The deepest gratitude to my supervisor Professor George Panayiotakis for offering me the opportunity to make this PhD and for his continuous support and guidance during all these years
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Biol. 50 2907-28 Walker D W 1971 Relativistic effects in low energy electron scattering from atoms Adv. Phys. 20 257-323 Yaffe M J 1995 Mammography (Bronjino J D, The Biomedical Engineering Handbook, CRC Press and IEEE Press) 972-89 Yaffe M J and Rowlands J A 1997 X-ray detectors for digital radiography Phys. Med. Biol. 42 1-39 Zentai G, Partain L, Pavlyuchkova R, Proano C, Breen B N, Taieb A, Dagan O, Schieber M, Gilboa H and Thomas J 2004 Mercuric iodide medical imagers for low exposure radiography and fluoroscopy Proc. SPIE 5368 200-10 Zentai G et al 2003 Mercuric iodide and lead iodide x-ray detectors for radiographic and fluoroscopic medical imaging Proc. SPIE 5030 77-91 Zhao B and Zhao W 2005 Temporal performance of amorphous selenium mammography detectors Med. Phys. 32 128-36 Zhao W, Blevis I, Germann S and Rowlands J A 1997 Digital radiology using active matrix readout of amorphous selenium: construction and evaluation of a prototype real-time detector Med. Phys. 24 1834-43 Zhao W, DeCrescenzo G, Kasap S O and Rowlands J A 2005 Ghosting caused by bulk charge trapping in direct conversion flat-panel detectors using amorphous selenium Med. Phys. 32 488-500 Zhao W and Law J 1998 Digital radiology using active matrix readout of amorphous selenium: Detectors with high voltage protection Med. Phys. 25 539-49 Zhao W and Rowlands J A 1997 Digital radiology using active matrix readout of amorphous selenium: theoretical analysis of detective quantum efficiency Med. Phys. 24 1819-33 Zhao W and Rowlands J A 1995 X-ray imaging using amorphous selenium: feasibility of a flat panel self-scanned detector for digital radiology Med. Phys. 22 1595-604 Abstract The quality of the mammographic image is directly related to its characteristics. The x-ray induced primary electrons inside the photoconductor of direct conversion digital flat panel mammographic detectors, comprise the primary signal which propagates in the material and forms the final signal (image). Consequently, the characteristics of the mammographic image strongly depend on the characteristics of the primary electrons. In this PhD thesis an investigation is carried out concerning the primary signal formation processes and the characteristics of primary electrons inside a-Se, a-As2Se3, GaSe, GaAs, Ge, CdTe, CdZnTe, Cd0.8Zn0.2Te, ZnTe, PbO, TlBr, PbI2 and HgI2, which are suitable photoconducting materials for direct detectors. In addition, for the case of a-Se a first study is made concerning the correlation between the characteristics of primary and final signal as well as the electric field distribution and the electron interaction mechanisms, two crucial parameters of a prospective model that would simulate the final signal formation. A Monte Carlo model that simulates the primary electron production inside the photoconductors mentioned, for a number of monoenergetic and polyenergetic x-ray spectra that cover the mammographic energies, has been developed. The model simulates the primary photon interactions (photoelectric absorption, coherent and incoherent