Development and Evaluation of a Stationary Head Computed Tomography Scanner

X-Ray Computed Tomography (CT) is a widely used 3D imaging technique, proving indispensable in the diagnosis of medical conditions and pathologies. However, virtually all of today’s state-of-the-art CT systems rely on a rotating gantry to acquire projections spanning up to 360 degrees around the head and/or body. By replacing the rotating source and detector with a stationary array of x-ray sources and line detectors, a CT scanner could be potentially constructed with a smaller footprint and faster scanning speed. The subject of this dissertation is the design, construction, and evaluation of a stationary head CT (s-HCT) scanner capable of diagnosis of stroke and head trauma patients in limited resource areas such as forward operating bases. By bringing the diagnostic CT scanning capability to the patient, survival rates could potentially be greatly improved through quicker delivery of appropriate treatments. The scanner is made possible by recent advances in technologies related to CT, including x-ray sensor technology, iterative reconstruction methods, and distributed x-ray sources. Recently, carbon nanotube (CNT) x-ray source arrays have been utilized in a number of medical and security applications. The unique electronic scanning ability afforded by these systems can removes the need for a rotating gantry, producing a stationary system which potentially is more mechanically robust and could provide diagnostic CT images in a smaller footprint, with little to no loss in image quality.The use of 3 linear x-ray source arrays naturally results in a triangular shape, representing a radical departure from a traditional (circular) source ring. The final construction of the prototype proves that circular objects can still be reconstructed accurately even though the geometry of the system is triangular. Furthermore, the prototype has been able to acquire all of the projection data in scan times comparable to those of commercial scanners (< 1min), indicating the CNT x-ray and s-HCT technologies are developed enough for clinical trials. As part of an initial evaluation, several objects are imaged in a phantom imaging study, with results demonstrating the temporal and spatial resolution, as well as the accuracy and noise associated with the 3D reconstruction output.Doctor of Philosoph

Design and development of a compact x-ray tube for stationary CT architecture

Multisource architectures enable sweeping one or more x-ray beams across the imaging field-of-view faster than physically moving a single x-ray source and/or a detector. Hence, these architectures are attractive for the applications in which temporal resolution plays an important role, for example, cardiac computed tomography (CT) or real-time CT. One of the recent developments in multisource architectures for CT imaging is stationary CT architecture, whereby two separate stationary arrays -- one for x-ray sources and one for detectors -- are utilized to sweep one or more x-ray beams along the gantry and acquire 360 degree projections. To have a stationary CT architecture and still acquire enough number of projections for a successful CT reconstruction, an array of closely spaced and individually addressable x-ray sources that are compact in size and are capable of producing x-ray pulses at a high frequency is required. This work is a part of a research continuum toward developing a compact x-ray tube potentially to be used in the said stationary CT architecture. The central hypothesis of the research conducted is that, a field emission (FE) based cold cathode and transmission type anode allow the size of an x-ray tube to be reduced, and still generate a required x-ray pulse. Specifically, this work entails the design of an electrostatic lens for electron focusing in a compact x-ray tube and the initial experimental studies of a prototype compact x-ray tube. Using particle-in-cell code, OOPIC PRO, electron field emission from CNT cathode and electron focusing by three types of electrostatic lenses--Single lens, Double lens, and Einzel lens--were simulated, compared, and studied for anode voltages of 30 kV p to 140 kV p. The first-generation prototype compact x-ray tube was developed and studied. The initial studies conducted to understand the performance of the prototype and its control parameters. After further optimization of size and testing, this compact x-ray tube design holds a potential to be used in a stationary CT architecture for improved temporal resolution --Abstract, page iv

Generalized-Equiangular Geometry CT: Concept and Shift-Invariant FBP Algorithms

With advanced X-ray source and detector technologies being continuously developed, non-traditional CT geometries have been widely explored. Generalized-Equiangular Geometry CT (GEGCT) architecture, in which an X-ray source might be positioned radially far away from the focus of arced detector array that is equiangularly spaced, is of importance in many novel CT systems and designs. GEGCT, unfortunately, has no theoretically exact and shift-invariant analytical image reconstruction algorithm in general. In this study, to obtain fast and accurate reconstruction from GEGCT and to promote its system design and optimization, an in-depth investigation on a group of approximate Filtered BackProjection (FBP) algorithms with a variety of weighting strategies has been conducted. The architecture of GEGCT is first presented and characterized by using a normalized-radial-offset distance (NROD). Next, shift-invariant weighted FBP-type algorithms are derived in a unified framework, with pre-filtering, filtering, and post-filtering weights. Three viable weighting strategies are then presented including a classic one developed by Besson in the literature and two new ones generated from a curvature fitting and from an empirical formula, where all of the three weights can be expressed as certain functions of NROD. After that, an analysis of reconstruction accuracy is conducted with a wide range of NROD. We further stretch the weighted FBP-type algorithms to GEGCT with dynamic NROD. Finally, the weighted FBP algorithm for GEGCT is extended to a three-dimensional form in the case of cone-beam scan with a cylindrical detector array.Comment: 31 pages, 13 figure

Theoretical and Experimental Evaluation of Spatial Resolution in a Variable Resolution X-Ray Computed Tomography Scanner

