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

Advancing the Clinical Potential of Carbon Nanotube-enabled stationary 3D Mammography

Scope and purpose. 3D imaging has revolutionized medicine. Digital breast tomosynthesis (DBT), also recognized as 3D mammography, is a relatively recent example. stationary DBT (sDBT) is an experimental technology in which the single moving x-ray source of conventional DBT has been replaced by a fixed array of carbon nanotube (CNT)-enabled sources. Given the potential for a higher spatial and temporal resolution compared to commercially-available, moving-source DBT devices, it was hypothesized that sDBT would provide a valuable tool for breast imaging. As such, the purpose of this work was to explore the clinical potential of sDBT. To accomplish this purpose, three broad Aims were set forth: (1) study the challenges of scatter and artifact with sDBT, (2) assess the performance of sDBT relative to standard mammographic screening approaches, and (3) develop a synthetic mammography capability for sDBT. Throughout the work, developing image processing approaches to maximize the diagnostic value of the information presented to readers remained a specific goal. Data sources and methodology. Sitting at the intersection of development and clinical application, this work involved both basic experimentation and human study. Quantitative measures of image quality as well as reader preference and accuracy were used to assess the performance of sDBT. These studies imaged breast-mimicking phantoms, lumpectomy specimens, and human subjects on IRB-approved study protocols, often using standard 2D and conventional 3D mammography for reference. Key findings. Characterizing scatter and artifact allowed the development of new processing approaches to improve image quality. Additionally, comparing the performance of sDBT to standard breast imaging technologies helped identify opportunities for improvement through processing. This line of research culminated in the incorporation of a synthetic mammography capability into sDBT, yielding images that have the potential to improve the diagnostic value of sDBT. Implications. This work advanced the evolution of CNT-enabled sDBT toward a viable clinical tool by incorporating key image processing functionality and characterizing the performance of sDBT relative to standard breast imaging techniques. The findings confirmed the clinical utility of sDBT while also suggesting promising paths for future research and development with this unique approach to breast imaging.Doctor of Philosoph

Development of a Stationary Chest Tomosynthesis System Using Carbon Nanotube X-ray Source Array

X-ray imaging system has shown its usefulness for providing quick and easy access of imaging in both clinic settings and emergency situations. It greatly improves the workflow in hospitals. However, the conventional radiography systems, lacks 3D information in the images. The tissue overlapping issue in the 2D projection image result in low sensitivity and specificity. Both computed tomography and digital tomosynthesis, the two conventional 3D imaging modalities, requires a complex gantry to mechanically translate the x-ray source to various positions. Over the past decade, our research group has developed a carbon nanotube (CNT) based x-ray source technology. The CNT x-ray sources allows compacting multiple x-ray sources into a single x-ray tube. Each individual x-ray source in the source array can be electronically switched. This technology allows development of stationary tomographic imaging modalities without any complex mechanical gantries. The goal of this work is to develop a stationary digital chest tomosynthesis (s-DCT) system, and implement it for a clinical trial. The feasibility of s-DCT was investigated. It is found that the CNT source array can provide sufficient x-ray output for chest imaging. Phantom images have shown comparable image qualities as conventional DCT. The s-DBT system was then used to study the ef- fects of source array configurations and tomosynthesis image quality, and the feasibility of a physiological gated s-DCT. Using physical measures for spatial resolution, the 2D source configuration was shown to have improved depth resolution and comparable in-plane res- olution. The prospective gated tomosynthesis images have shown substantially reduction of image blur associated with lung motions. The system was also used to investigate the feasibility of using s-DCT as a diagnosis and monitoring tools for cystic fibrosis patients. A new scatter reduction methods for s-DCT was also studied. Finally, a s-DCT system was constructed by retrofitting the source array to a Carestream digital radiography system. The system passed the electrical and radiation safety tests, and was installed in Marsico Hall. The patient trial started in March of 2015, and the first patient was successfully imaged.Doctor of Philosoph

High-quality computed tomography using advanced model-based iterative reconstruction

