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Simulating Medical Imaging X-Ray Tubes with Various Parameters Using BEAMnrc Monte Carlo Software

Open Journal of Radiology 2022年第3期12卷 125-141页

作者： Nikolaos Chatzisavvas Thanasis Koustas Georgios Karpetas Ioannis Valais Georgios Priniotakis dimitrios nikolopoulos Department of Industrial Design and Production Engineering University of West Attica Athens Greece Department of Medical Physics Medical School University of Patras Patra Greece Medical School University of Thessaly Larissa Greece Department of Biomedical Engineering University of West Attica Athens Greece

Context: Medical imaging has a wide range of applications in today’s society. Basic projectional radiography, CT scans, mammograms and a range of other advanced technologies all use x-rays to create a large number of examinations every day across the world. The most essential component of such medical equipment is the x-ray tube, which creates and produces x-rays. Objective: We describe and investigate an abstract model-geometry of a simple x-ray tube utilizing the open-source software package of BEAMnrc of the EGSnrcmp family, which is well validated by several studies over the years, for high and low energy photons generation. Methodology: Our research focuses on two different electron beam energies: 120 keV and 30 keV. The 120 keV is the typical energy for simple projectional radiographic exams and CT examinations, whereas the 30 keV is the typical energy of mammography. Results: Two different anode materials are used for each case, Gold (Au) and Tungsten (W) for 120 keV because these are the most common in projectional radiography and CT;Molybdenum (Mo) and Rhodium (Rh) for 30 keV because with these targets most mammography exams are carried out. The aim of this work is to show how the BEAMnrc software package can simulate effectively x-ray generation of low-energy photons which are utilized in modern medical imaging procedures. We describe useful information on anode-target characteristics, such as anode angle, anode material, and metal filter materials, based on previous quality studies even by using software other than BEAMnrc. Conclusion: We demonstrate that BEAMnrc can be efficiently used for Monte Carlo modeling of low-energy photons.

关键词： Bremsstrahlung Filter Monte Carlo Spectral Distribution X-Ray Tube

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A GATE Simulation Study of the Siemens Biograph DUO PET/CT System

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Open Journal of Radiology 2013年第2期3卷 56-65页

作者： dimitrios nikolopoulos Sofia Kottou Nikolaos Chatzisavvas Xenophon Argyriou Emannouel Vlamakis Panayiotis Yannakopoulos Anna Louizi Department of Engineering of Electronic and Computer Systems Technological Educational Institute (TEI) of Piraeus Athens Greece Department of Physics Chemistry and Material Science Technological Educational Institute (TEI) of Piraeus Athens Greece Medical Physics Department Medical School University of Athens Athens Greece

This is a GATE-simulation study of the Siemens Biograph DUO PET/CT system. It reports effects of changes in the thickness of the employed Lutetium Oxyorthosilicate(LSO) detectors. The PET/CT, a human body phantom and a cylindrical F-18 FDG source were simulated. Validation measurements were conducted. The results indicate that LSO thickness increase degrades spatial resolution, improves relative energy resolution from 9.0% to 11.3% and increases signal-to-noise-ratio from 0.81 to 1.17. Thicker LSO crystals present greater axial sensitivity so as the detection efficiency of PET would be significantly enhanced.

关键词： Monte-Carlo GATE PET Siemens Biograph DUO PET/CT

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Current surgical strategies for total arthroplasty in valgus knee

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World Journal of Orthopedics 2015年第6期6卷 469-482页

作者： dimitrios nikolopoulos Ioannis Michos George Safos Petros Safos Orthopaedic Department Central Clinic of Athens D’Orthopaedic Department Asklepeion Voulas General Hospital Orthopaedic Department Ikaria General Hospital

The majority of orthopaedic surgeons even currentlyagree that primary total arthroplasty in valgus knees with a deformity of more than ten degrees may prove challenging. The unique sets of bone and soft tissue abnormalities that must be addressed at the time of the operation make accurate axis restoration, component orientation and joint stability attainment a difficult task. Understanding the specific pathologic anatomic changes associated with the valgus knee is a prerequisite so as to select the proper surgical method, to optimize component position and restore soft-tissue balance. The purpose of this article is to review the valgus knee anatomical variations, to assess the best pre-operative planning and to evaluate how to choose the grade of constraint of the implant. It will also be underlying the up-to-date main approaches and surgical techniques be proposed in the English literature both for bone cuts and soft tissue management of valgus knees.

