Current brachytherapy dosimetry protocols assume that a physical source may be approximated by a point source. A new brachytherapy dosimetry protocol, recently proposed by the American Association of Physicists in Medicine Task Group 43, has the advantage of using functions derived solely from measurements performed in the medium and uses a more realistic source geometry than the point source approximation. The aim of this work is to obtain the dosimetric functions required by this new protocol for both a low and a high dose-rate Iridium-192 brachytherapy source through dose measurements in a water-equivalent phantom. / Dose measurements have been performed using lithium fluoride thermoluminescent detectors positioned in a polystyrene phantom at distances from the source that vary from 1 cm to 10 cm, with 1-cm intervals, and at angles that vary from 0$ sp circ$ to 170$ sp circ$ with 10$ sp circ$ intervals. / Our experimental results have clearly shown that the point-source approximation model can overestimate the dose to water, especially for the high dose-rate source, where we have found that differences between point-source estimates and exact measured values can differ by almost 30% for points along the longitudinal axis of the source.

Computer methods have proven very useful in many medical imaging studies and applications. The advantages of using computers for the analysis of the images of different modalities have raised the interest of researchers from different fields of science and medicine. / An image analysis application which aims to quantify Multiple Sclerosis (MS) lession in human brains based on Magnetic Resonance Imaging (MRI) has been developed. In this thesis, image analysis based on statistical theory applied in medical imaging studies is reviewed. A three-dimensional method has been developed to compare the classification probability distribution of two groups of subjects. Using random field theory, regional difference of two three-dimensional classification probability maps is assessed statistically. The method can be applied to the comparison of probability distributions of multiple sclerosis lesion in human brains under different conditions. The sensitivity of the method is presented, and it is shown that the method can effectively and reliably detect the regional difference between two populations. In addition, a novel algorithm is introduced to automatically register a T 2-weighted MRI into a T1-weighted MRI acquired from the same human brain.

Many of the problems faced in magnetic resonance imaging and angiography are due to hardware limitations of the scanners used. The use of multidimensional excitation pulses, and new, fast acquisition sequences such as echo-planar and spiral imaging, put demands on the gradient systems and although technology is progressing in these areas, performance must be carefully monitored to ensure artefact-free images. This thesis deals with four distinct aspects of magnetic resonance imaging (MRI). / A simple hardware modification to decrease the minimum achievable echo-time of our MRI scanner was designed, built, programmed and tested and was found to improve the quality of inflow angiograms significantly. Further improvements were demonstrated with the use of on-resonance (binomial) magnetization transfer saturation contrast enhancement pulses. / A method for measuring the k-space trajectories of gradient waveforms was adopted, modified, validated and used to measure the k-space trajectories of gradient waveforms used for selective excitation and spiral image acquisition. Distortion was observed even when gradient waveforms were designed within specifications of the manufacturer. / The literature reports the application of the k-space model, usually associated with image acquisition, to the design of multidimensional selective excitation pulses. This thesis demonstrates theoretically and experimentally the modification of the design procedure to compensate RF envelopes for distortion of the k-space trajectories of the accompanying gradient waveforms by using measured k-space data. / The correction of spiral image reconstruction algorithms to compensate for k-space trajectory distortion was also demonstrated theoretically and experimentally.

This thesis is concerned with the creation, verification and implementation of a computer programme for simulating photon transport in diagnostic radiology. The programme is based on the Monte Carlo technique in which probabilistic problems are solved using random numbers. For this reason mathematical number generators along with a few standard tests which enable the randomness of the numbers to be evaluated are introduced. A discussion on sampling from probability distributions is presented with emphasis on the physical aspects of the Monte Carlo technique applied to low energy photon transport. The validity of the computer programme is established by comparisons with previously published Monte Carlo work, with predictions from an analytical photon transport model and with experimental measurements. The application of Monte Carlo simulations to specific radiographic problems are illustrated by the modeling of percent depth doses.

