Carbon Nanotube Based Stationary X-ray Tomosynthesis Scanner for Detection of Breast Cancer

In this dissertation, my Ph.D. research during the past six years will be presented. My research mainly focused on design, fabrication and characterization of devices based on carbon nanotubes (CNT). Since the discovery of CNT in 1991, the research focus has gradually shifted from material synthesis and characterization to devices and applications. During early stages of my Ph.D. study, I worked on synthesizing CNT using laser ablation method, and fabricating CNT-AFM tips and magnetic wire MFM tips. During the second half of my Ph.D. study, my research focused more on design, fabrication, and testing the xray sources and imaging systems based on CNT, including both single-beam system (microCT) and multi-beam system (tomosysthesis). During the last two years, I worked on the project to develop a novel imaging system (Argus) using CNT x-ray source array for breast cancer detection. This is the first stationary digital breast tomosynthesis (DBT) imaging system in the world. My research result has shown great clinical potentials of these imaging systems using x-ray source based on CNTs. This dissertation is composed by five chapters. In chapter one, the physics behind xray and field emission theory will be reviewed. Chapter two covers discovery, property and synthesis of CNT. Methods to fabricate CNT emitters and their emission property will also be introduced there. Single and multi beam x-ray sources and applications will be discussed in chapter three. The design and characteristic test results are presented. The application includes micro-CT, tomosynthesis and multiplexing. Chapter four is dedicated to the Argus system, the first stationary digital breast tomosynthesis imaging system. At last, conclusion is given in chapter five

Characterization of a prototype for cone beam breast computed tomography

The project aims to experiment the Cone Beam Breast Computed Tomography technique using a standard digital mammography system. The work is focused on the definition of a protocol of quality measurements for the pre-clinical evaluation of the machine. The paper is developed in two parts. The first is specifically concerned with the methods used to define the image quality and dosimetry aspects specific for digital mammography devices. A complete characterization of the system has been performed according to the applicable IEC standards to assure the performances of the equipment and define the quality levels. Due to the lack of a quality control protocol dedicated to CBBCT mammography scanner, a new equivalent test procedure has been proposed. The second part of the paper is focused on the evaluation, through quantitative and visual analyzes, of the CBCT exam feasibility in the hardware and software conditions currently proposed by IMS Giotto. The prototype was in fact developed differing from the technical choices of competing companies and developed for a different intended use. The main difference with respect to the existing breast CT scanners is the possibility of performing on the same system the CBBCT scanning but also all the mammographic techniques. In this thesis, we aim to assess whether, in the current setup, considering a dosimetric range very close to that used in the clinic, the tests produce results that can be considered acceptable or at least indicative of the feasibility of the entire project from a commercial point of view. For this purpose, the final reconstruction images, obtained by two previously developed software, are analyzed

Optimising the benefits of spectral x-ray imaging in material decomposition

The extra energy information provided by spectral x-ray imaging using novel photon counting x-ray detectors may allow for improved decomposition of materials compared to conventional and dual-energy imaging. The information content of spectral x-ray images, however, depends on how the photons are grouped together. This thesis deals with the theoretical aspect of optimising material discrimination in spectral x-ray imaging. A novel theoretical model was developed to map the confidence region of material thicknesses to determine the uncertainties in thickness quantification. Given the thickness uncertainties, photon counts per pixel can be optimised for material quantification in the most dose efficient manner. Minimisation of the uncertainties enables the optimisation of energy bins for material discrimination. Using Monte Carlo simulations based on the BEAMnrc package, material decomposition of up to 3 materials was performed on projection images, which led to the validation of the theoretical model. With the inclusion of scattered radiation, the theoretical optima of bin border energies were accurate to within 2 keV. For the simulated photon counts, excellent agreement was achieved between the theoretical and the BEAMnrc models regarding the signal-to-noise ratio in a decomposed image, particularly for the decomposition of two materials. Finally, this thesis examined the implementation of the Medipix detector. The equalisation of pixel sensitivity variations and the processing of photon counting projection images were studied. Measurements using the Medipix detector demonstrated promising results in the charge summing and the spectroscopic modes of acquisition, even though the spectroscopic performance of the detector was relatively limited due to electronic issues known to degrade the equalisation process. To conclude, the theoretical model is sufficient in providing guidelines for scanning parameters in spectral x-ray imaging and may be applied on spectral projection measurements using e.g. the redesigned MedipixRX detector with improved spectroscopic performance, when it becomes available

