Radiation therapy is currently under investigation as a therapeutic option for the prevention and the treatment of restenosis following percutaneous coronary interventions. Restenosis after vessel injury is associated with a transient state of cell proliferation and bears many similarities with wound healing. It is known that ionizing radiation can delay or impair wound healing. One proposed approach uses the deployment of a radioactive stent and continuous low dose-rate treatment. Our experimental studies have been conducted to better evaluate the radiobiology of vascular cells and as a preamble to the design of an original radioactive stent based on a 45Ca-DTPA polymer-coated stent. / We performed morphometric analysis of human coronary segments with restenosis. Most of vessel area was occupied by atherosclerotic plaque and neointima accounted for a limited part. Adventitial and medial thicknesses were significantly thinner than the atherosclerotic plaque and neointima. The medial layer was the most cellular. Our results indicated that target volumes for endovascular catheter-based brachytherapy or radioactive stents remain small. After delivery of doses such as proposed in the current experience, only a few thousand cells would remain clonogenic. Depending of the residual growth stimulus, this would create a permanent impairment or a significant delay in the healing and the restenosis process. / We compared in vitro responses of human and pig fibroblasts, smooth muscle cells and endothelial cells exposed to high dose-rate gamma irradiation. Using clonogenic assays, growth inhibition and filter elution techniques, we found that human and pig cells have similar radiation responses. Our results suggest that vascular cell lines cover a broad range of radiosensitivities without specific differences between cell types. / We developed an experimental set-up to evaluate the in vitro effects of low energy beta-emitters on vascular cells. The complex is the 45Ca-diethylenetriaminepentaacetate (45Ca-DTPA) with a maximal energy of 255 KeV. There was a good correlation between calculated doses and doses measured by dosimeters placed at the bottom of petri dishes. However, it seems that doses administered to cell monolayers would be better evaluated by the dose calculated by the Medical Internal Radiation Dose (MIRD) instead of half the calculated dose, as previously suggested. / We evaluated the dose-rate effect of vascular smooth muscle cells exposed to low dose-rate irradiation with the 45Ca-DTPA complex. (Abstract shortened by UMI.)

The MicroPET R-4 scanner was designed for imaging small rodents such as mice and rats. In many cases its spatial resolution is not good enough to perform the required task. We have implemented an eccentric motion (commonly referred to as wobbling) which is applied to the bed during scanning. / The program which histograms the list-mode data was rewritten to increase the spatial sampling by incorporating the wobble position in the sinograms. The corrections for the dwell time, apparent crystal location, and crystal-pair efficiency are applied within the program. A series of scans were performed to decide the optimum wobble radius; it was found to be 1.50 mm. Another series of scans was performed during which a Na-22 source was moved 0.25 mm between scans with and without the optimal wobble motion. The peak-to-valley ratio between two Na-22 point sources 4.0 mm apart, improved from 1.75 in the conventional mode to 2.26 during wobbled scans applying a ramp filter. / The bed wobbling mechanism can he added to the microPET R4 or P4 scanners without any major changes to make wobble motion and without compromising any imaging modes. Implementing the wobble mechanism may present a cost-effective upgrade over a trade in or purchase of the higher performance scanner.

A clinically-useful treatment planning system for external photon beam radiotherapy must yield fast and accurate calculations of the dose distribution in the patient. The interface of the system should be "user friendly" and designed to minimize user work and errors. Visualization of the patient volume and dose calculation results should provide necessary formation without being confusing. A software module meeting these criteria has been implemented within the McGill Treatment Planning System (MPS). The MPS program is written in C code and compiled with CodeWarrior C/C++$ rm sp{TM}$ from MetroWerks Inc. The MPS program runs on the Apple Macintosh platform (either 68K or PowerPC series). This new software module permits dose calculation (modified Milan-Bentley method) and viewing of the patient volume in three dimensions. Patient anatomical data is acquired from computed tomography (CT) or magnetic resonance (MRI) images. The accuracy of the dose calculation has been verified through comparison of the calculated results with water tank and film measurements.

Introduction. The use of Functional Magnetic Resonance Imaging (fMRI) in studying central auditory pathways expanded our knowledge of the neurophysiology of hearing. In various studies, an array of diverse auditory stimuli has been used, including pure tones and words, in a "Block" design. Few studies, thus far, have been performed using an event-related design. None of these has used pure tone stimulation. / Subjects and methods. In eight out of 13 subjects enrolled and scanned in this project, data have been acquired and processed. Binaural pure tone stimulation at 2kHz and one higher frequency at 6, 8, or 12 kHz, at 90 dB SPL was presented to all subjects, using a stroboscopic design in an attempt to eliminate cortical response to scanning noise. In addition, an unusually long time of repetition (TR) interval of 10 seconds was utilized. The aim in data processing was to generate: (a) a Peak Height Map (PHM) with selection of areas of stimulation based on the maximal response; (b) a statistical map from the raw data of the images. / Results. No consistent morphology was detected in the BOLD event-related curves, corresponding to the percentage change from baseline over time. Such curves revealed significant oscillation with marked dips below baseline, highly suggestive of an irregular noise pattern. Comparing the curves to each other, no discernible shape or similarities were noted. / Conclusion. Attempting to extract both the temporal and spatial characteristics of the cortical response to pure tone stimuli resulted in a dispersal of data over many parameters, therefore "diluting" it. This, in turn, yielded a negative result where the responses were "drowned" revealing but a noise pattern.

