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The utilization of Computed Tomography in paediatric examinations constantly increases. During the procedure, a high amount of dose is delivered to children, which could be avoided. This study examined whether the selection of scanning parameters – tube voltage and tube current-time product- could be based on patient size instead of patient age or weight aiming, for dose reduction. The SRS78 spectrum processor software was employed to generate spectra with tube voltage between 80kVp and 120kVp and with tube current-time product between 50mAs and 165mAs. These spectra were attenuated by different thicknesses of polymethylmethacrylate (PMMA) phantoms. The simulation technique was validated with experimental measurements acquired on CTDI phantoms on a Siemens Somatom plus 4 scanner. The image quality was assessed in terms of noise, contrast and contrast-to-noise ratio (CNR). Furthermore the contrast of iodine, adipose tissue and cortical-bone relative to muscle were calculated in order to examine how the contrast of different materials was influenced when tube voltage changed. The data analysis shows that there is a definite relationship between image quality and the size of a patient. When exposure settings are kept constant, the level of noise, contrast and Contrast-to-Noise Ratio (CNR) depends on the size of the phantom. Noise is increased exponentially and contrast is reduced linearly as the size of the phantom is increased. CNR is markedly higher in small size phantoms. Moreover, when tube voltage was reduced the noise level was increased less in the small size phantoms and the contrast of high atomic number materials is reduced more when tube voltage is reduced. The CNR for high atomic number materials presents modest improvements when tube voltage is increased therefore examinations with contrast agents could be performed at lower tube voltages. Furthermore the high CNR in small size phantoms could be traded off with lower mAs. In particularly the mAs could be reduced by up to 95% while maintaining the same CNR as for adults resulting in dramatic dose reductions for children. Moreover, since Computed Tomography stands out from all the other X-ray techniques due to its ability to detect structures of similar densities the detect ability of low contrast details was investigated. The Catphan phantom and particularly the CTP515 module was employed. The phantom was scanned with the Siemens Somatom plus 4 scanner at 80kVp, 120kVp and 140kVp and with tube current-time product between 43mAs and 165mAs. The image quality was assessed subjectively and objectively. It is observed that when 120kVp and 140kVp are applied there are not sufficient differences on image quality which justify the selection of 140kVp in paediatric protocols. When 80kVp is applied structures with contrast lower than 10HU are not detected. Concerning mAs does not contribute to the detection of low contrast details except if it is combined with high tube voltages. However, mAs contribute to the visualisation of smaller in size details but above a threshold value, higher mAs does not serve any purpose and the value of 300mAs employ in many protocols is not justified. In conclusion, the reduction of dose during paediatric Computed Tomography examinations is more than probable since scanning parameters could be reduced without degradation of image quality. However in order to assure the reduction of dose without side effects, protocols must be constructed which will individualize the Computed Tomography examinations. That is, the optimum spectrum must be selected relative to the diagnostic task and the size of the patient. / -

