X-ray phase-contrast imaging has seen rapid development in recent decades due to its superior performance in imaging low-absorption objects, compared to traditional attenuation x-ray imaging. Having higher demand on coherence, x-ray phase-contrast imaging is performed mostly at synchrotrons. With the development of different imaging techniques, and the development of laboratory sources and x-ray optics, x-ray phase-contrast imaging can now be implemented on laboratory systems, which is promising and practical for broader range of applications. The subject of this thesis is the implementation, development and comparison of different laboratory phase-contrast methods using a liquid-metal-jet source. The three x-ray phase-contrast imaging methods included in this thesis are the propagation-, grating-, and speckle-based techniques. The grating-based method has been implemented on a laboratory system with a liquid-metal-jet source, which yields several times higher brightness than a standard solid-anode microfocus source. This allows shorter exposure time or a higher signal-to-noise ratio. The performance of the grating-based method has been experimentally and numerically compared with the propagation-based method, and the dose required to observe an object as a function of the object’s diameter has been investigated with simulations. The result indicates a lower dose requirement for the propagation-based method in this system but a potential advantage for the grating-based method to detect relatively large samples using a monochromatic beam. The speckle-based method, both the speckle-tracking and speckle-scanning techniques, has been implemented on a laboratory system for the first time, showing its adaptability to radiation of low temporal coherence. Tomography has been performed and shows the potential applications of this method on quantitative analysis on both absorption and phase information of materials. As a basis for further optimization and comparisons to other methods, the noise properties of the differential phase contrast of the speckle-based method have been studied and an analytical expression for the noise variance introduced, showing a similarity to the grating-based method. / Faskontrastavbildning med röntgenstrålning är en teknik som har utvecklats kraftigt de senaste årtiondena, eftersom den fungerar bättre än traditionella, absorptionsbaserade röntgenundersökningar för objekt med låg absorption. Den har dock höga krav på koherens, vilket gjort att den huvudsakligen används vid stora synkrotron-anläggningar. Tack vare utveckligen av nya avbildningstekniker, laboratoriekällor och röntgenoptik kan numera faskontrast användas även med laboratoriesystem, vilket är lovande då tekniken kan användas vid ett större antal olika tillämpningsområden Denna avhandling syftar till att tillämpa, utveckla och jämföra olika faskontrastmetoder i laboratoriemiljö, med en metallstråleröntgenkälla. De tre faskontrastmetoderna som behandlas i denna avhandling är propogation, gitter och speckelbaserad faskontrast. Den gitterbaserade metoden har implementerats i ett laboratoriesystem med en metallstrålekälla som ger flera gånger högre radians än en vanlig, fast mikrofokuskälla. Den högre radians en möjliggör kortare exponeringstider eller högre signal-brusförhållande. Den gitterbaserade tekniken har jämförts experimentellt och numeriskt med den propageringsbaserade metoden. Den strålningsdos som krävs för observera ett objekt, som funktion av dess diameter, har jämförts för de båda teknikerna, den här gången via simuleringar. Resultaten visar på en lägre strålningsdos för den propagationsbaserade tekniken i detta fall, men även att det finns en potentiell fördel för den gitterbaserade tekniken för något större objekt med monokromatisk röntgenstrålning. Speckelbaserade tekniker, nämare bestämt den som bygger på att spåra speckel och den som bygger på att scanna diffusorn, har för första gången implementerats i laboratoriemiljö. Därmed har visats att de fungerar även för strålning med låg tidskoherens. Tekniken har även använts för tomografi och visar möjliga tillämpningar inom kvantitativ analys av material. För att förenkla framtida optimeringar och jämförelser av tekniken med andra metoder, har brusegenskaperna för den speckelbaserade metoden studerats och visat sig likna den gitterbaserade metoden. / <p>QC 20160921</p>

<p>One of the challenges facing clinical practice today is intra-operative margin detection in breast conserving surgeries (BCS) or lumpectomy procedures. When a surgeon removes a breast tumor from a patient during a BCS procedure, the surgically excised tissue specimen is examined to see whether it contains a margin of healthy tissue around the tumor. A healthy margin of tissue around the tumor would indicate that the tumor in its entirety has been removed. On the other hand, if cancerous tissue is at the surface of the specimen, that would indicate that the tumor may have been transected during the procedure, leaving some residual cancerous tissue inside the patient. The most effective intra-operative real-time margin detection techniques currently used in clinical practice are frozen section analysis (FSA) and