Dual energy x-ray absorptiometry (DXA) provides accurate measurements of percent body fat (%BF) and lean body mass (LBM), however no reference standards currently exist using DXA-derived data. This study’s purpose was to develop reference data sets for DXA-derived %BF and LBM, and to characterize the agreement of obesity classifications between BMI (≥30 kg/m2) and %BF (≥25% for men and ≥30% for women). 2,761 subjects were scanned from 2003-2013 using either the GE Medical Systems Lunar Prodigy or Lunar iDXA. Normative reference tables displaying mean values and select percentiles were created for %BF and LBM across defined age groups for both genders. Mean %BF and LBM closely reflected data from the National Health and Nutrition Examination Survey across age groups in both genders. Agreements between BMI and %BF were 97% when identified as obese and 33% when identified as non-obese. Future research should consider creating a national registry for DXA-derived measurements. / School of Physical Education, Sport, and Exercise Science

The current thesis concerns Dual Energy Computed Tomography and specifically the physical principles and methods it is based on. Dual Energy CT offers the potential of not only anatomical, but also functional information from Computed Tomography (CT) exams. This is achieved by utilizing the energy dependence of X-rays’ attenuation within matter. In this way, materials are divided into those that are characterized by energy-dependent attenuation (strong spectral behavior), due to strong photoelectric effect contribution to total attenuation, and those that do not exhibit important photoelectric attenuation at radiological energies and therefore they attenuate X-rays in a much less energy dependent way. This information is useful for the identification of materials that, despite the fact that they are completely different as far as their chemical composition is concerned, they have the same or similar CT number values at a particular kVp level. The energy dependence of attenuation leads to the determination of a polychromatic linear attenuation coefficient. This coefficient may be approximated either by considering an equivalent monoenergetic attenuation coefficient that is characterized by the same half value layer as the the polyenergetic beam, or by a local linear attenuation coefficient that is determined by knowledge of the local x-ray spectrum. The energy dependence of attenuation is the cause of beam hardening effects. The basic fields where dual energy CT has become feasible and its current clinical applications are described in the thesis. The utility of DECT ranges from artifact elimination (beam hardening, metal artifacts) to tissue discrimination, material selective images and “conventional CT acquisition” equivalent images. The implementations of dual energy CT are also presented in the thesis and include consecutive scans at two different kVp values, fast kV-switching, dual source CT and dual layer CT. / Η παρούσα διπλωματική εργασία αφορά στην τομογραφία διπλής ενέργειας και ειδικότερα τις φυσικές αρχές και μεθόδους στις οποίες βασίζεται. H τομογραφία διπλής ενέργειας δίνει την προοπτική της απόκτησης όχι μόνο ανατομικής, αλλά και λειτουργικής πληροφορίας από τις εξετάσεις αξονικής τομογραφίας. Αυτό το επιτυγχάνει χρησιμοποιώντας την εξάρτηση της εξασθένησης των ακτίνων X μέσα στην ύλη από την ενέργεια των φωτονίων. Με αυτό τον τρόπο, τα υλικά διαχωρίζονται σε αυτά που εξασθενούν τα φωτόνια με πολύ διαφορετικό τρόπο σε διαφορετικές ενέργειες λόγω έντονου φωτοηλεκτρικού φαινομένου και σε αυτά που η ενεργειακή εξάρτηση του συντελεστή εξασθένησης τους είναι λιγότερο έντονη (λιγότερη συμμετοχή του φωτοηλεκτρικού φαινομένου στη συνολική εξασθένηση). Η πληροφορία αυτή είναι χρήσιμη για την ταυτοποίηση της σύστασης υλικών, που ενώ είναι εντελώς διαφορετικά σε χημική σύσταση, έχουν τον ίδιο αριθμό CT (CT number) σε συγκεκριμένη τάση λειτουργίας της πηγής ακτίνων X. Στην εκτεταμένη περίληψη αυτής της εργασίας, αρχικά γίνεται σύντομη αναφορά στην εξάρτηση του συντελεστή εξασθένησης από την ενέργεια και πώς αυτή επηρεάζει τον ορισμό του συντελεστή γραμμικής εξασθένησης για πολυχρωματικές ακτινοβολίες. Στη συνέχεια ακολουθεί μια συνοπτική περιγραφή υλοποιήσεων της τομογραφίας διπλής ενέργειας καθώς επίσης και των αλγορίθμων και των κλινικών εφαρμογών που είναι διαθέσιμες σήμερα. Τέλος, περνώντας στην περιγραφή του κυρίως μέρους της διπλωματικής, ακολουθεί συνοπτική περιγραφή της θεωρητικής και πειραματικής μελέτης της συμπεριφοράς διπλής ενέργειας του ιωδίου, του ασβεστίου, του οστού και υλικών που προσομοιάζουν το μαλακό και λιπώδη ιστό. Στη θεωρητική περιγραφή, η διπλωματική εστιάζει στις εξαρτήσεις του συντελεστή γραμμικής εξασθένησης, πρώτα από τα χαρακτηριστικά του ίδιου του υλικού (πυκνότητα, χημική σύσταση) και έπειτα από την ενέργεια των φωτονίων. Ο προβληματισμός σχετικά με την εξάρτηση του συντελεστή εξασθένησης με την ενέργεια έχει οδηγήσει συχνά σε τρόπους προσέγγισης και ορισμού ενός συντελεστή γραμμικής εξασθένησης που να ανταποκρίνεται σε πολυενεργειακές δέσμες, όπως αυτές που χρησιμοποιεί ο αξονικός τομογράφος. Ο συντελεστής γραμμικής εξασθένησης μιας πολυενεργειακής δέσμης από ένα συγκεκριμένο υλικό, προσδιορίζεται είτε μέσω ενός ισοδύναμου μονοενεργειακού συντελεστή εξασθένησης που να χαρακτηρίζεται από το ίδιο half value layer (HVL) με την πολυενεργειακή δέσμη είτε μέσω ενός «τοπικού» συντελεστή εξασθένησης. που για να προσδιοριστεί απαιτείται γνώση του φάσματος στην συγκεκριμένη θέση και στο συγκεκριμένο υλικό. Η εξάρτηση του συντελεστή εξασθένησης από την ενέργεια των φωτονίων ευθύνεται για τεχνήματα σκλήρυνσης δέσμης (beam hardening artifacts).