Biological effects and prostate motion were studied for prostate cancer patients treated with external beam radiotherapy. The prostate motion was determined using an ultrasound-based patient positioning system (BAT: B-mode Acquisition and Targeting) just after conventional patient setup. The changes in planned biological effects due to prostate motion were calculated for the prostate target organ and for the rectum and bladder normal tissues using TCP (tumor control probability) and NTCP (normal tissue complication probability) calculations, respectively. The prostate TCP calculations were carried out for both prostate PTV and GTV structures, whereas the rectum and bladder wall structures were used for the NTCP calculations.

Current challenges facing us in developing dedicated positron emission tomography (PET) systems for metabolic breast mammography (PEM) and small animal (ANIPET) are to achieve high spatial resolution (less than 2 mm) and high efficiency. It is also crucial to extend the sensitive areas of PEM detectors to their periphery in order to overcome the difficulty in imaging near a patient's chest wall. This limitation of the periphery dead region was revealed in the clinical trials of our previously developed PEM-I system. / In the new study, we developed prototype detectors by using position-sensitive photomultiplier tubes (PS-PMTS) and pixelated bismuth germanate (BGO) crystals with depth encoding scheme to detect and localize gamma rays. We used the following methods in crystal processing: (1) Crystal cutting---each crystal block was cut by diamond saw into small elements of 2.1 mm x 2.1 mm (2.2 mm pitch) on two opposite faces. The elements on one face of the block offset by half the crystal pitch from those on the opposite face in both X and Y dimensions. The depths of two layers were 11.5 mm and 6.5 mm, respectively. The middle solid space between the two layers was 2 mm. (2) Crystal polishing and encapsulating---The very roughly cut surfaces were chemically polished by acid etching method and the cut slots were encapsulated with an epoxy-compound mixture. (3) Crystal separating---the pseudo discrete crystal blocks were cut along the middle solid space into two discrete segments. (4) Crystal coupling---the two crystal segments were glued together and optically coupled to the PS-PMTs window. / We also developed front-end electronic circuits including high-voltage dividers, anode resistor chains, position readout circuits, and last-dynode timing circuits. Methods for combining four PS-PMTs with simple X+, X-, Y+, Y- outputs have been developed to further simplify the position recording. The detectors were constructed in the structure of arrays (two in the system)---modules (four in each array)---units (four in each module). The basic unit of one crystal and one PS-PMT was formed as field replaceable unit. The acquired list-mode data were analyzed with MATLAB and C. Different methods to generate distortion look-up-table were examined and evaluated. / Our new prototype detectors have spatial resolutions of 1.8 mm (vs. 2.8 mm in PEM-I), timing resolution of 10.3 ns (vs. 12 ns in PEM-I), and a field-of-view of 88 mm x 88 mm (vs. 64 mm x 56 mm in PEM-I). Our analysis shows that the design improves the spatial resolution, enhances the detector field-of-view, and significantly reduces the peripheral dead regions.

In a large multi-center study where many groups collaborate to produce data, the conditions under which the data are collected can vary significantly. In the case of the NIH funded MRI study of normal brain development (NIHPD), 6 centers across the US collect MRI scans of the developing child brain. In all, four different models of scanners from 2 manufacturers are used. The subject of this thesis is to quantify differences in acquired data that are due to scanner differences and thus enable improved biological study. / The aim of this study is to analyze the scanner-induced differences across sites using a specialized phantom recommended by the American College of Radiology (ACR). Together with the 7 tests listed by the ACR MRI accreditation manual, phantom data from each site can be analyzed for parameters that are characteristic of image quality. The measurements are done automatically using programs written at the MNI, that use MINC tools and the MNI autoreg package to compute the necessary parameters. / The results demonstrate that, in general, all data obtained complied with the 7 tests of the ACR MRI manual. Statistically significant variations over time and between sites were found in the data. However, the magnitude of the variations is of the order of 1%-2% indicating stable data acquisitions and comparability of data between sites.