In 3D positron emission tomography the scatter effect is a significant physical factor degrading image quality. The advancements in computing that occurred the last decades al lowed us to simulate the scatter coincidences fast and ef ficiently. The main concern now is how accurately do we simulate the scatter events. The scope of this project is the implementation and the evaluation of a scatter simulation algorithm that would be able to simulate the scat ter ef fect more precisely than the existing ones. One way to simulate the scatter distribut ion is with an algorithm, first published by Watson and Ollinger, that is based on the Klein-Nishina formula. These methods have been implemented taking into account only the single scatter events (where a photon scatters once in the at tenuation medium). Multiple scatter is generally taken into account by some scaling or filtering procedure. Their main advantages are short computational time and relatively good precision compared to previous more heuristical methods. Although these single scatter algorithms have been effective there are cases where their results are not that accurate. For low energy thresholds and large at tenuation mediums multiple scatter is increased. Given that a significant percentage of people are over -weight (thus the at tenuation medium has large volume) we should consider introducing multiple scatter events in our simulations. Moreover, the distribut ion of all scatter events is broader than the one of single scatter events therefore even if the single scatter distribution is scaled it will not match the total scatter one. In previous work by C. Tsoumpas et al, a new scatter simulation algorithm was developed that attempts to approximate the total scatter distribution by taking into account the case where the one annihilated photon is scattered twice and the case where both annihilated photons are scattered once. These two cases describe the double scatter events and by introducing them into our scatter simulation algorithm we aim to obtain a better approximation of the total scatter distribution. In this thesis we have improved this double scatter simulation algorithm in two important ways. When both annihilated photons scatter they acquire a favourable polarization direction with respect to each other and this influences thei r detect ion probabilities, especially when low energy photons are detected. In the algorithm that we implemented we considered this effect by using the polarized Klein-Nishina formula for this case. In addition, we investigated and validated the need to introduce extra solid angle factors in the implementation. The whole implementation is based on the STIR library (Software for Tomographic Image Reconstruction) written in the C++ programming language. Scatter events can also be simulated by Monte Carlo simulation packages such as SimSET. SimSET is a public domain package designed to simulate positron emission tomography (PET) (and Single Photon Emission Tomography) and was used extensively in this project. Monte Carlo packages because of their ability to exclude any unknown physical parameter they can simulate physical processes like the ones that take place in PET very accurately. Thus they were essential for the evaluation of our scatter correction algorithm. The reason why Monte Carlo packages are not used inclinical practice instead of the model-based methods is that they demand a large computational time. Besides Monte Carlo packages we also per formed a series of experimental scans in order to evaluate our scatter simulation algorithm. The tomograph used for the experiments was the ECAT 962 used in a 3D mode. / -

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Στη συγκεκριμένη διατριβή περιλαμβάνεται όλη η απαραίτητη θεωρία της σεισμικής τομογραφίας όπως και ιστορική αναδρομή των μεθόδων που προηγήθηκαν αυτής που παρουσιάζεται στην ανα χείρας μελέτη. Γίνεται σύγκριση των μεθόδων ειδικά σε δεδομένα από επιφανειακά πειράματα και μελετάται η αντίδραση των αλγορίθμων σε περιπτώσεις εντόνου αντιθέσεως στη ταχύτητα των σεισμικών κυμάτων, αναλυτική μεθοδολογία ανάλυσης αβεβαιοτήτων με στατιστικές μεθόδους, θεωρητική ανάπτυξη του ευθέους αλλά και του αντιστρόφου προβλήματος, χρήση για πρώτη φορά σε ανάλογα προβλήματα των αλγορίθμων Kohonen, μεθοδολογία βέλτιστης λήψης δεδομένων σε δυσχερείς συνθήκες και τέλος λήψη, ανάλυση, ποιοτικός έλεγχος, επεξεργασία και ερμηνεία ενός μεγάλου όγκου σεισμικών δεδομένων, αλλά και ορισμένων γεωηλεκτρικών που έδρασαν συνεπικουρικά, πάνω σε ένα γεωτεχνικό πρόβλημα. Η καινοτομία της διατριβής έγκειται στην εισαγωγή γεωμετρίας Finsler για την επίλυση των προβλημάτων που δημιουργούνται στα επιφανειακά προβλήματα από τις έντονες αντιθέσεις των σεισμικών κυμάτων. / In this PhD we include all the necessary theory on seismic tomography as well as an historical review of older methods. We compare these methods especially on shallow experiments data, we compare the uncertainties with statistical methods, we develop new methods on forward and inverse problem, we use also Kohonen algorithms. The data we gathered apart from seismic ones were also downhole and electrical data in order to compare the validity of the method. We also applied Finsler geometry in order to overpass the problems that occur in shallow experiments.