关键词： Valgus knee Arthroplasty Balancing soft tissue Knee surgical approaches Tibial tubercle osteotomy

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Investigation of proper modeling of very dense granular flows in the recirculation system of CFBs

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Particuology 2012年第6期10卷 699-709页

作者： Aristeidis nikolopoulos Nikos nikolopoulos Nikos Varveris Sotirios Karellas Panagiotis Grammelis Emmanuel Kakaras Laboratory of Steam Boilers and Thermal Plants Thermal Engineering SectorSchool of Mechanical EngineeringNTUAAthensGreece Centre for Research and Technology Hellas Institute for Solid Fuels Technology and ApplicationsAthens Gr-15310Greece

The aim of this paper is the development of new models and] or the improvement of existing numerical models, used for simulating granular flow in CFB （circulating fluidized bed） recirculation systems. Most recent models follow the TFM （two-fluid model） methodology, but they cannot effectively simulate the inter-particle friction forces in the recirculation system, because the respective stress tensor does not incorporate compressibility of flow due to change of effective particle density. As a consequence, the induced normal and shear stresses are not modeled appropriately during the flow of the granular phase in the CFB recirculation system. The failure of conventional models, such as that of yon MisesJCoulomb, is mainly caused by false approximation of the yield criterion which is not applicable to the CFB recirculation system. The present work adopts an alternative yield function, used for the first time in TFM Eulerian modeling. The proposed model is based on the Pitman-Schaeffer-Gray-Stiles yield criterion. Both the temporal deformation of the solid granular phase and the repose angle that the granular phase forms are more accurately simulated by this model. The numerical results of the proposed model agree well with experimental data, implying that frictional forces are efficiently simulated by the new model.

关键词： Euler-EulerCFBRepose angleFull-loop simulationGranular viscosityYield criterion

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P-Tree Structures and Event Horizon： Efficient Event-Set Implementations

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Journal of Computer Science & Technology 2006年第1期21卷 19-26页

作者： Katerina Asdre Stavros D. nikolopoulos Department of Computer Science University of Ioannina GR-45110 Ioannina Greece

This paper describes efficient data structures, namely the Indexed P-tree, Block P-tree, and Indexed-Block P-tree （or/P-tree, BP-tree, and IBP-tree, respectively, for short）, for maintaining future events in a general purpose discrete event simulation system, and studies the performance of their event set algorithms under the event horizon principle. For comparison reasons, some well-known event set algorithms have been selected and studied, that is, the Dynamic-heap and the P-tree algorithms. To gain insight into the performance of the proposed event set algorithms and allow comparisons with the other selected algorithms, they are tested under a wide variety of conditions in an experimental way. The time needed for the execution of the Hold operation is taken as the measure for estimating the average time complexity of the algorithms. The experimental results show that the BP-tree algorithm and the IBP-tree algorithm behave very well with the event set of all the sizes and their performance is almost independent of the stochastic distributions.

关键词： discrete-event simulation event set algorithms hold model event horizon data structures heap P-tree P-tree structures

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Protective and defensive roles of non-glandular trichomes against multiple stresses:structure-function coordination

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Journal of Forestry Research 2020年第1期31卷 1-12页

作者： George Karabourniotis Georgios Liakopoulos Dimosthenis nikolopoulos Panagiota Bresta Laboratory of Plant Physiology Department of Crop ScienceAgricultural University of Athens

