SNR Spectra as a Quantitative Model for Image Quality in Polychromatic X-Ray Imaging

In polychromatic x-ray imaging for nondestructive testing, material science or medical applications, image quality is usually a problem of detecting sample structure in noisy data. This problem is typically stated this way: As many photons as possible need to be detected to get a good image quality. We instead propose to use the concept of signal detection, which is more universal. In signal detection, it is the sample properties which are detected. Photons play the role of information carriers for the signal. Signal detection for example allows modeling the effects which polychromaticity has on image quality. $\mathit{SNR}$ spectra (= spatial $\mathit{SNR}$) are used as a quantity to describe if reliable signal detection is possible. They include modulation transfer and phase contrast in addition to noisiness effects. $\mathit{SNR}$ spectra can also be directly measured, which means that theoretical predictions can easily be tested. We investigate the effects of signal and noise superposition on the $\mathit{SNR}$ spectrum and show how selectively not detecting photons can increase the image quality

Empirical and theoretical investigation of the noise performance of indirect detection, active matrix flatâ panel imagers (AMFPIs) for diagnostic radiology

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134855/1/mp7919.pd

Segmented phosphors: MEMSâ based high quantum efficiency detectors for megavoltage xâ ray imaging

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134924/1/mp4774.pd

Signal, noise power spectrum, and detective quantum efficiency of indirectâ detection flatâ panel imagers for diagnostic radiology

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135120/1/mp8243.pd

DEVELOPMENT AND CHARACTERIZATION OF A HIGH ENERGY PHASE CONTRAST X-RAY IMAGING SYSTEM PROTOTYPE

The field of mammography receives constant research attention focused on improving the balance between the benefits of cancer screening and the risks of harmful radiation to the patient. As a result, numerous advancements have been made throughout the history of mammography, which have not only improved the ability to detect cancer at an earlier stage, but also to diagnose previously undetectable cancer. Numerous clinical trials have demonstrated the decrease in mortality rates. Due to the potential for saving lives, along with the recent public concerns regarding radiation dose, significant research attention remains focused on investigating methods for further improving the detection capabilities and reducing the radiation dose. However, the similar absorption characteristics of normal and malignant tissue present a challenge in differentiating between them using conventional x-ray imaging. The current method for providing higher image quality involves utilizing anti-scatter grids and operating at much lower x-ray energies than other radiography fields, both of which result in an increased radiation dose. An emerging technology called phase contrast imaging, which is based not only on absorption but also the effects produced by x-ray phase changes, holds the potential to increase the x-ray energy and remove the grid without compromising the image quality, which could reduce the patient dose and thus benefit the field of mammography. Preliminary studies in phase contrast imaging at the same energy as conventional imaging have indicated the ability to reduce the radiation dose without negatively impacting the diagnosis capabilities. However, existing challenges in clinical implementation have prevented the technology from further progress.The goal of the research presented in this dissertation comprises a thorough investigation of the potential of high energy phase contrast imaging to overcome these challenges and further reduce the radiation dose without decreasing the detection ability. Following an introductory chapter, Chapter 2 presents a detailed description of the necessary methods required to perform the dissertation research. The methods are separated into four categories: image quality, statistical methods, phase contrast imaging, and radiation dose. Chapters 3 through 6 encompass four preliminary studies accomplished to demonstrate a thorough understanding of the research methods, as well as to evaluate the feasibility of the research and corresponding motivation in the medical imaging field. The development and preliminary feasibility investigation of a high energy phase contrast imaging system prototype is presented in Chapter 7, followed by an image quality comparison to high and low energy conventional imaging with similar entrance exposures in Chapter 8. Chapter 9 presents a comprehensive image quality and dose comparison of high energy phase contrast and low energy conventional imaging. Finally, the summary and discussion of results are presented in Chapter 10, along with planned research direction for future studies.This dissertation encompasses numerous original contributions, perhaps the most significant of which were the demonstration of the ability of phase contrast imaging to deliver acceptable image quality for detection and diagnosis at higher x-ray energies than investigated previously, as well as the comprehensive comparison of high energy phase contrast imaging with low energy conventional imaging. These results clearly demonstrate the ability of phase contrast imaging to sustain the image quality improvement at high x-ray energies and for clinical thicknesses without an increase in the radiation dose. In addition, each of the preliminary studies involved the development of novel methods or techniques to improve existing procedures. First, the step-by-step optimization of the MTF algorithm presented in Chapter 4 was an original approach, which also included the application of new methods to several of the steps, resulting in an optimized algorithm with significantly improved accuracy. Next, Chapter 5 presented the development of a quantitative method to determine the error contributed to any calculated result by each of the represented components, as well as a new method for calculating the magnification factor that considerably reduces the error, especially for clinical systems. Chapter 6 presented the novel application of the existing method of beam hardening to reduce the radiation dose without affecting the detection capability, which holds the potential to greatly benefit mammography and related fields.The research presented in this dissertation is a strong indication of the potential of high energy phase contrast imaging to dramatically benefit x-ray imaging fields such as mammography by improving the ability to detect and diagnose diseases at earlier stages or when previously undetectable without increasing the radiation dose. The ability to improve the capability to diagnose disease without increasing the risk of harmful radiation to the patient would significantly improve the balance between the risks and benefits of cancer screening, which holds the potential to revolutionize the fields of x-ray imaging and lower mortality rates

