  The determination of relative stopping power (RSP) via proton computed
tomography (pCT) of a patient is dependent in part on the knowledge of the
incoming proton kinetic energies; the uncertainty in these energies is in turn
determined by the proton source -- typically a cyclotron. Here we show that
reducing the incident proton beam energy spread may significantly improve RSP
determination in pCT. We demonstrate that the reduction of beam energy spread
from the typical 1.0% (at 70MeV) down to 0.2%, can be achieved at the proton
currents needed for imaging at the Paul Scherrer Institut 230MeV cyclotron.
Through a simulated pCT imaging system, we find that this effect results in RSP
resolutions as low as 0.2% for materials such as cortical bone, up to 1% for
lung tissue. Several materials offer further improvement when the beam
(residual) energy is also chosen such that the detection mechanisms used
provide the optimal RSP resolution.