touch-prep cytology. These methods have been shown to possess inconsistent accuracy, which result in 20% to 30% of BCS patients being called back for a repeat BCS procedure to remove the residual tumor tissue. In addition these techniques have been shown to be time-consuming--requiring the operating room team to have to wait at least 20 minutes for the results. Therefore, there is a need for accurate and faster technology for intra-operative margin detection. </p><p>In this dissertation, we describe an x-ray coherent scatter imaging technique for intra-operative margin detection with greater accuracy and speed than currently available techniques. The method is based on cross-sectional imaging of the differential coherent scatter cross section in the sample. We first develop and validate a Monte Carlo simulation of coherent scattering. Then we use that simulation to design and test coherent scatter computed tomography (CSCT) and coded aperture coherent scatter spectral imaging (CACSSI) for cancerous voxel detection and for intra-operative margin detection using (virtual) clinical trials. Finally, we experimentally implement a CACSSI system and determine its accuracy in cancer detection using tissue histology. </p><p>We find that CSCT and CACSSI are able to accurately detect cancerous voxels inside of breast tissue specimens and accurately perform intra-operative margin detection. Specifically, for the task of individual cancerous voxel detection, we show that CSCT and CACSSI have AUC values of 0.97 and 0.94, respectively. Whereas for the task of intra-operative margin detection, the results of our virtual clinical trials show that CSCT and CACSSI have AUC values of 0.975 and 0.741, respectively. The gap in spatial resolution between CSCT and CACSSI affects the results of intra-operative margin detection much more than it does the task of individual cancerous voxel detection. Finally, we also show that CSCT would require on the order of 30 minutes to create a 3D image of a breast cancer specimen, whereas CACSSI would require on the order of 3 minutes. </p><p>These results of this work show that coherent scatter imaging has the potential to provide more accurate intra-operative margin detection than currently used clinical techniques. In addition, the speed (and therefore low scan duration: 3 min) of CACSSI, along with its ability to automatically classify cancerous tissue for margin detection means that coherent scatter imaging would be much more cost-effective than the clinical techniques that require up to 20 minutes and a trained pathologist. With the cancerous voxel detection accuracy of a 0.94 AUC and scan time of on the order of 3 minutes demonstrated for coherent scatter imaging in this work, coherent scatter imaging has the potential to reduce healthcare costs for BCS procedures and rates of repeat BCS surgeries. The accuracy for CACSSI can be considerably improved to match CSCT accuracy by improving its spatial resolution through a number of techniques: incorporating into the CACSSI reconstruction algorithm the ability to differentiate noise from high frequency signal so that we can image with higher frequency coded aperture masks; implementing a 2D coded aperture mask with a 2D detector; or acquiring additional angles of projection data.</p> / Dissertation

No description available.

The term“Digital X-ray Imaging" refers to a variety of technologies that electronically capture x-ray images. Once captured the images may be electronically processed, stored, displayed and communicated. Digital imaging has the potential to overcome weaknesses inherent in traditional screen-film imaging, with high detection efficiency, high dynamic range and the capability for contrast enhancement. Image processing also makes possible innovative techniques such as computer-aided diagnosis, tomosynthesis, dual-energy imaging, and digital subtraction imaging. Several different approaches to digital imaging are being studied, and in some cases, have been developed and are being marketed. Common to all these approaches are a number of technological and medical issues to be resolved. One of the technological issues is the optimal pixel size for any particular image sensor technology. In general, the spatial resolution of the digital image is limited by the pixel size. Unfortunately while reducing pixel size improves spatial resolution this comes at the expense of signal to noise ratio (SNR). In a scintillator-charge-coupled device (CCD) system, the signal can be increased by improving the efficiency of the scintillator or by reducing noise. This study used a very low noise CCD to determine if image quality, as indicated by the modulation transfer function (MTF), the noise power spectrum (NPS) and the detective quantum efficiency (DQE), could be maintained while reducing pixel size. Two scintillators, one a commonly used radiographic screen the other a thallium doped cesium iodide scintillator, were used and the results compared. The results of this study show that image quality can be maintained as pixel size is reduced and that high DQE can be attained and maintained over a wide range of spatial frequencies with a well designed scintillator.