Computed Tomography (CT) is an essential technology for the treatment, diagnosis, and study of disease, providing detailed three-dimensional images of patient anatomy. While CT image quality and resolution has improved in recent years, many clinical tasks require visualization and study of structures beyond current system capabilities. Model-Based Iterative Reconstruction (MBIR) techniques offer improved image quality over traditional methods by incorporating more accurate models of the imaging physics. In this work, we seek to improve image quality by including high-fidelity models of CT physics in a MBIR framework. Specifically, we measure and model spectral effects, scintillator blur, focal-spot blur, and gantry motion blur, paying particular attention to shift-variant blur properties and noise correlations. We derive a novel MBIR framework that is capable of modeling a wide range of physical effects, and use this framework with the physical models to reconstruct data from various systems. Physical models of varying degrees of accuracy are compared with each other and more traditional techniques. Image quality is assessed with a variety of metrics, including bias, noise, and edge-response, as well as task specific metrics such as segmentation quality and material density accuracy. These results show that improving the model accuracy generally improves image quality, as the measured data is used more efficiently. For example, modeling focal-spot blur, scintillator blur, and noise iicorrelations enables more accurate trabecular bone visualization and trabecular thickness calculation as compared to methods that ignore blur or model blur but ignore noise correlations. Additionally, MBIR with advanced modeling typically outperforms traditional methods, either with more accurate reconstructions or by including physical effects that cannot otherwise be modeled, such as shift-variant focal-spot blur. This work provides a means to produce high-quality and high-resolution CT reconstructions for a wide variety of systems with different hardware and geometries, providing new tradeoffs in system design, enabling new applications in CT, and ultimately improving patient care

Investigations into the links between the generation of X-rays in X-ray Computed Tomography and shifts in extracted areal surface texture parameters

The discovery of X-rays revolutionised medical imaging, never before had a technique offered a method of non-invasively imaging the human body. X-ray computed tomography (XCT) built on this, allowing not only the imaging but the reconstruction of a scanned object into a 3D volume to be realised. For both fields the primary use in the formative years was medical imaging, for which the technologies were uniquely suited. Both however were also adopted as means of non-destructive evaluation (NDE) for engineering applications. This largely occurred for the same reasons the technologies thrived in the medical field. X-rays can be used to image the internal structure of objects without line of sight, due to their penetrative capabilities. Both X-ray imaging and XCT are however not without drawback. Most obviously X-rays are a form of ionising radiation, this was unknown in the early years of using X-rays as a medical imaging tool and led to significant numbers of illness and fatality amongst the early pioneers. Another issue highlighted by early research into X-rays and XCT was the low resolution and long processing times. Whilst between the first X-ray to the first use of XCT these had both improved, the issues remain prevelant. Today XCT has been adopted into a wide range of industrial engineering disciplines, including porosity analysis and metrology. Current research in these fields works to develop specialist methods and parameters to further advance XCT’s usefulness as a tool for the non-destructive analysis. The use of XCT in fields such as metrology, has led to the need to establish traceability and an understanding of error sources in XCT. Traceability is the ability to trace measurements taken back to a pre-existing standard. For most metrology applications this standard is the meter. Current research has highlighted that extracted areal surface texture parameters show a level of variation not seen in other measurement methods.This thesis investigates how instability in several key processes of the generation of X-rays may propagate through to extracted areal surface texture parameters taken using XCT. The filament is a key component in the generation of X-rays and a consumable that requires changing on a semi-regular basis to ensure continued function of the XCT. Responsible for the emission of electrons that are used to generate X-rays it is superheated to allow for thermionic emission to occur, this causes its degradation over time. Theoretically as the cross-sectional area of a part emitting electrons is altered so with the electron beam emitted. This is investigated with several studies assessing if a pattern could be established between the filament’s age and extracted parameter results. It is shown that no pattern could be established though variation in extracted surface parameters is seen throughout the filament’s life, with larger jumps present if the filament is changed.The changing of a filament is a procedure after which the machine requires refocusing, this process is investigated, and it is shown that by refocusing the machine without changing the filament a similar variation to that noted after a filament change is achieved. To quantify this a method of measuring the focal spot of a XCT is required, several methods were considered and one selected as a base from which a novel method was developed. The novel method was developed to allow for the measurement of the focal spot alongside a surface artefact. The method proved viable returning data in line with literature and existing methods. The thesis presents work showing the development of this method and its application in measuring both the machine’s focal spot and a surface artefact simultaneously. Measured focal spot diameter is shown to shift and links to the variation noted in areal surface texture parameters are presented.The work shown in this thesis was carried out to investigate how variables inherent to XCT propagate into extracted areal surface texture parameters, the methods developed are applicable to any cone beam XCT with little alteration. Results presented show that users of cone beam XCT should take into consideration the effects of variation in the XCT process when performing non-destructive evaluation. The work also highlights the need for further work in developing XCT for surface metrology