scattering), as well as the atomic deexcitations (fluorescent photon production, Auger and Coster-Kronig electron emission). In addition, a mathematical formulation has been developed for the drifting of primary electrons of a-Se in vacuum under the influence of a capacitor’s electric field and the electron characteristics on the collecting electrode are being studied. The formulation is based on the Newton’s equations of motion and the theorem for kinetic energy change. Furthermore, a code has been developed that calculates the distribution of the electric potential inside a-Se, using an existing analytical solution, the boundary values of our case and certain numerical calculation methods. Finally, the structure and the mathematical formulation of a model that would simulate the electron interactions inside a-Se have been developed. An existing model has been reexamined and enriched with certain theoretical considerations and simulation formalisms. It has been found that for all materials and energies the energy distributions of backwards escaping primary photons resemble the shape of the incident spectrum, while this is not the case for primary photons that escape forwards. Furthermore, the characteristic feature in the primary electron energy distributions for PbI2 and HgI2 is the atomic deexcitation peaks. For the rest of materials the photoelectrons produced from primary photon absorption can also influence the shape of the distributions. The primary electrons prefer to be ejected forwards. In the mammographic energy range, the percentage of electrons being forwards ejected is approximately 60 % with the most probable polar angles ranging from 50o to 70o. In addition, the electrons prefer to be emitted at two lobes around ö=0 and ö=ð. At the practical mammographic energies (15-40 keV) a-Se, a-As2Se3 and Ge have the minimum azimuthal uniformity whereas CdZnTe, Cd0.8Zn0.2Te and CdTe the maximum one. The electron spatial distributions are affected from scatter and the emission of fluorescent photons. The distributions for a-Se, a-As2Se3, GaSe, GaAs, Ge, PbO and TlBr are almost independent on the polyenergetic spectrum while those for CdTe, CdZnTe, Cd0.8Zn0.2Te, ZnTe, PbI2 and HgI2 have a spectrum dependence. In the practical mammographic energy range and at this primitive stage of primary electron production, a-Se has the best inherent spatial resolution. For all the investigated materials and incident energies, the majority of primary electrons is produced within the first 300 ìm from detector’s surface. PbO has the minimum bulk space in which electrons can be produced whereas CdTe has the maximum one. In all materials and incident energies, except for Eµ §30 keV in a-Se, a-As2Se3, GaSe, GaAs and Ge (light materials), photons escape backwards whereas the overwhelming majority is fluorescent photons. The escaping of fluorescent photons and the atomic deexcitation are the factors that affect the primary electron production. The number of primary electrons increases at energies higher than the K edges of light materials, Cd and Te K edges as well as Pb, Hg and Tl L edges where the fluorescent photon escaping decreases and their absorption is followed by long atomic deexcitation cascades. For Eµ §30 keV in the light materials, the number of forwards escaping