Collagen scaffolds are porous structures which are used in bioreactors and in a wide range of tissue engineering applications. In these contexts, the scaffolds may be subjected to conditions in which fluid is forced through the structure and the scaffold is simultaneously compressed. It is clear that fluid transport within collagen scaffolds, and the inter-relationships between permeability, scaffold structure, fluid pressure and scaffold deformation are of key importance. However, these relationships remain poorly understood. In this thesis, a series of isotropic collagen structures were produced using a freeze-drying technique from aqueous slurry concentrations 0.5, 0.75 and 1 wt%, and fully characterised using X-ray micro-tomography and compression testing. It was found that collagen wt% influenced structural parameters such as pore size, porosity, relative density and mechanical properties. Percolation theory was used to investigate the pore interconnectivity of each scaffold. Structures with lower collagen fraction resulted in larger percolation diameters, but lower mechanical stiffness. Aligned collagen scaffolds were also produced by altering the freeze-drying protocol and using different types of mould materials and designs. It was found that a polycarbonate mould with stainless base resulted in vertically aligned structures with low angular variation. When compared with isotropic scaffolds from slurry of the same concentration, aligned scaffolds had a larger percolation diameter. Tortuosity was used as a mathematical tool to characterise the interconnected pathways within each porous structure. The effect of the size of the region of interest (ROI) chosen and the size of the virtual probe particle used in the analysis on the values of tortuosity calculated were determined and an optimised calculation methodology developed. Increasing the collagen fraction within isotropic scaffolds increased the tortuosity, and aligned structures had smaller tortuosity values than their isotropic counterparts. Permeability studies were conducted using two complementary experimental rigs designed to cover a range of pressure regimes and the results were compared with predictions from mathematical models and computational simulations. At low pressures, it was found that the lower collagen fraction structures, which had more open morphologies, had higher permeabilities. Alignment of the structure also enhanced permeability. The scaffolds all experienced deformation at high pressures resulting in a restriction of fluid flow. The lower collagen fraction scaffolds experienced a sharper decrease in permeability with increased pressure and aligned structures were more responsive to deformation than their isotropic counterparts. The inter-relationships between permeability, scaffold structure, fluid pressure and deformation of collagen scaffolds were explored. For isotropic samples, permeability followed a broad $(1- \epsilon)^2$ behaviour with strain as predicted by a tetrakaidecahedral structural model, with the constant of proportionality changing with collagen fraction. In contrast, the aligned structures did not follow this behaviour with the permeability dropping much more sharply in the early stages of compression. Open-cell polyurethane (PU) foams, sometimes used as dressings in wound healing applications, are often compared with collagen scaffolds in permeability models and were used in this thesis as a comparison structure. The foam had a higher permeability than the scaffolds due to its larger pore sizes and higher interconnectivity. In the light of the effects of compression on permeability, the changes in porous structure with compression were explored in isotropic and aligned 0.75 wt% scaffolds. Unlike the fluid flow experiments, these experiments were carried out in the dry state. Deformation in simple linear compression and in step-wise compression was studied, and the stress relaxation behaviour of the scaffolds characterised. A methodology was developed to characterise the structural changes accompanying compression using X-ray micro-tomography with an in situ compression stage. The methodology accounted for the need for samples to remain unchanged during the scan collection period for stable image reconstruction. The scaffolds were studied in uniaxial compression and biaxial compression and it was found that pore size and percolation diameter decreased with increasing compressive strain, while the tortuosity increased. The aligned structure was less affected than the isotropic at low compressions, in contrast to the results from the permeability study in which the aligned structure was more responsive to strain. This suggests that the degree of hydration may affect the structural changes observed. The insights gained in this study of the inter-relationships between microstructure, fluid dynamics and deformation in collagen scaffolds are of relevance to the informed design of porous structures for medical applications.