Design and Characterization of a High-resolution Cardiovascular Imager

Fluoroscopic imaging devices for interventional radiology and cardiovascular applications have traditionally used image-intensifiers optically coupled to either charge-coupled devices (CCDs) or video pick-up tubes. While such devices provide image quality sufficient for most clinical applications, there are several limitations, such as loss of resolution in the fringes of the image-intensifier, veiling glare and associated contrast loss, distortion, size, and degradation with time. This work is aimed at overcoming these limitations posed by image-intensifiers, while improving on the image quality. System design parameters related to the development of a high-resolution CCD-based imager are presented. The proposed system uses four 8 x 8-cm three-side buttable CCDs tiled in a seamless fashion to achieve a field of view (FOV) of 16 x 16-cm. Larger FOVs can be achieved by tiling more CCDs in a similar manner. The system employs a thallium-doped cesium iodide (CsI:Tl) scintillator coupled to the CCDs by straight (non-tapering) fiberoptics and can be operated in 78, 156 or 234-microns pixel pitch modes. Design parameters such as quantum efficiency and scintillation yield of CsI:Tl, optical coupling efficiency and estimation of the thickness of fiberoptics to provide reasonable protection to the CCD, linearity, sensitivity, dynamic range, noise characteristics of the CCD, techniques for tiling the CCDs in a seamless fashion, and extending the field of view are addressed. The signal and noise propagation in the imager was modeled as a cascade of linear-systems and used to predict objective image quality parameters such as the spatial frequency-dependent modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE). The theoretical predictions were compared with experimental measurements of the MTF, NPS and DQE of a single 8 x 8-cm module coupled to a 450-microns thick CsI:Tl at x-ray beam quality appropriate for cardiovascular fluoroscopy. The measured limiting spatial resolution (10% MTF) was 3.9 cy/mm and 3.6 cy/mm along the two orthogonal axes. The measured DQE(0) was ~0.62 and showed no dependence with incident exposure rate over the range of measurement. The experimental DQE measurements demonstrated good agreement with the theoretical estimate obtained using the parallel-cascaded linear-systems model. The temporal imaging properties were characterized in terms of image lag and showed a first frame image lag of 0.9%. The imager demonstrated the ability to provide images of high and uniform spatial resolution, while preserving and potentially improving on DQE performance at dose levels lower than that currently used in clinical practice. These results provide strong support for potential adaptation of this type of imager for cardiovascular and pediatric angiography

