Acquisition of proton computed tomography images

1 . A method of proton computed tomography, comprising:

providing a proton computed tomography (CT) scanner;

configured to deliver a scanning proton pencil beam and to rotate relative to an object;

for a proton pencil beam of a first known entrance energy, scanning the pencil beam over the object at a plurality of gantry angles and, for each pencil-beam position of a scan pattern at each gantry angle, measuring on a fluorescent scintillator screen located in air downstream of the object a one-dimensional intensity profile of an exit proton beam by acquiring, with an optical camera or an electronic portal imaging device of a type used for quality assurance of proton therapy beams, an image of the scintillator screen and determining a beam-width parameter σ_exit from a Gaussian fit to the one-dimensional intensity profile, wherein σ_exit is computed from a full width at half maximum (FWHM) of the fit;

using a Monte Carlo model of proton transport to generate, for the first entrance energy, a correlation between σ_exit and residual beam energy for proton beams traversing objects of different thickness and composition and, for each respective pencil-beam path, determining a total energy loss along the path by a applying the correlation to the measured σ_exit for that path;

reconstructing, from the total-energy-loss values for the pencil-beam paths, a first three-dimensional map of relative stopping power of the object corresponding to the first entrance energy;

repeating the scanning, measuring, determining and reconstructing steps for at least a second entrance proton energy different from the first entrance energy to obtain a second three-dimensional map of relative stopping power of the object; and

comparing the first and second three-dimensional maps of relative stopping power to determine, for voxels of the object, information about material composition based on a dependence of σ_exit on entrance energy;

wherein the method determines the total energy loss without measuring a Bragg-peak depth using an ion-chamber array.

2 . The method of claim 1 , wherein, for a constant entrance energy, σ_exit depends on the composition and thickness of the material traversed along a beam path.

3 . The method of claim 2 , further comprising:

performing the measuring of the one-dimensional intensity profile recited in claim 1 at a plurality of entrance energies by conducting separate scans for each pencil-beam path, and, for each beam path, using the variation of σ_exit with entrance energy to obtain additional information about the material along the path.

4 . A method comprising:

providing a proton computed tomography (CT) scanner;

configured to deliver a scanning proton pencil beam and to rotate relative to an object;

for a proton pencil beam of a first known entrance energy entering the object, inserting a fluorescent scintillator screen in air downstream of the object and acquiring a beam profile of an exit proton beam by recording, with an optical camera or an electronic portal imaging device of a type used for quality assurance of proton therapy beams, an image of the fluorescent scintillator screen, and determining a sigma (σ_exit) for the exit beam by determining a full width at half maximum (FWHM) of a Gaussian distribution of a one-dimensional intensity profile of the beam recorded on the image;

using a Monte Carlo model of proton transport to generate, for the first entrance energy, a correlation between σ_exit and exiting energy of the beam for proton beams traversing objects of different thickness and composition;

scanning the pencil beam over the object in a plane and over 360 degrees around the object, using the fluorescent scintillator screen and the optical camera or electronic portal imaging device to measure σ_exit of each pencil beam at the exit, and

determining a total energy loss for each beam path from the correlation;

repeating the scanning, measuring, and determining steps for at least a second entrance energy different from the first entrance energy so as to obtain, for each beam path, σ_exit values as a function of entrance energy; and

using differences in a dependence of σ_exit on entrance energy to determine, for voxels along the beam paths, information about material composition including atomic number and mass number;

wherein determining the total energy loss is performed without measuring a Bragg-peak depth using an ion-chamber array.

5 . The method of claim 4 further comprising:

determining a relative stopping power in each voxel along the beam path.

6 . The method of claim 4 , further comprising:

reconstructing, from the total energy losses determined for the pencil-beam paths, a three-dimensional map of relative stopping power of the object to obtain a relative stopping power value for each voxel.