Peter Michael Edic

CatSim: a new computer assisted tomography simulation environment

We present a new simulation environment for X-ray computed tomography, called CatSim. CatSim provides a research platform for GE researchers and collaborators to explore new reconstruction algorithms, CT architectures, and X-ray source or detector technologies. The main requirements for this simulator are accurate physics modeling, low computation times, and geometrical flexibility. CatSim allows simulating complex analytic phantoms, such as the FORBILD phantoms, including boxes, ellipsoids, elliptical cylinders, cones, and cut planes. CatSim incorporates polychromaticity, realistic quantum and electronic noise models, finite focal spot size and shape, finite detector cell size, detector cross-talk, detector lag or afterglow, bowtie filtration, finite detector efficiency, non-linear partial volume, scatter (variance-reduced Monte Carlo), and absorbed dose. We present an overview of CatSim along with a number of validation experiments.

Multi-source inverse geometry CT: a new system concept for x-ray computed tomography

Third-generation CT architectures are approaching fundamental limits. Spatial resolution is limited by the focal spot size and the detector cell size. Temporal resolution is limited by mechanical constraints on gantry rotation speed, and alternative geometries such as electron-beam CT and two-tube-two-detector CT come with severe tradeoffs in terms of image quality, dose-efficiency and complexity. Image noise is fundamentally linked to patient dose, and dose-efficiency is limited by finite detector efficiency and by limited spatio-temporal control over the X-ray flux. Finally, volumetric coverage is limited by detector size, scattered radiation, conebeam artifacts, Heel effect, and helical over-scan. We propose a new concept, multi-source inverse geometry CT, which allows CT to break through several of the above limitations. The proposed architecture has several advantages compared to third-generation CT: the detector is small and can have a high detection efficiency, the optical spot size is more consistent throughout the field-of-view, scatter is minimized even when eliminating the anti-scatter grid, the X-ray flux from each source can be modulated independently to achieve an optimal noise-dose tradeoff, and the geometry offers unlimited coverage without cone-beam artifacts. In this work we demonstrate the advantages of multi-source inverse geometry CT using computer simulations.

Coronary Artery Angiography Using Multislice Computed Tomography Images

Multislice CT scanners are the newest class of CT scanners and they have not one but many detectors. These scanners can acquire up to 4 slices of data from the body in the same time it takes a single-slice CT scanner to acquire one. Multislice CT allows for rapid cardiac imaging during a single breath-hold. A multislice scanner operated in helical mode provides information that can be used to reconstruct 3D cardiac images in arbitrary phases of the cardiac cycle. A 71-year-old man with hypertension, hypercholesterolemia, and known aortic and peripheral vascular disease was imaged with a LightSpeed 4-slice, multislice CT scanner (GE Medical Systems). Ten minutes before the cardiac scan, the patient received intravenous contrast material (150 mL of 300 mgI/mL) for a CT study of his abdomen. The cardiac …

CT Image Contrast of High-Z Elements: Phantom Imaging Studies and Clinical Implications

PURPOSE To quantify the computed tomographic (CT) image contrast produced by potentially useful contrast material elements in clinically relevant imaging conditions. MATERIALS AND METHODS Equal mass concentrations (grams of active element per milliliter of solution) of seven radiodense elements, including iodine, barium, gadolinium, tantalum, ytterbium, gold, and bismuth, were formulated as compounds in aqueous solutions. The compounds were chosen such that the active element dominated the x-ray attenuation of the solution. The solutions were imaged within a modified 32-cm CT dose index phantom at 80, 100, 120, and 140 kVp at CT. To simulate larger body sizes, 0.2-, 0.5-, and 1.0-mm-thick copper filters were applied. CT image contrast was measured and corrected for measured concentrations and presence of chlorine in some compounds. RESULTS Each element tested provided higher image contrast than iodine at some tube potential levels. Over the range of tube potentials that are clinically practical for average-sized and larger adults-that is, 100 kVp and higher-barium, gadolinium, ytterbium, and tantalum provided consistently increased image contrast compared with iodine, respectively demonstrating 39%, 56%, 34%, and 24% increases at 100 kVp; 39%, 66%, 53%, and 46% increases at 120 kVp; and 40%, 72%, 65%, and 60% increases at 140 kVp, with no added x-ray filter. CONCLUSION The consistently high image contrast produced with 100-140 kVp by tantalum compared with bismuth and iodine at equal mass concentration suggests that tantalum could potentially be favorable for use as a clinical CT contrast agent.

Opportunities for new CT contrast agents to maximize the diagnostic potential of emerging spectral CT technologies.

