The symplectic sp(4) algebra provides a natural framework for studying proton-neutron (pn) and like-nucleon pairing correlations as well as higher-J pn interactions in nuclei when protons and neutrons occupy the same shell. While these correlations manifest themselves most clearly in the binding energies of 0+ ground states, they also have a large effect on the spectra of excited isobaric analog 0+ states. With a view towards nuclear structure applications, a fermion realization of sp(4) is explored and its q-deformed extension, sp(4)q, is constructed for single and multiple shells. The su(2)(q) substructures that enter are associated with isospin symmetry and with identical-particle and pn pairing. We suggest a non-deformed as well as a q-deformed algebraic descriptions of pairing for even-A nuclei of the mass 32 < A < 164 region. A Hamiltonian with a symplectic dynamical symmetry is constructed and its eigenvalues are fit to the relevant Coulomb corrected experimental 0+ state energies in both the classical and deformed cases. While the non-deformed microscopic theory yields results that are comparable to other models for light nuclei, the present approach succeeds in providing a reasonable estimate for interaction strength parameters as well as a detailed investigation of isovector pairing, symmetry energy and symmetry breaking effects. It also reproduces the relevant ground and excited 0+ state energies and predicts some that are not yet measured. The model successfully interprets fine features driven by pairing correlations and higher-J nuclear interactions. In a classification scheme that is inherent to the sp(4) algebraic approach, a finite energy difference technique is used to investigate two-particle separation energies, irregularities found around the N = Z region, and like-particle and pn isovector pairing gaps. The analysis identifies a prominent staggering behavior between groups of even-even and odd-odd nuclides that is due to discontinuities in the pairing and symmetry terms. While the classical limit of the theory provides good overall results, the analysis also shows that q-deformation can be used to gain a better understanding of higher-order effects in the interaction within each individual nucleus.

Satellite and rover remote sensing of planetary regolith surfaces, in the form of thermal infrared emissivity spectra taken at nadir and off-nadir angles of emergence from the surface, requires use of theoretical models for interpretation of constituent grain physical properties. However, such models have remained in stasis in recent years, with nearly a ten-year gap in significant advances. To date, no radiative transfer model (semiempirical, exact, or hybrid solution) has been able to adequately predict the nadir emissivity behavior of simple mineral assemblages. Few measurements have been attempted in the laboratory or field regarding directional emissivity effects of planetary regoliths; such measurements are necessary for modeling and interpreting directional emissivity effects that are clearly present in the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) and Mars Exploration Rover mini-TES datasets. The research goals of this dissertation directly involve the extraction of information on two major dust microphysical properties: particle size and packing fraction. Results of a theoretical model are compared to laboratory-measured thermal infrared (wavenumber = 2000-200 cm-1) emissivities for micron-sized quartz particles. This work shows that Mie theory, a widely used but poor approximation to irregular grain shape, fails to produce the single scattering properties needed to arrive at the desired laboratory emissivity values and also illustrates shortcomings of popular dense packing correction methods. Through numerical experiments, I provide evidence that, assuming RT methods work given sufficiently well-quantified inputs, assumptions about the scatterer itself constitute the most crucial aspect of modeling nadir emissivity values. Also included in the dissertation are detailed laboratory investigations used to obtain realistic and quantifiable input parameters to the theoretical model, i.e., particle size distribution and particle shape. Nadir and directional emissivity comparison datasets obtained in the laboratory and in the field at Mars terrestrial analog sites are presented to set the stage for modeling directional emissivity. Future directions (e.g., how to incorporate nonspherical particle shapes into the model) are briefly discussed.

The PET/CT scanner has been recognized as a powerful diagnostic imaging modality in oncology and radiation treatment planning. Traditionally, PET has been used for quantitative analysis, and diagnostic interpretations of PET images greatly relied on a nuclear medicine physicians experience and knowledge. The PET data set represents a positron emitters activity concentration as a gray scale in each pixel. The assurance of the quantitative accuracy of the PET data is critical for diagnosis and staging of disease and evaluation of treatment. The standard uptake value (SUV) is a widely employed parameter in clinical settings to distinguish malignant lesions from others. SUV is a rough normalization of radioactive tracer uptake where normal tissue uptake is unity. The PET scanner is a sensitive diagnostic method to detect small lesions such as lymph node metastasis less than 1 cm in diameter, whereas the CT scanner may be limited in detecting these lesions. The accuracy of quantitation of small lesions is critical for predicting prognosis or planning a treatment of the patient. PET/CT uses attenuation correction factors obtained from CT scanner data sets. Non-biological materials such as metals and contrast agents are recognized as a factor that leads to a wrong scaling factor in the PET image. We challenge the accuracy of the quantitative method that physicians routinely use as a parameter to distinguish malignant lesions from others under clinical settings in commercially available CT/PET scanners. First, we verified if we could recover constant activity concentration throughout the field of view for small identical activity concentration sources. Second, we tested how much the CT-based attenuation correction factor could be influenced by contrast agents. Third, we tested how much error in quantitation could be introduced by object size. Our data suggest that the routine normalization process of the PET scanner does not guarantee an accurate quantitation of discrete uniform activity sources in the PET/CT scanner. Also, activity concentrations greatly rely on an objects dimensions and object size. A recovery correction factor is necessary on these quantitative data for oncological evaluation to assure accurate interpretation of the activity concentration. Development of parameters for quantitation other than SUV may overcome SUVs inherent limitations reflecting patient-specific physiology and the imaging characteristics of individual scanners.