OPTICAL GEOMETRY CALIBRATION METHOD FOR COMPUTED TOMOGRAPHY AND APPLICATIONS OF COMPACT MICROBEAM RADIATION THERAPY

Digital tomosynthesis is a type of limited angle tomography that allows for 3D information reconstructed from a set of X-ray projection images taken at various angles using an X-ray tube, a mechanical arm to rotate the tube, and a digital detector. Tomosynthesis reconstruction requires the knowledge of the precise location of the detector with respect to each X-ray source. Current clinical tomosynthesis methods use a physically coupled source and detector so the geometry is always known and is always the same. This makes it impractical for mobile or field operations. We demonstrated a free form tomosynthesis and free form computed tomography (CT) with a decoupled source and detector setup that uses a novel optical method for accurate and real-time geometry calibration. We accomplish this by using a camera to track the motion of the source relative to the detector. A checkerboard pattern is positioned on or next to the detector using an extension arm in such a way that the pattern will not move relative to the detector. A camera is mounted on the source in a way that the pattern is visible during imaging and will not move relative to the source. The image of the pattern captured by the camera is then used to determine the relative camera/pattern position and orientation by analyzing the pattern distortion. This allows for accurate, real time geometry calibration of the X-ray source relative to the detector. This method opens the doors for inexpensive upgrades to existing 2D imaging systems and an even more exciting application of a mobile, hand-held CT imaging system.Doctor of Philosoph

High energy x-ray implementation of phase contrast and dark field imaging

X-ray phase contrast and dark field imaging are emerging imaging modalities which provide significantly enhanced visibility of details classically considered “x-ray invisible” and complementary information on a sample’s micro-structure, respectively. To date they have been successfully implemented in a series of applications at low x-ray energy, but their translation to higher x-ray energies is still, to some extent, problematic. Yet the ability to perform phase contrast and dark field imaging at high x-ray energy would have a series of significant implications in various applications, medical or otherwise. This thesis work investigates this option through a combination of modelling and experimental work. Particular attention has been dedicated to the behaviour of the optical elements (x-ray masks) that make phase contrast and dark field possible at high energy, which required the design of new methods of their implemention into simulation models. The modeling results have been validated first through a pilot experiment at a synchrotron facility, then in a series of lab experiments. Results clearly indicate that implementations of phase contrast and dark field imaging at high x-ray energy exist, however particular care must be taken in the design and fabrication of the masks; moreover, a series of parasitic effects which are absent at lower energies appear, which this thesis work describes and against which it suggests mitigation solutions

A dual modality, DCE-MRI and x-ray, physical phantom for quantitative evaluation of breast imaging protocols

The current clinical standard for breast cancer screening is mammography. However, this technique has a low sensitivity which results in missed cancers. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) has recently emerged as a promising technique for breast cancer diagnosis and has been reported as being superior to mammography for screening of high-risk women and evaluation of extent of disease. At the same time, low and variable specificity has been documented in the literature as well as a rising number of mastectomies possibly due to the increasing use of DCE-MRI. In this study, we developed and characterized a dual-modality, x-ray and DCE-MRI, anthropomorphic breast phantom for the quantitative assessment of breast imaging protocols. X-ray properties of the phantom were quantitatively compared with patient data, including attenuation coefficients, which matched human values to within the measurement error, and tissue structure using spatial covariance matrices of image data, which were found to be similar in size to patient data. Simulations of the phantom scatter-to-primary ratio (SPR) were produced and experimentally validated then compared with published SPR predictions for homogeneous phantoms. SPR values were as high as 85% in some areas and were heavily influenced by the heterogeneous tissue structure. MRI properties of the phantom, T1 and T2 relaxation values and tissue structure, were also quantitatively compared with patient data and found to match within two error bars. Finally, a dynamic lesion that mimics lesion border shape and washout curve shape was included in the phantom. High spatial and temporal resolution x-ray measurements of the washout curve shape were performed to determine the true contrast agent concentration as a function of time. DCE-MRI phantom measurements using a clinical imaging protocol were compared against the x-ray truth measurements. MRI signal intensity curves were shown to be less specific to lesion type than the x-ray derived contrast agent concentration curves. This phantom allows, for the first time, for quantitative evaluation of and direct comparisons between x-ray and MRI breast imaging modalities in the context of lesion detection and characterization