The introduction of spectral CT imaging in the form of fast clinical dual-energy CT enabled contrast material to be differentiated from other radiodense materials, improved lesion detection in contrast-enhanced scans, and changed the way that existing iodine and barium contrast materials are used in clinical practice. More profoundly, spectral CT can differentiate between individual contrast materials that have different reporter elements such that high-resolution CT imaging of multiple contrast agents can be obtained in a single pass of the CT scanner. These spectral CT capabilities would be even more impactful with the development of contrast materials designed to complement the existing clinical iodine- and barium-based agents. New biocompatible high-atomic number contrast materials with different biodistribution and X-ray attenuation properties than existing agents will expand the diagnostic power of spectral CT imaging without penalties in radiation dose or scan time.

A Fourier-domain algorithm for total-variation regularized phase retrieval in differential X-ray phase contrast imaging

Phase retrieval in differential X-ray phase contrast imaging involves a one dimensional integration step. In the presence of noise, standard integration methods result in image blurring and streak artifacts. This work proposes a regularized integration method which takes the availability of two dimensional data as well as the integration-specific frequency-dependent noise amplification into account. In more detail, a Fourier-domain algorithm is developed comprising a frequency-dependent minimization of the total variation orthogonal to the direction of integration. For both simulated and experimental data, the novel method yielded strong artefact reduction without increased blurring superior to the results obtained by standard integration methods or regularization techniques in the image domain.

Multisource X-Ray and CT: Lessons Learned and Future Outlook

Distributed X-ray sources open the way to innovative system concepts in X-ray and computed tomography. They offer promising opportunities in terms of system performance, but they pose unique challenges in terms of source and system technologies. Several academic and industrial teams have proposed a variety of concepts and developed some unique prototypes. We present a broad review of multisource systems. We also discuss X-ray source components and challenges. We close with our perspective on the future prospects of multisource imaging.

Energy dispersive X-ray diffraction spectral resolution considerations for security screening applications

Energy dispersive X-ray diffraction (EDXRD) is a very effective method for explosive and narcotic threat detection in baggage screening. The XRD profiles arise from the molecular interference when X-rays are coherently scattered by a substance. The accurate identification of the target material depends on the ability to detect and resolve the peaks present in the coherent scatter profiles. A high-energy resolution High Purity Germanium (HPGe) detector is therefore generally used in such type of systems. To evaluate the suitability of cost-effective room-temperature semiconductor detectors for next-generation baggage screening systems, an assessment of the minimal requirements for the system resolution is required. In this study a hybrid Monte Carlo code has been modified to account for the molecular interference function that gives rise to the coherent scatter signature. A model for a realistic response function for Cadmium Zinc Telluride (CZT) detectors is then used to convolve the spectral output. This simulation tool is then used to assess the system design features and their influence on spectral resolution.

Design and image quality results from volumetric CT with a flat-panel imager

Preliminary MTF and LCD results obtained on several volumetric computed tomography (VCT) systems, employing amorphous flat panel technology, are presented. Constructed around 20-cm x 20-cm, 200-mm pitch amorphous silicon x-ray detectors, the prototypes use standard vascular or CT x-ray sources. Data were obtained from closed-gantry, benchtop and C-arm-based topologies, over a full 360 degrees of rotation about the target object. The field of view of the devices is approximately 15 cm, with a magnification of 1.25-1.5, providing isotropic resolution at isocenter of 133-160 mm. Acquisitions have been reconstructed using the FDK algorithm, modified by motion corrections also developed by GE. Image quality data were obtained using both industry standard and custom resolution phantoms as targets. Scanner output is compared on a projection and reconstruction basis against analogous output from a dedicated simulation package, also developed at GE. Measured MTF performance is indicative of a significant advance in isotropic image resolution over commercially available systems. LCD results have been obtained, using industry standard phantoms, spanning a contrast range of 0.3-1%. Both MTF and LCD measurements agree with simulated data.

Atomic number resolution for three spectral CT imaging systems

The material specificity of computed tomography is quantified using an experimental benchtop imaging system and a physics-based system model. The apparatus is operated with different detector and system configurations each giving X-ray energy spectral information but with different overlap among the energy-bin weightings and noise statistics. Multislice, computed tomography sinograms are acquired using dual kVp, sequential source filters or a detector with two scintillator/photodiodes layers. Basis-material and atomic number images are created by first applying a material decomposition algorithm followed by filtered backprojection. CT imaging of phantom materials with known elemental composition and density were used for model validation. X-ray scatter levels are measured with a beam-blocking technique and the impact to material accuracy is quantified. The image noise is related to the intensity and spectral characteristics of the X-ray source. For optimal energy separation adequate image noise is required. The system must be optimized to deliver the appropriate high mA at both energies. The dual kVp method supports the opportunity to separately engineer the photon flux at low and high kvp. As a result, an optimized system can achieve superior material specificity in a system with limited acquisition time or dose. In contrast, the dual-layer and sequential acquisition modes rely on a material absorption mechanism that yields weaker energy separation and lower overall performance.