Breast cancer is the most commonly diagnosed cancer among Canadian women; x-ray mammography is the leading screening technique for early detection. This work introduces a semi-automated technique for analyzing mammographic x-ray images to measure their degree of suspiciousness for containing abnormalities. The designed system applies the discrete wavelet transform to parse the images and extracts statistical features that characterize an images content, such as the mean intensity and the skewness of the intensity. A naïve Bayesian classifier uses these features to classify the images, achieving sensitivities as high as 99.5% for a data set containing 1714 images. To generate confidence levels, multiple classifiers are combined in three possible ways: a sequential series of classifiers, a vote-taking scheme of classifiers, and a network of classifiers tuned to detect particular types of abnormalities. The third method offers sensitivities of 99.85% or higher with specificities above 60%, making it an ideal candidate for pre-screening images. Two confidence level measures are developed: first, a real confidence level measures the true probability that an image was suspicious; and second, a normalized confidence level assumes that normal and suspicious images were equally likely to occur. The second confidence measure allows for more flexibility and could be combined with other factors, such as patient age and family history, to give a better true confidence level than assuming a uniform incidence rate. The system achieves sensitivities exceeding those in other current approaches while maintaining reasonable specificity, especially for the sequential series of classifiers and for the network of tuned classifiers.

Breast cancer is the most commonly diagnosed cancer among Canadian women; x-ray mammography is the leading screening technique for early detection. This work introduces a semi-automated technique for analyzing mammographic x-ray images to measure their degree of suspiciousness for containing abnormalities. The designed system applies the discrete wavelet transform to parse the images and extracts statistical features that characterize an images content, such as the mean intensity and the skewness of the intensity. A naïve Bayesian classifier uses these features to classify the images, achieving sensitivities as high as 99.5% for a data set containing 1714 images. To generate confidence levels, multiple classifiers are combined in three possible ways: a sequential series of classifiers, a vote-taking scheme of classifiers, and a network of classifiers tuned to detect particular types of abnormalities. The third method offers sensitivities of 99.85% or higher with specificities above 60%, making it an ideal candidate for pre-screening images. Two confidence level measures are developed: first, a real confidence level measures the true probability that an image was suspicious; and second, a normalized confidence level assumes that normal and suspicious images were equally likely to occur. The second confidence measure allows for more flexibility and could be combined with other factors, such as patient age and family history, to give a better true confidence level than assuming a uniform incidence rate. The system achieves sensitivities exceeding those in other current approaches while maintaining reasonable specificity, especially for the sequential series of classifiers and for the network of tuned classifiers.

