The availability of accurately aligned, whole body, functional PET images has a significant impact on the diagnosis of malignant disease and on identifying and localizing metastasis. Gamma ray attenuation correction is essential in all quantitative PET studies. / The object of this study was to explore the possibility of using beta-gamma coincidence as an attenuation correction technique in order to improve transmission scan image quality. / This study consisted of testing and implementing a beta-gamma attenuation correction technique on an animal PET scanner. In its final form the system uses 68Ge sources enclosed in plastic scintillator cylinders coupled to PMTs. The detection of positrons is activated by the energy loss in the scintillator medium. This system is used in coincidence with one of the animal PET scanner's BGO crystal detectors in order to acquire transmission scans.

<p> The NPDGamma Experiment is measuring the parity-violating correlation A_&gamma; between neutron spin and gamma momentum in the radiative capture of a polarized cold neutron beam on a cryogenic liquid parahydrogen target. This measurement is expected to give insight into theories that incorporate the weak interaction into what is primarily a strongly interacting system. This dissertation discusses the operation and characterization of the liquid hydrogen target, including the calibration of the instrumentation that monitors the state of the hydrogen. An important consideration is the fact that for safety reasons the instrumentation in direct contact with the hydrogen is limited, and so a detailed understanding of the target design and of the properties of hydrogen is required in order to interpret the state of the system. For this experiment, it is essential that the hydrogen be kept mostly in the para state in order to prevent the beam from being significantly depolarized before capture. Since the uncatalyzed conversion rate is slow, an ortho-para conversion loop is used to promote conversion from the room temperature orthohydrogen fraction to the fraction associated with the temperatures of the cryogenic vessel. In addition to the calibration and characterization studies, a method is introduced for placing an empirical limit on the deviation of the orthohydrogen fraction inside the vessel from the desired level associated with the temperature of the ortho-para conversion catalyst and vessel. This method, which does not require precise knowledge of the parahydrogen cross section, involves observing the transmission of the beam through the target while the rate of flow of hydrogen through the ortho-para conversion loop is changed. In addition to the studies of the hydrogen target, this dissertation discusses a calibration of some ³He ion chambers that monitor the flux of the neutron beam and that are used to perform beam transmission measurements. This calibration, which involves a study of the noise inherent in the signal due to neutron capture, does not involve comparison to a separate calibrated detector.</p>

<p>In high-energy collisions of gold nuclei at the Relativistic Heavy Ion Collider (RHIC) and of lead nuclei at the Large Hadron Collider (LHC), a new state of matter known as the Quark-Gluon Plasma (QGP) is formed. This strongly-coupled, deconfined state of quarks and gluons represents the high energy-density limit of quantum chromodynamics. The QGP can be probed by high-momentum quarks and gluons (collectively known as partons) that are produced in hard scatterings early in the collision. The partons traverse the QGP and fragment into collimated "jets" of hadrons. Studies of parton energy loss within the QGP, or medium-induced jet quenching, can lead to insights into the interactions between a colored probe (a parton) and the colored medium (the QGP).</p><p> Two analyses of jet quenching in relativistic heavy ion collisions are presented here. In the jet-hadron analysis, the distributions of charged hadrons with respect to the axis of a reconstructed jet are investigated as a function of azimuthal angle and transverse momentum (<i>p</i>_T). It is shown that jets that traverse the QGP are softer (consisting of fewer high-<i>p</i>_T fragments and more low-<i>p</i>_T constituents) than jets in <i>p+p</i> collisions. There are also indications that the shapes of the distributions of charged hadrons about the jet axis are modified by interactions with the QGP. The results are quantitatively consistent with two models of medium-induced radiative parton energy loss.</p><p> A measurement of jet <i>v</i>₂, defined as the correlation between reconstructed jets and the reaction plane or 2<i>^nd</i>-harmonic participant plane (approximated by the 2<i>^nd</i>-harmonic event plane), provides information about the medium-induced pathlength-dependence of parton energy loss. The event plane is reconstructed with detectors at forward pseudorapidity in order to reduce the artificial jet &ndash; event plane bias, which results from jet fragments being included in the event plane calculation. A non-zero jet <i>v</i>₂ is measured, indicating that more jets are reconstructed with a higher energy in-plane compared to out-of-plane, which demonstrates that the parton energy loss depends on the length of the parton's path through the QGP.</p><p> The data analyzed here were collected in [special characters omitted] = 200 GeV Au+Au and <i>p+p</i> collisions at the STAR detector at RHIC. A novel method for measuring jet <i>v</i>₂ is also proposed and tested in simulation.</p>

Two ultrasound systems were studied to investigate the effects of positional and volumetric prostate variations on dosimetry over the course of external radiation therapy. A 2D system, currently used at the Montreal General Hospital for patient repositioning, was compared to a 3D system invented recently. Prostate variations were quantified from ultrasound images acquired daily during a 2003 clinical study. A method was devised to introduce ultrasound information in a Monte Carlo Treatment Planning System previously developed at McGill. Patient repositioning was evaluated for both systems using dose-volume histograms of Voxel Monte Carlo dose calculation. Repositioning with the 3D system, neglecting volume changes, was found to bring the target dose to within 1 % of the planned dose, rather than the 12 % of the clinical 2D system. However, when considering the varying 3D volumes, the dose could only be corrected to within 7 %. These results indicate that the 3D system provides not only a more accurate assessment of prostate displacements, but also volumetric information that significantly affects the dosimetry.