A variable resolution x-ray (VRX) computed tomography (CT) scanner can image objects of various sizes with greatly improved spatial resolution. The scanner employs an angulated discrete detector and achieves the resolution boost by matching the detector angulation to the scanner field of view (FOV) determined by the size of an object being imaged. A comprehensive evaluation of spatial resolution in an experimental version of the VRX CT scanner is presented in this dissertation. Two components of this resolution were evaluated – the pre-reconstruction spatial resolution, described by the detector presampling modulation transfer function (MTF), and the post-reconstruction spatial resolution, given by the scanner reconstruction MTF. The detector presampling MTF was modeled by the Monte Carlo simulation and measured by the moving-slit method. The modeled results showed the increase in the maximum cutoff frequency (in the detector plane) from 1.53 to 53.64 cycles per mm (cy/mm) as the scanner FOV decreased from 32 to 1 cm. The measured results supported the modeling, except for the small FOVs (below 8 cm), where the MTF could not be measured up to the cutoff frequency due to the focal-spot limitation. The scanner reconstruction MTF was measured by the special-phantom method. The measured results demonstrated the increase in the average cutoff frequency (in the object plane) from 2.44 to 4.13 cy/mm as the scanner FOV decreased from 16 to 8 cm. The MTF could not be measured at the FOVs other than 8 and 16 cm, due to the calibration-reconstruction inaccuracies and, again, the focal-spot limitation. Overall, the evaluation confirmed the potential value of the VRX CT scanner and produced results important for its further development

Image Reconstruction for Stationary Head CT with CNT X-ray Source Arrays

The rapid diagnosis of traumatic brain injuries is of paramount importance in resource-poor environments. Current CT systems are highly complex instruments with rapid rotation of the gantry, which limits the acquisition time and introduces motion artifacts. The invention of carbon nanotube x-ray source arrays has enabled the development of novel imaging systems, including stationary tomosynthesis and stationary computed tomography with fast data acquisition, mechanically robust structures, and reduced image blur from source-detector motion. In this work, we explore the feasibility of building a s-HCT system using commercially available CNT linear x-ray sources and build a data processing package for image reconstruction and analysis for the designed system. At the initial stage of developing the system, this work involved both simulation and basic experimentation study. A data processing and image reconstruction package was developed to investigate the optimal system configuration of the s-HCT and the best approach to achieve high-fidelity images. System configurations were assessed qualitatively and quantitatively from both sinogram coverage and reconstructed image quality perspectives. The final implementation of the prototype s-HCT system was evaluated and improved on its performance for head imaging. Our simulation studies suggest that a s-HCT system using three CNT x-ray source arrays could achieve sufficient sinogram coverage to generate reconstructed images with accurate CT numbers and detectability on low-contrast features. Iterative reconstruction algorithms were implemented for reduced-projection reconstruction. Such a system prototype has been implemented in a benchtop setting. 3D volumetric data can be acquired and reconstructed with the proposed data processing package for the s-HCT system. Preliminary evaluations indicated the system could generate images with good uniformity, high spatial resolution, and detectability on high-resolution features. Improvements on system noise were undertaken to achieve better image quality and detection on low-resolution soft tissues in head imaging. This work includes a comprehensive comparison of stationary CT to standard imaging approaches. Geometry configurations for head CT utilizing linear CNT source tubes were first investigated with a specifically developed data processing package. Under the guidance of simulation results, a proof-of-concept system is first developed and characterized. A clinical setup of the system is under construction and has been approved by IRB for clinical trials.Doctor of Philosoph

STATIONARY DIGITAL TOMOSYNTHESIS: IMPLEMENTATION, CHARACTERIZATION, AND IMAGE PROCESSING TECHNIQUES

The use of carbon nanotube cathodes for x-ray generation was pioneered and perfected by our team in the Applied Nanotechnology Laboratory at the University of North Carolina at Chapel Hill. Over the past decade, carbon nanotube (CNT) field emission x-ray source technology has matured and translated into multiple pre-clinical and clinical devices. One of the most prominent implementations of CNT x-ray technology is a limited angle tomography method called tomosynthesis, which is rapidly emerging in clinical radiography. The purpose of this project is two-fold, to develop and characterize to the latest iteration, stationary intraoral tomosynthesis, and develop a low-dose, effective scatter reduction technique for breast and chest tomosynthesis. The first portion of this project was to develop and evaluate a new quasi-3D imaging modality for dental imaging. My work consists of experiments which dictated the design parameters and subsequent system evaluation of the dedicated s-IOT clinical prototype system currently installed in the UNC Department of Oral and Maxillofacial Radiology clinic in the School of Dentistry. Experiments were performed in our lab to determine optimal source array geometry and system configuration. The system was fabricated by our commercial partner then housed in our research lab where I performed initial characterization and assisted with software development. After installation in the SOD, I performed additional system characterization, including source output validation, dosimetry, and quantification of resolution. The system components and software were refined through a rapid feedback loop with the engineers involved. Four pre-clinical imaging studies have been performed in collaboration with several dentists using phantoms, extracted teeth, and cadaveric dentition. I have generated an operating manual and trained four dental radiologists in the use of the s-IOT device. The system has now been vetted and is ready for patient use. The second portion of this project consists of hardware development and implementation of an image processing technique for scatter correction. The primary sampling scatter correction (PSSC) is a beam pass technique to measure the primary transmission through the patient and calculate the scatter profile for subtraction. Though developed for breast and chest tomosynthesis, utilization in mammography and chest radiography are also demonstrated in this project. This dissertation is composed of five chapters. Chapters one and two provide the basics of x-ray generation and a brief history of the evolution of carbon nanotube x-ray source technology in our lab at UNC. Chapter three focuses on stationary intraoral tomosynthesis. The first section provides background information on dental radiology and project motivation. Sections 3.2 and 3.3 detail my work in benchtop feasibility and optimization studies, as well as characterization and evaluation of the clinical prototype. Chapter four introduces scatter in imaging, providing motivation for my work on primary sampling scatter correction (PSSC) image processing method, detailed in chapter five.Doctor of Philosoph