Innovative detection methods for radiation hardness

The thesis deals with new methods for the characterization of ion beams and detection of radiation used in radiation hardness applications, namely charged particles, X- and gamma-radiation and neutrons. As far as the detection of charged particles, X- and gamma- rays the radiochromic films, dosimeters intensively employed in medical physics, were found suitable for these purposes. The calibration of radiochromic films was carried out with the law that describe the physical phenomenon of the film darkening. On this line the independence of the response of a kind of film to incident radiation type, energy and dose rate was demonstrated. These results were crucial for the full dosimetry characterization of a 90-Sr/90-Y beta source, recently proposed as irradiation source for Total Ionizing Dose tests as alternative to the well-established 60-Co source. Furthermore, since standard methods of reading of radiochromic films do not allow real-time dosimetry, the design, development and related tests of a new opto-electronic-based real-time radiochromic film reader is presented in this thesis. Owing to the wide employment of radiochromic films in the applications and to the potential diffusion on the market, a National Patent was filed in January 2018 through the INFN Tech-Transfer. The problem of neutron detection and production has been addressed at two charged particle accelerators. In particular, for the first time a neutron beam line was implemented at the IBA 18/18 medical cyclotron of University of Bern and the test of a new prototype of polysiloxane-based scintillator was carried out at the tandem accelerator of Laboratori Nazionali del Sud (LNS) in Catania. All these topics are discussed in this thesis and in dedicated publications on international scientific journals

Novel Semiconducting Materials and Thin Film Technologies for High Energy Radiation Detection

Nowadays the development of real-time ionizing radiation detection system operating over large areas is crucial. Increasing quest for flexible, portable, low cost and low power consumption sensors pushed the scientific community to look for alternative materials and technologies able to fulfill these new requirements. In this thesis the potentiality of organic semiconductors and metal oxides as material platforms for novel ionizing radiation sensors is demonstrated. In particular, organic semiconductors are human tissue-equivalent and this represents a unique and desirable property for the development of dosimeters to be employed in the medical field. The ionizing radiation sensors described in this thesis have been designed, fabricated and characterized during my PhD research and are realized onto polymeric foils leading to flexible devices operating at low voltages, in ambient condition and able to directly detect X-rays, gamma-rays and protons. Following the study of the properties and of the mechanisms of interaction between the radiation and the active layers of the sensors, several strategies have been adopted to enhance the efficiency of these detectors. X-rays dosimeters based on organic semiconductors have been realized presenting record sensitivity values compared with the state of the art for large area radiation detection. The unprecedentedly reported performance led to the possibility to testing these devices in actual medical environments. Moreover, the proof-of-principle demonstration of a dosimetric detection of proton beams by organic-based sensors is reported. Finally, a new sensing platform based on metal oxides is introduced. Combining the advantages of amorphous high mobility oxide semiconductors with a multilayer dielectric, novel devices have been designed, capable of providing a sensitivity one order of magnitude higher than the one shown by the standard RADFETs. Thanks to their unique properties, these sensors have been integrated with a wireless readout system based on a commercial RFID tag and its assessment is presented

