X-ray mammography cannot always distinguish between benign and malignant breast lesions. This leads to unnecessary biopsies, costs, and stress for the patient. Positron Emission Mammography (PEM) provides images of increased glucose metabolism in malignant tumours compared with healthy tissue. After injection of a radioactively-labelled glucose analog, cancerous tumours appear as bright spots on the breast image. / Quantitative analysis of PEM images consists in comparing the amount of activity absorbed in both breasts of a patient. Based on ROC analysis of 15 subjects, an asymmetry of 10% in the number of counts detected from each breast was taken as a sign of cancer. The application of the count asymmetry method seems to results in a 22% improvement of PEM accuracy (from 64% to 86%). It is particularly useful for the detection of big or diffuse tumours. Quantitative data will also provide tools for future applications of PEM technology, such as follow-up of patients after cancer therapy.

In modern radiotherapy, 3D conformal dose distributions are achieved using several beam ports each having pre-calculated planar distributions of photon beam intensity. The intensity matrix for a given beam port is generated by independent motion of the leaves of a multileaf collimator (MLC). In this thesis, we have used the step-and-shoot approach to intensity-modulated beam delivery, the safest and most popular approach at the moment. The first component of this thesis was to write a leaf sequence algorithm to control the MLC fitted to our Clinac 2300 C/D linear accelerator. Our algorithm is more efficient than other published step-and-shoot type algorithms, and takes into account the MLC transmission, MLC penumbra, and change in scatter conditions with field size. / Although sophisticated means to calculate and deliver these spatially-modulated beams have been developed by our group as well as by other medical physics research centres, means to verify their actual delivery are definitely the most problematic at the moment, making equipment and treatment quality assurance difficult to enforce. The second (and major) component of this thesis has been to investigate the use of a new portal imaging device for dosimetric verification purposes. We show that an electronic portal imaging device of the scanning liquid ionization chamber type yields images which, once calibrated from a previously-determined calibration curve, provide highly-precise planar maps of the incident dose rate distribution. For verification of an intensity-modulated beam delivered in the segmented approach with an MLC, a portal image is acquired for each subfield of the leaf sequence. Subsequent to their calibration, the images are multiplied by their respective associated monitor unit settings, and summed to produce a planar dose distribution at the measurement depth in phantom. The excellent agreement of our portal imager measurements with calculations of our treatment planning system and measurements with a one-dimensional beam profiler attests to the usefulness and relative simplicity of this method for the planar verification of intensity-modulated fields, which are produced in the segmented approach on a computerized linear accelerator equipped with an MLC.

This thesis presents the development and implementation of a quantitative multi-slice cerebral perfusion imaging technique using magnetic resonance imaging. An acquisition sequence capable of acquiring up to 9 slices was designed and implemented into two final pulse sequences: an interleaved perfusion/BOLD (blood oxygenation level dependent) sequence and a perfusion-only sequence. A number of practical imaging issues were addressed and resolved, including the design of an appropriate inversion pulse for labelling of arterial spins, spatial offsetting of this pulse for use in the arterial spin labelling technique chosen for implementation, and the design of various saturation pulses necessary for quantification of the technique. Experimental validation of the quantitative multi-slice perfusion technique was performed by measuring visual cortex cerebral blood flow (CBF) values in a group of 8 subjects using a block-design visual stimulus paradigm. Results indicated good sequence stability and CBF measurements agreed well with quantitative values found in the literature.

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.

From our everyday life, we know that our hearts beat with a rhythm which is not perfectly periodic. Even an isolated spontaneously beating cardiac cell, devoid of neural, hormonal, and intracardiac regulatory input, does not beat perfectly regularly. I investigate the hypothesis that the beat-to-beat fluctuations in transmembrane potential of spontaneously beating cardiac cells are due to stochastic gating of the ionic channels in the cell membrane. / Recordings of transmembrane potential from small clusters of spontaneously beating 7-day-old embryonic chick ventricular cells were analyzed to characterize the voltage waveform and the regularity of beating. I constructed a deterministic Hodgkin-Huxley-type ionic model which reproduces spontaneous activity in our experimental recordings, as well as the experimental results of applying various ion channel blockers (D-600, almokalant, and Ba2+). The model consists of six currents: a calcium current (ICa), three potassium currents (IKs, I Kr, IK1), a background current ( Ib), and a seal-leak current (I seal). / The deterministic Hodgkin-Huxley-type model was then reformulated into a stochastic single-channel model. The single-channel model reproduces the irregularity of beating seen experimentally: e.g. the coefficient of variation of interbeat interval was 4.4% vs. 3.9% in the clusters. In the model, IKs is the current giving the major contributions to fluctuations in interbeat interval. / Phase resetting of the spontaneous activity of cardiac pacemaker cells by a brief stimulus pulse was simulated in Hodgkin-Huxley-type models and single-channel models of slow-upstroke (central) and fast-upstroke (peripheral) rabbit sinoatrial node cells. In the Hodgkin-Huxley-type models the phase-resetting response is continuous, but can be extremely delicate in the fast-upstroke model, in that a tiny difference in the stimulus timing can change the stimulus response from a delayed action potential to an advanced one. Therefore, the noise in the fast-upstroke single-channel model can cause a stimulus with fixed amplitude and fixed timing to have widely different effects: sometimes it will induce an action potential but in other cases it will delay an action potential, as seen previously in experiments on cardiac preparations.

Magnetic resonance imaging (MRI) can be used to image diffusion in liquids, such as water in brain structures. Molecular diffusion can be isotropic or anisotropic, depending on the fluid's environment, and can therefore be characterized by a scalar, D, or by a tensor, D, in the respective cases. For anisotropic environments, the eigenvector of D corresponding to the largest eigenvalue indicates the preferred direction of diffusion. / This thesis describes the design and implementation of diffusion tensor imaging on a clinical MRI system. An acquisition sequence was designed and post-processing software developed to create diffusion trace images, scalar anisotropy maps, and anisotropy vector maps. A number of practical imaging problems were addressed and solved, including optimization of sequence parameters, accounting for flow effects, and dealing with eddy currents, patient motion, and ghosting. Experimental validation of the sequence was performed by calculating the trace of the diffusion tensor measured in various isotropic liquids. The results agreed very well with the quantitative values found in the literature, and the scalar anisotropy index was also found to be correct in isotropic phantoms. Anisotropy maps, showing the preferred direction of diffusion, were generated in human brain in vivo. These showed the expected white matter tracts in the corpus callosum.

The stringent demands for accuracy in radiosurgery impinge on all aspects of the radiosurgical procedure. A computer program, used to determine target coordinates from planar stereotactic image pairs, has been developed. Digital subtraction angiography images, obtained with a localizer attachment affixed to the stereotactic frame, are analyzed with the program for the localization of certain malformations. During treatment set-up verification, the center of the radiation distribution is determined using portal linear accelerator images of the collimated radiosurgery beam, with the patient in the final treatment position and without removal of the radiosurgical collimator. A custom portal localizer attachment has been developed in-house for this purpose. Localization results obtained with a test phantom show a maximum deviation of 0.4 mm from the known target locations. The program has also been shown to be effective in detecting a misalignment between a localized target and the treatment radiation center during set-up verification.

Thesis (Ph.D.)--University of California, San Francisco, 2008. / Source: Dissertation Abstracts International, Volume: 69-06, Section: B, page: 3512. Adviser: Barbara J. Drew.

Thèse (Ph.D.)--Université de Sherbrooke (Canada), 2007. / Titre de l'écran-titre (visionné le 1 février 2007). In ProQuest dissertations and theses. Publié aussi en version papier.

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