abstract: Computed tomography (CT) is one of the essential imaging modalities for medical diagnosis. Since its introduction in 1972, CT technology has been improved dramatically, especially in terms of its acquisition speed. However, the main principle of CT which consists in acquiring only density information has not changed at all until recently. Different materials may have the same CT number, which may lead to uncertainty or misdiagnosis. Dual-energy CT (DECT) was reintroduced recently to solve this problem by using the additional spectral information of X-ray attenuation and aims for accurate density measurement and material differentiation. However, the spectral information lies in the difference between two low and high energy images or measurements, so that it is difficult to acquire the accurate spectral information due to amplification of high pixel noise in the resulting difference image. In this work, a new model and an image enhancement technique for DECT are proposed, based on the fact that the attenuation of a high density material decreases more rapidly as X-ray energy increases. This fact has been previously ignored in most of DECT image enhancement techniques. The proposed technique consists of offset correction, spectral error correction, and adaptive noise suppression. It reduced noise, improved contrast effectively and showed better material differentiation in real patient images as well as phantom studies. / Dissertation/Thesis / Ph.D. Bioengineering 2011

abstract: Coronary heart disease (CHD) is the most prevalent cause of death worldwide. Atherosclerosis which is the condition of plaque buildup on the inside of the coronary artery wall is the main cause of CHD. Rupture of unstable atherosclerotic coronary plaque is known to be the cause of acute coronary syndrome. The composition of plaque is important for detection of plaque vulnerability. Due to prognostic importance of early stage identification, non-invasive assessment of plaque characterization is necessary. Computed tomography (CT) has emerged as a non-invasive alternative to coronary angiography. Recently, dual energy CT (DECT) coronary angiography has been performed clinically. DECT scanners use two different X-ray energies in order to determine the energy dependency of tissue attenuation values for each voxel. They generate virtual monochromatic energy images, as well as material basis pair images. The characterization of plaque components by DECT is still an active research topic since overlap between the CT attenuations measured in plaque components and contrast material shows that the single mean density might not be an appropriate measure for characterization. This dissertation proposes feature extraction, feature selection and learning strategies for supervised characterization of coronary atherosclerotic plaques. In my first study, I proposed an approach for calcium quantification in contrast-enhanced examinations of the coronary arteries, potentially eliminating the need for an extra non-contrast X-ray acquisition. The ambiguity of separation of calcium from contrast material was solved by using virtual non-contrast images. Additional attenuation data provided by DECT provides valuable information for separation of lipid from fibrous plaque since the change of their attenuation as the energy level changes is different. My second study proposed these as the input to supervised learners for a more precise classification of lipid and fibrous plaques. My last study aimed at automatic segmentation of coronary arteries characterizing plaque components and lumen on contrast enhanced monochromatic X-ray images. This required extraction of features from regions of interests. This study proposed feature extraction strategies and selection of important ones. The results show that supervised learning on the proposed features provides promising results for automatic characterization of coronary atherosclerotic plaques by DECT. / Dissertation/Thesis / Ph.D. Bioengineering 2013