Development of a single photon counting computed tomography system using MPGDs

Desenvolvimento de um sistema de tomografia computorizada de contagem de fotão único usando MPGDsThe development of computed tomography systems with energy resolving detectors is a current challenge in medical physics and biomedical engineering. A computed tomography system of this kind allows getting complementary informations relatively to conventional systems, that can help the medical diagnosis, being of great interest in medicine. The work described in this thesis is related to the development of a computed tomography system using micropattern gaseous detectors, which allow storing, simultaneously, information about the interaction position and the energy of each single photon that interacts with the detector. This kind of detectors has other advantages concerning the cost and characteristics of operation when compared with solid state detectors. Tomographic acquisitions were performed using a MicroHole & Strip Plate based detector, which allowed reconstructing cross-sectional images using energy windows, applying the energy weighting technique and performing multi-slice and tri-dimensional reconstructions. The contrast-to-noise ratio was improved by 31% by applying the energy weighting technique, comparing with the corresponding image obtained with the current medical systems. A prototype of a computed tomography with flexibility to change the detector was developed, making it possible to apply different detectors based on Thick-COBRA. Several images acquired with these detectors are presented and demonstrate their applicability in X-ray imaging. When operating in NeCH4, the detector allowed a charge gain of 8 104, an energy resolution of 20% (full width at half maximum at 8 keV), a count rate of 1 106 Hz/mm2, a very stable operation (gain fluctuations below 5%) and a spacial resolution of 1.2 mm for an energy photon of 3.6 keV. Operating the detector in pure Kr allowed increasing the detection efficiency and achieving a charge gain of 2 104, an energy resolution of 32% (full width at half maximum at 22 keV), a count rate of 1 105 Hz/mm2, very stable operation and a spatial resolution of 500 m. The software already existing in the group was improved and tools to correct geometric misalignments of the system were also developed. The reconstructions obtained after geometrical correction are free of artefacts due to the referred misalignments.O desenvolvimento de sistemas de tomografia computorizada que incorporem detetores com resolução em energia é um desafio atual em física médica e engenharia biomédica. Um sistema de tomografia computorizada espetral permite obter informações complementares comparativamente a um sistema convencional, que podem auxiliar no diagnóstico médico, sendo por isso de grande interesse em medicina. O trabalho exposto nesta tese prende-se com o desenvolvimento de um sistema de tomografia usando detetores gasosos microestruturados que permitem, simultaneamente, ter informação da posição de interacção e da energia de cada fotão que interage com o detetor. Este tipo de detetores possui ainda outras vantagens relativamente a custo ou características de funcionamento quando comparados com detetores de estado sólido. Foram realizadas aquisições tomográficas usando um detetor baseado numa MicroHole & Srip Plate que permitiu reconstruir imagens utilizando diferentes gamas de energia, aplicar técnicas de ponderação em energia e fazer pela primeira vez reconstrução multi-corte e obter imagens tri-dimensionais. Aplicando a técnica de ponderação em energia foi possível melhorar a relação contraste-ruído em 31% comparativamente à imagem correspondente aquela obtida nos actuais sistemas médicos. Posteriormente, foi desenvolvido um protótipo de um sistema de tomografia computorizada com flexibilidade para alterar o detetor, tornando possível utilizar vários detetores baseados na microestrutura Thick-COBRA. São apresentadas várias imagens adquiridas com estes detetores que evidenciam a sua aplicabilidade em imagiologia por raio-X. A operar no meio gasoso NeCH4 o detetor permitiu um ganho de 8 104, uma resolução em energia de 20% (largura a meia altura a 8 keV), uma taxa de contagem de 1 106 Hz/mm2, um funcionamento muito estável (variações de ganho inferiores a 5%) e uma resolução espacial de 1.2 mm para fotões de 3.6 keV. A operar em Kr puro foi possível aumentar a eficiência de deteção e alcançar um ganho de 2 104, uma resolução em energia de 32% (largura a meia altura a 22 keV), uma taxa de contagem de 1 105 Hz/mm2, um funcionamento também bastante estável e uma resolução espacial de 500 m. O software já existente no grupo para reconstrução de imagem foi melhorado e foram ainda desenvolvidas ferramentas para correcção de desalinhamentos geométricos do sistema. As reconstruções obtidas após correção geométrica surgem livres de artefactos originados pelos referidos desalinhamentos

