  Proton computed tomography (pCT) is a novel medical imaging modality for
mapping the distribution of proton relative stopping power (RSP) in medical
objects of interest. Compared to conventional X-ray computed tomography, where
range uncertainty margins are around 3.5%, pCT has the potential to provide
more accurate measurements to within 1%. This improved efficiency will be
beneficial to proton-therapy planning and pre-treatment verification. A
prototype pCT imaging device has recently been developed capable of rapidly
acquiring low-dose proton radiographs of head-sized objects. We have also
developed an advanced, fast image reconstruction software based on distributed
computing that utilizes parallel processors and graphical processing units. The
combination of fast data acquisition and fast image reconstruction will enable
the availability of RSP images within minutes for use in clinical settings. The
performance of our image reconstruction software has been evaluated using data
collected by the prototype pCT scanner from several phantoms.
