D. C. Williams

The most likely path of an energetic charged particle through a uniform medium.

Presented is a calculation of the most likely path for a charged particle traversing a uniform medium and suffering multiple-Coulomb scattering when the entrance and exit positions and angles are known. The effects of ionization energy loss are included and the results are verified using Monte Carlo simulation. The application to proton computed tomography is discussed.

Reconstruction for proton computed tomography by tracing proton trajectories: A Monte Carlo study

Proton computed tomography (pCT) has been explored in the past decades because of its unique imaging characteristics, low radiation dose, and its possible use for treatment planning and on-line target localization in proton therapy. However, reconstruction of pCT images is challenging because the proton path within the object to be imaged is statistically affected by multiple Coulomb scattering. In this paper, we employ GEANT4-based Monte Carlo simulations of the two-dimensional pCT reconstruction of an elliptical phantom to investigate the possible use of the algebraic reconstruction technique (ART) with three different path-estimation methods for pCT reconstruction. The first method assumes a straight-line path (SLP) connecting the proton entry and exit positions, the second method adapts the most-likely path (MLP) theoretically determined for a uniform medium, and the third method employs a cubic spline path (CSP). The ART reconstructions showed progressive improvement of spatial resolution when going from the SLP [2 line pairs (lp) cm(-1)] to the curved CSP and MLP path estimates (5 lp cm(-1)). The MLP-based ART algorithm had the fastest convergence and smallest residual error of all three estimates. This work demonstrates the advantage of tracking curved proton paths in conjunction with the ART algorithm and curved path estimates.

Density resolution of proton computed tomography

Conformal proton radiation therapy requires accurate prediction of the Bragg peak position. Protons may be more suitable than conventional x-rays for this task since the relative electron density distribution can be measured directly with proton computed tomography (CT). However, proton CT has its own limitations, which need to be carefully studied before this technique can be introduced into routine clinical practice. In this work, we have used analytical relationships as well as the Monte Carlo simulation tool GEANT4 to study the principal resolution limits of proton CT. The noise level observed in proton CT images of a cylindrical water phantom with embedded tissue-equivalent density inhomogeneities, which were generated based on GEANT4 simulations, compared well with predictions based on Tschalars theory of energy loss straggling. The relationship between phantom thickness, initial energy, and the relative electron density resolution was systematically investigated to estimate the proton dose needed to obtain a given density resolution. We show that a reasonable density resolution can be achieved with a relatively small dose, which is comparable to or even lower than that of x-ray CT.

Issues in Proton Computed Tomography

We report progress on a feasibility study of Proton Computed Tomography and Proton Transmission Radiography for applications in treatment planning and patient positioning for proton radiation therapy. We analyzed data from proton transmission studies through a hollow aluminum cylinder taken with a telescope of silicon detectors with very high spatial and good energy resolution. In addition, we explored the usefulness of applying a cut on the angular divergence of the transmitted beam in a GEANT4 simulation study.

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.

Prototype tracking studies for proton CT

As part of a program to investigate the feasibility of proton computed tomography, the most likely path (MLP) of protons inside an absorber was measured in a beam experiment using a silicon strip detector set-up with high position and angular resolution. The locations of 200 MeV protons were measured at three different absorber depth of PolyMethylMethAcrylate-PMMA (3.75, 6.25 and 12.5 cm) and binned in terms of the displacement and the exit angle measured behind the absorber. The observed position distributions were compared to theoretical predictions showing that the location of the protons can be predicted with an accuracy of better than 0.5 mm

Construction and initial operation of a proportional wire detector for use in a Cerenkov ring imaging system

The final version of the multiwire single-electron detector for the Cerenkov ring imaging device of the Stanford Linear Collider Detector is described. Recent research-and-development efforts to define the design parameters are reported. These were concerned with computer simulations necessary for the detector design; wire aging and its solution; surface resistivity and voltage breakdown of G-10 in a TMAE environment; corona studies for various gases; wire breaking in the spark; measured mechanical characteristics of 7- mu m carbon wires; wire-stretching technique for 7- mu m carbon wires; and wire tension measuring for th final detector. The details of the geometry of the detector and experimental tests with the detector itself are discussed. >

Signal-to-noise in silicon microstrip detectors with binary readout

We report the results of a beam test at KEK using double-sided AC-coupled silicon microstrip detectors with binary readout, i.e., a readout where the signals are discriminated in the front-end electronics and only the hit location as kept. For strip pitch between 50/spl mu/ and 200/spl mu/, we determine the efficiency and the noise background as function of threshold setting. This allows us to reconstruct the Landau pulse height spectrum and determine the signal/noise ratio. In addition, the threshold/noise ratio necessary for operation with low occupancy is determined. >

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.

Development of a low noise preamplifier for the detection and position determination of single electrons in a Cerenkov ring imaging detector by charge division

A preamplifier using a low-noise dual-gate MOSFET front end has been designed, built, and tested. It performs well, having a noise level of about 500 electrons RMS at a shaping time of 65 ns. It is linear over its entire range to better than 1%. It has been used to detect single photoelectrons in a Cerenkov ring imaging detector. A single channel contains preamp, RC-CR shaper, gain adjustment, driver, and calibration circuitry. The circuit is described in detail, and results of noise and linearity measurements are presented. >