Comparing Image Quality in Phase Contrast subμ\mu X-Ray Tomography -- A Round-Robin Study

How to evaluate and compare image quality from different sub-micrometer (sub$\mu$) CT scans? A simple test phantom made of polymer microbeads is used for recording projection images as well as 13 CT scans in a number of commercial and non-commercial scanners. From the resulting CT images, signal and noise power spectra are modeled for estimating volume signal-to-noise ratios (3D SNR spectra). Using the same CT images, a time- and shape-independent transfer function (MTF) is computed for each scan, including phase contrast effects and image blur ($\mathrm{MTF_{blur}}$). The SNR spectra and MTF of the CT scans are compared to 2D SNR spectra of the projection images. In contrary to 2D SNR, volume SNR can be normalized with respect to the object's power spectrum, yielding detection effectiveness (DE) a new measure which reveals how technical differences as well as operator-choices strongly influence scan quality for a given measurement time. Using DE, both source-based and detector-based sub$\mu$ CT scanners can be studied and their scan quality can be compared. Future application of this work requires a particular scan acquisition scheme which will allow for measuring 3D signal-to-noise ratios, making the model fit for 3D noise power spectra obsolete

Noise reduction and spatial resolution in CT imaging with the ASIR iterative reconstruction algorithm at different doses and contrasts – a phantom study

Aims and objectives The aim of this study was to quantitatively assess noise reduction and spatial resolution in computed tomography (CT) imaging with the ASIR (Adaptive Statistical Iterative Reconstruction, GE Healthcare) reconstruction algorithm at different kVp, mAs and contrasts. Methods and materials Acquisitions of the Catphan-504 phantom were performed on a PET/CT scanner (Discovery-710, GE Healthcare). CT images were reconstructed using both filtered back projection (FBP) and ASIR with different percentages of reconstruction (20%, 40%, 60%, 80%, 100%). The image noise was estimated for different values of scanning parameters (i.e. tube-load, kilovoltage, pitch, slice thickness). Then, 3D/2D/1D noise power spectrum was estimated. Also, spatial resolution was assessed by obtaining the modulation transfer function (MTF) for a wide range of scanning parameters values and different contrast objects by the circular Edge Spread Function method (using CTP404 modulus) and the Point Spread Function method (using CTP528 modulus). . Results Image noise decreased (up to 50% as compared to FBP) with increasing the percentage of ASIR reconstruction (behaviour more relevant for higher spatial frequencies). Only for low tube load (<56 mAs) and low contrast objects (polistirene with respect to PMMA) acquisitions, MTF analysis showed that ASIR-reconstructed images were characterized by an appreciable reduction in spatial resolution, when compared to FBP-reconstructed images. Conclusion When compared to FBP, ASIR allows a relevant noise reduction without appreciably affecting image quality, except for very low dose and contrast acquisitions

Development and Application of Computational Tools for the Study and Optimization of Variable Resolution X-ray (VRX) Computed Tomography Scanners

The overall goal of this project was to develop and apply important computerized aids for the design and implementation of Variable Resolution X-ray (VRX) CT scanners developed at the University of Tennessee, Memphis. VRX scanners take advantage of the “projective compression” principle that allows the same device to image objects of very different sizes with the same level of detail by adjusting the field of view and the reconstruction resolution. The first part of this project aimed to develop a set of computational tools specifically tailored for the design, implementation and study of VRX scanners. This included creating a reconstruction algorithm that takes into account the unique geometries of the different VRX systems that have been designed, along with improving the calibration algorithm needed to ensure a proper reconstruction. It also included the development of a computer model of VRX scanners that is an invaluable tool for the development and study of these devices. The second part of the project was composed of a small series of experiments in which the computational tools developed proved to be fundamental in the analysis and evaluation of some aspects of VRX imaging. This included a comparison of the performance of different targeting VRX geometries in terms of spatial and contrast resolution, and a study of the effect of the VRX angle on the severity of common artifacts in single-arm images

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

Quantitative Analysis of Three-Dimensional Cone-Beam Computed Tomography Using Image Quality Phantoms

In the clinical setting, weight-bearing static 2D radiographic imaging and supine 3D radiographic imaging modalities are used to evaluate radiographic changes such as, joint space narrowing, subchondral sclerosis, and osteophyte formation. These respective imaging modalities cannot distinguish between tissues with similar densities (2D imaging), and do not accurately represent functional joint loading (supine 3D imaging). Recent advances in cone-beam CT (CBCT) have allowed for scanner designs that can obtain weight-bearing 3D volumetric scans. The purpose of this thesis was to analyze, design, and implement advanced imaging techniques to quantify image quality parameters of reconstructed image volumes generated by a commercially-available CBCT scanner, and a novel ceiling-mounted CBCT scanner. In addition, imperfections during rotation of the novel ceiling-mounted CBCT scanner were characterized using a 3D printed calibration object with a modification to the single marker bead method, and prospective geometric calibration matrices