photons increases, due to the escaping primary photons, and becomes higher than the number of photons that escape backwards. Furthermore, the primary electron production is additionally affected by the escaping of primary photons that decreases the number of electrons. a-Se has the minimum number of primary electrons produced in the practical mammographic energy range. The energy distributions of primary electrons of a-Se that reach the collecting electrode are shifted at slightly higher energies with a small change in their shape. Most of electrons are collected at t<5 x 10-12 s. The majority of electrons is collected at the point of x-ray incidence whereas the xy spatial distributions have two opposing lobes around y=0 as well as a ring of an approximate radius of 2 mm. The azimuthal angles are not affected by the electron drifting while all electrons have polar angles è>ð/2 with the most probable polar angle being è=1.92 rad ~ 111o. Conclusively, insights are gained into the physics of primary electron production that lead to the investigation of the primary electron characteristics, which strongly influence the characteristics of the final image, and the factors which affect them. The results that concern the electron characteristics on the collecting electrode for the case of a-Se give, at a first approximation, the dependence of the characteristics of the final signal on the characteristics of the primary signal. Nevertheless, a complete simulation of the signal propagation inside the photoconductor’s bulk should be developed in order to derive conclusive remarks on the correlation of primary and final signal characteristics. The basis of developing such a simulation model can be found on the formulations presented for the electric field distribution and the electron interactions inside a-Se. Åêôåíçò Ðåñéëçøç Ç ìáóôïãñáößá åßíáé ìÝ÷ñé êáé óÞìåñá ç ðéï óçìáíôéêÞ êáé åõñÝùò äéáäåäïìÝíç áðåéêïíéóôéêÞ ôå÷íéêÞ ôïõ ãõíáéêåßïõ óôÞèïõò. Ç ìáóôïãñáöéêÞ åéêüíá ðñÝðåé íá åßíáé éêáíÞ ü÷é ìüíï íá áðïêáëýðôåé ðïëý ìéêñÝò äéáöïñÝò óýíèåóçò êáé ðõêíüôçôáò éóôïý, áëëÜ ôáõôü÷ñïíá ôçí ðáñïõóßá ôùí áðïôéôáíþóåùí ïé ïðïßåò Ý÷ïõí Ýíá ôõðéêü ìÝãåèïò ãýñù óôá 100 ìm. Åßíáé åìöáíÝò üôé åßíáé áðáñáßôçôï ôüóï ç áíôßèåóç åéêüíáò üóï êáé ç äéáêñéôéêÞ éêáíüôçôá íá äéáôçñïýíôáé óå õøçëÜ åðßðåäá åíþ ôáõôü÷ñïíá ï èüñõâïò íá ðáñáìÝíåé ðåñéïñéóìÝíïò. Åðéðñüóèåôá, ëüãù ôùí êéíäýíùí ïé ïðïßïé õðÜñ÷ïõí êáôÜ ôçí ÷ñÞóç éïíôéæïõóþí áêôéíïâïëéþí, ç äüóç óôï ìáóôü ðñÝðåé íá åßíáé ôüóï ÷áìçëÞ üóï ëïãéêÜ ìðïñåß íá åðéôåõ÷èåß (ALARA). Óôçí ðñïóðÜèåéá íá ðñáãìáôïðïéçèïýí ïé ðáñáðÜíù áíôéêåéìåíéêïß óôü÷ïé üðùò åðßóçò íá âåëôéùèåß ç åõáéóèçóßá êáé åéäéêüôçôá ôçò ìáóôïãñáöéêÞò äéáäéêáóßáò, ãåãïíüò ôï ïðïßï èá åðÝôñåðå ìßá ðïéü áêñéâÞò êáé ðïéü Ýãêáéñç äéÜãíùóç ôïõ êáñêßíïõ ôïõ ìáóôïý, ç Ýñåõíá åóôéÜæåé: (á) óôç ëåãüìåíç äéÜãíùóç CAD (Computer Aided Diagnosis), ç ïðïßá ó÷åôßæåôáé ìå ôçí åöáñìïãÞ ôå÷íéêþí åðåîåñãáóßáò êáé áíÜëõóçò åéêüíáò áëëÜ êáé ìç÷áíéêÞò üñáóçò óå øçöéïðïéçìÝíåò ìáóôïãñáöéêÝò åéêüíåò, êáé (â) óôç âåëôéóôïðïßçóç ôçò ðïéüôçôáò åéêüíáò ìå ôáõôü÷ñïíç ìåßùóç ôçò äüóçò óôïí ìáóôü ìå ôïí ó÷åäéáóìü êáé ôçí åêëÝðôõíóç åîåéäéêåõìÝíïõ ìáóôïãñáöéêïý åîïðëéóìïý êáé ôïí ðñïóäéïñéóìü ôùí âÝëôéóôùí ëåéôïõñãéêþí ðáñáìÝôñùí åíüò ìáóôïãñÜöïõ. ÐáñÜ ôï ãåãïíüò üôé ç Ýñåõíá CAD ðáñïõóéÜæåé