruction formula is surprisingly
simple compared with existing Fourier-domain reconstruction formulae. It also
yields a straightforward numerical implementation that is robust and two orders
of magnitude more computationally efficient than filtered backprojection
algorithms.