Statistical image reconstruction for quantitative computed tomography

Statistical iterative reconstruction (SIR) algorithms for x-ray computed tomography (CT) have the potential to reconstruct images with less noise and systematic error than the conventional filtered backprojection (FBP) algorithm. More accurate reconstruction algorithms are important for reducing imaging dose and for a wide range of quantitative CT applications. The work presented herein investigates some potential advantages of one such statistically motivated algorithm called Alternating Minimization (AM). A simulation study is used to compare the tradeoff between noise and resolution in images reconstructed with the AM and FBP algorithms. The AM algorithm is employed with an edge-preserving penalty function, which is shown to result in images with contrast-dependent resolution. The AM algorithm always reconstructed images with less image noise than the FBP algorithm. Compared to previous studies in the literature, this is the first work to clearly illustrate that the reported noise advantage when using edge-preserving penalty functions can be highly dependent on the contrast of the object used for quantifying resolution. A polyenergetic version of the AM algorithm, which incorporates knowledge of the scanner’s x-ray spectrum, is then commissioned from data acquired on a commercially available CT scanner. Homogeneous cylinders are used to assess the absolute accuracy of the polyenergetic AM algorithm and to compare systematic errors to conventional FBP reconstruction. Methods to estimate the x-ray spectrum, model the bowtie filter and measure scattered radiation are outlined which support AM reconstruction to within 0.5% of the expected ground truth. The polyenergetic AM algorithm reconstructs the cylinders with less systematic error than FBP, in terms of better image uniformity and less object-size dependence. Finally, the accuracy of a post-processing dual-energy CT (pDECT) method to non-invasively measure a material’s photon cross-section information is investigated. Data is acquired on a commercial scanner for materials of known composition. Since the pDECT method has been shown to be highly sensitive to reconstructed image errors, both FBP and polyenergetic AM reconstruction are employed. Linear attenuation coefficients are estimated with residual errors of around 1% for energies of 30 keV to 1 MeV with errors rising to 3%-6% at lower energies down to 10 keV. In the ideal phantom geometry used here, the main advantage of AM reconstruction is less random cross-section uncertainty due to the improved noise performance

Rapid 3D Phase Contrast Magnetic Resonance Angiography through High-Moment Velocity Encoding and 3D Parallel Imaging

abstract: Phase contrast magnetic resonance angiography (PCMRA) is a non-invasive imaging modality that is capable of producing quantitative vascular flow velocity information. The encoding of velocity information can significantly increase the imaging acquisition and reconstruction durations associated with this technique. The purpose of this work is to provide mechanisms for reducing the scan time of a 3D phase contrast exam, so that hemodynamic velocity data may be acquired robustly and with a high sensitivity. The methods developed in this work focus on the reduction of scan duration and reconstruction computation of a neurovascular PCMRA exam. The reductions in scan duration are made through a combination of advances in imaging and velocity encoding methods. The imaging improvements are explored using rapid 3D imaging techniques such as spiral projection imaging (SPI), Fermat looped orthogonally encoded trajectories (FLORET), stack of spirals and stack of cones trajectories. Scan durations are also shortened through the use and development of a novel parallel imaging technique called Pretty Easy Parallel Imaging (PEPI). Improvements in the computational efficiency of PEPI and in general MRI reconstruction are made in the area of sample density estimation and correction of 3D trajectories. A new method of velocity encoding is demonstrated to provide more efficient signal to noise ratio (SNR) gains than current state of the art methods. The proposed velocity encoding achieves improved SNR through the use of high gradient moments and by resolving phase aliasing through the use measurement geometry and non-linear constraints.Dissertation/ThesisPh.D. Bioengineering 201

Qualitative analysis of a microtomographic apparatus and measurement of the bone tissue density with reference to microgravity conditions

