Stephen Peggs

Toward proton computed tomography

Proton therapy, long regarded as a superior method of radiation therapy, is now becoming more cost effective and is being used in a number of clinical centers around the world. In light of this development the use of the proton beam itself should be considered for the most accurate method of treatment planning. X-ray computed tomography (XCT), which is widely available, has been used for the treatment planning for proton therapy. The basic interactions of XCT in matter are fundamentally different than those of the protons. Thus, the resulting density map from XCT is only an approximation of the true density map for proton therapy. Progress in proton computed tomography (pCT) is presented in this work. The experimental requirements for pCT are examined, and data analysis and Monte Carlo simulations are used to estimate the feasibility of pCT as an imaging modality.

Coupling and Decoupling in Storage Rings

The accidental or intentional introduction of solenoids and skew quadrupoles into a storage ring couples the transverse motion unless the lattice is trimmed. Coupling can cause, for example, irreversible emittance blowup at injection (protons), resonance excitation, flat beam blowup in collision (electrons), and can destructively modify the beam-beam equations of motion. Apart from these effects themselves not many diagnostics are available to guide compensation attempts. This paper first develops a matrix description of coupling, then analyzes the compensation of a lattice insertion, and goes on to show how random errors can be handled close to a coupling resonance. The global decoupling of random errors in the SPS, achieved by the observation of eigenfrequencies on a spectrum analyzer, is described. Rough estimates show that global compensation usually makes further (local) compensation unnecessary.

Design of a proton computed tomography system for applications in proton radiation therapy

Proton computed tomography (pCT) has the potential to improve the accuracy of dose calculations for proton treatment planning, and will also be useful for pretreatment verification of patient positioning relative to the proton beam. A design study was performed to define the optimal approach to a pCT system based on specifications for applications in proton therapy. Conceptual and detailed design of a pCT system is presented; the system consists of a silicon-based particle tracking system and a crystal calorimeter to measure energy loss of individual protons. We discuss the formation of pCT images based on the reconstruction of volume electron density maps and the suitability of analytic and statistical algorithms for image reconstruction.

Reconstruction for proton computed tomography: a Monte Carlo study

Advantages of proton computed tomography (pCT) have been recognized in the past. However, the quality of a pCT image may be limited due to the stochastic nature of the proton path inside the object. In this work, we report a preliminary study on reconstruction of pCT image with improved path estimation. A set of Monte Carlo simulations was carried out with the GEANT4 program, and reconstructed by filtered backprojection method. Simulations with different density contrast settings were compared, and spatial resolution around 0.5 mm for the highest contrast phantom was achieved, which is comparable to that of X-ray CT image. Further improvement by utilizing the statistical properties of proton transport is expected and is under progress.

Reconstruction for proton computed tomography: a practical approach

Proton computed tomography (CT) has important implications for both image-guided diagnosis and radiation therapy. For diagnosis, the fact that the patient dose committed by proton CT compares favorably with that delivered by traditional X-ray CT, for the same density resolution and contrast, may be exploited in dose-critical clinical settings. Proton CT is also the most appropriate imaging method to perform planning and verification of proton-based radiation treatment, since proton stopping power maps constructed by table-based transformation of X-ray CT images only render approximate stopping power estimates. In proton CT, sharp features become blurred by the phenomenon of multiple Coulomb scattering (MCS), resulting in a resolution of around 3 to 5 mm. Studies showed that the spatial resolution of proton radiography and CT can be improved to about 1-2 mm by tracking individual protons in coincidence as they enter and exit the imaged object. This paper describes a new practical implementation of this approach. We first bin the captured protons into slots of similar tracks. Optionally, proton energy statistics can be collected within each bin to obtain further parameters for tissue characterization. The envelope of path uncertainty due to MCS can be be modeled as a banana-shaped curve. The 3D reconstruction, using either filter-backprojection or iterative algorithms, can be performed rapidly on graphics hardware, using a slice blurring technique to model the MCS uncertainty curve.