The first goal of this study was to develop experimental tools to estimate the blur and noise characteristics of X-ray CT systems. We evaluated the noise power spectrum (NPS) using the averaged periodogram technique. As predicted by theory, varying the image reconstruction filter produced a shift in the frequency content while slice thickness only affected the amplitude of the noise. / In order to quantify the blur, we developed and evaluated two non-linear parametric models of the point spread function (PSF): the Gaussian and damped cosine models. We fitted these models to images of spatially distributed point sources and thus quantified the pattern of shift-variance. We found that the system produced a rotating blur and observed a loss of PSF radial symmetry as the input point source moved away from the center of the field of view. We validated the use of point sources by comparing non-parametric PSF estimates obtained with this input to those found using a correlation-based technique (Wiener-Hopf equation). We gained insight into the design of the input signal, which consisted of pseudo-randomly located holes, through an exhaustive simulation. / The second goal was to investigate how this information could be used to process CT images. We formulated and evaluated a coordinate transformation for shift-invariant restoration of CT images. We developed a simple evaluation procedure which proved beneficial in delimiting the usefulness and detecting limitations of the method. We also formulated a number of recommendations regarding the use of the threshold and Laplacian of a Gaussian segmentation operators taking the shape of the PSF and of the NPS into account.

The effect of a single exposure of He-Ne laser on collagen content was studied in vivo using laboratory animals. A standardized 1.5 cm full thickness skin incision was made on the dorsum of 24 Sprague-Dawley rats. The rats were divided into six equal size groups, three groups functioning as controls and three groups functioning as experimental. The experimental animals were irradiated with He-Ne laser four days post-incision for 16 minutes to deliver a total dose of 5.99 J/cm$ sp2$. On days 6, 10 and 14 post-incision, control and experimental animals had the healing wounds excised for analysis of collagen content by assaying spectrophotometrically the hydroxyproline content within the tissue samples. / The results of the hydroxyproline content were analyzed using independent-groups t-test and analysis of variance. It was demonstrated that even though the mean values of hydroxyproline within the experimental groups were higher than the respective control groups, no statistical significance could be attributed to this observation.

Quantification of physiological functions with positron emission tomography requires knowledge of the arterial radioactivity concentration. Automated blood sampling systems increase the accuracy of this measurement, particularly for short-lived tracers such as oxygen-15, by reducing the sampling interval to a fraction of a second. They, however, require correction for tracer delay between the arterial puncture site and the external radiation detector (external delay), and for the tracer bolus distortion in the sampling catheter (external dispersion). / We have evaluated and implemented the "Scanditronix" automated blood sampling system and measured its external delay and dispersion. PET studies of cerebral blood flow and oxygen metabolism using simultaneous manual and automated blood sampling were analyzed and compared. We show that the results obtained with automated blood sampling are more reliable than those based on manual sampling. We also present suggestions to further improve the reliability of quantitative PET studies based on automated blood sampling.

Space constraints often present the greatest challenge in determining the lay-out of specific radiotherapy installations. To fulfill these constraints high density concrete is often employed in construction. We have undertaken a study to establish the transmission of primary, leakage and scatter radiation through a particular type of high density concrete. In the course of this study various characteristics of leakage and scatter radiation from a 10 MV linac have also been investigated. The results of these studies are reported in this thesis. / To establish the correct chamber configuration required for the characterization of the transmission factors for primary, leakage and scatter radiation and to determine the scatter-to-incidence dose ratio a, we have measured the build-up curves and verified the validity of the inverse square law for these different radiations. The build-up data for the radiation scattered from a humanoid phantom at different angles from the primary axis have shown that even at low scatter angles little to no build-up is required for scatter radiation measurements.

Atherosclerotic narrowing of the carotid arteries in asymptomatic individuals is associated with an increased risk of cerebrovascular ischemic events. The prevention of stroke in this context requires an understanding of the natural history of cerebrovascular atherosclerosis and the availability of an effective means for the detection and follow-up of carotid stenosis. / Methodological standards for the conduct and reporting of natural history studies of asymptomatic carotid disease are proposed and used to carry out a structured methodological appraisal of the literature. The findings suggest that greater attention should be paid to the design and statistical analysis of natural history studies. / A secondary analysis of the data from a natural history study of asymptomatic carotid disease is undertaken for the purpose of evaluating the prognostic usefulness of repeated duplex ultrasound examinations. Multivariable Cox proportional hazards models with fixed and time-dependent covariates are used to evaluate the predictive ability of carotid stenosis measurements obtained by duplex ultrasound at enrollment and during follow-up. The results of the analysis do not support the use of routine carotid ultrasound in clinical practice to determine prognosis for unselected patients with asymptomatic carotid disease.

The problem of geometrical distortion in MR images is addressed in the context of the applicability of stereotactic techniques. For this purpose, the distortion of phantom images is measured at various readout bandwidths and the spatial linearity is evaluated in view of the use of a stereotactic frame. The presence of a contribution to the overall distortion of non-linear magnetic gradients is shown from the data, although the distortion observed seems to be mostly attributable to the main field inhomogeneity. The specific problems of distortion of the fiducial markers due to bulk magnetic susceptibility effects is addressed. The occurrence of such effects is characterized with the help of imaging, and the role of the phenomenon on proper target localization is demonstrated. In addition, a method of bypassing the detrimental aspect of these effects is presented. / Various distortion correction approaches are discussed, and their benefits and drawbacks are evaluated. In the light of this discussion, a recently reported correction method is then presented. This method allows the calculation of an image free from geometrical and intensity distortion from the combined effect of main field inhomogeneity, susceptibility effects and chemical shift.