A Monte Carlo simulation system was enhanced in order to analyze photon transport in tapered geometries. This system was used to evaluate the performance of new tapered collimator designs, for multi-slice positron emission tomography (MS-PET) and positron volumetric imaging systems (PVI-Systems). For each imaging modality, the performance of the new collimators was compared to those of the current collimators of choice. Collimators were evaluated based upon their: (1) trues detection efficiencies; (2) scatter detection efficiencies; (3) scatter fractions; and (4) total singles to trues events ratios. In MS-PET, collimators with a diamond-like cross section--double tapered collimators--were introduced and their performance compared to those of single tapered and cylindrical collimators. Double tapered collimators yielded better performance than single tapered ones. In PVI systems, external tapered collimators were simulated and their performance compared to those of cylindrical ones. External tapered collimators did not improve the scanner performance.

An efficient and productive radiation treatment planning (RTP) system must make use of both appropriate visualization techniques and good user interface design. The suitability of several visualization techniques have been examined in the context of 3-D radiation treatment planning. These techniques include wire frame, surface rendering, volume rendering and a subset of volume rendering: reformatting of data. A rudimentary computerized RTP system was written using the most appropriate visualization techniques examined earlier. These techniques were used to display the anatomical data acquired from computed tomography (CT) scanners, the beam position within the anatomy, and finally, the dose distributions resulting from the entered plan. The program was written in ANSI C and runs on a Silicon Graphics Personal Iris UNIX workstation. The system makes use of effective user interface tools and efficient code which results in an efficient and interactive system. The accuracy of the system is verified by comparing dose profiles obtained with film dosimetry and from the computer calculations.

Functional magnetic resonance imaging (fMRI) is a technique allowing the study of hemodynamic changes in the brain. Having a number of advantages over positron emission tomography (PET), the current gold standard, fMRI appears like an attractive alternative to study brain function. However, the measurements obtained with fMRI have not been rigorously validated. / This thesis describes a study comparing cerebral blood flow (CBF) changes measured using a flow-sensitive alternating inversion recovery (FAIR) fMRI perfusion method, to the ones obtained using PET. We scanned 10 healthy normal volunteers under identical experimental conditions during presentation of 4 levels of visual stimulation and one level of hypercapnia. The CBF changes were compared in 4 regions-of-interest. / Good correspondence was found in the locations of the CBF changes. FAIR CBF changes had a higher signal-to-noise ratio and presented a monotonic increase with stimulation intensity, absent in PET measurements. FAIR measurements were correlated to PET but slightly lower. Statistical analysis of the data did not show that FAIR measurements were significantly different from PET ones.

Often in neurosurgical planning a dual echo acquisition is performed that yields proton density (PD) and T2-weighted images to evaluate edema near a tumor or lesion. The development of vessel segmentation algorithms for PD images is of general interest since this type of acquisition is widespread and is entirely non-invasive. Whereas vessels are signaled by black blood contrast in such images, extracting them is a challenge because other anatomical structures also yield similar contrasts at their boundaries. / In this thesis we present a novel multi-scale geometric flow for segmenting vasculature from PD images which can also be applied to the easier cases of MR angiography data or Gadolinium enhanced MRI. The key idea is to first apply Frangi's vesselness measure [Frangi et al. (1998)] to find putative centerlines of tubular structures along with their estimated radii. This multi-scale measure is then distributed to create a vector field which is orthogonal to vessel boundaries so that the flux maximizing flow algorithm of Vasilevskiy and Siddiqi (2002) can be applied to recover them. We carry out a qualitative validation of the approach on PD, MR angiography and Gadolinium enhanced MRI volumes and suggest a new way to visualize the segmentations in 2D with masked projections. We also validate the approach quantitatively on a data set consisting of PD, phase contrast (PC) angiography and time of flight (TOF) angiography volumes, all obtained for the same subject. A significant finding is that over 80% of the vasculature recovered in the angiographic data sets is also recovered from the PD volume. Furthermore, over 25% of the vasculature recovered from the PD volume is not detectable in the TOF angiographic data. / Thus, the technique can be used not only to improve upon results obtained from angiographic data but also as an alternative when such data is not available.