A técnica de mamografia por dupla energia tem se mostrado uma ferramenta eficiente para aumentar a visualização e detecção de microcalcificações, uma vez que permite, por meio da combinação de imagens obtidas com energias diferentes (baixa e alta energia), suprimir o ruído estrutural. Por outro lado, pesquisas recentes sugerem que esta técnica permite a obtenção de imagens quantitativas das espessuras de microcalcificações e frações glandulares presentes nos tecidos mamários. O presente trabalho tem como objetivo, o estudo e otimização de imagens quantitativas obtidas através da técnica de mamografia por dupla energia. Este estudo foi realizado por simulação computacional, utilizando o código PENELOPE (Penetrarion and Energy Loss of Positrons and Electrons), o qual foi adaptado para fornecer imagens mamográficas digitais e dados relevantes para o estudo da formação da imagem, bem como fornecer as grandezas dosimétricas de interesse nesta técnica. Para isto, a geometria de um típico exame clínico foi simulado,considerando desde o equipamento mamográfico digital (fonte, placa compressora, suporte e detector), até a mama (pele, gordura, tecidos glandulares e microcalcificações). A validação das adaptações ao código foi feita através da comparação entre os valores de razão espalhado-primário (SPR) e dose glandular normalizada (DgN) gerados neste trabalho, com aqueles presentes na literatura. Finalmente, todos os elementos da cadeia da formação de imagens quantitativas foram implementados e avaliados, consistindo de (i) um algoritmo de pré-processamento, que permite corrigir a contribuição do espalhamento nas imagens, com desempenho superior a 95%; (ii) um algoritmo de pós-processamento nas imagens combinadas (baixa e alta energia) que permite corrigir o aumento do ruído decorrente do pré-processamento e da combinação das imagens, com desempenho de 90%. Os resultados de otimização foram adquiridos para diversas características de mama (espessuras de 2 a 6 cm e frações glandulares entre 25 e 75%) e avaliados para diferentes tamanhos de calcificações. As combinações ótimas de energias (baixa;alta) para mamas com espessuras de 2, 4 e 6 cm foram (16;62), (19;56) e (22;56) keV, respectivamente. As combinações escolhidas como ótimas conseguiram estimar as espessuras das microcalcificações com exatidão superior a 92% e a fração glandular com exatidão superior a 95%. / The dual-energy mammography technique has proven to be an efficient tool to increase the visualization and detection of microcalcifications, since it allows, by combining images obtained with different energies (low and high energy), suppress the structural noise. Moreover, recent researches suggests that this technique would allow obtaining quantitative images of microcalcifications thicknesses or glandular fractions present in the breast tissues. This current work aims the study and optimization of quantitative images obtained through dual-energy mammography technique, and was carried out by computer simulation using the PENELOPE code (Penetrarion and Energy Loss of Positron and Electrons), wich was adapted to provide digital mammographic images, and relevant data used in the study of the formation of images and provide the dosimetric quantities of interest in this technique. For this, the model of a typical clinical examination was simulated, from considering the digital mammography equipment (source, compressing plate holder and detector), until the breast (skin, fat, glandular tissues and calcifications). The validation of code adaptation was evaluated by comparing the values of scatter-primary ratio (SPR) and normalized glandular dose (DgN) acquired in this work, with the data in the literature. Finally, all elements of the data chain of the quantitative images were implemented and evaluated, which consist of (i) a pre-processing algorithm that corrects the scattering contribution in the images, with a performance superior to 95%; (ii) a post-processing algorithm in the combined images (low and high energy) that allows to correct the increased noise from the pre-processing and from the combination of the images, with a performance of 90%. The optimization results were obtained for different breast characteristics and evaluated in different calcifications sizes. The optimum combinations of energies (low;high) for breasts with thicknesses of 2, 4 and 6 cm were (16;62), (19;56) and (22;56) keV , respectively. The combinations chosen as optimal was able to estimate the thickness of microcalcifications with an accuracy higher than 92% and the glandular fraction with an accuracy upper to 95% .