Application of X-ray Grating Interferometry to Polymer/Flame Retardant Blends in Additive Manufacturing

X-ray grating interferometry is a nondestructive tool for visualizing the internal structures of samples. Image contrast can be generated from the absorption of X-rays, the change in phase of the beam and small-angle X-ray scattering (dark-field). The attenuation and differential phase data obtained complement each other to give the internal composition of a material and large-scale structural information. The dark-field signal reveals sub-pixel structural detail usually invisible to the attenuation and phase probe, with the potential to highlight size distribution detail in a fashion faster than conventional small-angle scattering techniques. This work applies X-ray grating interferometry to the study of additively manufactured polymeric objects. Additively manufactured bunnies made from single material—acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA)—were studied by grating-based X-ray interferometric two-dimensional imaging and tomography. The dark-field images detected poor adhesion in the plane perpendicular to the build plate. Curvature analysis of the sample perimeter revealed a slightly higher propensity to errors in regions of higher curvature. Incorporation of flame-retardant molecules to near-surface regions of otherwise flammable objects through the fused deposition modeling additive manufacturing technique was also explored. The anticipated advantage was efficient use of the flame retardants while keeping them away from the surface for safety. To determine heat propagation effects, two-dimensional grating-based interferometry imaging at LSU CAMD was used to study heated samples. The focus was on the dark-field signals to highlight voids and gaps arising from layer delamination or gasification of chemical components. The resulting differential phase and dark-field x images were tainted by fringes attributed to inaccuracies in the grating-step position. Attempts to correct this will be presented. Interferometric tomography was also carried out on the heated samples using the W. M. Keck interferometric system at LSU. Grating-based interferometry was also used to probe scattering structure sizes of heated samples. Comparison of the data with the conventional small-angle x-ray scattering technique, SAXS, is being pursued. The results obtained so far from the above-mentioned experimental works are presented in this document

Objective Characterization of In-line Phase Contrast X-ray Imaging Prototype Using a Mid-energy Beam

The purpose of the research presented in this thesis is the characterization of an in-line phase contrast x-ray imaging prototype operating at mid-energy, and to compare the data to that of high energy imaging with the same prototype. The prototype of interest has already been well characterized for high energy in-line phase contrast imaging. High energy imaging was the primary research focus for this prototype because a technique called phase retrieval required high energy phase contrast images to be implemented properly. Phase retrieval recovers a map of the phase shifts detected within a phase contrast image. The phase retrieval algorithm used by partners of this lab group has historically relied on high energy imaging due to its dependence on the phenomenon known as phase-attenuation duality. That algorithm is now under development for mid-energy x-ray imaging as well. Though this research does not involve phase retrieval directly, it is still necessary to characterize the prototype for mid-energy phase contrast imaging, before phase retrieval at mid-energy can be evaluated. The research presented in this thesis involved investigations into three image quality metrics. First, images taken of an edge device were taken to calculate the modulation transfer function. An angle resolution pattern device was then imaged to corroborate the cutoff frequency indicated by the first study. Images were then obtained with no object but with a virtual detector to calculate the noise power spectrum. Finally, the first and second studies’ results were used to calculate the detective quantum efficiency of the prototype system. The study focused on images obtained with a source potential of 60kV, the results from which were compared to those from images obtained with source potentials of 90kV and 120kV. A micro focus x-ray source was used with a CMOS based flat panel detector. The source-to-object and source-to-image distances were set such that a magnification of 2.2 was introduced. Results indicate that the prototype system’s detective quantum efficiency at 60kVp was higher than it was for 90kVp and 120kVp. The mid-energy phase contrast imaging technique has potential for offering high detectability with lower dose to a patient for applications such as breast cancer diagnosis, as compared to current, conventional mammographic procedures.