Different techniques have been developed and used to evaluate dose distribution calculation accuracy and dose delivery reproducibility as a part of patient-specific IMRT QA e.q. film dosimetry, ionization chambers, and diode arrays. To verify that the calculated dose distribution is delivered accurately during treatment, film dosimetry is usually used. The accuracy and reproducibility of film optical density as an indicator of dose is influenced by several variables, including the chemical processing and scanning conditions. This study investigates the possibility to use a desktop computed radiography (CR) system for patient-specific intensity modulated radiation therapy (IMRT) quality assurance (QA). A study was done at Mary Bird Perkins Cancer Center, Baton Rouge, LA; where phantom IMRT plans are calculated using an ADAC Pinnacle³ treatment planning system. A Kodak ACR-2000i system is used for the study together with Kodak flexible phosphor screens (plates). In this study, 778 CR plate exposures were done. Several tests were performed including evaluation of the CR plate response dependency when exposed to changes in either setup or scan conditions. Calibration curves were generated for three different energies: 4 MV, 6 MV and 10 MV. Using these calibration curves, the CR plates response and behavior as an IMRT tool was analyzed using 10 different patients IMRT plans for each energy with approximately 7 fields per patient. Analysis of film was done with commercial IMRT analysis software. Analysis of CR plate data was done in IDL (Research Systems, Inc.), with programs written in house, and included several separate algorithms including automatic image registration. This algorithm uses the Fourier-Mellin transform for automatic image registration. It was found that CR plates showed generally good agreement with the planned values with some significant over-response in the low dose regions, which can be reduced by filtration and improved calibration curves. In view of the results presented, a CR system stands as a potentially fast and practical tool for IMRT patient-specific treatment QA.

We present results from investigations of the nonlinear development of the secular bar-mode instability that is driven by gravitational radiation-reaction (GRR) forces in rotating neutron stars. Our fully three-dimensional hydrodynamical simulations have shown that, in the absence of any competing viscous effects, initially uniformly rotating axisymmetric n=1/2 polytropic stars with a ratio of rotational to gravitational potential energy greater than the critical limit are driven by GRR forces to a bar-like structure, as predicted by linear theory. The pattern frequency of the bar slows to nearly zero, that is, the bar becomes almost stationary as viewed from an inertial frame of reference as GRR forces remove energy and angular momentum from the star. In this "Dedekind-like" state, rotational energy is stored as motion of the fluid in highly noncircular orbits inside the bar. However, in a very short time after its formation, the bar loses its initially coherent structure as the ordered flow inside the bar is disrupted by what appears to be a purely hydrodynamical, short-wavelength, "shearing" type instability. The gravitational waveforms generated by such an event are determined, and an estimate of the detectability of these waves is presented. Our results also suggest that since a Dedekind-like configuration is susceptible to this turbulent instability, the long believed evolutionary path of a secularly unstable star driven by gravitational radiation toward the Dedekind ellipsoid, becomes questionable.

PET/CT scans are frequently used for radiation treatment planning (RTP). Our work demonstrates a practical approach for validating the PET/CT dataset for RTP. We tested this QA process on a Reveal HD PET/CT scanner. The phantom used is a TGM2 ISIS QA phantom, a 14 cm acrylic cube with a central bore for object inserts. It has four different built-in inserts for electron density verification. 22Na seeds are inserted into the pinholes at the side of the cube. PET/CT images of the phantom with 22Na seeds are acquired and fused in the scanner Syngo fusion software. Registration of the PET/CT dataset is visualized by raising the lower threshold of the PET images to reduce the 22Na point sources to a few pixels and comparing it with the CT images of 22Na seeds. Geometric scaling accuracy of the pixels is verified by measuring the dimension of the cube in x, y and z axes. The HU values of four electron density verification inserts are measured and compared with manufacturer specified HU values. These QA tests are repeated in the RTP software after importing the PET/CT dataset. A quantitative analysis of registration error and geometric scaling accuracy of pixels are verified independently using MATHEMATICA. The resolution of the PET scanner was determined by measuring the FWHM of capillary tube sources inserted in a Styrofoam block based on the NEMA-2 protocol. Minor misalignment of the fused images was detected in the scanner (~1 mm) while the imported dataset in the RTP system showed a major misalignment (~6 mm) when fused by auto fusion software. The maximum geometric scaling errors of object sizes were observed in the z direction (5.2% decrease) in the scanner and the scaling errors were less in the RTP software (2.9% decrease). The greatest HU errors in the CT image compared with expected HU values were observed in the bone density insert (28% increase) in the scanner and all HU values for different inserts were shifted up by a constant value in the RTP system. The resolution of the PET scanner was comparable to the manufacturers specification.