åîáéñåôéêÞ ðñüïäï, ç åðéôõ÷ßá ôçò åîáñôÜôáé áðü ôçí ðïéüôçôá ôçò ìáóôïãñáöéêÞò åéêüíáò ç ïðïßá ëáìâÜíåôáé óôïí áíé÷íåõôÞ åéêüíáò. Ï áíé÷íåõôÞò åéêüíáò åßíáé Ýíáò áðü ôïõò ðéï êáèïñéóôéêïýò ðáñÜãïíôåò ôçò áðïôåëåóìáôéêüôçôáò ôçò ìáóôïãñáöéêÞò äéáäéêáóßáò. Ç ìáóôïãñáößá ìå óõóôÞìáôá öéëì-åíéó÷õôéêÞò ðéíáêßäáò ðáñáìÝíåé ìÝ÷ñé êáé óÞìåñá ç ðéï äéáäåäïìÝíç ôå÷íéêÞ. Ìåôáîý Üëëùí, ðñïóöÝñåé êáëÞ áðåéêüíéóç äïìþí ÷áìçëÞò áíôßèåóçò ìå åìöáíÞ üñéá. Åíôïýôïéò, ôá óõóôÞìáôá áõôÜ å÷ïõí ðåñéïñéóìÝíï åýñïò Ýêèåóçò (1:25) åíþ ïé ìÜæåò êáé ïé ìéêñïáóâåóôþóåéò, óçìáíôéêÝò åíäåßîåéò ýðáñîçò êáñêßíïõ, äýóêïëá áðåéêïíßæïíôáé óå ðõêíïýò ìáóôïýò. Ç ðñüóöáôç Ýñåõíá Ýäåéîå üôé ç øçöéáêÞ ìáóôïãñáößá ðñïóöÝñåé âåëôéùìÝíç ðïéüôçôá åéêüíáò óõãêñéôéêÜ ìå ôá óõóôÞìáôá öéëì-åíéó÷õôéêÞò ðéíáêßäáò êáèþò åðßóçò êáëýôåñç êâáíôéêÞ áðïäïôéêüôçôá (quantum efficiency) êáé åõêïëüôåñç ëÞøç, åðåîåñãáóßá êáé áðïèÞêåõóç åéêüíáò. Åðéðñüóèåôá, ïé öùôïáãþãéìïé áíé÷íåõôÝò åíåñãïý ìÞôñáò áðïäåéêíýïíôáé íá õðåñÝ÷ïõí ôùí öùôïäéåãåéñüìåíùí öùóöüñùí (photostimulable phosphors) êáé ôùí óõóêåõþí óõæåõãìÝíïõ öïñôßïõ (Charge Coupled Devices Þ CCDs). Åéäéêüôåñá, ïé öùôïáãþãéìïé áíé÷íåõôÝò åíåñãïý ìÞôñáò Üìåóçò ìåôáôñïðÞò ðáñÝ÷ïõí âåëôéùìÝíç êâáíôéêÞ áðïäïôéêüôçôá, ìåéùìÝíç áóÜöåéá êáé õøçëÞ äéáêñéôéêÞ éêáíüôçôá. Óôïõò Üìåóïõò áíé÷íåõôÝò åíåñãïý ìÞôñáò, Ýíáò öùôïáãùãüò ìåôáôñÝðåé Üìåóá ôéò ðñïóðßðôïõóåò áêôßíåò × óå íÝöïò öïñôßùí ôï ïðïßï ïëéóèáßíåé êÜôù áðü ôçí åðßäñáóç çëåêôñéêïý ðåäßïõ ðñïò ôá çëåêôñüäéá üðïõ êáé óõëëÝãåôáé ó÷çìáôßæïíôáò ôçí ìáóôïãñáöéêÞ åéêüíá. Ùò åê ôïýôïõ, ôï öùôïáãþãéìï õëéêü åßíáé Ýíáò áðü ôïõò ðéï óçìáíôéêïýò ðáñÜãïíôåò óå áõôÜ ôá óõóôÞìáôá. Ôï Üìïñöï óåëÞíéï (a-Se) åßíáé Ýíá áðü ôá êáôáëëçëüôåñá õëéêÜ êõñßùò ëüãù ôçò éêáíüôçôÜò ôïõ íá áíáðôýóóåôáé óå ìåãÜëåò åðéöÜíåéåò ìå ïìïéïãåíÞ ÷áñáêôçñéóôéêÜ áðåéêüíéóçò êáé ëüãù ôçò õøçëÞò åíäïãåíïýò äéáêñéôéêÞò ôïõ éêáíüôçôáò. Ðáñüëáõôá, ôï õëéêü áõôü ðÜó÷åé áðü ðåñéïñéóìÝíç éêáíüôçôá áðïññüöçóçò áêôßíùí × êáé ìåéùìÝíç åõáéóèçóßá. ÕëéêÜ üðùò ôá a-As2Se3, GaSe, GaAs, Ge, CdTe, CdZnTe, Cd0.8Zn0.2Te, ZnTe, PbO, TlBr, PbI2 êáé HgI2 éêáíïðïéïýí êÜðïéá áðü ôá ÷áñáêôçñéóôéêÜ åíüò éäáíéêïý áíé÷íåõôÞ êáé Ýôóé åßíáé õðïøÞöéá óáí åíáëëáêôéêÞ ëýóç ãéá áõôïý ôïõ åßäïõò ôá óõóôÞìáôá áðåéêüíéóçò. Ìåôáîý áõôþí, ôá ðïëõêñõóôáëëéêÜ CdTe, CdZnTe, Cd0.8Zn0.2Te, PbO, PbI2 êáé HgI2 üðùò êáé ôï Üìïñöï a-As2Se3 åßíáé ïé êáëýôåñïé äõíáôïß õðïøÞöéïé êõñßùò ëüãù ôïõ ãåãïíüôïò üôé ìðïñïýí íá áíáðôõ÷èïýí óå ìåãÜëåò åðéöÜíåéåò. Áðï ôçí Üëëç ôá êñõóôáëëéêÜ GaSe, GaAs, Ge, ZnTe êáé TlBr áíáðôýóóïíôáé óå ðåñéïñéóìÝíåò åðéöÜíåéåò ìå âÜóç ôéò ðáñïýóåò ôå÷íéêÝò êáé Ýôóé åßíáé ëéãüôåñï êáôÜëëçëá. Åíôïýôïéò ïé ôå÷íéêÝò áíÜðôõîçò âåëôéþíïíôáé. Directory: jspui -> bitstream bitstream -> College day annual report bitstream -> Review of coastal ecosystem management to improve the health and resilience of the Great Barrier Reef World Heritage Area bitstream -> A. gw student and alumni numbers summary 3 bitstream -> Advances in Imaging from the first x-ray Images bitstream -> Clustering Microarray Data within Amorphous Computing Paradigm and Growing Neural Gas Algorithm bitstream -> Навчальний посібник Для студентів економічних І правових спеціальностей немовних вузів Суми двнз "уабс нбу" 2014 Download 0.55 Mb. Share with your friends:
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