<p>Research using mice and rats has gained interest because they are robust test beds for clinical drug development and are used to elucidate disease etiologies. Blood vessel visualization and blood flow measurements are important anatomic and physiologic indicators to drug/disease stimuli or genetic modification. Cardio-pulmonary blood flow is an important indicator of heart and lung performance. Small animal functional imaging provides a way to measure physiologic changes minimally-invasively while the animal is alive, thereby allowing for multiple measurements in the same animal with little physiologic perturbation. Current methods of measuring cardio-pulmonary blood flow suffer from some or all of these limitations-they produce relative measurements, are limited to global or whole animal or organ regions, do not provide vasculature visualization, limited to a few or singular samples per animal, are not able to measure acute changes, or are very invasive or requires animal sacrifice. The focus of this work was the development of a small animal x-ray imaging system capable of minimally invasive real-time, high resolution vascular visualization, and cardio-pulmonary blood flow measurements in the live animal. The x-ray technique used was digital subtraction angiography (DSA). This technique is a particularly appealing approach because it is easy to use, can capture rapid physiological changes on a heart beat-to-beat basis, and provides anatomical and functional vasculature information. This DSA system is special because it was designed and implemented from the ground up to be optimized for small animal imaging and functional measurements. This system can perform: 1) minimally invasive in vivo blood flow measurements, 2) multiple measurements in the same animal in a rapid succession (every 30 seconds-a substantial improvement over singular measurements that require minutes to acquire by the Fick method), 3) very high resolution (up to 46 micron) vascular visualization, 4) quantitative blood flow measurements in absolute metrics (mL/min instead of arbitrary units or velocity) and relative blood volume dynamics from discrete ROIs, and 5) relative mean transit time dynamics on a pixel-by-pixel basis (100 µm x 100 µm). The end results are 1) anatomical vessel time course images showing the contrast agent flowing through the vasculature, 2) blood flow information of the live rat cardio-pulmonary system in absolute units and relative blood volume information at discrete ROIs of enhanced blood vessels, and 3) colormaps of relative transit time dynamics. This small animal optimized imaging system can be a useful tool in future studies to measure drug or disease modulated blood flow dynamics in the small animal.</p> / Dissertation

The current thesis is focused on the study of tomosynthesis techniques applied on breast imaging, in order to improve the detection of breast lesions. Breast Tomosynthesis (BT) is a pseudo-three-dimensional (3D) x-ray imaging technique that provides reconstructed tomographic images from a set of angular projections taken in a limited arc around the breast, with dose levels similar to those of a two-view conventional mammography. Simulation studies and clinical trials suggest that BT is very useful for imaging the breast in an attempt to optimize the detection and characterization of lesions particularly in dense breasts and has the potential to reduce the recall rate. Reconstruction algorithms and acquisition parameters are critical for the quality of reconstructed slices. The aim of this research is to explore tomosynthesis modalities for breast imaging and evaluate them against existing mammographic techniques as well as to investigate the effect of reconstruction algorithms and acquisition parameters on the image quality of tomosynthetic slices. A specific aim and innovation of the study was to demonstrate the feasibility of combining BT and monochromatic radiation for 3D breast imaging, an approach that had not been studied thoroughly yet. For the purposes of this study a computer-based platform has been developed in Matlab incorporating reconstruction algorithms and filtering techniques for BT applications. It