On modelling the kinestatic charge detector for digital radiographic diagnostic and portal imaging.

The kinestatic charge detection (KCD) principle has been a digital radiography technique for more than a decade. The advances of the KCD technique have gone from diagnostic imaging to portal imaging. However, little work has been done on understanding the selection of key KCD parameters and relationships between them. In the present study, an engineering model was established that could be used to optimize the placements of key parameters in terms of KCD system mechanical design. In the proposed KCD engineering model, the basic energy conservation law was applied to the process of ion transmission. It allows for the computation of the KCD design parameters such as the optimum grid placement, high voltage board tilt angle and grid wire space, as well as to provide recommendations on high voltage board and electric potentials and their ratio.

A variable resolution x-ray detector for computed tomography: I. Theoretical basis and experimental verification.

A computed tomography imaging technique called variable resolution x-ray (VRX) detection provides detector resolution ranging from that of clinical body scanning to that of microscopy (1 cy/mm to 100 cy/mm). The VRX detection technique is based on a new principle denoted as "projective compression" that allows the detector resolution element to scale proportionally to the image field size. Two classes of VRX detector geometry are considered. Theoretical aspects related to x-ray physics and data sampling are presented. Measured resolution parameters (line-spread function and modulation-transfer function) are presented and discussed. A VRX image that resolves a pair of 50 micron tungsten hairs spaced 30 microns apart is shown.

A variable resolution x-ray detector for computed tomography: II. Imaging theory and performance.

A computed tomography (CT) imaging technique called variable resolution x-ray (VRX) detection provides variable image resolution ranging from that of clinical body scanning (1 cy/mm) to that of microscopy (100 cy/mm). In this paper, an experimental VRX CT scanner based on a rotating subject table and an angulated storage phosphor screen detector is described and tested. The measured projection resolution of the scanner is > or = 20 lp/mm. Using this scanner, 4.8-s CT scans are made of specimens of human extremities and of in vivo hamsters. In addition, the system's projected spatial resolution is calculated to exceed 100 cy/mm for a future on-line CT scanner incorporating smaller focal spots (0.1 mm) than those currently used and a 1008-channel VRX detector with 0.6-mm cell spacing.

CT reconstruction by using the MLS-ART technique and the KCD imaging system--I: low-energy X-ray studies.

We investigated the use of the kinestatic charge detector (KCD) combined with the multilevel scheme algebraic reconstruction technique (MLS-ART) for X-ray computer tomography (CT) reconstruction. The KCD offers excellent detective quantum efficiency and contrast resolution. These characteristics are especially helpful for applications in which a limited number of projections are used. In addition, the MLS-ART algorithm offers better contrast resolution than does the conventional convolution backprojection (CBP) technique when the number of projections is limited. Here we present images of a Rando-head phantom that was reconstructed by using the KCD and MLS-ART. We also present, for comparison, the images reconstructed by using the CBP technique. The combination of MLS-ART and the KCD yielded satisfactory images after just one or two iterations.

Enhanced X-Ray Detectors Using Polar Dopants for KCD Digital Radiography.

The goal of this study is to develop high resolution imaging detectors with applications in digital radiography and computed tomography. A physical treatment aimed at a better understanding of the line-spread function response of kinestatic charge detector (KCD) gas media, using dopants with permanent electric dipoles, is presented. Experimental results were obtained by operating a KCD krypton-filled detector at pressures up to 60 atm and constant electric field-to-gas density ratio doped with small amounts of polar or nonpolar polyatomic molecules with low or high ionization potential. The results clearly indicate that the addition of dopants having both low ionization potential and high dipole moment significantly enhance the imaging signal quality. An analysis of the experimental results aimed at providing a plausible interpretation of the reported observations is offered.

Line spread function study of kinestatic charge detectors operating at high gas pressures.

A systematic study of the line spread function (LSF) in the drift direction of a high-pressure ionization chamber for x-ray detection and imaging is presented. Experimental results, obtained by operating a KCD krypton-filled detector at pressures up to 60 atm and constant electric field-to-gas pressure ratio, indicate that the width of the LSF increases with the drift distance and decreases with increasing pressure, both effects being quite large. The hypothesis of this paper is that, at sufficiently high pressures, formation of clusters of molecular ions with a unique or narrowed mobility distribution take place by means of energy exchange mechanisms. Therefore, the LSF of the ionic signal becomes narrower and the FWHM of the ionic signal improves significantly with increasing gas pressure. This research is aimed at investigating methods to improve the spatial resolution as part of the development of a large field-of-view prototype digital radiographic scanner operating on kinestatic charge detection principles.

Engineering aspects of a kinestatic charge detector.

The engineering aspects of a nine-channel digital radiographic system developed for bioimaging research, based on high gas pressure ionography and kinestatic principles, are presented. The research imaging system uses a pulsed x-ray beam which allows one to study simultaneously the ionic signal characteristics at 10 different ionization sites along the drift axis. This research imaging detector system allows one to investigate methods to improve the detection and image quality parameters as part of the development of a large scale prototype medical imaging system.

Entrance window design parameters for high-pressure gas x-ray imaging detectors.

Gas ionization x-ray detectors operating at pressures up to 100 atm offer inherently high spatial and contrast resolution. However, incorporating the detector x-ray entrance window in a conventional pressure vessel designed for such pressures can result in high primary beam loss in the window and a much reduced overall detective quantum efficiency. The design of a gas chamber cover plate for a strip beam detector which mechanically isolates the x-ray entrance window from the lateral tensile stresses in the chamber body is described. A number of test windows of this design, varying in three geometric parameters-thickness, window curvature, and fillet radius-were fabricated from wrought aluminum [6061-T651 ] and subjected to pressures of up to 400 atm for the purpose of selecting an optimum window for a prototype digital x-ray imaging detector. The experimental data indicate that windows can be designed for a detector admitting a 1.0 cm wide x-ray beam that have rupture pressures exceeding 500 atm while maintaining x-ray transmittances of as much as 93.4% for a 120 kVp tungsten anode spectrum.