Radiosurgery is characterized by high radiation doses, delivered via small diameter radiation beams in a single session, placing stringent requirements on the numerical and spatial accuracy of dose delivery to the target volume within the brain. In this thesis, physical and clinical aspects of radiosurgery are discussed, including a method for the production of cylindrical dose distributions with rectangular beams using cylindrical dynamic rotation. / The measurements of radiosurgical x-ray beam parameters are presented. Monte Carlo simulations determine that a measured increase depth of dose maximum with increasing field size is a result of primary dose deposition in phantom for small diameter beams. / An analytical representation based on a curve-fitting process is developed to parametrize radiosurgical x-ray beam percentage depth doses as a function of depth in phantom, field diameter and beam energy using bi-exponential and polynomial functions. / Measurements of dose in the build-up region of x-ray beams ranging from 1 x 1 cm$ sp2$ to 30 x 30 cm$ sp2$ show that the depth of dose maximum increases rapidly with increasing field size at small fields, reaches a maximum around 5 x 5 cm$ sp2$ and then gradually decreases with increasing field size for large fields. Monte Carlo simulations attribute effect observed at large fields to the scatter contamination of the primary beam from the linac head. This scatter contamination is measured by a half-block technique and further experiments show that it consists of electrons originating in the flattening filter of the linac.

An electret radiation dosimeter for long-term personnel monitoring is described. The design of this prototype (a modified parallel-plate ionization chamber) and the associated isothermal electret charging technique are presented. In the charging process, an external voltage causes ions created in air by the passage of radiation to move towards, and become trapped on, a dielectric (e.g., Mylar, Teflon) that covers the measuring electrode, forming an electret. Once the external voltage is removed, the field across the sensitive volume is produced by the electret charge, such that during subsequent irradiation, ions opposite in sign to those on the electret surface are attracted to the electret thus depleting the charge layer in an amount proportional to the exposure. Further irradiation releases the remaining charge on the electret which is measured with an electrometer. This technique allows the electret to be charged, used in the field, and discharged in situ, without dismantling the dosimeter as is required with other electret dosimeters relying on corona charging or other forming methods. Calibration, energy dependence, exposure range, and guard-ring effects of the dosimeter are discussed. This electret dosimeter may prove to be a viable alternative to film dosimeters and TLDs, and is inherently superior because the measuring medium is air.

The spin-spin and spin-lattice relaxation times of protons on polymers, T$ sb{ rm 1p}$ and T$ sb{ rm 2p}$, respectively, have been used to probe the absorbed dose of irradiated polymer solutions in which radiation-induced changes in polymer molecular weight, M$ sb{ rm n}$, occur. The M$ sb{ rm n}$ dependencies of T$ sb{ rm 1p}$ and T$ sb{ rm 2p}$, and of the water proton T$ sb{ rm 1w}$ for solutions of poly(ethylene oxide) (PEO) in D$ sb2$O and H$ sb2$O are presented. T$ sb{ rm 1p}$ and T$ sb{ rm 1w}$ are independent of M$ sb{ rm n},$ and T$ sb{ rm 2p}$ varies with M$ sb{ rm n}$ according to a specific inverse power dependence until low M$ sb{ rm n}$ when T$ sb2$ saturation occurs. The dose dependence of T$ sb{ rm 1p}$ and T$ sb{ rm 2p}$ measured for dilute solutions of PEO in D$ sb2$O reflects the dependence of M$ sb{ rm n}$ on dose. A novel semi-empirical model is proposed for the dose dependence of T$ sb{ rm 2p}$ which incorporates the measured M$ sb{ rm n}$ power dependence of T$ sb{ rm 2p}$ into a theoretical expression of the dose dependence of the M$ sb{ rm n}$. This expression is based on previous bulk polymer work and has been modified to hold for polymers in solution. The model can be fitted well to the T$ sb{ rm 2p}$ data measured for different doses, and the values of the fitting parameters agree with those expected from independent measurements. Practical aspects of the NMR/polymer dosimetry technique are also addressed.