An interactive 3D target localisation and delineation tool has been developed for radiosurgery planning of arteriovenous malformations (AVMs). With this system, magnetic resonance (MR), MR angiography (MRA) and computed tomography (CT) volumes can be fused in stereotactic space. Stereotactic angiography (SA) images can be linked to the MRA volume by recovering the SA acquisition geometry. The MRA and SA images can be correlated (1) by ray-tracing through the MRA volume with the recovered SA acquisition geometry and overlaying the images onto the SA images and (2) by localising the AVM onto a volume rendered representation of the MRA with a 3D cursor and projecting its position onto the SA images. Target contours can then be drawn on the MRA/MR/CT images and simultaneously projected onto the SA images. / The plans of patients who had previously undergone radiosurgery at our institution employing SA images for localisation and MR images for delineation were investigated. MRA datasets were also acquired at the time of MR scanning employing the 3D TOF technique. Some ray-traced MRA images correlated well visually with the SA images, others presented inconsistencies which suggest that MRA should be used only as complement to SA images. / The role of the different modalities (M-RA, MR and SA) in the definition of target volumes is investigated by defining the target contours with different combinations of modalities within the interactive system. The target volumes drawn with different modalities were compared to a reference volume, drawn using MRA, MR and SA images, and presented underestimation and overestimation of target volumes ranging from 20% to 92% and from 3% to 40%. / The dosimetric implications of image fusion for target delineation are investigated by retrospective evaluation of the dose coverage of the reference target volume by the original treatment plan. Target coverage inferior to 60% of the reference target volumes by the original treatment plans was obtained for the patients. Treatment plan optimisation was performed to evaluate the possible dosimetric improvements resulting from image fusion for AVM target delineation. The plans were improved by employing three, eight and four isocentres for the 3 patients, and resulted in target coverage equal or superior to 98% for all three patients.

Magnetic resonance imaging (MRI) is conventionally used for macroscopic qualitative observations. However, increasingly there is a need for quantitative MRI measures, which may lead to enhanced detection sensitivity. Two quantitative techniques that may be used to make neuroanatomical inferences about a population or between different populations are magnetization transfer ratio (MTR) and voxel-based morphometry (VBM). / VBM involves the statistical analysis of smoothed segmented white or gray matter maps to reflect increases or decreases in the probability of classifying a voxel as either white or gray matter. MTR provides a measure of the interaction of water and semi-solids within tissue, and thus is indicative of its macromolecular density and microstructural integrity. An MTR group analysis may detect variations of these semi-solid tissue characteristics within or between populations. / This thesis investigates the relationship between information attained from VBM and MTR population studies carried out in the context of the Saguenay Youth Study. Additionally, through this study, the effects of age and gender on brain neuroanatomy are explored using the above techniques. The observed age and gender VBM and MTR effects were consistent with existing literature, but also offered new findings. Overall, applying MTR in conjunction with VBM allows for further insight into the origins of specific anatomical changes, and the discovery of areas that undergo within-tissue development without corresponding white or gray matter volume changes.

Real-time ultrasonic imaging systems have been available for more than fifty-five years and are becoming an important tool in the practice of modern medicine. During this time much has occurred to the basic architecture and functions of these clinical systems and their beamformers, which are, in many ways, the most important components of these systems. Throughout most of the 30 years of real time imaging, analog beamformers have been the mainstay of all ultrasonic instruments. But at the present time the industry is undergoing a major shift toward digital beamformation with the introduction of several commercial systems. The thesis will look at the evolution of digital ultrasound beamformers, some of the changes that have occurred and will discuss current trends in beamformer design. Typical analog and digital beam formers have been simulated and their performance compared in terms of beam width, side lobe levels and signal-to-noise ratio. Also the effect of apodization on images have been examined. Experimental investigations have been carried out to compare delay-sum-add and synthetic aperture imaging on different phantoms. The technical challenges in digital beamformation win be reviewed, as also the constraints introduced by today's market place. Finally, the future of digital beamformation in the context of advances in computer and microelectronics technologies is discussed.