is fully parameterized and has a modular architecture for easy addition of new algorithms. Simulations studies with the XRayImaging Simulator and experimental work at ELETTRA Synchrotron facilities in Trieste, Italy, have been performed using software and complex hardware phantoms, of realistic shape and size, consisting of materials mimicking the breast tissue. The work has been carried out in comparison to conventional BT and mammography and demonstrates the feasibility of the studied new technique and the potential advantages of using BT with synchrotron modality for the detection of breast low- and high-contrast breast lesions such as masses and microcalcifications (μCs). Evaluations of both simulation and experimental tomograms demonstrated superior visibility of all reconstructed features using appropriately optimized filtered algorithms. Moreover, image quality and evaluation metrics are improved with extending the acquisition length for the masses. The visualization of μCs was found less sensitive to this parameter due to their high inherent contrast. Breast tomosynthesis shows advantages in visualizing features of small size within phantoms of increased thickness and especially in bringing into focus and localizing low-contrast masses hidden in a highly heterogeneous background with superimposed structures. Monochromatic beams can result in better tissue differentiation and in combination with BT can lead to improvement of features’ visibility, better detail and higher contrast. Monochromatic BT provided improved image quality at lower incident exposures, compared to conventional mammography, concerning mass detection and visibility of borders, which is important for their characterization, especially when they are spiculated. Overall it has been proved that while reducing the radiation dose, monochromatic beams combined with BT, result in an improvement of image quality. These findings are encouraging for the development of a tomosynthesis system based on monochromatic beams. / Η συγκεκριμένη διατριβή εστιάζει στη μελέτη των τεχνικών της τομοσύνθεσης όπως αυτές εφαρμόζονται στην απεικόνιση του μαστού, με στόχο την βελτίωση της ανίχνευσης των αλλοιώσεων του μαστού. Η τομοσύνθεση του μαστού είναι μια τεχνική ψευδό-τρισδιάστατης απεικόνισης με ακτίνες-χ που ανακατασκευάζει τομογραφικές εικόνες χρησιμοποιώντας μια σειρά προβολικών λήψεων υπό διαφορετικές γωνίες σε περιορισμένο τόξο γύρω από το μαστό και με δόσεις ακτινοβολίας παρόμοιες με εκείνες που απαιτούνται για τις δύο τυπικές λήψεις της κλασικής μαστογραφία. Μελέτες προσομοίωσης και κλινικές δοκιμές δείχνουν πως η τομοσύνθεση του μαστού βελτιώνει την απεικόνιση του μαστού, με αποτέλεσμα την καλύτερη ανίχνευση των αλλοιώσεων ειδικά σε πυκνούς μαστούς και αναμένεται ότι η εφαρμογή της θα μπορούσε να μειώσει την ανάγκη επανάληψης της εξέτασης. Οι αλγόριθμοι ανακατασκευής και οι παράμετροι λήψης των προβολικών εικόνων είναι μεγάλης σημασίας για την ποιότητα των ανακατασκευασμένων τομογραφικών εικόνων. Ο στόχος αυτής της έρευνας είναι να μελετήσει τεχνικές που βασίζονται στην τομογραφική απεικόνιση του μαστού και να τις συγκρίνει με υπάρχουσες τεχνικές μαστογραφίας καθώς και να διερευνήσει την επίδραση των αλγορίθμων ανακατασκευής και των παραμέτρων λήψης στην ποιότητα της ανακατασκευασμένης τομογραφικής εικόνας. Ένας συγκεκριμένος στόχος και καινοτομία αυτής της μελέτης ήταν να διερευνήσει πιθανά πλεονεκτήματα και να επιδείξει την σκοπιμότητα του συνδυασμού της τομοσύνθεσης του μαστού με μονοχρωματική ακτινοβολία που παράγεται από σύγχροτρον για την τρισδιάστατη απεικόνιση του μαστού, μία προσέγγιση που δεν είχε ακόμα μελετηθεί εκτενώς. Για τις ανάγκες αυτής της μελέτης αναπτύχθηκε στο Matlab μια πλατφόρμα που ενσωματώνει αλγορίθμους ανακατασκευής και τεχνικές φιλτραρίσματος για τομοσύνθεση μαστού. Η εφαρμογή είναι πλήρως παραμετροποιημένη και σχεδιασμένη ώστε να είναι εύκολη η προσθήκη νέων αλγορίθμων. Προσομοιώσεις με τη χρήση του προσομοιωτή XRayImagingSimulator καθώς και πειραματικές μελέτες στις εγκαταστάσεις σύγχροτρον ELETΤRA, στην Τεργέστη της Ιταλίας έχουν πραγματοποιηθεί, με χρήση απλών και σύνθετων ομοιωμάτων μαστού, μιμούμενα τις ιδιότητες του ιστού του