The complexity of components fabricated in today's industry is ever increasing. This increase is partly due to market pressure but it is also a result from progress in fabrication technologies that opens up new possibilities. The increased use of additive manufacturing and multi-material systems, especially, has driven the complexity of parts to new heights. The new complex material systems brings benefits in many areas such as; mechanical properties, weight optimisation, and sustainability. However, the increased complexity also makes material integrity investigations and dimensional control more difficult. In additive manufacturing, for example, internal features can be fabricated which cannot be seen or measured with conventional tools. There is thus a need for non-destructive inspection methods that can measure these geometries. Such a method is X-ray computed tomography. Computed tomography utilizes the X-rays ability to penetrate material to create 3D digital volumes of components. Measurements and material investigations can be performed in these volumes without any damage to the investigated component. However, computed tomography in material science is still not a fully mature method and there are many uncertainties associated with the investigation technique. In the work presented in this thesis geometries fabricated by various additive manufacturing processes have been investigated using computed tomography. Also in this work, a dual-energy computed tomography tool has been developed with the aim to increase the measurement consistency of computed tomography when investigating complex geometries and material combinations. / MultiMatCT

Made available in DSpace on 2014-06-12T23:02:48Z (GMT). No. of bitstreams: 2 arquivo8577_1.pdf: 1277931 bytes, checksum: 3fa83084d4bb4f85991aa2e7f000febd (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2006 / A obesidade é uma excessiva acumulação de energia sob a forma de gordura corporal com prejuízos à saúde. O aumento da prevalência de obesidade em diferentes populações leva a acreditar que esta condição clínica caminha para ser a mais importante causa de doença crônica no mundo. O objetivo deste trabalho foi o de avaliar a importância da obesidade em especial a central e seus fatores de risco à saúde em homens adultos jovens por meio de revisão da literatura, bem como, realizar um estudo comparativo entre duas técnicas de avalição da composição corporal: a Antropometria e o Dual Energy X-ray absorptiometry (DXA). O delineamento do estudo foi do tipo transversal com seleção aleatória. Participaram 45 indivíduos com idades entre 20 e 30 anos, distribuídos em três grupos segundo o Índice de Massa Corporal (IMC), nas categorias peso normal, pré-obeso e obeso I. O referido estudo originou uma nova equação de predição para a gordura centralizada utilizando o perímetro abdominal (Pab) e a gordura do tronco medida pelo DXA em homens adultos jovens, permitindo ainda classificar os referidos indivíduos em risco à saúde de acordo com o Z-score usando a quantidade de gordura estimada pela referida equação

The complexity of components fabricated in today's industry is ever increasing. This increase is partly due to market pressure but it is also a result from progress in fabrication technologies that opens up new possibilities. The increased use of additive manufacturing and multi-material systems, especially, has driven the complexity of parts to new heights. The new complex material systems brings benefits in many areas such as; mechanical properties, weight optimisation, and sustainability. However, the increased complexity also makes material integrity investigations and dimensional control more difficult. In additive manufacturing, for example, internal features can be fabricated which cannot be seen or measured with conventional tools. There is thus a need for non-destructive inspection methods that can measure these geometries. Such a method is X-ray computed tomography. Computed tomography utilizes the X-rays ability to penetrate material to create 3D digital volumes of components. Measurements and material investigations can be performed in these volumes without any damage to the investigated component. However, computed tomography in material science is still not a fully mature method and there are many uncertainties associated with the investigation technique. In the work presented in this thesis geometries fabricated by various additive manufacturing processes have been investigated using computed tomography. Also in this work, a dual-energy computed tomography tool has been developed with the aim to increase the measurement consistency of computed tomography when investigating complex geometries and material combinations. / MultiMatCT