Computed Thomography is a relatively new field in the area of non destructive imaging.It allows to reconstruct the internal structure of opaque objects without destroy them. This is a great advantage compared to conventional microscopy techniques, any optical or electronic microscope, in fact, provides information on the internal structure of samples only if samples are properly processed and sectioned. Information about three-dimensional structure could be obtained by the image of a surface or a combination of several thin slices, but in both cases information cannot be certain since methods of cutting and preparation can dramatically change the structure of the sample. Microcomputed tomography, commonly referred to as µCT, like conventional computed tomography is based on the collection of projections of X rays through a specimen and the application of tomographic principles to reconstruct the 3-D structure of the specimen. Itis based on the interaction of X-rays with matter. The attenuation ofX-rays, passingthrough an object, is dependent on thedensity and atomic number of the object under investigation. This radiation is converted in a radiographic image of the object. Images obtained from different angles are analyzed by analgorithm called Filter back projection in order to reconstruct a virtual slice through the object. When different consecutive slices are reconstructed, a 3D visualizationcan be obtained. The term "micro" denotes a scanning system much higher in resolution than conventional clinical scanners. Clinical tomographic scanners may have resolutions on the order of a millimeter or less. However, high-resolution µCT scanners may have resolutions below five microns. The high resolution of this system makes it useful in the analysis of small objects such as trabecular bone samples. Trabecularbone consists of a complicated three-dimensional network of plates and rods, arranged ina lattice-like network.The architectural parameters of trabecular bone could be strongly influenced by aging or bone diseases such as osteoarthritis or osteoporosis. Until recently, information about thesestructural parameters of trabecular bone were only available by histomorphometry, adestructive procedure limited to two-dimensional analysis. Nowadays Micro-CT, because of its capability to allow three-dimensional and non destructive analysis, found largeapplications in pre-clinical bone research.The increasing incidence and prevalence of bone pathologies on the population, increases the interest of improve an accurate bone characterization by Micro-CT. Micro-CT system, object of this study is the Skyscan 1072, located at the Technology and Health Department of the Italian National Institute of Health.One of the goal of this research is set at optimizing the system for the analysis of bone samples. The first part is dedicated on determining the resolution of the system. The performance of an imaging system is usually described by the measurement of its Modulation Transfer Function or MTF whichgives a description of how much contrast at a specific spatial frequency is maintained by the imaging process.The second part of this study is focused on the process of images reconstruction, fundamental in a Micro-CT analysis. Micro-CT images are affected by several artifacts which will be widely discussed in the following chapters. One of the most difficult artifact is beam hardening. It depends on the polychromatic X-ray tube used in these systems. The X-rays beam investing the sample is composed of X-rays with a spectrum of different energies. The attenuation of an X-ray depends on its energy, the lowestX-ray energies are preferentially absorbed. Assuming that the grey level of CT images corresponds to the linear coefficientof attenuation, which is constants depending on the material, because of the beam hardening, the attenuationof a given material is not strictly proportional to its thickness. This implies visual distortions on the images and the consequent origin of quantitative problems. In order to better understand the effect of beam hardening on Micro-CT images, the filtered back projection algorithm will be implemented in LabVIEW (Version 8.2). The Skyscan 1072 allows to correct the effect of beam hardening during the process of images reconstruction by the definition of a proper parameter. In order to define the correct value of this parameter for a bone sample analysis, a comparison between the results of both the algorithm implemented and the Skyscan reconstruction software will be evaluated. After the optimization of the system for bone analysis, nineteen trabecular bone samples, extracted from femoral heads of eight patients subject to a hip arthroplasty surgery, will be analyzed. The main problem of bone analysis by micro-CT is the processing of the reconstructed cross-sections images for the sample morphometric analysis. The post-processing of the images for the morphometric characterization usually requires a process named binarization of the images which consists on the definition of a threshold value of grey-level, necessary to distinguish bone from background. The choice of this value is a crucial task since a standard method doesn’t exist. Moreover, the inhomogeneity of bone causes another problem during the binarization process. Binarization associates each pixel of the image to bone or air, not considering that each pixel can be composed by both of them. This effect is called Partial Volume Effect and it affects especially pixels at the edges of the analyzed sample. In order to avoidproblems related to the binarization, the main goal of this study is the evaluation of a new method for the histomorphometric analysis of bone sample from the direct processing of the greylevel histogram of the images. Finally, the last part of this research will be dedicated on the remodeling process of bone. The remodeling of bone is an important research topic because of its importance in the study of bone pathologies such as osteoporosis. Osteoporosis is a bone disorder characterized by an inadequate amount and faulty structure of bone, resulting in fractures from relatively minor trauma. It leads to a bone mineral density (BMD) reduction, a bone microarchitecture deterioration and an alteration of the amount and variety of proteins in bone. Aging is the main factor of osteoporosis incidence but in the last years, another factor related to long-duration spaceflight, has been considered. Because of the difficult in reproducing in-vivo space conditions, the development of numerical models is a good alternative for the remodeling process study