HYPR-spectral photoacoustic CT for preclinical imaging

We have designed and built a prototype PCT (photoacoustic CT) scanner suitable for small animal imaging that acquires a sparse set of 128 photoacoustic, radial "projections" uniformly distributed over the surface of a hemisphere in response to optical absorption from a tunable, pulsed NIR (near-infrared) laser. Acquisition of a denser set of projections is achieved by rotating the hemispherical array about its vertical axis and acquiring additional, interleaved projections. Each detector in the array is a 3-mm diameter, piezo-composite with a center frequency of 5 MHz and 70% bandwidth. Spatial resolution is < 300 mu m and nearly isotropic, owing to the array geometry. Preliminary results acquired at half of the allowable laser power and with no system optimizations show a low contrast sensitivity sufficient to detect a 350 nM concentration of a NIR-absorbing organic dye embedded in 12.5 mm of soft tissue. This scanner design will allow our group to take advantage of HYPR (HighlY constrained backPRojection) reconstruction techniques, which can significantly improve temporal (or spectral) resolution, without sacrificing signal-to-noise or spatial resolution. We will report how these accelerated reconstruction techniques can be implemented with this PCT scanner design. Using this approach, we may be able to achieve 100-ms temporal resolution for dynamic studies throughout a 20-mm-diameter imaging volume.