Investigations in x-ray computed tomography polyacrylamide gel dosimetry

Polyacrylamide gels (PAGs) are radiosensitive materials currently under development for use as three dimensional (3D) dosimeters in radiation therapy. Dose information is recorded in the gels and extracted through imaging. X-ray computed tomography (CT) has emerged as a promising gel imaging method due to a change in gel density that occurs upon irradiation. The accessibility of CT technology to cancer hospitals makes CT read-out clinically attractive, however the technique remains of limited clinical use due in part to poor dose resolution. This thesis investigates the use of CT for extracting dose information from PAG with an overall goal to improve achievable dose resolution. Thesis results are divided into three studies: a gel compositional study, a study of noise and dose resolution and a digital filtering study. The first study investigates the effects of gel composition on PAG CT dose response and the underlying density change. Systems for irradiating and imaging gels are designed and tested and dose response reproducibility is established. Results indicate dramatic variation in CT dose response sensitivity and range with gel composition. A model is developed to describe gel density change with dose, revealing two fundamental properties of the density to dose response: the density change that occurs per unit polymer yield is highest for gels with low and high concentrations of crosslinking molecule (%C) and the dose response sensitivity is linearly dependent on the total concentration of monomer in the gel. The second study investigates strategies for minimizing noise in x-ray CT polymer gel dosimetry and assesses system performance. Specifically, the effects of phantom design, scanning technique and image voxel size on image noise are investigated. This work leads to the establishment of a method of predicting image noise for any given CT imaging protocol. Image uniformity is also assessed, in the context of noise levels in gel dosimetry. The effect of scanning protocol on imaging time is established and the dose resolution achievable with an optimized system is calculated given voxel size and imaging time constraints. These results, when compared with published values for MRI and optical CT gel dosimetry indicate that CT dose resolution (e.g. 5%, 1 x 1 x 3 mm³ voxels), is still not at the level of the best MRI or optical CT techniques, however fast imaging times makes the rapid acquisition of volumetric data most feasible with x-ray CT. The third study investigates the potential of image filtering for improved dose resolution in CT gel dosimetry. CT image noise is characterized as Gaussian distributed and independent of signal strength and niters for reducing spatially invariant noise are investigated: mean, median, midpoint, adaptive mean, alpha-trimmed mean, sigma mean and a relatively new filter called SUSAN. The filters are tested on a CT image of a PAG irradiated with a clinically relevant dose distribution. Filter performance varies greatly in both achieved dose resolution and affects on the spatial distribution of dose. The ADAPTIVE and SUSAN filters provide the best overall performance, more than halving the dose resolution without significantly distorting the spatial distribution of dose. In summary, this thesis provides new insight into the fundamental nature of PAG density to dose response, develops strategies for minimizing image noise and quantifies system performance and demonstrates that digital image filtering is an effective tool to provide additional improvements to dose resolution.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

X-ray computed tomography for performing polymer gel dosimetry : a feasibility study

Radiation therapy treatment of cancer is increasingly concerned with delivering dose distributions that conform to the tumour volume. For verification of treatment planning computer dose calculations, these conformal therapies demand an accurate, sensitive, high resolution three dimensional (3D) dosimeter. Polymer gels are novel, inherently 3D, tissue equivalent radiation dosimeters. Traditionally, dose distributions recorded in polymer gel are read out using magnetic resonance imaging (MRI). Presented here is a feasibility study on a new 3D dosimetry technique that uses x-ray computed tomography (CT) to read dosimetric information from polymer gels. The technique exploits a gel density change that occurs in response to ionizing radiation. This study has three main goals: 1) to develop a protocol for producing quality CT polymer gel images; 2) to evaluate the nature and reproducibility of the dosimeter's CT number (JVcr)-dose response; and 3) to compare this technique with MRI polymer gel dosimetry. A quantitative discussion of the density changes occurring in the gel in response to ionization radiation is also provided. Experiments are conducted using a PAG (polyacrylamide and gelatin) gel dosimeter irradiated with four intersecting 10 M V photon beams. The NCT -dose response is found to be linear and reproducible over the range of 200 to 1OOOcGy. At room temperature the response is (8.7 ± 0.3)xl0⁻³ NCT/cGy resulting in a limited dose resolution, - 100 cGy. Gel temperature during imaging is determined to have only a small effect, 0.3%/°C, on the dose response. Spatial resolution is 0.5 mm in the image plane and 10mm-1mm (depending on noise requirements) in the third dimension. Despite the low dose resolution, preliminary results indicate this technique provides accurate localization of high dose regions and, given the availability and speed of CT imaging, has the potential to be a valuable and practical tool for radiation therapy clinics.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Daily quality assurance phantom for ultrasound image guided radiation therapy

A simple phantom was designed, constructed, tested, and clinically implemented for daily quality assurance (QA) of an ultrasound-image-guided radiation therapy (US-IGRT) system, the Restitu Ultrasound system (Resonant Medical, Montreal, QC). The phantom consists of a high signal echogenic background gel surrounding a low signal hypoechoic egg-shaped target. Daily QA checks involve ultrasound imaging of the phantom and segmenting of the embedded target using the automated tools available on the US-IGRT system. This process serves to confirm system hardware and software functions and, in particular, accurate determination of the target position. Experiments were conducted to test the stability of the phantom at room temperature, its tissue-mimicking properties, the reproducibility of target position measurements, and the usefulness of the phantom as a daily QA device. The phantom proved stable at room temperature, exhibited no evidence of bacterial or fungal invasion in 9 months, and showed limited desiccation (resulting in a monthly reduction in ultrasound-measured volume of approximately 0.2 cm3). Furthermore, the phantom was shown to be nearly tissue-mimicking, with speed of sound in the phantom estimated to be 0.8 % higher than that assumed by the scanner calibration. The phantom performs well in a clinical setting, owing to its light weight and ease of operation. It provides reproducible measures of target position even with multiple users. At our center, the phantom is being used for daily QA of the US-IGRT system with clinically acceptable tolerances of ±1 cm3 on target volume and ±2 mm on target position. For routine daily QA, this phantom is a good alternative to the manufacturer-supplied calibration phantom, and we recommended that that larger phantom be reserved for less frequent, more detailed QA checks and system calibration

A Hybrid Direct Search and Model-Based Derivative-Free Optimization Method with Dynamic Decision Processing and Application in Solid-Tank Design

A derivative-free optimization (DFO) method is an optimization method that does not make use of derivative information in order to find the optimal solution. It is advantageous for solving real-world problems in which the only information available about the objective function is the output for a specific input. In this paper, we develop the framework for a DFO method called the DQL method. It is designed to be a versatile hybrid method capable of performing direct search, quadratic-model search, and line search all in the same method. We develop and test a series of different strategies within this framework. The benchmark results indicate that each of these strategies has distinct advantages and that there is no clear winner in the overall performance among efficiency and robustness. We develop the Smart DQL method by allowing the method to determine the optimal search strategies in various circumstances. The Smart DQL method is applied to a problem of solid-tank design for 3D radiation dosimetry provided by the UBCO (University of British Columbia—Okanagan) 3D Radiation Dosimetry Research Group. Given the limited evaluation budget, the Smart DQL method produces high-quality solutions