A novel Fourier technique for solving a wide variety of boundary value problems is introduced. The technique, called Fourier projection, is based on the geometric properties of vector calculus operators in reciprocal space. Fourier projection decomposes arbitrary vector fields into collections of irrotational and/or divergenceless dipole subfields. For well-posed problems, Fourier projection algorithms can calculate unknown field values from a knowledge of primary sources and boundary conditions. Specifically, this technique is applied to several problems associated with biomagnetic imaging, including volume current calculations and equivalent surface current solutions. In addition, a low-cost magnetic field mapping system designed to aid reconstruction algorithm development is described.

This dissertation explores the application of objective assessment of image quality (OAIQ) to hardware evaluation for both linear shift-variant and nonlinear imaging systems. We define our task to be the detection of a known signal in either a uniform or structured background. In particular, we study the detection of signals in digital radiography and optical tomography. In digital radiography, current figures of merit are based on stationarity assumptions on the data. The Hotelling observer as we compute it does not make such assumptions. We quantify, from a detection-theoretic perspective, the errors incurred by using stationarity assumptions for nonstationary digital data. We find that by using Monte Carlo methods, the Hotelling observer carries over to the nonlinear setting, and we use it to study spatially varying detectability in optical tomography. In optical tomography there are several data types that can be used to detect signals. Using our methodology, we quantify the information content of those data types. Our results show that information content depends on the type of signal and background as well as how deep the signal is in the tissue. This type of analysis is meant to guide experimental techniques to be suited for the desired detection task.

In this dissertation we explore theoretical and computational methods to investigate Bayesian ideal observers for performing signal-detection tasks. Object models are used to take into account object variability in image backgrounds and signals for the detection tasks. In particular, lumpy backgrounds (LBs) and Gaussian signals are used for various paradigms of signal-detection tasks. Simplified pinhole imaging systems in nuclear medicine are simulated for this work. Markov-chain Monte Carlo (MCMC) methods that estimate the ideal observer test statistic, the likelihood ratio, for signal-known-exactly (SKE) tasks, where signals are nonrandom, are employed. MCMC methods are extended to signal-known-statistically (SKS) tasks, where signals are random. Psychophysical studies for the SKE and SKS tasks using non-Gaussian and Gaussian distributed LBs are conducted. The performance of the Bayesian ideal observer, the human observer, and the channelized-Hotelling observer for the SKE and SKS tasks is compared. Human efficiencies for both the SKE tasks and SKS tasks are estimated. Also human efficiencies for non-Gaussian and Gaussian-distributed LBs are compared for the SKE tasks. Finally, the theory of the channelized-ideal observer (CIO) is introduced to approximate the performance of the ideal observer by the performance of the CIO in cases where the channel outputs of backgrounds and signals are non-Gaussian distributed. Computational approaches to estimate the CIO are investigated.

No description available.

Commercial positron emission tomography scanners that use block detectors have additional blurring on spatial resolution, referred to as block effect. We studied the origin of the block effect, using experiments in which all other blurring effects were minimized and precisely determined. Bismuth germanate crystals (1 mm width) and a small (1 mm) 68Ge source were used to probe the spatial resolution of a CTI HR+ block detector and two precise translation stages to move detectors. Coincidence aperture functions for crystals in the block and for single crystals were compared. The central crystals in the block showed an additional blurring of 0.8 mm whereas the edge ones showed no additional blurring. The apparent centroids of the crystals in the block are not located at the geometric centers, which gives errors in the reconstruction algorithm assumed uniform sampling. Our results suggest that the additional blurring in scanners with block detectors is not only due to the use of block detectors.

No description available.

A new model is proposed for the calculation of relative depth doses for diagnostic radiology using a direct photon transport/ray tracing technique which incorporates both primary and first scatter dose. The x-ray spectra are generated from computer algorithms based on the Birch and Marshall semi-empirical model; the spectra are established by matching calculated and measured transmission data. The algorithms for the generation of x-ray spectra and for the calculation of depth doses are described. Relative depth doses are determined for a number of radiographic techniques. The calculations are compared with measured and published depth doses; the agreement is very good for tube voltages below 90 kV$ sb{ rm p}$. It is suggested that relative integral doses obtained from relative depth doses give an accurate representation of risk reductions obtained with different radiological techniques. The integral dose reductions predicted by the model are within 8.5% of those from measured data.

