Harry E. Martz

Calculation of the rotational centers in computed tomography sinograms

An efficient method for accurately calculating the center-of-rotation, or projection center, for parallel computed tomography projection data, or sinograms, is described. This method uses all the data in the sinogram to estimate the center by a least-squares technique and requires no previous calibration scan. The method also finds the objects center-of-mass without reconstructing its image. Since the method uses the measured data, it is sensitive to noise in the measurements, but that sensitivity is relatively small compared to other techniques. Examples of its use on simulated and actual data are included. For fan-beam data over 360 degrees , two related methods are described to find the center in the presence or absence of a midline offset. >

Quantitative characterization of inertial confinement fusion capsules using phase contrast enhanced x-ray imaging

Current designs for inertial confinement fusion capsules for the National Ignition Facility consist of a solid deuterium–tritium (D–T) fuel layer inside of a copper doped beryllium, Be(Cu), shell. Phase contrast enhanced x-ray imaging is shown to render the D–T layer visible inside the Be(Cu) shell. Phase contrast imaging is experimentally demonstrated for several surrogate capsules and validates computational models. Polyimide and low density divinyl benzene foam shells were imaged at the Advanced Photon Source synchrotron. The surrogates demonstrate that phase contrast enhanced imaging provides a method to characterize surfaces when absorption imaging cannot be used. Our computational models demonstrate that a rough surface can be accurately characterized using phase contrast enhanced x-ray images.

Computed tomography systems and their industrial applications

Abstract x-Ray computed axial tomography (CT) provides cross-sectional views of materials, components, and assemblies for industrial non-destructive evaluation. We have applied CT imaging to quantitatively measure the 3-D distribution ogf x-ray attenuation at reasonably high resolutions. In our industrial x-ray CT-studies, we have centered on two technical approaches: a first-generation translate/rotate CT system that consist of well-collimated (∼ 0.55 mm) photon source detector, and a third-generation scanner that uses a fluoroscopy detector.

Segmentation of artifacts and anatomy in CT metal artifact reduction

PURPOSE Metal objects present in x-ray computed tomography (CT) scans are accompanied by physical phenomena that render CT projections inconsistent with the linear assumption made for analytical reconstruction. The inconsistencies create artifacts in reconstructed images. Metal artifact reduction algorithms replace the inconsistent projection data passing through metals with estimates of the true underlying projection data, but when the data estimates are inaccurate, secondary artifacts are generated. The secondary artifacts may be as unacceptable as the original metal artifacts; therefore, better projection data estimation is critical. This research uses computer vision techniques to create better estimates of the underlying projection data using observations about the appearance and nature of metal artifacts. METHODS The authors developed a method of estimating underlying projection data through the use of an intermediate image, called the prior image. This method generates the prior image by segmenting regions of the originally reconstructed image, and discriminating between regions that are likely to be metal artifacts and those that are likely to represent anatomical structures. Regions identified as metal artifact are replaced with a constant soft-tissue value, while structures such as bone or air pockets are preserved. This prior image is reprojected (forward projected), and the reprojections guide the estimation of the underlying projection data using previously published interpolation techniques. The algorithm is tested on head CT test cases containing metal implants and compared against existing methods. RESULTS Using the new method of prior image generation on test images, metal artifacts were eliminated or reduced and fewer secondary artifacts were present than with previous methods. The results apply even in the case of multiple metal objects, which is a challenging problem. The authors did not observe secondary artifacts that were comparable to or worse than the original metal artifacts, as sometimes occurred with the other methods. The accuracy of the prior was found to be more critical than the particular interpolation method. CONCLUSIONS Metals produce predictable artifacts in CT images of the head. Using the new method, metal artifacts can be discriminated from anatomy, and the discrimination can be used to reduce metal artifacts.

X-Ray Imaging Of Cryogenic Deuterium-Tritium Layers In A Beryllium Shell

Solid deuterium-tritium (D-T) fuel layers inside copper-doped beryllium shells are robust inertial confinement fusion fuel pellets. This paper describes the first characterization of such layers using phase-contrast x-ray imaging. Good agreement is found between calculation and experimental contrast at the layer interfaces. Uniform solid D-T layers and their response to thermal asymmetries were measured in the Be(Cu) shell. The solid D-T redistribution time constant was measured to be 28 min in the Be(Cu) shell.

Segmentation of artifacts and anatomy in CT metal artifact reduction - eScholarship