Assessment and optimisation of digital radiography systems for clinical use

Digital imaging has long been available in radiology in the form of computed tomography (CT), magnetic resonance imaging (MRI) and ultrasound. Initially the transition to general radiography was slow and fragmented but in the last 10-15 years in particular, huge investment by the manufacturers, greater and cheaper computing power, inexpensive digital storage and high bandwidth data transfer networks have lead to an enormous increase in the number of digital radiography systems in the UK. There are a number of competing digital radiography (DR) technologies, the most common are computer radiography (CR) systems followed by indirect digital radiography (IDR) systems. To ensure and maintain diagnostic quality and effectiveness in the radiology department appropriate methods are required to evaluate and optimise the performance of DR systems. Current semi-quantitative test object based methods routinely used to examine DR performance suffer known short comings, mainly due to the subjective nature of the test results and difficulty in maintaining a constant decision threshold among observers with time. Objective image quality based measurements of noise power spectra (NPS) and modulation transfer function (MTF) are the ‘gold standard’ for assessing image quality. Advantages these metrics afford are due to their objective nature, the comprehensive noise analysis they permit and in the fact that they have been reported to be relatively more sensitive to changes in detector performance. The advent of DR systems and access to digital image data has opened up new opportunities in applying such measurements to routine quality control and this project initially focuses on obtaining NPS and MTF results for 12 IDR systems in routine clinical use. Appropriate automatic exposure control (AEC) device calibration and a reproducible measurement method are key to optimising X-ray equipment for digital radiography. The uses of various parameters to calibrate AEC devices specifically for DR were explored in the next part of the project and calibration methods recommended. Practical advice on dosemeter selection, measurement technique and phantoms were also given. A model was developed as part of the project to simulate CNR to optimise beam quality for chest radiography with an IDR system. The values were simulated for a chest phantom and adjusted to describe the performance of the system by inputting data on phosphor sensitivity, the signal transfer function (STF), the scatter removal method and the automatic exposure control (AEC) responses. The simulated values showed good agreement with empirical data measured from images of the phantom and so provide validation of the calculation methodology. It was then possible to apply the calculation technique to imaging of tissues to investigate optimisation of exposure parameters. The behaviour of a range of imaging phosphors in terms of energy response and variation in CNR with tube potential and various filtration options were investigated. Optimum exposure factors were presented in terms of kV-mAs regulation curves and the large dose savings achieved using additional metal filters were emphasised. Optimum tube potentials for imaging a simulated lesion in patient equivalent thicknesses of water ranging from 5-40 cm thick for example were: 90-110kVp for CsI (IDR); 80-100kVp for Gd2O2S (screen /film); and 65-85kVp for BaFBrI. Plots of CNR values allowed useful conclusions regarding the expected clinical operation of the various DR phosphors. For example 80-90 kVp was appropriate for maintaining image quality over an entire chest radiograph in CR whereas higher tube potentials of 100-110 kVp were indicated for the CsI IDR system. Better image quality is achievable for pelvic radiographs at lower tube potentials for the majority of detectors however, for gadolinium oxysulphide 70-80 kVp gives the best image quality. The relative phosphor sensitivity and energy response with tube potential were also calculated for a range of DR phosphors. Caesium iodide image receptors were significantly more sensitive than the other systems. The percentage relative sensitivities of the image receptors averaged over the diagnostic kV range were used to provide a method of indicating what the likely clinically operational dose levels would be, for example results suggested 1.8 µGy for CsI (IDR); 2.8 µGy for Gd2O2S (Screen/film); and 3.8 µGy for BaFBrI (CR). The efficiency of scatter reduction methods for DR using a range of grids and air gaps were also reviewed. The performance of various scatter reduction methods: 17/70; 15/80; 8/40 Pb grids and 15 cm and 20 cm air gaps were evaluated in terms of the improvement in CNR they afford, using two different models. The first, simpler model assumed quantum noise only and a photon counting detector. The second model incorporated quantum noise and system noise for a specific CsI detector and assumed the detector was energy integrating. Both models allowed the same general conclusions and suggest improved performance for air gaps over grids for medium to low scatter factors and both models suggest the best choice of grid for digital systems is the 15/80 grid, achieving comparable or better performance than air gaps for high scatter factors. The development, analysis and discussion of AEC calibration, CNR value, phosphor energy response, and scatter reduction methods are then brought together to form a practical step by step recipe that may be followed to optimise digital technology for clinical use. Finally, CNR results suggest the addition of 0.2 mm of copper filtration will have a negligible effect on image quality in DR. A comprehensive study examining the effect of copper filtration on image quality was performed using receiver operator characteristic (ROC) methodology to include observer performance in the analysis. A total of 3,600 observations from 80 radiographs and 3 observers were analysed to provide a confidence interval of 95% in detecting differences in image quality. There was no statistical difference found when 0.2 mm copper filtration was used and the benefit of the dose saving promote it as a valuable optimisation tool