Permanent Low Dose Rate (LDR) brachytherapy is mostly used for the treatment of prostate cancer and is also used for breast cancer treatment. The dosimetry is made using the TG-43 protocol in which the interseed and the tissue-composition effects are ignored. The interseed effect is the impact of the presence of the seeds on the dosimetry. By ignoring that effect, the Dose Volume Histogram (DVH) is right-shifted and dose analysis parameters such as the D90 are overestimated. The tissue-composition effect is due to the presence of materials different from water around the seeds. In prostate tissue, the DVH is right shifted when this effect is ignored and the dosimetry is made using the TG-43 formalism. In breast tissue the DVH is left shifted when the tissue-composition effect is ignored. The tissue-composition effect is more important in breast tissue than in prostate tissue, so that parameters like the D90 are greatly underestimated by doing the dosimetry for a breast permanent LDR brachytherapy treatment using the TG-43 protocol. / La curiethérapie permanente à faible débit de dose est surtout utilisée pour traiter le cancer de la prostate et est aussi utilisée pour le traitement du cancer du sein. La dosimétrie est faite en utilisant le formalisme du TG-43 dans lequel l'effet intergrain et l'effet de composition sont ignorés. L'effet intergrain est l'impact de la présence des grains sur la dosimétrie. Quand cet effet est ignoré, le DVH (Dose Volume Histogram) est décalé vers la droite et les paramètres dosimétriques comme la D90 sont surestimés. L'effet de composition est dû à la présence de matériel différent de l'eau autour des grains. Dans du tissus prostatique, le DVH est décalé vers la droite quand cet effet est ignoré et que la dosimétrie est faite avec le protocole TG-43. Dans le sein, le DVH est décalé vers la gauche quand l'effet de composition est ignoré. L'effet de composition est plus important pour le sein que pour la prostate. Conséquemment, les paramètres comme la D90 sont grandement sous-estimés en faisant la dosimétrie pour un traitement du sein en curiethérapie permanente à faible débit avec le protocole TG-43.

Clinical use of shaped mega-voltage photon beams in the treatment of cancer has become an essential component of contemporary treatment with an ever-increasing reliance on plans that include computer controlled temporal and spatial shaping of the beam aperture. One such dynamic technique involves the use of the Enhanced Dynamic Wedge (EDW), which modulates the radiation dose across one axis of a rectangular field via the movement of a collimator jaw under computer control. The majority of clinical applications of this technique employ a field that is either symmetric and centered upon the central axis of the beam or involves a "half-blocked" field extending from the central axis out some distance. However there are many situations in which an off-axis (asymmetric) field would be optimal (e.g. for extremities or plans with multiple planning target volumes are irradiated from a single gantry angle) so it is necessary to understand the effect of moving the center of the EDW shaped field off the axis. In this study we have investigated the variance between the treatment planning system and the dose predicted according to the current clinical model. A procedure has been devised to correct for these variances and bring the predictions into agreement with one another (and with measurements in phantom). / L'usage clinique de rayons de photon de méga-tension formés dans le traitement de cancer est devenu un composant essentiel de traitement contemporain avec une confiance jamais-qui augmente sur les projets qui incluent l'ordinateur moulage contrôlé, temporel et spatial de l'ouverture de rayon. Une telle technique dynamique implique l'usage du Enhanced Dynamic Wedge (EDW), qui module la dose de rayonnement à travers un axe d'un champ rectangulaire via le mouvement d'une mâchoire de collimator sous le contrôle informatique. La majorité d'applications cliniques de cette technique emploie un champ qui est ou symétrique et centré sur l'axe central du rayon ou implique un le champ moitié-bloqué s'étendant de l'axe central hors quelque distance. Cependant il y a beaucoup de situations dans lesquelles un d'-axe (asymétrique) le champ serait optimal (par ex pour les extrémités ou les projets avec les volumes de cible de planification de multiple sont irradié d'un angle de portique seul) si c'est nécessaire de comprendre l'effet de déménagement du centre du champ en forme d'EDW de l'axe. Dans cette étude nous avons examiné la variance entre le traitement planifiant le système et la dose prédite selon le modèle clinique actuel. Une procédure a été conçue pour rectifier ces variances et amène les prédictions dans l'accord avec l'un l'autre (et avec les mesures dans le fantôme). fr

The goal of this project is to enhance the teaching and the self study of diagnostic and radiotherapy x-ray imaging by creating an interactive educational software package, ImaSim, based on a simulation environment. Various imaging modalities found in a radiology or radiation oncology department have been included. ImaSim aims at faithfully reproducing the physics behind these modalities while keeping the operation simple and straightforward. Photons simulated fall into the energy range encompassing radiology and radiation oncology. The user can interactively vary many parameters related to image formation. ImaSim enables a user to quickly demonstrate and study principles associated with the creation of a radiological image in a classroom or in a self-learning setting. Many imaging phenomena can be studied with the aid of ImaSim. This work, by rendering accurate image creation easily accessible, has the potential to enhance textbook based teaching and heighten student interest in medical photon imaging. / L'objectif de ce projet consiste en l'amélioration de l'enseignement de l'imagerie médicale par la création d'un logiciel interactif, ImaSim, basé sur un environnement de simulation. La plupart des modalités d'imagerie médicale propres à un département de radiologie ou radio oncologie se retrouvent dans ImaSim. ImaSim vise à préserver une utilisation simple tout en modélisant adéquatement les aspects physiques associés aux modalités d'imagerie. Les photons générés tombent dans la gamme d'énergie propre à la radiologie et à la radio oncologie. Plusieurs paramètres liés à la formation d'images peuvent être variés interactivement par l'utilisateur. ImaSim permet donc d'étudier les principes associés à la création d'une image radiologique. Plusieurs phénomènes peuvent êtres étudiés à l'aide d'ImaSim. Ce projet a le potentiel de complémenter l'enseignement traditionnel de l'imagerie médicale.