As superficial structures,non-glandular trichomes,protect plant organs against multiple biotic and abiotic *** protective and defensive roles of these epidermal appendages are crucial to developing organs and can be attributed to the excellent combination of suitable structural traits and chemical reinforcement in the form of phenolic compounds,primarily fl *** the formation of trichomes and the accumulation of phenolics are interrelated at the molecular *** the early stages of development,non-glandular trichomes show strong morphological similarities to glandular ones such as the balloon-like apical cells with numerous *** later developmental stages,and during secondary wall thickening,phenolics are transferred to the cell walls of the *** to the diff use deposition of phenolics in the cell walls,trichomes provide protection against UV-B radiation by behaving as optical fi lters,screening out wavelengths that could damage sensitive *** from strong visible radiation is also aff orded by increased surface light refl ***,the mixtures of trichome phenolics represent a superfi-cial chemical barrier that provides protection against biotic stress factors such as herbivores and *** the cells of some trichomes die at maturity,they can modulate their quantitative and qualitative characteristics during development,depending on the prevailing conditions of the external biotic or abiotic *** fact,the structure and chemical constituents of trichomes may change due to the particular light regime,herbivore damage,wounding,water stress,salinity and the presence of heavy ***,trichomes represent dynamic protective structures that may greatly aff ect the outcome of many plant–environment interactions.

关键词： Non-glandular trichomes Phenolics Flavonoids Protection Defence Biotic stress Abiotic stress

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Statins and the risk of colorectal cancer: An updated systematic review and meta-analysis of 40 studies

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World Journal of Gastroenterology 2014年第7期20卷 1858-1870页

作者： Theodore Lytras Georgios nikolopoulos Stefanos Bonovas Department of Occupational and Industrial Hygiene National School of Public Health11521 AthensGreece National Development and Research Institutes Inc.New YorkNY 10010United States Hellenic Centre for Disease Control and Prevention 15123 AthensGreece Department of Pharmacology School of MedicineUniversity of Athens11527 AthensGreece Laboratory of Drug Regulatory Policies IRCCS Mario Negri Institute for Pharmacological Research20156 MilanItaly

AIM: To investigate the association between statin use and colorectal cancer risk, we conducted an updated meta-analysis of published studies.

关键词： 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors Statins Colorectal cancer Systematic review Meta-analysis Cancer chemoprevention

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Planning the hepatitis C virus elimination in Cyprus:A modeling study

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World Journal of Gastroenterology 2021年第31期27卷 5219-5231页

作者： Ilias Gountas Ioanna Yiasemi Evi Kyprianou Christos Mina Chrysanthos Georgiou Petros Katsioloudes Andri Kouroufexi Anna Demetriou Elena Xenofontos Georgios nikolopoulos Medical School University of CyprusNicosia 1678Cyprus Cyprus Monitoring Centre Cyprus National Addictions AuthorityNicosia 1678Cyprus Nicosia General Hospital Nicosia 1678Cyprus Evangelistria Medical Centre Nicosia 1678Cyprus Ministry of Health Pharmaceutical ServicesNicosia 1678Cyprus Ministry of Health Health Monitoring UnitNicosia 1678Cyprus Department of Internal Medicine Limassol General HospitalLimassol 4131Cyprus

BACKGROUND Hepatitis C virus(HCV)infection is a major global public health *** the Republic of Cyprus,the estimated prevalence of chronic hepatitis C(CHC)among the general population is 0.6%,while the CHC prevalence among people who inject drugs(PWID)is estimated at 46%.Direct-acting antivirals that can eliminate HCV are not yet widely available in the Republic of ***,when direct-acting antivirals become available,a long-term strategic plan to guide elimination efforts will be needed to maximize the effect of *** To determine the programmatic targets to eliminate HCV in the Republic of *** A dynamic,stochastic,individual-based model of HCV transmission,disease progression,and cascade of care was calibrated to data from *** model stratifies the population into the infected general population and the PWID population.A variety of test,prevention,and treatment strategies concerning the general population,PWID,or both were *** time horizon of the analysis was until *** Under the status quo scenario,the model predicted that 75(95%confidence interval(CI):60,91)and 575(95%CI:535,615)liver-related deaths and new infections would occur by 2034,*** an expanded treatment program,without screening interventions,would cause modest outcomes regarding CHC prevalence(16.6%reduction in 2034 compared to 2020)and liverrelated deaths(10 deaths would be prevented compared to the status quo scenario by 2034).Implementing a test and treat strategy among the general population but without any intervention in the PWID population would suffice to meet the mortality target but not the incidence *** achieve HCV elimination in Cyprus,3080(95%CI:3000,3200)HCV patients need to be diagnosed and treated by 2034(2680 from the general population and 400 from PWID),and harm reduction coverage among PWID should be increased by 3%per year(from 25%in 2020 to 67%in 2034).CONCLUSION Elimination of HCV is a demanding pub