Atomistic aspects of dynamic fracture in amorphous and nanostructured silica are herein studied via Molecular dynamics (MD) simulations, ranging from a million to 113 million atom system. The MD simulations were performed on massivelly parallel computers using highly efficient multi-resolution algorithms. Crack propagation in these systems is accompanied by nucleation and growth of nanometer scale cavities up to 20 nm ahead of the crack front. Cavities coalesce and merge with the advancing crack to cause mechanical failure. Recent AFM studies in silica glasses confirm this scenario of fracture [1]. The morphology of the fracture surfaces is studied by calculating the height-height correlation function. The MD simulation finds the first roughness exponent (æ=0.5). Simulations of amorphous nanostructured silica reveal pore nucleation ahead of the crack front, and the crack front meandering around the nanoparticles and merging with those pores.

Purpose: To ascertain if PET image data of a positron tracer can be used for the quantitative description of dose distribution in support of direct prostate seed dosimetry. Materials and Methods: Simulated brachytherapy seeds were constructed containing trace amounts of a positron emitter, F-18, such that all annihilation events took place in the encapsulation wall. An acrylic prostate phantom containing these seeds was imaged with a GE Discovery ST PET/CT scanner in 2D and 3D acquisition modes and several image reconstruction methods. The PET scan data was used as the input for Monte Carlo calculation of dose distribution due to the F-18. This dose distribution was then compared to computations wherein the source was restricted to the encapsulation wall. This was done to determine if the measured data could be used to accurately compute the annihilation dose, which in turn would be used to compute the therapeutic dose due to known seed activity. Results: Examination of the dose distributions indicates a close agreement between the measured data and theoretical calculations for certain cases. We found that 2D acquisition with OSEM reconstruction resulted in a maximum difference in transaxial dose distribution of 15% in a single voxel, and a mean difference of 4% for the remaining voxels. However, the mean discrepancy between dose computations based on the ideal source versus PET based source is within or close to the Monte Carlo error of 2% to 4%. These results do not reflect any optimized acquisition protocol that may further reduce the observed differences. Conclusions: This work indicates there is potential for using PET data for the proposed link between the therapeutic brachytherapy dose and the dose due to a trace amount of encapsulated positron emitter, as developed by Sajo and Williams. Because this method does not require explicit information on seed locations, clinical implementation of this technique could significantly reduce the time needed for post-implant evaluation, and several of the uncertainties and limitations inherent in current prostate brachytherapy dosimetry.

Radiotherapy treatment planning utilizing PET and CT is rapidly gaining acceptance in oncology. A limiting factor of the dual modality is the PET/CT alignment. A small error in PET/CT alignment may result in giving large doses of radiation to healthy tissues as a result of poor treatment planning. For this purpose, regular quality assurance testing of PET/CT must be performed. Separate QA procedures and phantoms have been developed for the two different modalities. In particular, many existing phantoms cannot be used for both modalities, which is a requirement for evaluating PET/CT alignment. Our goal is to evaluate several existing phantom designs to evaluate their utility for checking PET/CT alignment. The three phantoms investigated are a Gammex 464 phantom, a Triple-Line Source PET phantom, and a Hot Sphere PET phantom. The PET phantoms are unmodified the Gammex 464 phantom is modified to perform PET/CT alignment. The Gammex 464 phantom is typically used for routine quality assurance of CT scanners. Several CT parameters are determined with this phantom before and after modification. Then PET/CT alignment testing is performed using this modified CT phantom and the two other phantoms. Three methods have been used for analyzing the PET/CT images to measure the PET/CT alignment errors. The methods are the Manual method which calculates the alignment error from hand-drawn profiles, the Maximum-Pixel Value method which measures the error based on the pixel value of the objects in the PET/CT images, and the Curve-fitting method, which measures the alignment error by getting the best fit values for the object profiles. The Curve-fitting method also estimates the PET resolution from apparent size of objects in the phantoms. Our PET/CT alignment data and results suggest that the Maximum-Pixel Value method for the modified phantom with acrylic insert is a good choice for measuring the PET/CT alignment error, providing a reasonable balance between computational analysis effort and measurement precision.

Positron emission tomography (PET) and computed tomography (CT) are well established and powerful tools for medical diagnostics but even integrated PET/CT scanner images still lack the necessary quality and resolution that would make medical diagnoses flawless. In this thesis experiments were performed to statistically determine the effect that image acquisition parameters have upon diagnostic accuracy. Images from different PET/CT scanners were assessed by comparing subject human diagnostic accuracy from a sample of both professional and student volunteers. The assessment results were compared to the objective NEMA-standards performance data provided by the manufacturers for each scanner. The data analysis method is the receiver operating characteristic (ROC) curves. We hypothesize that human performance in making accurate diagnoses from PET images correlates with the system performance. The data shows that human diagnostic performance correlates to spatial resolution and sensitivity of the PET imaging systems.