Development of a Monte Carlo Simulation Method for use in investigating CT (Computed Tomography) Mammography

The development of new digital mammography techniques such as dual-energy imaging, tomosynthesis and CT mammography will require investigation of optimal camera design parameters and optimal imaging acquisition parameters. One tool that is useful for this purpose is Monte Carlo simulation. This study presents a methodology for generating simulated images from a CsI-based, flat-panel imager model and for estimating the normalized glandular dose to the uncompressed breast in CT mammography. The simulation uses the GEANT 3 Monte Carlo code to model x-ray transport and absorption within the CsI scintillator, and the DETECT-II code to track optical photon spread within a columnar model of the CsI scintillator. The Monte Carlo modeling of x-ray transport and absorption within the CsI was validated by comparing to previously published values for the probability of a K-shell interaction, the fluorescent yield, the probability of a K-fluorescent emission, and the escape fraction describing the probability of a K x-ray escaping the scintillator. To validate the combined (GEANT 3 coupled with DETECT-II) Monte Carlo approach to form simulated images, comparison of modulation transfer functions (MTFs) and system sensitivity (electrons/mR/pixel) obtained from simulations were compared to empirical measurements obtained with different x-ray spectra and imagers with varying CsI thicknesses. By varying the absorption and reflective properties of the columnar CsI used in the DETECT-II code, good agreement between simulated MTFs and system sensitivity and empirically measured values were observed. The Monte Carlo software was also validated for dosimetry by comparing results of the linear attenuation coefficient values and the normalized glandular dose (DgN) values of the compressed breast, to those reposted in the literature. The normalized glandular dose was then estimated for three different sizes of the uncompressed breast with a homogeneous composition of adipose and glandular tissue. Further, fit equations of the normalized glandular dose curves were also generated using MATLAB. These equations can be used to replicate the dose for the three sizes of the breast and three compositions of the adipose and glandular tissue. In addition, images displaying energy deposition maps are presented to better understand the spatial distribution of dose in CT mammography

Performance evaluation of detectors for digital radiography

To date the hospital radiological workflow is completing a transition from analog to digital technology. Since the X-rays digital detection technologies have become mature, hospitals are trading on the natural devices turnover to replace the conventional screen film devices with digital ones. The transition process is complex and involves not just the equipment replacement but also new arrangements for image transmission, display (and reporting) and storage. This work is focused on 2D digital detector’s characterization with a concern to specific clinical application; the systems features linked to the image quality are analyzed to assess the clinical performances, the conversion efficiency, and the minimum dose necessary to get an acceptable image. The first section overviews the digital detector technologies focusing on the recent and promising technological developments. The second section contains a description of the characterization methods considered in this thesis categorized in physical, psychophysical and clinical; theory, models and procedures are described as well. The third section contains a set of characterizations performed on new equipments that appears to be some of the most advanced technologies available to date. The fourth section deals with some procedures and schemes employed for quality assurance programs