Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1998. / Vita. / Includes bibliographical references (p. 129-135). / The relation between myocardial kinematics and underlying architectural components is the key to understanding the functional design of the ventricular myocardium. This thesis develops a completely noninvasive method, registered diffusion and strain MRI, to acquire information about myocardial architecture and myocardial strain under identical in-vivo conditions. This noninvasive methodology solves important limitations of existing methods all of which require myocardial dissection. It provides metrically correct data of myocardial structure and myocardial function without postmortem distortion. Further, it can be applied to living humans and allows examinations of multiple time horizons, essential to the study of normal development and disease. To provide a valid MR methodology to study myocardial structure and structure-function relations in living humans, we focus on the three steps most essential to achieving this goal: 1) validate the correspondence between diffusion MRI and myocardial architecture, particularly the fiber and sheet organizations; 2) develop a practical method of measuring myocardial diffusion in vivo; 3) show that data obtained by registered diffusion and strain MRI can be employed to address important questions about myocardial structure-function relations. To validate the ability of diffusion MRI to map myocardial architecture, we show, with a novel printing technique, that the deviation of sheet orientations is within MR noise from those in the cow heart specimens. The correspondence between directions of greatest diffusivity and fiber orientations is also verified by the consistency of architectural patterns in MRI of the cadaver heart with those reported in histology. To measure myocardial diffusion in vivo, a robust MR method is developed. In the normal heart that has the synchronous contraction, we show that the strain effect is negligibly small at time points relative to which the mean strain over one cardiac cycle equals zero: "sweet spots." Using this fact, we localize the sweet spots and show that the depicted myocardial fiber architecture agrees with the ex-vivo results. Using registered diffusion and strain MRI, we obtain first quantitative maps of fiber and sheet dynamics in human hearts. Anatomically, MRI shows the classic pattern of fiber helix angles, namely a smooth transmural variation from a left-handed helix at the epicardium to a right-handed helix at the endocardium. It also shows a septum-versus-free-wall polarization of sheet orientations, a pattern recently documented in canine hearts. Analysis of conjoint data of diffusion and strain gives a clear picture of myocardial structure-function relations: 1) systolic fiber shortening, 11±3% relative to end-diastole, is exceptionally uniform across the wall; 2) cross-fiber shortening has a steep transmural slope; it is produced by a linear variation of angles between fibers and directions of principal shortening against wall depth (from 0 at the epicardium to 900 at the endocardium). Moreover, MRI shows two new findings: 1) there is no difference in fiber shortening between trabecular and compact myocardium; 2) sheet orientations are optimized to maximize sheet shear. In conclusion, registered diffusion and strain MRI can map myocardial structure and structure-function relations practically and reliably in living human subjects. The noninvasive and spatially resolved characteristics of this methodology will facilitate investigation of myocardial mechanics in human disease. / by Wen-Yih Isaac Tseng. / Ph.D.

Computed tomography (CT) is an invaluable diagnostic tool in current medical practice. Unfortunately, the radiation dose imparted during a CT scan can be significant. This thesis seeks to develop, verify, and validate appropriate computational methods for computing this dose accurately and efficiently. The components of the model are the nuclear data, transport methods, and computer codes. Monte Carlo transport methods are employed primarily for their ability to accurately capture most of the relevant physical phenomena. Deterministic transport methods are subsequently verified and validated. The work is divided into three stages: experimental, verification, and validation. The experimental stage involves gathering high-fidelity data to aid in the validation procedures. Multiple radiation detection devices are employed to give greater certainty to the results. In addition, an important task is gathering data using a geometrically simplified phantom which is easier to model than the detailed Rando phantom. Towards this end, a CTDI FDA phantom is imaged. Exposure and dose measurements were taken in air and in the phantom center and periphery. The second stage, verification, involves the testing of the deterministic model for correctness of the methodology and the physics data, i.e. cross section library. Primarily, there are a few key assumptions which must be tested. The first is the importance of the secondary electron transport. Using Monte Carlo methods, it is found that the transport is unimportant for the accurate computation of the dose deposition distribution given the relatively low energy photons produced by x-rays tubes employed in CT scan machines. This makes the deterministic transport calculations much simpler. Next, the discretization of space, energy, and angle in the deterministic model is examined to ensure sufficient refinement capable of delivering accurate results. The Monte Carlo method is an excellent complement to deterministic methods, serving as reference as though it were an actual experiment, thus allowing the testing of these issues in a straightforward and highly controlled manner. In each discretization, the deterministic model proved capable, although some flux spectrum results differed by fifteen percent or more, mostly a result of the multigroup cross section set. Finally, after ensuring that the deterministic model was functioning as expected, a comparison was made of the simulations to the experimentally measured data. This was the most difficult of the tasks, in great part because of the lack of precise knowledge of detailed information concerning some of the parameters comprising the experimental setup. However, much effort was placed into conforming the simulations to the experiment as closely as possible. The ratio of exposures in the CTDI FDA phantom periphery-to-center is computed to within experimental uncertainty of about ten percent, while the absolute computed exposures have greater errors. The absolute exposures differed from the measured values by less than 35 percent.

