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.

An on-line beam to target portal verification technique has been developed for stereotactic radiosurgery. Conventional radiosurgery employs a stereotactic frame in order to obtain sufficient spatial accuracy in dose delivery. Frame based verification methods attempt to ensure accurate target positioning with respect to the frame but they do not account for possible movement of the frame with respect to the anatomy and isocenter. We account for this possibility by superimposing digitally reconstructed radiographs (DRRs) over orthogonal edge detected digital portal image pairs. By developing a process for interactively manipulating the CT-data in three dimensions (rotations and translations) new DRRs are generated and overlaid with orthogonal portal images. This method is able to account for ambiguities in matching due to rotations and translations outside the imaging plane because of the availability of DRRs at any possible orientation. This matching procedure is performed using only the anatomy and is used in tandem with a fiducial marker array attached to the stereotactic frame. The method is evaluated using portal images simulated from patient CT-data and then tested using a radiographic head phantom. Results show that repositioning precision of the system is at the level required by stereotactic radiosurgery.

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.

This study was designed as an investigation of the role of changes in muscle force and changes in muscle length on the EMG for the Tibialis Anterior (TA). / Using surface electrodes we examined the EMG for 4 contraction levels at 5 ankle positions over 60$ sp circ$ of ankle rotation. The change in median frequency with muscle length identified a significant shift in the power spectrum to lower frequencies with increasing muscle length. / To further investigate our results we performed three other experiments: First, using X-rays to identify the relative change in distance between two intramuscular wire electrodes we found the change in TA muscle length for this study to be 15% over the 60$ sp circ$ of ankle rotation. Second, to test for synergist contamination we used fine wire electrodes in the Extensor Digitorum Longus and the Peroneus. We found no evidence to support significant contamination. Third, we examined the role of smaller electrodes with a smaller interelectrode distance on our findings. The EMG showed drastic changes with even a slight shift in electrode position most likely due to the large number of innervation zones. / Therefore, the results indicate a shift in the power spectrum with a change in muscle length. In addition, surface EMG results are heavily dependent on the innervation zones and on the electrode geometry, all of which are important considerations in developing the EMG as an accurate diagnostic tool.

In this thesis, the use of inhomogeneity corrections in intensity modulated radiotherapy (IMRT) inverse treatment planning is investigated. Firstly, the dosimetric consequences of CT beam hardening artifacts present on images used for treatment planning are estimated and found to be of little clinical significance (<1% dose difference). Secondly, experiments to evaluate the PEREGRINE Monte Carlo system (Nomos, Cranberry, PA) are undertaken for a 6 MV photon beam. The use of inhomogeneity corrections in clinical treatment planning is assessed for five clinical head and neck cancer cases. The cases are planned with the CORVUS optimization module and the dose distributions are then calculated with CORVUS and PEREGRINE in order to compare the two calculation techniques with emphasis on how each method handles tissue inhomogeneities. The plans are assessed in terms of dose, dose-volume distributions and the biological indices of TCP and NTCP. On average, PEREGRINE calculates a 1% lower mean dose to the GTV and a 2% lower mean dose to the CTV compared to the CORVUS calculations with EPL inhomogeneity corrections. In the last part of this work, quality assurance (QA) measurements are performed for a clinical case to investigate how the CORVUS and PEREGRINE calculations agree with the dose measurements on a QA phantom.

