This study examines the potential of a multisource x-ray system to reduce cone beam artifacts in a dedicated breast CT acquisition geometry. A breast CT scanner (Doheny), built at our institution, was used to demonstrate the potential of multiple x-ray sources in a single x-ray tube housing. Both 3 focal spot and 5 focal spot thermionic systems were physically simulated in this study. The x-ray tube is mounted on a vertical actuator on the breast CT system gantry, allowing the single x-ray source to be positioned at different vertical locations in the field of view. Five acquisition geometries were used to acquire raw cone beam CT data with the x-ray source locations placed at 2 cm intervals. Data was collected using a 15-cm tall Defrise phantom. The individual acquisitions of raw CT data were reconstructed using filtered back projection, aligned and summed. The reconstructed CT volume data set using three sources and five sources were compared to that produced from a single source. Both multi-source datasets demonstrated less visible cone beam artifact, and the contrast clearly improved. The resolvable field of view in the vertical direction was extended by 50% when comparing the one source to the three source geometry and extended by 120% when comparing the one source to the five source geometry. This physical simulation of a multisource x-ray CT system successfully demonstrated that a reduction in cone beam CT artifacts could be achieved using a multi-source x-ray tube on a breast CT scanner.
A stationary x-ray source for tomographic medical imaging is being developed. The source is based upon a matrix addressable array of microfabricated cold cathodes. The characteristics of the x-ray source for breast imaging applications are being quantified. Emission currents exceeding 300 mA from 1 sqmm cathodes have been observed and currents exceeding 100 mA are now routine. X-ray focal spots on the order of 0.2 mm in diameter have been produced with currents of 25 mA @ 25 kV dc.
In this paper we report recent results from an ongoing program designed to develop a fundamental understanding of the effects of materials, vacuum deposition parameters, and post fabrication processing on the performance of field-emitter arrays for displays. Molybdenum and silicon have been the materials of choice for first generation displays, and have produced acceptable results for the first trials. However, investigations of other emitter materials such as diamond- like-carbon (DLC) and zirconium carbide (ZrC) have produced intriguing improvements in emission performance. In addition in situ processes such as coating of molybdenum and silicon emitters with alternate materials and aggressive emitter- surface cleaning processes such as hydrogen-plasma cleaning and emission-stimulated desorption by high-current pulses, have also been shown to be beneficial. It has also been shown that when using the Spindt emitter fabrication process the emitter cone can be tailored to a preferred shape by appropriate materials selection and manipulation of the emitter deposition parameters. Finally, it is shown that the details of the emitter tip shape can have an impact on the performance of the emitter due to the dynamics of temperature and field-induced surface diffusion during cathode operation. Emitter tips of the same material, operated in the same environment and at the same emission levels can behave very differently depending on the details of the emitter-tip geometry.
Microfabricated field emitter arrays have attracted interest for a variety of applications. The most prominent of these applications are flat panel displays, microwave amplifiers, x- ray tubes, electron beam probes, ionizers for vacuum pressure gauges, mass spectrometers, and electronic charge management on spacecraft. From a commercial point of view, the most exciting application has been flat panel displays, while high frequency applications are the most challenging with respect to cathode performance. Displays require attention to issues related to economic high-volume production, very low-voltage operation, and a very high level of uniformity over large areas with a low emission current loading. Microwave and other high frequency applications require small areas, with high tip packing density and the highest possible current loading per tip. Ionization and charge management applications require moderate emission performance, but present special problems with regard to stability and lifetime in relatively harsh environments. Designing an emitter array to meet the requirements of any of these applications involves dealing with lithography issues concerning emitter size and packing density; materials issues as they relate to fabrication processes; stability and lifetime issues with regard to hostile environments, and electronic properties such as dielectric constant, resistivity, and work function of the emitter tip; and the cost of large-scale production.
Two inexpensive and extremely accurate methods for fabricating miniature 10 - 50 kV and 0.5 - 10 kV electron beam columns have been developed: `slicing,' and `stacking.' Two or three miniature columns could be used to perform a 20 nm or better alignment of an x-ray mask to a substrate. An array of miniature columns could be used for rapid wafer inspection and high throughput electron beam lithography. The column fabrication methods combine the precision of semiconductor processing and fiber optic technologies to create macroscopic structures consisting of charged particle sources, deflecting and focusing electrodes, and detectors. The overall performance of the miniature column also depends on the emission characteristics of the micromachined electron source which is currently being investigated.