Devices and techniques for the characterization of inverse Compton sources

Innovative intense monochromatic x/ -ray sources are of great interest in the scientific community. A large number of applications in basic and applied physics research, as well as in different science fields, require an intense, monochromatic or quasi-monochromatic, tunable radiation source. Synchrotron radiation is optimal for low energy applications (< 100 keV) but the size and cost of synchrotron facilities prevent a large-scale spread of this kind of source, that is fundamental for applications such as routine clinical diagnostic. Moreover, synchrotron light is not suitable in the case of high energy applications (> 1 MeV), needed primarily for nuclear physics experiments, due to limitations on the maximum energy obtainable for monochromatic beams with synchrotron light. Alternative sources that can overcome such limitations are those based on inverse Compton interaction, which permit to obtain compact and cost-effective sources for low energy applications and can provide monochromatic collimated beam in the high energy range. Inverse Compton is the process in which a photon interacts with a relativistic electron, in this case the electron can transfer a fraction of its energy in the collision, resulting in a backscattered photon with an increased energy. This process can be used to produce hard x/ -rays by the backscattering of low-energy laser photons by a relativistic electron beam. A radiation source based on this interaction is usually called an inverse Compton source, alternatively, it can be called Thomson source when the energies involved allow a classical description of the process, as in the case of Thomson scattering. The work described in this dissertation concerns the devices and techniques developed to perform a characterization of inverse Compton sources. In particular, the work is focused on two major projects: BEATS2 experiment and ELI-NP-GBS proposal of E-Gammas collaboration. BEATS2 is an experiment funded by Istituto Nazionale di Fisica Nucleare (INFN) aimed to study medical applications, specially to mammographic imaging, of the SL-Thomson source of SPARC-LAB at the INFN-LNF that will be commissioned in the first half of 2013. E-Gammas is an international collaboration composed by several Universities and Institutions including: INFN and Universit`a di Roma La Sapienza, in Italy, Universitè de Paris Sud and IN2P3/CNRS, in France, and ASTeC of STFC, in UK. The collaboration is aimed to the preparation of a Technical Design Report for the ELI-NP Gamma Beam System (ELI-NP-GBS) to be commissioned by the end of 2016. This Gamma Beam System will be a high energy inverse Compton source, included in the Extreme Light Infrastructure - Nuclear physics (ELI-NP), an European project dedicated to the development of laser beams and the generation of high intensity gamma beams for frontier research in nuclear physics

Risk of radiation-induced cancer from screening mammography

Background and Objectives: When the benefits and risks of mammography are considered, the risk of radiation-induced cancer is calculated only for the breast using the mean glandular dose (MGD). Whilst MGD is a useful concept, it has many limitations. This thesis aims to establish a novel method to determine and convey radiation risk from full field digital mammography (FFDM) screening using lifetime effective risk. Method: For effective risk calculations, organ doses as well as examined breast MGD are required. Screening mammography was simulated by exposing a breast phantom for cranio-caudal and medio-lateral oblique for each breast using 16 FFDM machines. An anthropomorphic dosimetry phantom loaded with thermo-luminescent detectors (TLDs) was positioned in contact with the breast phantom to simulate the client’s body. Once the risk per individual was calculated, total effective lifetime risk across 48 worldwide screening programmes was calculated. The total effective risk data sets were analysed to establish a regression model to predict the effective risk of any screening programme. Graphs were generated to extrapolate the total effective risk of any screening programme of specific screening commencement age and frequency considering the MGD differences of different FFDM machines. Since the highest radiation dose after examined breast was received by contralateral breast, the effect of a contralateral breast lead shield on effective risk was also investigated. Results: Large differences in the effective lifetime risk exist between worldwide screening programmes. The effective lifetime risk varied from approximately 50 cases/106 to more than 1000 cases/106. These differences were mainly attributed to the commencement age and frequency of screening. Since tissue radio-sensitivity reduces with age, the cessation age of screening mammography does not result in a noteworthy effect on the total effective risk. The use of contralateral breast shield reduces the total effective risk by about 1.5% for most worldwide screening programmes.Conclusion: A novel method has been proposed to assess radiation-induced cancer risk from FFDM screening which considers the radiation dose received by all body tissues in addition to the examined breast. Using effective risk, the data is more likely to be understandable by screening clients and referring clinicians, unlike MGD which is not readily available or understandable by the general populace. This novel method and the data are compatible with the incoming European Commission legislation about giving the patient information on radiation risk