Electron beams are used to treat superficial lesions in radiation oncology by taking advantage of the sharp dose fall-off and the limited range of the particles in tissue. The irregular shape of individual tumors, however, often requires custom made shielding in order to geometrically shape the radiation field to the target, while minimizing the dose to surrounding tissues. In many institutions, low melting alloy or lead cutouts are used for electron beam shaping. In this work, electron dosimetry beam parameters such as percentage depth dose (PDD), outputs, and beam profiles, were measured with ten different electron beams from two linear accelerators. The dependence of beam characteristics on field size and shape, particularly for small cutouts, was investigated. In addition, this project examined different methods for measuring electron PDDs, including film densitometry, ion chambers, and diode dosimetry. / The work presented here demonstrates that the depth dose effect is significant when one of the field dimensions of the cutout is less than R p, the practical range of electrons. For these cutouts, it was observed that both PDD and outputs vary significantly due to the lack of lateral electronic equilibrium. As the cutout becomes smaller, the depth of dose maximum (dmax) shifts towards the surface, while the output at dmax decreases. Therefore, it is crucial that PDDs and outputs are either measured or calculated for small field electron cutouts used in the clinical setting.

In PET activation studies, brain function is detected by identifying regional differences in tracer concentration between images acquired while a subject(s) executes different tasks. When imaging volunteers or patients, the differences produced by executing higher-order cognitive processes have magnitudes similar to those of the noise found in the measurements. Therefore, the development of techniques that enhance the signal to noise (S/N) of activation images is of great importance. / The work described here was performed to investigate and evaluate three strategies designed to enhance the S/N in activation images of cognitive function. / First, the applicability of switched protocols to the study of cognitive paradigms was analysed. Switched protocols enhance the S/N of subtracted images by manipulating tracer kinetics during the washout phase of the input function. This allows the acquisition of images up to 4-min long. Activation foci detected with switched and standard rCBF protocols were compared using two language tasks. The switched protocol improved the signal statistical significance up to 38% by reducing image noise. / Second, a protocol designed to augment the S/N enhancements yielded by switched protocols was evaluated. This protocol enhances the S/N of H 215O bolus PET activation images by altering the relative distribution of tracer in the uptake and washout phases of the input function. The protocol combines task-switching with the release of a large pool of activity-free blood (cold-bolus). The combined protocol was first tested using computer simulations of the uptake of H215O into the brain and then evaluated by imaging subjects performing two language tasks. In the PET experiments, activation foci obtained using the combined protocol had significantly higher t-statistic values than equivalent foci detected using a conventional switched protocol (mean improvement 36%). The improvements resulted from increases in signal magnitude. / Third, the performance of the ECAT EXACT HR + 3D PET scanner when employed to measure brain function using H215O bolus activation protocols in single sessions was evaluated. Using two sensory tasks and rCBF measurements, the scanner Performance was studied at different count-rates. Optimal dose fractionation strategies for one session imaging studies employing standard rCBF, switched, and cold-bolus/switched protocols were determined.

<p> Fluorescence molecular imaging will have an important clinical impact in the area of guided oncology surgery, where emerging technologies are poised to provide the surgeon with real-time molecular information to guide resection, using targeted molecular probes. The development of advanced surgical systems has gone hand in hand with probe development, and both aspects are analyzed in this work. A pulsed-light fluorescence guided surgical (FGS) system has been introduced to enable video rate visible light molecular imaging under normal room light conditions. The concepts behind this system design are presented and performance is compared with a commercial system in both phantom and <i> in vivo</i> animal studies using PpIX fluorescence. </p><p> The second critical advance in the emergence of these technologies has been the development of targeted near infrared (NIR) probes. A small, engineered three-helix protein was analyzed for imaging of glioma tumors. The blood brain barrier affects delivery of probes and the superior delivery of a smaller targeted protein (anti-EGFR Affibody) as compared to a full sized antibody is shown using a murine model, <i>ex vivo</i> tissue slices and a commercial imaging system. This small targeted probe is examined further for its possible application in FGS using the pulsed light imaging system. A concentration sensitivity analysis to determine the lower bounds on concentration needed for effective imaging is performed with this culminating analysis carried out in a murine orthotopic glioma tumor model.</p>