Sciences & Société
Soutenance de thèse : Feriel KHELLAF
List-mode proton CT reconstruction
Doctorante : Feriel KHELLAF
Laboratoire INSA : CREATIS
Ecole doctorale : ED160 Electronique, Electrotechnique, Automatique
Proton therapy is used for cancer treatment to achieve better dose conformity. Proton therapy treatment planning systems require knowledge of the stopping power map of the patient to compute the dose. In clinical practice, this map is generated through a conversion from X-ray CT Hounsfield units to proton stopping power relative to water (RSP). This calibration generates uncertainties as photon and proton physics are different, which leads to the use of safety margins. In order to reduce uncertainties, proton CT (pCT) was proposed as a planning imaging modality to directly reconstruct the RSP. Protons undergo multiple Coulomb scattering (MCS) inducing non-linear paths, thus making the pCT reconstruction problem different from that of X-ray CT. The objective of this thesis is to improve image quality of pCT list-mode reconstruction. The use of a most likely path (MLP) formalism for protons to account for the effects of MCS has improved the spatial resolution in pCT. The first contribution of this thesis is a study on proton paths in heteregeneous media: the accuracy of the MLP was evaluated against a Monte Carlo generated path in different heterogeneous configurations. Results in terms of spatial, angular, and energy distributions were analyzed to assess the impact on reconstruction. The second contribution is a 2D directional ramp filter used for pCT data reconstruction. An intermediate between a filtered backprojection and a backproject-filter approach was proposed, based on the extension of the ramp filter to two dimensions. An expression for a band-limited 2D version of the ramp filter was derived and tested on simulated pCT data. Finally, the last contribution is a comparison of direct reconstruction algorithms in terms of spatial resolution and RSP accuracy. Five algorithms were tested to reconstruct different simulated phantoms. Results were compared between reconstruction from data acquired using idealized or realistic trackers.
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