Several methods of reprocessing are currently available to separate recyclable materials from spent nuclear fuel. Electrochemical processing, also known as pyroprocessing, represents a non-aqueous method of reprocessing that uses high temperature molten-salt based electrochemical technology. This method provides several advantages over conventional aqueous processing with respect to proliferation resistance. With electrochemical processing there is no pure plutonium separation and the presence of large decay heat and high radiation barriers dissuades diversion attempts. As the current electrochemical processing relies on a batch operation, the total throughput of the system is inherently limited and nuclear materials accounting is difficult due to the nonhomogeneous nature of the process. This results in much larger uncertainties in the total amount of material processed compared to the aqueous UREX+ or PUREX processes. Continuous electrochemical processing was considered as a way to address these concerns. The objective of this research was to investigate the feasibility of a continuous electrochemical processing operation to achieve the desired separation performance by using computer based simulation. The conceptual design of the continuous electrochemical processing includes two separate stages in a molten salt medium. First, a pure uranium deposit is collected at a solid cathode during the uranium extraction stage. When the amount of plutonium in electrorefiner becomes comparable or higher than the amount of uranium in the electrorefiner, a liquid cathode is employed to extract both uranium and plutonium in the second stage. In this approach, molten salt, as the material carrier, flows through the electrorefiner while chopped spent fuel is continuously fed into the system. Simulations of electrochemical reactions at the electrode surfaces were based on the kinetic modeling capability of a time-dependent code, REFIN. Based on a screening study performed for the most significant process parameters over a broad range of values, a functional combination of initial uranium and plutonium concentrations at the anode and in the molten salt was determined for continuous operation. This dictated the use of a higher concentration of uranium than plutonium at the anode and a lower concentration of uranium than plutonium in the molten salt. Furthermore, using design of experiment technique for computers, a refinement of initial concentrations was performed to maximize the total throughput and minimize the operational time. The flow velocity profiles and chemical concentration distributions of elements in molten salt have been determined through three dimensional Computational Fluid Dynamics simulations using ANSYS CFX. This approach resulted in the need to evaluate the diffusion layer thickness at the cathode â molten salt interface, an important parameter for the electrochemical process. Computer based simulations of the continuous electrochemical processing concept presented in this study have provided an indication that electrochemical processing could be a viable technology for closing the nuclear fuel cycle.

Diffraction Enhanced Imaging (DEI) with x-ray radiation provided by a synchrotron source has been shown to provide good image contrast at lower radiation dose for materials with small x-ray attenuation coefficient As a result, DEI has received significant interest for digital mammography and other medical imaging applications. However, deployment of a synchrotron source at a medical facility is not currently feasible due to its size and costs. Consequently, a compact x-ray source capable of delivering x-ray intensities and beam collimation similar to a synchrotron accelerator is desirable. A wide beam area x-ray source has been suggested as a possible alternative to a synchrotron source, with the x-rays generated by electron bombardment of a suitable target material. Previous research work demonstrated a prototype scale cylindrical shaped oxygen free copper target with a layer of molybdenum to be feasible from an engineering perspective. An industrial size DEI facility requires a scale-up of the proof-of-principle design. The x-ray flux necessary for high image quality implies significant heat loading on the x-ray source. Safe operation of a full scale DEI facility is reliant upon a thermal management solution capable of rejecting this heat. An active target cooling system has been proposed and its performance has been evaluated through CFD simulation. The design ensures the maximum target temperature is maintained at reasonable levels and coolant boiling is not reached under the most demanding operating conditions.