We quantified confidence intervals (CIs) for T-scores for the lumbar spine and hip and determined the practical effect (impact on diagnosis) of variability around the T-score cutpoint of -2.5. Using precision data from the literature for GE Lunar Prodigy dual-energy X-ray absorptiometry (DXA) systems, the 95% CI for the T-score was ±0.23 at the lumbar spine (L1-L4), ± 0.20 at the total hip, and ±0.41 at the femoral neck. Thus, T-score variations of ±0.23 or less at the spine, ±0.20 at the total hip, and ±0.41 at the femoral neck are not statistically significant. When diagnosing osteoporosis, T-scores in the interval -2.3 to -2.7 for spine or total hip (after rounding to conform to guidelines from the International Society for Clinical Densitometry) and -2.1 to -2.9 for femoral neck are not statistically different from -2.5. Better precision values resulted in smaller 95% CIs. This concept was applied to actual clinical data using Hologic DXA systems. The study cohort comprised 2388 white women with either normal or osteopenic spines in whom the densitometric diagnosis of osteoporosis would be determined by hip T-scores. When evaluating actual patient T-scores in the range -2.5 ± 95% CI, we found that the diagnosis was indeterminate in approximately 12% of women when T-scores for femoral neck were used and in 4% of women when T-scores for total hip were used, with uncertainty as to whether the classification was osteopenia or osteoporosis. We conclude that precision influences the variability around T-scores and that this variability affects the reliability of diagnostic classification.

Introduction: We determined the prevalence of left-right differences in hip bone mineral density (BMD) by dual-energy x-ray absorptiometry (DXA) and the resultant consequence, namely: the frequency at which patients would be classified differently if lumbar spine and only one hip (rather than both hips) were measured. Methods: This was a retrospective DXA scan reanalysis of 3012 white women ≥50 yrs who had scans of both hips using Hologic DXA systems. The difference between left and right hips was considered significant if it exceeded the least significant change (LSC) for any of three hip subregions (total hip, femoral neck, trochanter). The number of women with osteoporosis in both hips, the left hip only, or the right hip only was determined by lowest T-score from total hip, femoral neck, or trochanter. Results: Despite high left-right correlations of subregion BMD, significant left-right differences in BMD were common: the difference exceeded the LSC for 47% of women at total hip, 31% at femoral neck, and 56% at trochanter. Left-right differences in BMD that exceeded the LSC affected the percent agreement of left-right hip classification: for all women irrespective of spine status, there was 77% classification (diagnostic) agreement in hip pairs in which the left-right hip BMD difference exceeded the LSC versus 87% agreement in which LSC was not exceeded (significant difference in proportions, P<0.0001). The greatest risk of different classification would occur in women with normal spines as the diagnosis might be determined by hip T-scores. Using L1-4 lumbar spine T-scores, 1229 women were normal at the spine. Twenty-four (2%) were osteoporotic at both hips. However, 12 women (1%) were osteoporotic only in the left hip (significantly different from zero, P<0.001) and 11 (1%) only in the right hip (P<0.001); of these 23 women, the difference in BMD between the osteoporotic hip and the contralateral hip exceeded the LSC in 16 (70% of those with osteoporosis in only one hip). Using L1-4 lumbar spine T-scores, 1159 women were osteopenic at the spine. Of these, 126 (11%) were osteoporotic at both hips, 54 (5%) only in the left hip (P<0.001), and 42 (4%) only in the right hip (P<0.001); of these 96 women, the difference in BMD between the osteoporotic hip and the contralateral hip exceeded the LSC in 56 (58% of those with osteoporosis in only one hip). Conclusions: A statistically significant number of women with osteoporosis are potentially classified differently when scanning only one hip as a result of the high prevalence of left-right differences in BMD. Although the percentages are low, the total number of women affected may be large. From a public health perspective, the practice of scanning both hips could potentially identify more women with osteoporosis and may help prevent future hip fractures. © 2006 International Osteoporosis Foundation and National Osteoporosis Foundation.