Objective: To characterize the association between longitudinal changes in fasting serum glucose and changes in flourodeoxyglucose Positron Emission Tomography (FDG PET) measurements of regional cerebral metabolic rate for glucose (rCMRgl) in brain regions preferentially affected by Alzheimer's disease (AD). A secondary objective was to investigate whether higher fasting serum glucose levels are associated with lower rCMRgl in younger adults within these same AD relevant brain areas. Methods: For the primary study, baseline, interim, and 4.4 ± 1.0-year follow-up fasting serum glucose and PET CMRgl were analyzed in 80 cognitively unimpaired, non-diabetic, 61.5 ± 5 year-old persons with a first-degree family history of AD, including 38 carriers and 42 non-carriers of the apolipoprotein E (APOE) ε 4 allele. An automated brain-mapping algorithm was used to characterize associations between changes in fasting serum glucose levels and changes in rCMRgl. Longitudinal changes in fasting serum glucose levels and their correlation with changes in six pre-selected neuropsychological test measures of memory, attention and processing speed were also assessed with linear regression. The secondary study included a cross sectional sample of 31 cognitively unimpaired, non-diabetic participants, 31.2 ±5.4 years of age. General linear model-based voxel-wise analyses were performed to examine the correlation between fasting serum glucose and rCMRgl. Results: In the primary study of older adults, average fasting serum glucose levels increased over longitudinal measurement, and changes in these levels were inversely associated with longitudinal CMRgl changes in the vicinity of brain regions preferentially affected by AD (p<0.05, corrected for multiple comparisons). Fasting serum glucose was also inversely associated with performance on a measure of visuospatial memory (p<0.05, corrected for multiple comparisons). In the younger sample, fasting serum glucose levels were inversely associated with rCMRgl in left frontal pole and right primary visual cortex regions (p<.05, corrected for multiple comparisons).Conclusions: In older adults, fasting serum glucose increases across time and is inversely related to rCMRgl in AD relevant regions and to visual memory test scores. This relationship between serum glucose and regional brain metabolism may begin in metabolically sensitive areas at a younger age.

This study investigates three high resolution optical imaging modalities to better detect and diagnose skin cancer. The ideal high resolution optical imaging system can visualize pre-malignant tissue growth non-invasively with resolution comparable to histology. I examined 3 modalities which approached this goal. The first method examined was high magnification microscopy of thin stained tissue sections, together with a statistical analysis of nuclear chromatin patterns termed Karyometry. This method has subcellular resolution, but it necessitates taking a biopsy at the desired tissue site and imaging the tissue ex-vivo. My part of this study was to develop an automated nuclear segmentation algorithm to segment cell nuclei in skin histology images for karyometric analysis. The results of this algorithm were compared to hand segmented cell nuclei in the same images, and it was concluded that the automated segmentations can be used for karyometric analysis.The second optical imaging modality I investigated was Optical Coherence Tomography (OCT). OCT is analogous to ultrasound, in which sound waves are delivered into the body and the echo time and reflected signal magnitude are measured. Due to the fast speed of light and detector temporal integration times, low coherence interferometry is needed to gate the backscattered light. OCT acquires cross sectional images, and has an axial resolution of 1-15 µm (depending on the source bandwidth) and a lateral resolution of 10-20 µm (depending on the sample arm optics). While it is not capable of achieving subcellular resolution, it is a non-invasive imaging modality. OCT was used in this study to evaluate skin along a continuum from normal to sun damaged to precancer. I developed algorithms to detect statistically significant differences between images of sun protected and sun damaged skin, as well as between undiseased and precancerous skin.An Optical Coherence Microscopy (OCM) endoscope was developed in the third portion of this study. OCM is a high resolution en-face imaging modality. It is a hybrid system that combines the principles of confocal microscopy with coherence gating to provide an increased imaging depth. It can also be described as an OCT system with a high NA objective. Similar to OCT, the axial resolution is determined by the source center wavelength and bandwidth. The NA of the sample arm optics determines the lateral resolution, usually on the order of 1-5 µm. My effort on this system was to develop a handheld endoscope. To my knowledge, an OCM endoscope has not been developed prior to this work. An image of skin was taken as a proof of concept. This rigid handheld OCM endoscope will be useful for applications ranging from minimally invasive surgical imaging to non-invasively assessing dysplasia and sun damage in skin.