F-18 is a short-lived radioisotope commonly used in Positron Emission Tomography (PET). This radionuclide is typically produced through the O-18(p,n)F-18 reaction by proton bombardment of O-18 enriched water. Thermosyphon batch targets have been proposed as a means to increase F-18 production due to their enhanced heat rejection capabilities. These boiling targets have been operated with up to 3.2 kW of beam power with manageable O-18 enriched water volumes. The primary purpose of this work has been to develop a fundamental approach to target design from a modeling perspective, and to implement this approach to design new thermosyphon targets with enhanced production capabilities. Computational methods have been developed to predict target thermal performance and have been validated with experimental test data from the Duke University Medical Cyclotron and the Wisconsin Medical Cyclotron. These methods have been used to design a new production target for the Duke cyclotron with enhanced F-18 production capabilities. Low volume test targets have been successfully operated at the Wisconsin cyclotron with beam powers in excess of the desired 1.6 kW.

Single phase and boiling batch water targets are the most common designs for the cyclotron production of 18F via the 18O(p,n)18F reaction. Thermosyphon targets have design and operating characteristics which enables higher power operation than conventional boiling targets of like size. Experiments and calculations were performed in order to characterize the performance of a 1.3 cc tantalum [18F]Target. The test target led to the development of a variety of computational techniques as well as experimental methods that will be used in future target design and optimization. Computational methods include several applications of Monte Carlo Radiation Transport as well as Finite Element Analysis. In addition, experimental thermal hydraulic and radiochemical analyses were performed.

Single phase and boiling batch water targets are the most common designs for the cyclotron production of 18F via the 18O(p,n)18F reaction. Thermosyphon targets have design and operating characteristics which enable higher power operation than conventional boiling targets of like size. Experimental thermosyphon target systems demonstrated the feasibility of high intensity irradiation via bottom pressurized operation. An effective experimental characterization platform was developed and utilized in parallel with computational modeling efforts to further improve designs. A control strategy was also developed to provide a simple and robust means of remote target operation. Clinical production systems were designed and deployed at two facilities.

North Carolina State University in conjunction with Bruce Technologies Inc. is developing recirculating water targets for the cyclotron production of high yields of 18F fluoride for PET radiopharmaceuticals. Flourine-18 is commonly produced through proton irradiation of 18O enriched water by the 18O(p,n)18F reaction. Heat deposited in the target fluid by the proton beam is proportional to the 18F produced, thus production is often limited by the targets ability to reject heat. For power levels above 3 kW, boiling batch targets with local cooling can become impractical due to excessive 18O water volumes. One potential solution is a recirculating target system where the target water velocity is sufficient to prevent boiling. In this design the heated fluid travels through an external heat exchanger of sufficient capacity to remove the heat, and then through a pump which returns the cooled fluid to the target. A high-flow/low-volume pump and a high-capacity/low-volume heat exchanger are essential to the overall performance of the recirculating target. In this work, two different types of heat exchangers are considered. Laboratory testing was conducted on a small shell and tube heat exchanger that removed nearly 6 kW of heat at flows provided by a miniature regenerative turbine pump. Laboratory testing was also conducted on a small cross flow heat exchanger with measured performance of 7.4 kW and predicted peak performance approaching 10 kW.

Slant-path buildup factors for photons between 1 keV and 10 MeV for nine radiation shielding materials (air, aluminum, concrete, iron, lead, leaded glass, polyethylene, stainless steel, and water) are calculated with the most recent cross-section data available using Monte Carlo and discrete ordinates methods. Discrete ordinates calculations use a 244-group energy structure that is based on previous research at Los Alamos National Laboratory (LANL), but extended with the results of this thesis, and its focused studies on low-energy photon transport and the effects of group widths in multigroup calculations. Buildup factor calculations in discrete ordinates benefit from coupled photon/electron cross sections to account for secondary photon effects. Also, ambient dose equivalent (herein referred to as dose) buildup factors were analyzed at lower energies where corresponding response functions do not exist in literature. The results of these studies are directly applicable to radiation safety at LANL, where the dose modeling tool Pandemonium is used to estimate worker dose in plutonium handling facilities. Buildup factors determined in this thesis will be used to enhance the code?s modeling capabilities, but should be of interest to the radiation shielding community.

Strontium catalysis may be an important aspect of a novel method of power production involving hydrino reactions. These reactions allow for the release of energy from hydrogen atoms as its electron falls to an energy state below the ground state. The thermal behavior of He-H plasmas with and without Sr present was examined. The plasmas were modeled as capacitors using the Drude model for the dielectric constant. The Joule heating of the plasmas was determined and compared with differential thermal energy to see what effect the addition of strontium had on the thermal output. Most results were inconclusive, but it was clear that plasmas with 1.01% H had a marked increase in thermal output when Sr was added. This means that there may be strontium catalysis of the hydrino process in this case and further experimentation should be done to rule out other explanations for this increase.