Use of a low ionization potential dopant in a kinestatic charge detector: experimental findings.

The broadening of the line spread function (LSF) in the drift direction with increasing drift distance in the kinestatic charge detector is substantially reduced when small amounts (less than 1%) of trimethylamine [(CH3)3 N] are added to the x-ray detection medium (krypton or xenon). The LSF of a mixture of Kr and 0.01% trimethylamine (TMA) was measured as a function of distance at 15, 25, and 35 atm absolute pressure. The full width at half-maximum (FWHM) of the LSF was reduced from about 1.0 mm to less than 0.5 mm at a drift distance of 4.0 mm for the three pressures. The LSF's of mixtures of xenon and TMA at concentrations ranging from 0.0004% to 0.4% in one run and 0.06% to 4.0% in a second run were measured at a constant pressure of 20 atm. The FWHM of the LSF was reduced from 0.6 to 0.4 mm at 4.0 mm for the xenon measurements. The optimum concentration of TMA in Xe was found to be in the neighborhood of 0.1%. The use of TMA reduced the drift distance-dependent LSF broadening to the level expected from ionic diffusion, space charge repulsion, and electric field nonuniformity, and it may be possible to reduce the 0.4-mm FWHM plateau through the use of an improved Frisch grid design. Observation of negative charge carriers showed that electron attachment increases with increasing TMA concentration, although this could be caused by impurities in the TMA. The implications of these results are discussed in terms of extending the maximum drift distance attainable in a kinestatic charge detector.

Space charge effects in a kinestatic charge detector.

The effects of space charge in a kinestatic charge detector (KCD) were examined using computer solutions to Poisson's equation. The KCD is a strip-beam parallel-plate drift chamber used for digital radiography. It was assumed that there is negligible electron attachment, i.e. there are no negative ions formed. The ionization rate per mA as a function of x-ray interaction depth was calculated for a detector filled with xenon at 25.3 x 10(5) Pa. Solution of Thomson's equations gave the positive ion density at the cathode, also as a function of depth. Water filtration values ranging from 0 to 30 cm were used in order to estimate the range of ion density values expected in a clinical KCD. The case of steady-state x-ray illumination was simulated for ionization rates less than the zero field limit (above which space charge changes the polarity of the electric field). Line spread responses were found for varying ionization rates to show the effect of space charge due to electric field distortion on the spatial resolution performance in the drift direction. The effect of imaging ideal edges with a KCD was calculated and the expected output signal was plotted for densities up to the zero field limit. Space charge dependence on the selection of KCD design and operating parameters is discussed. Because of the dependence of the KCD drift-direction spatial resolution on the uniformity of the electric field, space charge effects impose an upper limit on the detector entrance exposure and define the dynamic range of the device.

Kinestatic charge detection.

A new technique for detecting ionizing radiation in two dimensions, called "kinestatic charge detection," is proposed and analyzed. This technique is useful when the signal photons must be integrated, as in computed tomography and digital radiography, rather than counted, as in nuclear medicine imaging. A generic treatment of the technique with gas-, liquid-, or solid-state radiation detectors is presented. A theoretical analysis is given of the fundamental physical parameters required for kinestatic charge detection to be successful.

The effect of varying spatial resolution on the detectability of diffuse pulmonary nodules. Assessment with digitized conventional radiographs.

Detection and discrimination of miliary disease and small pulmonary nodules were tested at six levels of spatial resolution varying between 0.3 and 2.5 l.p./mm, and receiver operating characteristic (ROC) curves were generated. The test images were obtained by digitizing selected normal and abnormal standard film/screen radiographs. Although there was no statistically significant difference in observer performance between 0.5 and 2.5 l.p./mm, the findings may have clinical relevance. Further observer detection studies of different disease processes are planned to help formulate performance standards for an electric digital radiographic device for chest imaging.

Intravenous angiography using scanned projection radiography: preliminary investigation of a new method.

The use of digital subtraction techniques combined with fluoroscopy has rekindled interest in arteriography using intranvenous injections of contrast media. A new method is proposed for intravenous angiography in which an x-ray source and xenon detector array from a computed tomographic (CT) scanner are used to scan a region of interest to produce projection image. In order to provide adequate visualization of small concentrations of iodine in blood vessels, a subtraction scheme is used to remove the contribution from overlapping soft tissue and bone. Initial experiments with a temporal subtraction algorithm on phantoms have demonstrated the ability to image simulated blood vessels of 1.7-mm diameter containing dilute diatrizoate with an iodine concentration of 3.7 mg/cc, at an exposure of less than 100 mR. Vascular structures 5-8 mm in diameter have been imaged in dogs with iodine concentrations of less than 37 mg/cc using temporal subtraction. Principal advantages of the method over other film or fluoroscopic subtraction techniques are: 1) wide dynamic range an low noise of the (CT) detectors, providing excellent iodine sensitivity; 2) high scatter rejection; and 3) efficient utilization of x-ray dose.

The use of contrast -- detail -- dose evaluation of image quality in a computed tomographic scanner.

Limiting-perception measurements of hole patterns in computed tomography reconstructions of a low-contrast phantom are used to generate contrast -- detail -- dose diagrams for various contrasts, hole diameters, dose, and system modulation transfer functions (MTF). These curves are then divided into MTF-dominated, transition, and noise-dominated regions. The low-contrast perceptibility and image noise are parameterized and compared. A limiting-perception factor is difined and is shown to be nearly independent of contrast, hole diameter, dose, and MTF in the noise-dominated region.