We describe the geometric and dosimetric characteristics of a device developed to modify the penumbra of an electron beam and thereby improve the dose uniformity in the overlap region when fields are abutted. The device is a Lipowitz metal block placed on top of the electron applicator's insertion plate and positioned to stop part of the electron beam. The air-scattered electrons beyond the block increase the penumbra width from about 1.4 to 2.7-3.4 cm. The modified penumbra is broad and almost linear at all depths for the 9 MeV and 12 MeV electron beams used in this study. Film dosimetry was used to obtain beam profiles and isodose distributions. Without the penumbra generator, lateral setup errors of 2 to 3 mm may introduce dose variations of up to 20% in the junction region. Similar setup errors cause less than 5% dose variations when the penumbra generator is used to match the fields.

A new paradigm for the characterization of structure appearance is proposed, based on a combination of grey-level intensity data and a shape descriptor derived from a priori Principal Components Analysis of 3D deformation vector fields. Generated without external intervention, it extends more classical, 2D manual landmark-based shape models. Application of this novel concept leads to a method for the segmentation of medial temporal lobe structures from brain magnetic resonance images. The strategy employed for segmentation is similar to that used in other appearance-based approaches, while the resulting output data is identical to ANIMAL, a non-linear registration and segmentation technique. The proposed method was tested on a data set of 80 normal subjects for which manual and ANIMAL segmentated structures were available. Experimental results demonstrated the robustness and flexibility of this method. Segmentation accuracy, measured by overlap statistics, is marginally lower (<2%) than ANIMAL, while processing time is 6 times faster. Finally, the applicability of this concept towards shape deformation analysis is presented.

The brain can be explored while a subject executes different tasks. Such techniques are commonly referred as activation studies. The regions of the brain involved in a particular task can be located by comparing regional differences in tracer concentration to a control state. Group of neurons in the brain form complex logical circuits and their activities increase while they interpret a stimulation. These neuronal activities require glucose and oxygen and these substances travel in the blood following hemodynamic rules. Many factors influence the hemodynamic response induced by brain activation (FRIBA), but certain aspects of its temporal behavior are still unclear. A special technique was implemented with an ECAT EXACT HR+ (CTI/Siemens) to fulfil this lack in PET activation studies. The new HR+ has this 3D option, which can help to evaluate the FRIBA with a tracer (11C-CO) that remains in the blood vessels to measure the cerebral blood volume (CBV) differences. Only CBV studies can appreciate the temporal course of the FRIBA in PET. The resulting images are, however, very noisy and many pitfalls are present during their analysis. (Abstract shortened by UMI.)

The block effect in Positron Emission Tomography (PET) is the degradation in spatial resolution that is seen when there are more scintillation crystals than photomultiplier tubes in a detector. / We measured the block effect in detectors from three PET scanners: the Siemens-CTI HR+, the GE Advance, and the Siemens-CTI HI-REZ. In the CTI HR+ the block effect was 0.7 mm in the central crystals, and negligible for the edge crystals. In the GE Advance the block effect was 0.6 mm for the central crystals, and 0.5 mm for the edge crystals. In the CTI HI-REZ detector the block effect varied from 0.5 mm to 2.1 mm depending on the crystal location. / The effect of changing the depth of the first point of gamma-ray interaction in the block detectors was examined. The gamma-ray interaction depth was varied, and the event position in the crystal identification image was examined. If the events were positioned inconsistently with gamma-ray interaction depth, this might be a cause of the block effect. / In the CTI HR+ detector and the CTI HI-REZ detector, the positioning of events in the crystal identification image was dependent both on the crystal depth of the first point of interaction and the crystal location. In the GE Advance block detector the positioning of events in the crystal identification image was consistent for varying gamma-ray interaction depth, and crystal position. / In crystals where the positioning of events is not consistent with interaction depth, the block effect is larger. In the GE Advance block detector, another cause of the block effect may exist.

Electron arc therapy is the treatment modality for superficial tumors involving large curved surfaces. At the McGill University Health Centre, the characteristic angle beta concept was developed for the dosimetry and treatment planning of electron arc therapy. In this work, this concept was verified in dynamic mode with the new generation linac of Varian(TM), the Clinac-21 EX. / Radial PDDs were measured for beta angles varying from 60º--120º of electron arc beams with energies of 6 MeV, 9 MeV, and 12 MeV and 2-D isodose distributions of electron arc beams were studied as well. Dose penumbral regions exist at the beam start area and stop area, and the size of these regions is a function of the beam energy and the field width. When shielded with a tertiary collimator, the size of the penumbral regions and the dose levels in the penumbral regions were reduced significantly, and it is sufficient for the shielding to cover exactly the start field and the stop field of the arc beam in a clinical setting. / The dose rate of the electron arc beam was investigated. It was found that the dose rate at the depth of dose maximum linearly increased with the arc beam field width and was independent of the arc angle.

A commercial electron dose calculation software (Eclipse TM) implementation based on the Macro Monte Carlo algorithm has been introduced. Eclipse™ initial configurations were performed for all available electron beam energies 6, 9, 12, 16, and 20 MeV. We evaluated the electron Monte Carlo (eMC) module of the Eclipse™ using a verification data set comprised of depth dose curves, profiles, Relative Output Factors (ROF), and 2-0 dose distributions in the transverse plane in a homogeneous phantom. The verification data set was comprised of measurements performed for combinations of 6, 9, 12, 16, and 20 MeV beam energies with five standard field sizes and thirteen irregularly shaped fields under three phantom setups. The phantom setups were normal beam incidence at source-to-surface distance (SSO) = 100cm, normal incidence with extended SSO=11 Ocm, and oblique incidence with extended SSO=11 Ocm. Calculations were performed in a digital phantom with the maximum number of particles accepted by Eclipse ™ with a grid spacing that was no larger than approximately one-tenth the distal falloff distance of the electron depth dose curve from 80% to 20% of the maximum dose. Overall, the agreement between the calculated dose distributions and measured on es was good for fields larger than 2 cm to better than 3% dose difference and 3 mm distance-to-agreement.