Dynamically delivered intensity modulated beams (IMBs) pose unique verification problems that may be addressed with the use of integrating continuous 3D dosimeters such as gel based Fricke dosimeters. Accurate knowledge of the ability of these dosimeters to measure adequately and precisely the delivered dose is a prerequisite for their clinical use. The magnetic properties of the ferrous and ferric ions present in the gel based Fricke dosimeter after its irradiation are the basis for the use of magnetic resonance imaging (MRI) in the measurement of dose. This thesis presents the investigation of a 3D gel based Fricke dosimetry system (Fricke-gel). A software system is developed and spin-lattice relaxation rate (R1) images are computed from MR images of irradiated Fricke-gel phantoms in order to quantify the dosimetric uncertainties resulting from the MR imaging system, from the gel itself, as well as from the external parameters. The sensitivity and the minimum detectable dose of the Fricke-gel dosimeter are determined. Validation of the dosimeter's capacity to measure dose distributions is made through measurement of percent depth dose curves (PDD's), and field profiles (open and wedged). An example of clinical utilisation of the Fricke-gel dosimeter is presented. Dose distributions are evaluated visually by 3D software tools and quantitatively analyzed by dose-volume histograms. Results show a good correlation between the Fricke-gel measured dose distributions and treatment planning software dose calculations.

Dose predictions by Monte-Carlo (MC) techniques could alleviate the measurement load required in linac commissioning and clinical radiotherapy practice, where small or irregular electron fields are routinely encountered. In particular, this study focused on the MC calculation of cutout factors for clinical electron beams. A MC model for a Varian linac CL2300C/D was built and validated for all electron energies and applicators. A MC user code for simulation of irregular cutouts was then developed and validated. Supported by a home-developed graphical user interface, it determines in situ cutout factors and depth dose curves for arbitrarily shaped electron fields and collects phase space data. Overall, the agreement between simulations and measurements was excellent for fields larger than 2 cm. / The MC model was also used to calculate dose distributions with the fast MC code XVMC in CT images of phantoms of clinical interest. These dose distributions were compared to dose calculations performed by the pencil-beam algorithm-based treatment planning system CadPlan and verified against measurements. Good agreement between calculations and measurements was achieved with both systems for phantoms containing 1-dimensional heterogeneities, provided a minimal quality of the CT images. In phantoms with 3-dimensional heterogeneities however, CadPlan appeared unable to predict the dose accurately, whereas MC provided with a more satisfactory dose distribution, despite some local discrepancies.

Liquid ionization chambers have characteristics that can remedy some of the drawbacks of air-filled ionization chamber dosimetry: large sensitive volumes, fluence perturbations, and energy dependence. However, high ion recombination rates can be a significant problem in liquid chambers. In this work, we have investigated properties of a new liquid chamber, called the GLIC-03 (Guarded Liquid Ionization Chamber), including chamber stability, reproducibility, and establishing recombination corrections. The response varied by less than 1% over 10 hours, and was reproducible within 1.5% of the mean over different liquid fills. Recombination corrections were established, and were small for low dose rates and high voltages. The establishment of these characteristics allowed us to compare measurements of the GLIC-03 in a region of charged particle disequilibrium to those made with a diamond detector. Results show the GLIC-03's suitability as a high resolution detector.

This treatise examines the ICRP's new respiratory tract model and its implementation into G scENMOD, a program that facilitates internal dosimetric calculation for the body. The inclusion of the ICRP66 model into G scENMOD improves the radiation dose estimates to the respiratory tract. The accuracy of this implementation was confirmed by validating G scENMOD's output against results published in ICRP66.

To optimize delivery of radiation therapy treatments, accurate knowledge of absorbed dose in the clinical beam is essential. In this work we investigated issues related to the use of different types of ionization chambers for accurate radiation dosimetry. We determined values of Pwall at 60Co for plane-parallel chambers and used them to derive the conversion factors kecal k'R 50 or kQ for a 20 MeV electron beam. Our results agreed within 0.4% with kQ values given in the IAEA TRS-398 protocol, but differences of up to 1.8% were found between our kecalk' R50 values and those in the AAPM TG-51 protocol. We also investigated the behavior of two liquid-filled ionization chambers in megavoltage photon beams, examining stability, ion recombination and beam quality dependence. Methods for determining and correcting for recombination were investigated. Preliminary results show that the energy dependence of a liquid-filled chamber can typically be limited to less than 1% for megavoltage photon beams.