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An Evaluation of a Prototype Proton Computed Tomography Scanner


Since the 1990s, the number of clinical proton therapy facilities around the world has been growing exponentially. Because of this, and the lack of imaging support for proton therapy in the treatment room, a renewed interest in proton radiography and computed tomography (CT) has emerged. This imaging modality was largely abandoned in the 1970s and '80s in favor of the already successful x-ray CT, for reasons including long acquisition times and inadequate spatial resolution. Protons are particularly useful for radiotherapy because of their well-defined range in matter and their favorable energy profile which facilitates greater conformality than other radiotherapies; however, in order to realize the full potential of proton radiotherapy, the range of protons in the patient must be precisely known.

Presently, proton therapy treatment planning is accomplished by taking x-ray CTs of the patient and converting each voxel into proton relative stopping power with respect to water (RSP) via a stoichiometrically-acquired calibration curve. However, since there is no unique relationship between Hounsfield values and RSP, this procedure has inherent uncertainties of a few percent in the proton range, requiring additional distal uncertainty margins in proton treatment plans. In contrast to x-ray CT, proton CT measures the RSP of an object directly,

eliminating the need for Hounsfield-value-to-RSP conversion.

In the prototype proton CT scanner that we have developed, a low-intensity beam of 200 MeV protons traverses a patient, entirely, and stops in a downstream energy/range detector. The entry and exit vectors of each proton are measured in order to determine a most-likely path of the proton through the object, and the response of the energy/range detector is converted to the water-equivalent path length of each proton in the object. These measurements are made at many angles between 0 and 360 degrees in order to reconstruct a three-dimensional map of proton RSP in the object.

In this dissertation, I present the fully operational prototype proton CT scanner that our collaboration has recently developed. The performance of the proton tracking system, data-acquisition system and the energy/range detector system is discussed in terms of speed, efficiency, and accuracy. In addition, the process by which proton CT images are reconstructed is described and evaluated in terms of the fidelity of the reconstructed RSP values and the spatial resolution. Finally, the present outlook on proton CT is evaluated and suggestions are proposed for the future directions of proton CT in terms of the design of the next generation system and improvements to image reconstruction.

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