Hugo R. Schelin

Computerized tomography with high-energy proton beams: tomographic image reconstruction from computer-simulated data

The use of protons instead of X-rays for computerized tomography (CT) studies has potential advantages, especially for medical applications in proton treatment planning. However, the spatial resolution of proton CT is limited by multiple Coulomb scattering (MCS). We used the Monte Carlo simulation tool GEANT4 to study the resolution achievable with different experimental arrangements of a proton CT scanner. The passage of a parallel 200MeV proton beam through a virtual cylindrical aluminum phantom with 50mm external diameter was simulated. In our study, the phantom contained a set of cylindrical holes with diameters ranging from 4mm to 0.5mm. The GEANT4 simulation consisted of a series of 180 projections at 2 degree intervals with 350 proton track histories for each one. The filtered back projection algorithm was used to reconstruct a 2D tomographic image of phantom.

Evaluation of Escherichia coli cells damages induced by ultraviolet and proton beam radiation

Prokaryote cells were exposed to ultra violet (UVc) radiation and to proton beams in order for the induced effects to be studied. Morphological and physiological alterations occurred in Escherichia coli (E. coli) cells exposed to the beams were investigated. The measurements using UVc radiation were made at the Biology Department of CEFET-PR while the measurements using proton beams were made at the Pelletron Accelerator

Evaluation of Fetal Dose in Breast Radiotherapy With Shielding and Wedges

The fetus may be seriously affected by the peripheral dose when a pregnant woman undergoes breast radiotherapy. To quantify this dose, a humanoid phantom was irradiated at the left breast, simulating breast radiotherapy. The phantom, consisting of an adapted manikin (with internal and external materials having densities close to that of water), was irradiated in a 6-MeV X-ray linear accelerator (Clinac 600CD). A shield of blocks and lead slabs was placed around the abdominal region of the phantom. Two types of wedges were used to modulate the beam: a physical wedge and the enhanced dynamic wedge (EDW), both with 30° angulation. Using a cylindrical ionization chamber of 0.6 cm 3 positioned in the phantoms fetal region, at the end of breast treatment, the peripheral doses were 3.90-48.67 cGy with the physical wedge and 1.75-13.78 cGy with the EDW. The doses were 3.5 times greater when the physical wedge was used due to scattering by its attenuator material and the increase in the leakage radiation intensity. The use of a shield around the abdominal region of the phantom and the use of the EDW during simulation of the treatment reduced the peripheral doses to tolerable limits from ICRP and AAPM.

The density measurements in pCT imaging

In existing proton treatment centers, dose calculations are performed based on x-ray computerized tomography (CT). Alternatively, the therapeutic proton beam could be used to collect the data for treatment planning via proton CT (pCT). With the development of medical proton gantries, first at Loma Linda University Medical Center and now in several other proton treatment centers, it is of interest to continue the early pCT investigations of the 1970s and the early 1980s. From that time, the basic idea of the pCT method has advanced from average energy loss measurements to an individual proton tracking technique. This reduces the image degradation due to multiple Coulomb scattering. Thereby, the central pCT problem shifts to the fidelity of the physical information obtained about the scanned patient, which will be used for proton treatment planning. The accuracy of relative electron density distributions extracted from pCT images was investigated in this work using continuous slowing down approximation (CSDA) and water-equivalent-thickness (WET) concepts. Analytical results were checked against Monte Carlo simulations, which were obtained with SRIM2003 and GEANT4 Monte Carlo software packages. The range of applications and the sources of absolute errors are discussed.

Studies of electrical properties of polyaniline irradiated by X-rays

Recent development and studies of physical and chemical properties of the conducting polymers have been calling the interest of many scientists. Among this group of materials, the polyaniline (PANI) deserves special attention because of its high chemical and thermal stability under normal conditions, simple process of polymerization and low cost. This polymer is also known due to its strongly pronounced and well studied electrochemical characteristics. In the present work, the variations of the electrical resistance of polyaniline caused by X-rays radiation have been studied. The polymer has been synthesized by electrochemical method in stainless steel and graphite electrodes. The data concerning the electrical resistance of PANI were measured just after the synthesis. This has been performed in the water solution of sulfuric acid within monomer of aniline where the polyaniline has been submitted to the applied voltages in the range from 200mV to 600mV. Polymeric samples have been irradiated using the X-rays with the energy 50keV and 100keV. After the irradiation, the electrical measurements have been repeated. The whole experimental procedure was performed several times. The obtained results have clearly shown the potential of polyaniline as a dose sensitive material that could be applied as a radiation sensor.

