Compact Proton Beam Energy Modulator

A proton beam imaging system includes: a proton beam generator to generate a proton beam; a proton beam modulator through which the proton beam passes positioned between the proton beam generator and an image target; and a proton beam detector positioned to detect the proton beam existing the image target; wherein the proton beam modulator comprises: a rotating wheel having an axis of rotation positioned so that the proton beam passes through the axis of rotation and the axis of rotation is perpendicular to the proton beam; a first modulating portion comprising a first material portion and a second material portion through which a proton beam passes; and a second modulating portion comprising a third material portion and a fourth material portion through with the proton beam passes; wherein the first and second wedges are positioned opposite each other on the rotating wheel.