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Improved proton computed tomography by dual modality image reconstruction
Author(s) -
Hansen David C.,
Petersen Jørgen Breede Baltzer,
Bassler Niels,
Sørensen Thomas Sangild
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4864239
Subject(s) - imaging phantom , proton therapy , iterative reconstruction , physics , particle therapy , cone beam computed tomography , scanner , image resolution , monte carlo method , helical scan , tomography , image guided radiation therapy , nuclear medicine , optics , medical imaging , beam (structure) , mathematics , computed tomography , computer science , artificial intelligence , radiology , statistics , medicine , magnetic tape , tape recorder , acoustics
Purpose: Proton computed tomography (CT) is a promising image modality for improving the stopping power estimates and dose calculations for particle therapy. However, the finite range of about 33 cm of water of most commercial proton therapy systems limits the sites that can be scanned from a full 360° rotation. In this paper the authors propose a method to overcome the problem using a dual modality reconstruction (DMR) combining the proton data with a cone‐beam x‐ray prior.Methods: A Catphan 600 phantom was scanned using a cone beam x‐ray CT scanner. A digital replica of the phantom was created in the Monte Carlo code Geant4 and a 360° proton CT scan was simulated, storing the entrance and exit position and momentum vector of every proton. Proton CT images were reconstructed using a varying number of angles from the scan. The proton CT images were reconstructed using a constrained nonlinear conjugate gradient algorithm, minimizing total variation and the x‐ray CT prior while remaining consistent with the proton projection data. The proton histories were reconstructed along curved cubic‐spline paths.Results: The spatial resolution of the cone beam CT prior was retained for the fully sampled case and the 90° interval case, with the MTF = 0.5 (modulation transfer function) ranging from 5.22 to 5.65 linepairs/cm. In the 45° interval case, the MTF = 0.5 dropped to 3.91 linepairs/cm For the fully sampled DMR, the maximal root mean square (RMS) error was 0.006 in units of relative stopping power. For the limited angle cases the maximal RMS error was 0.18, an almost five‐fold improvement over the cone beam CT estimate.Conclusions: Dual modality reconstruction yields the high spatial resolution of cone beam x‐ray CT while maintaining the improved stopping power estimation of proton CT. In the case of limited angles, the use of prior image proton CT greatly improves the resolution and stopping power estimate, but does not fully achieve the quality of a 360° proton CT scan.