  Low-dose Proton Computed Tomography (pCT) is an evolving imaging modality
that is used in proton therapy planning which addresses the range uncertainty
problem. The goal of pCT is generating a 3D map of Relative Stopping Power
(RSP) measurements with high accuracy within clinically required time frames.
Generating accurate RSP values within the shortest amount of time is considered
a key goal when developing a pCT software. The existing pCT softwares have
successfully met this time frame and even succeeded this time goal, but
requiring clusters with hundreds of processors.
  This paper describes a novel reconstruction technique using two Graphics
Processing Unit (GPU) cores, such as is available on a single Nvidia P100. The
proposed reconstruction technique is tested on both simulated and experimental
datasets and on two different systems namely Nvidia K40 and P100 GPUs from IBM
and Cray. The experimental results demonstrate that our proposed reconstruction
method meets both the timing and accuracy with the benefit of having reasonable
cost, and efficient use of power.