关键词： Mathematical modelling Projections Hepatitis C virus elimination Directacting antivirals Screening campaigns

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Watermarking Images in the Frequency Domain by Exploiting Self-Inverting Permutations

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Journal of Information Security 2013年第2期4卷 80-91页

作者： Maria Chroni Angelos Fylakis Stavros D. nikolopoulos Department of Computer Science University of Ioannina Ioannina Greece

In this work we propose efficient codec algorithms for watermarking images that are intended for uploading on the web under intellectual property protection. Headed to this direction, we recently suggested a way in which an integer number w which being transformed into a self-inverting permutation, can be represented in a two dimensional (2D) object and thus, since images are 2D structures, we have proposed a watermarking algorithm that embeds marks on them using the 2D representation of w in the spatial domain. Based on the idea behind this technique, we now expand the usage of this concept by marking the image in the frequency domain. In particular, we propose a watermarking technique that also uses the 2D representation of self-inverting permutations and utilizes marking at specific areas thanks to partial modifications of the image’s Discrete Fourier Transform (DFT). Those modifications are made on the magnitude of specific frequency bands and they are the least possible additive information ensuring robustness and imperceptiveness. We have experimentally evaluated our algorithms using various images of different characteristics under JPEG compression. The experimental results show an improvement in comparison to the previously obtained results and they also depict the validity of our proposed codec algorithms.

关键词： Watermarking Techniques Image Watermarking Algorithms Self-Inverting Permutations 2D Representations of Permutations Encoding Decoding Frequency Domain Experimental Evaluation

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Management of patients after recovering from acute severe biliary pancreatitis

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World Journal of Gastroenterology 2016年第34期22卷 7708-7717页

作者： Georgia Dedemadi Manolis nikolopoulos Ioannis Kalaitzopoulos George Sgourakis Department of Surgery Sismanoglio-Amalia Fleming Hospital Department of Surgery Furness General Hospital

Cholelithiasis is the most common cause of acute pancreatitis,accounting 35%-60% of cases. Around 15%-20% of patients suffer a severe attack with high morbidity and mortality rates. As far as treatment is concerned,the optimum method of late management of patients with severe acute biliary pancreatitis is still contentious and the main question is over the correct timing of every intervention. Patients after recovering from an acute episode of severe biliary pancreatitis can be offered alternative options in their management,including cholecystectomy,endoscopic retrograde cholangiopancreatography(ERCP) and sphincterotomy,or no definitive treatment. Delaying cholecystectomy until after resolution of the inflammatory process,usually not earlier than 6 wk after onset of acute pancreatitis,seems to be a safe policy. ERCP and sphincterotomy on index admission prevent recurrent episodes of pancreatitis until cholecystectomy is performed,but if used for definitive treatment,they can be a valuable tool for patients unfit for surgery. Some patients who survive severe biliary pancreatitis may develop pseudocysts or walled-off necrosis. Management of pseudocysts with minimally invasive techniques,if not therapeutic,can be used as a bridge to definitive operative treatment,which includes delayed cholecystectomy and concurrent pseudocyst drainage in some patients. A management algorithm has been developed for patients surviving severe biliary pancreatitis according to the currently published data in the literature.

关键词： Biliary pancreatitis Cholecystectomy Endoscopic retrograde cholangiopancreatography Sphincterotomy Pseudocyst Walled-off necrosis

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