μαστού, με ρεαλιστικό μέγεθος και σχήμα. Οι μελέτες έχουν πραγματοποιηθεί σε σύγκριση με την τυπική τομοσύνθεση μαστογραφία και δείχνουν πόσο εφικτή είναι η νέα τεχνική και τα δυνητικά πλεονεκτήματα της τομοσύνθεσης του μαστού με χρήση μονοχρωματικής ακτινοβολίας για την εύρεση χαμηλής και υψηλής αντίθεσης αλλοιώσεων όπως μάζες και μικροαποτιτανώσεις. Εκτιμήσεις των τομογραφικών εικόνων που έχουν προκύψει τόσο από προσομοιώσεις όσο και από πειράματα δείχνουν βελτιωμένη απεικόνιση όλων των ανακατασκευασμένων στοιχείων ενδιαφέροντος με χρήση κατάλληλων βελτιστοποιημένων φίλτρων. Επιπλέον, η ποιότητα της εικόνας βελτιώνεται με τη διεύρυνση του τόξου λήψης για τις μάζες, ενώ η απεικόνιση των μικροαποτιτανώσεων βρέθηκε να είναι λιγότερο ευαίσθητη σε αυτή τη παράμετρο λόγω της υψηλότερης αντίθεσης που έχουν σε σχέση με τον περιβάλλοντα φυσιολογικό ιστό του μαστού. Η τομοσύνθεση του μαστού φάνηκε να έχει πλεονεκτήματα στην απεικόνιση αλλοιώσεων μικρού μεγέθους και πιο συγκεκριμένα στο να διακρίνει και να ανιχνεύει χαμηλής αντίθεσης μάζες, μέσα σε πυκνούς μαστούς με έντονα ετερογενή σύσταση, μετριάζοντας τα προβλήματα επικάλυψης. Η μονοχρωματική ακτινοβολία μπορεί να προσφέρει καλύτερη διαφοροποίηση των ιστών του μαστού και σε συνδυασμό με την τομοσύνθεση μπορεί να οδηγήσει στην βελτίωση της απεικόνισης των αλλοιώσεων και στην παραγωγή εικόνων με καλύτερη λεπτομέρεια και υψηλότερη αντίθεση. Γενικά βρέθηκε ότι η μονοχρωματική τομοσύνθεση του μαστού παρέχει βελτιωμένη ποιότητα εικόνας, σε σύγκριση με την κλασική μαστογραφία, όσον αφορά την ανίχνευση όγκων και την ορατότητα των περιγραμμάτων τους, που είναι σημαντική για τον χαρακτηρισμό των μαζών, ειδικά όταν δεν έχουν καλώς καθορισμένα όρια. Συνολικά η μελέτη αυτή έδειξε ότι ακόμα και με μικρότερη δόση ακτινοβολίας, η χρήση μονοχρωματικής ακτινοβολίας σε συνδυασμό με την τομοσύνθεση του μαστού, έχουν ως αποτέλεσμα την βελτίωση της εικόνας, γεγονός που είναι ενθαρρυντικό για την ανάπτυξη ενός συστήματος τομοσύνθεσης βασισμένο σε ακτίνες-χ μονοχρωματικής δέσμης.

Modern image guided radiation therapy involves the use of an isocentrically mounted imaging system to take radiographs of a patient's position before the start of each treatment. Image guidance helps to minimize errors associated with a patients setup, but the radiation dose received by patients from imaging must be managed to ensure no additional risks. The Varian On-Board Imager (OBI) (Varian Medical Systems, Inc., Palo Alto, CA) does not have an automatic exposure control system and therefore requires exposure factors to be manually selected. Without patient specific exposure factors, images may become saturated and require multiple unnecessary exposures. A software based automatic exposure control system has been developed to predict optimal, patient specific exposure factors. The OBI system was modelled in terms of the x-ray tube output and detector response in order to calculate the level of detector saturation for any exposure situation. Digitally reconstructed radiographs are produced via ray-tracing through the patients' volumetric datasets that are acquired for treatment planning. The ray-trace determines the attenuation of the patient and subsequent x-ray spectra incident on the imaging detector. The resulting spectra are used in the detector response model to determine the exposure levels required to minimize detector saturation. Images calculated for various phantoms showed good agreement with the images that were acquired on the OBI. Overall, regions of detector saturation were accurately predicted and the detector response for non-saturated regions in images of an anthropomorphic phantom were calculated to generally be within 5 to 10 % of the measured values. Calculations were performed on patient data and found similar results as the phantom images, with the calculated images being able to determine detector saturation with close agreement to images that were acquired during treatment. Overall, it was shown that the system model and calculation method could potentially be used to predict patients' exposure factors before their treatment begins, thus preventing the need for multiple exposures. / Graduate / 0760 / 0574 / 0756