Excitation of discrete levels in 54Fe and 56Fe nuclei by means of (e, e′) reactions

The excitation of discrete levels in 54Fe and 56Fe nuclei by means of (e, e′) reactions is studied at excitation energies of up to 8 MeV over the momentum-transfer range between 0.6 and 1.7 fm−1. An unconventional procedure of multipole analysis is used in experimental-data processing. Data on the reduced probability of transitions and their multipolarity are obtained for 12 low-lying levels of 54Fe and 10 levels of 56Fe. Five levels in 54Fe and three levels in 56Fe are observed for the first time in (e, e′) reactions. There is no information about two of them in the present-day database on discrete levels.

Computed tomography with a low-intensity proton flux: results of a Monte Carlo simulation study

Conformal proton radiation therapy requires accurate prediction of the Bragg peak position. This problem may be solved by using protons rather than conventional x-rays to determine the relative electron density distribution via proton computed tomography (proton 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 GEANT4 simulations were validated by comparing them to predictions of the Bethe Bloch theory and Tschalars theory of energy loss straggling, and were found to be in good agreement. The relationship between phantom thickness, initial energy, and the relative electron density uncertainty was systematically investigated to estimate the number of protons and dose needed to obtain a given density resolution. The predictions of this study were verified by simulating the performance of a hypothetical proton CT scanner when imaging a cylindrical water phantom with embedded density inhomogeneities. We show that a reasonable density resolution can be achieved with a relatively small number of protons, thus providing a possible dose advantage over x-ray CT.

Validation of Geant4 on Proton Transportation for Thick Absorbers: Study Based on Tschalär Experimental Data

Imaging techniques using protons as incident particles are currently being developed to substitute X-ray computer tomography and nuclear magnetic resonance methods in proton therapy. They deal with relatively thick targets, like the human head or trunk, where protons lose a significant part of their energy, however, they have enough energy to exit the target. The physical quantities important in proton imaging are kinetic energy, angle and coordinates of emerging proton from an absorber material. In recent times, many research groups use the Geant4 toolkit to simulate proton imaging devices. Most of the available publications about validation of Geant4 models are for thin or thick absorbers (Bragg Peak studies), that are not consistent with the contour conditions applied to proton imaging. The main objective of this work is to evaluate the kinetic energy spectrum for protons emerging from homogeneous absorbers slabs comparing it to the experimental results published by Tschalär and Maccabee, in 1970. Different models (standard and detailed) available on Geant4 (version 9.6.p03) are explored taking into account its accuracy and computational performance. This paper presents a validation for protons with incident kinetic energies of 19.68 MeV and 49.10 MeV. The validation results from the emerging protons kinetic energy spectra show that: (i) there are differences between the reference data and the data produced by different processes evoked for transportation and (ii) the validation energies are sensitive to sub-shell processes.

Evaluation of Geant4 to describe proton transportation for thick absorbers

Imaging techniques using protons are actively being developed currently to substitute X-ray computer tomography and nuclear magnetic resonance method in proton therapy. They deals with relatively thick targets like the human head or trunk where protons lose the major part of their energy, however they having enough energy to exit the target. The physical quantities important in proton imaging are proton exit energy, angle and coordinates. Nowadays many research groups are using The Geant4 toolkit for the development of proton imaging devices. However the most of available publications about validation of the Geant4 models are or for thin absorbers, or for energy deposition of completely absorbed protons (Bragg Peak), what are not important characteristics in proton imaging. The main objective of this work is to validate different models available on Geant4 (version 9.6.p03) taking into account its accuracy and computational performance from the viewpoint of proton imaging. This paper presents the comparison against experimental data published by Tschalär and Maccabee (for incident kinetic energies for protons of 19.68 MeV and 49.10 MeV). The results show that for the validation energies the spectra of the kinetic energy of the emerging protons: (i) there are differences between the validation and the different processes invoked for transportation and (ii) the validation energies are sensitive to sub-shell processes.

Validation of Geant4 for scattered spectrum from pediatric exams for dosimetry of occupational workers applied to radiological protection

Fluoroscopic barium meal (BM) studies are largely used around the world and very common to be performed in children. The medical and occupational exposures are important parameters that need to be investigated to evaluate the radiological effects. Calibration of the dosimeter used for exposure measurement is usually done with primary X-ray beam. The difference in energy spectra of primary and scattered radiation can influence obtained results. The main objective of this work is to evaluate the X-ray scattered spectra by different pediatric phantoms (simulation of patients subjected to BM procedures) to calculate an energy correction factor to the absorbed energy by the thermoluminescent dosimeters (TLD). To perform this evaluation, the TLDs were positioned over three areas in two occupational workers eyes, thyroid and hands. The Geant4 toolkit was used to define the spectra that reached TLDs and made possible to correct entrance surface air kerma. The present work was developed in two stages: (i) evaluation of scattered spectra by different standard phantoms (newborn, 1 year old, 5 years old and 10 years old); (ii) definition of the energy correction factor to the absorbed energy by each TLD. The results of this work show that Geant4 is a good toolkit to these analysis making possible to generate a correction factor considering a primary spectrum of 60 keV.