PURPOSE Metal objects present in x-ray computed tomography (CT) scans are accompanied by physical phenomena that render CT projections inconsistent with the linear assumption made for analytical reconstruction. The inconsistencies create artifacts in reconstructed images. Metal artifact reduction algorithms replace the inconsistent projection data passing through metals with estimates of the true underlying projection data, but when the data estimates are inaccurate, secondary artifacts are generated. The secondary artifacts may be as unacceptable as the original metal artifacts; therefore, better projection data estimation is critical. This research uses computer vision techniques to create better estimates of the underlying projection data using observations about the appearance and nature of metal artifacts. METHODS The authors developed a method of estimating underlying projection data through the use of an intermediate image, called the prior image. This method generates the prior image by segmenting regions of the originally reconstructed image, and discriminating between regions that are likely to be metal artifacts and those that are likely to represent anatomical structures. Regions identified as metal artifact are replaced with a constant soft-tissue value, while structures such as bone or air pockets are preserved. This prior image is reprojected (forward projected), and the reprojections guide the estimation of the underlying projection data using previously published interpolation techniques. The algorithm is tested on head CT test cases containing metal implants and compared against existing methods. RESULTS Using the new method of prior image generation on test images, metal artifacts were eliminated or reduced and fewer secondary artifacts were present than with previous methods. The results apply even in the case of multiple metal objects, which is a challenging problem. The authors did not observe secondary artifacts that were comparable to or worse than the original metal artifacts, as sometimes occurred with the other methods. The accuracy of the prior was found to be more critical than the particular interpolation method. CONCLUSIONS Metals produce predictable artifacts in CT images of the head. Using the new method, metal artifacts can be discriminated from anatomy, and the discrimination can be used to reduce metal artifacts.

Fabrication of Double Shell Targets with a Glass Inner Capsule Supported by SiO2 Aerogel for Shots on the Omega Laser in 2006

Abstract Indirectly driven double shell implosions are being investigated as a possible noncryogenic path to ignition on the National Ignition Facility. Lawrence Livermore National Laboratory has made several technological advances that have produced double shell targets that represent a significant improvement to previously fielded targets. The inner capsule is supported inside the ablator shell by SiO2 aerogel with a nominal density of 50 mg/cm3. The aerogel is cast around the inner capsule and then machined concentric to it. The seamless sphere of aerogel containing the embedded capsule is then assembled between the two halves of the ablator shell. The concentricity between the two shells has been improved to less than 1.5 μm. The ablator shell consists of two hemispherical shells that mate at a step joint that incorporates a gap with a nominal thickness of 0.1 μm. Using a new flexure-based tool holder that precisely positions the diamond cutting tool on the diamond turning machine, step discontinuities on the inner surface of the ablator of less than 0.5 μm have been achieved. New methods have been used to comprehensively characterize each of the targets using high-resolution x-ray imaging systems.

Computerized tomography studies of concrete samples

Abstract X-ray computerized tomography (CAT or CT) is a sophisticated imaging technique that provides cross-sectional views of materials, components and assemblies for industrial nondestructive evaluation (NDE). We have studied the feasibility of using CT as an inspection tool for reinforced concrete and the use of multi-energy, linear, attenuation techniques to deduce variations in density (ρ) and/or atomic number (Z) that could be caused by varying the types of concrete mixes and/or compaction in the concrete itself. To perform this study, we designed and built a prototype medium-/high-energy (200- to 2000 keV) CT scanner — ZCAT — to image small concrete samples (± 30 cm in diameter and ± 75 cm in height) with a spatial resolution of about 2 mm. We used ZCAT to quantitatively inspect a 20 cm concrete cube with 1.27 cm diameter reinforcing bars (rebars) and to measure p and/or Z variations in a 20 cm diameter concrete cylinder. We describe the ZCAT scanner design, some of its physical limitations and the data-acquisition parameters used in our study. Our results and those of others [1,2] show that CT can be used to inspect reinforced concrete and to distinguish material p and/or Z variations within concrete.

Nuclear waste drum characterization with 2 MeV x-ray and gamma-ray tomography

This paper describes the progress of a multi-modality (three detector type) system for x-rays and gamma-rays developed for the waste inspection tomography (WIT) program. WIT provides mobile semi-trailer mounted nondestructive examination (NDE) and assay (NDA) for nuclear waste drum characterization. WIT uses various computed tomography (CT) methods for both NDE and NDA of nuclear waste drums. Without opening waste drums, WIT inspects and characterizes radioactive waste, including low level (LLW), transuranic (TRU), and mixed waste. With externally transmitted x-ray NDE techniques, WIT has the ability to identify high density waste materials like heavy metals, define drum contents in 2D and 3D space, quantify free liquid volumes through density and x-ray attenuation coefficient discrimination, and measure drum wall thickness. With waste emitting gamma-ray NDA techniques, WIT can locate gamma emitting radioactive sources in 2D and 3D space, identify gamma emitting isotopic species, identify gamma emitting isotopic species, identify the extermal activity approximations, and provide the data needed for waste classification as LLW or TRU.

Potential of Computed Tomography for inspection of aircraft components

Computed Tomography (CT) using penetrating radiation (x- or gamma-rays) can be used in a number of aircraft applications. This technique results in 3D volumetric attenuation data that is related to density and effective atomic number. CT is a transmission scanning method that must allow complete access to both sides of the object under inspection; the radiation source and detection systems must surround the object. This normally precludes the inspection of some large or planar (large aspect ratio) parts of the aircraft. However, we are pursuing recent limited-data techniques using object model information to obtain useful data from the partial information acquired. As illustrative examples, we describe how CT was instrumental in the analysis of particular aircraft components. These include fuselage panels, single crystal turbine blades, and aluminum-lithium composites.