Over the past several years, experiments in adaptive optics involving multiple natural and laser guide stars have been carried out at the 1.55 m Kuiper telescope and the 6.5 m MMT telescope. The astronomical imaging improvement anticipated from both ground-layer and tomographic adaptive optics has been calculated. Ground-layer adaptive optics will reduce the effects of atmospheric seeing, increasing the resolution and sensitivity of astronomical observations over wide fields. Tomographic adaptive optics will provide diffraction-limited imaging along a single line of sight, increasing the amount of sky coverage available to adaptive optics correction.A new facility class wavefront sensor has been deployed at the MMT which will support closed-loop adaptive optics correction using a constellation of five Rayleigh laser guide stars and the deformable F/15 secondary mirror. The adaptive optics control loop was closed for the first time around the focus signal from all five laser signals in July of 2007, demonstrating that the system is working properly. It is anticipated that the full high-order ground-layer adaptive optics loop, controlled by the laser signals in conjunction with a tip/tilt natural guide star, will be closed in September 2007, with the imaging performance delivered by the system optimized and evaluated.The work here is intended to be both its own productive scientific endeavor for the MMT, but also as a proof of concept for the advanced adaptive optics systems designed to support observing at the Large Binocular Telescope and future extremely large telescopes such as the Giant Magellan Telescope.

This dissertation explores subduction zone-related deformation both on short time scales in the form of subduction zone earthquakes and over larger time and geographical scales in the form of subduction rollback or detachment of the subducting lithosphere. The study presented here is composed of two parts. First, we analyzed the source-rupture processes of the April 1, 2007 Solomon Islands Earthquake (Mw=8.1) using a body-wave inversion technique. Our analysis indicated that the earthquake ruptured approximately 240 km of the southeast Pacific subduction zone in two sub-events.In the second part of this study, we used shear-wave splitting analysis to investigate the effects of the subducting African lithosphere on the upper-mantle flow field beneath the Anatolian Plate in the Eastern Mediterranean region. Our shear-wave splitting results are consistent with relatively uniform southwest-directed flow towards the actively southwestward-retreating Aegean slab. Based on spatial variations in observed delay times we identified varying flow speeds beneath Anatolia and we attribute this variation to the differential retreat rates of the Aegean and the Cyprean trenches.Finally, we used teleseismic P-wave travel-time tomography to image the geometry of the subducting African lithosphere beneath the Anatolia region. Our tomograms show that the subducting African lithosphere is partitioned into at least two segments along the Cyprean and the Aegean trenches. We observed a gap between the two segments through which hot asthenosphere ascends beneath the volcanic fields of western Anatolia. Our results show that the Cyprean slab is steeper than the Aegean slab. We inferred that this steep geometry, in part, controls the flow regime of asthenosphere beneath Anatolia causing variations in flow speeds inferred from shear-wave splitting analysis.

More and more researchers and clinicians are looking to molecular sensing to predict how cells will behave, seeking the answers to questions like "will these tumor cells become malignant?" or "how will these cells respond to chemotherapy?" Optical methods are attractive for answering these questions because optical radiation is safer and less expensive than alternative methods, such as CT which uses X-ray radiation, PET/SPECT which use gamma radiation, or MRI which is expensive and only available in a hospital setting. In this dissertation, three distinct optical methods are explored to detect at the molecular level: optical coherence tomography (OCT), laser-induced fluorescence (LIF), and optical polarimetry. OCT has the capability to simultaneously capture anatomical information as well as molecular information using targeted contrast agents such as gold nanoshells. LIF is less useful for capturing anatomical information, but it can achieve significantly better molecular sensitivity with the use of targeted fluorescent dyes. Optical polarimetry has potential to detect the concentration of helical molecules, such as glucose. All of these methods are noninvasive or minimally invasive.The work is organized into four specific aims. The first is the design and implementation of a fast, high resolution, endoscopic OCT system to facilitate minimally invasive mouse colon imaging. The second aim is to demonstrate the utility of this system for automatically identifying tumor lesions based on tissue microstructure. The third is to demonstrate the use of contrast agents to detect molecular expression using OCT and LIF. The last aim is to demonstrate a new method based on optical polarimetry for noninvasive glucose sensing.