David P. Trauernicht

System considerations in CCD‐based x‐ray imaging for digital chest radiography and digital mammography

System modeling is used to investigate the effect of various system parameters on the image quality in CCD-based x-ray imaging systems. The systems considered consist of a typical phosphor-based scintillating screen coupled to a CCD through lens or fiberoptic taper. Two applications, chest radiography and mammography, are analyzed. For each application typical system characteristics and operating conditions are used to determine the detective quantum efficiency (DQE) as a function of spatial frequency, optical collection efficiency, optical demagnification factor, and electronic noise. The DQE is modeled by extending the analysis for storage phosphor systems. The calculations are done for typical exposure conditions (0.25 mR for chest and 10.0 mR for mammography); however, the exposure effects are also discussed. It was found that a reasonable DQE can be obtained for both applications through each coupling approach; however, the demagnification requirements and electronic noise limitations are more stringent for the digital mammography application.

The Measurement Of Conversion Noise In X-Ray Intensifying Screens

A significant source of noise in screen-film radiography results from the variation in light output of the screen for absorption of x-rays of equal energy. Two methods are described in the literature for measuring the statistics of the number of light quanta emitted for each absorbed x-ray. The coincidence method registers x-ray events by detecting temporally correlated light quanta. It is limited in its ability to see events having a small number of quanta and can be biased by non-Poisson photomultiplier dark events. The pulse height method uses pulse shaping to produce pulses whose height is proportional to the number of quanta. In this case small pulses may be lost in order to discriminate against photomultiplier dark counts. We present a new method, based on the synchronous detection of a chopped x-ray source, which has the potential to avoid these shortcomings. Analytical methods necessary to remove the effects of unwanted x-ray energy components (such as backscatter radiation from our x-ray fluorescent targets) are discussed which provide stable estimates of the first and second moments of the light emission statistics. The method is then used to obtain a new set of measurements of the light emission statistics (including the mean light output and Swank I factor) for samples of Kodak Lanex x-ray intensifying screens for mean incident x-ray energies of 14.4, 17.8, 27.9, 35.2, 42.1, 49.8, and 59.6 keV.

A new method for deflecting liquid microjets

A new method is reported for deflecting a microscopic jet emanating from a nozzle away from the nozzle’s axis of symmetry. It relies on putting energy into the jet through an asymmetric heater embedded in the nozzle. This novel phenomenon is probed theoretically. It is shown that jet deflection is set by the competition among three effects. Two of these can be attributed to the variation with temperature of surface tension and the third to that of viscosity. Whether the contact line is fixed or free is shown to profoundly impact the extent of jet deflection at a given flow rate.

Improved spatial resolution in flat-panel imaging systems

Results of an investigation into the limiting spatial resolution of a flat-panel amorphous silicon (a-Si:H) X-ray imaging system are reported. The system was comprised of a 127 micrometer pixel pitch a-Si:H array used in conjunction with an overlying Gd2O2S:Tb (GOS) phosphor screen. The pre- sampled modulation transfer function (psMTF) of the system was measured at diagnostic X-ray energies and compared to the value predicted from a knowledge of the spatial resolution of the individual system components. A reproducible drop in the measured psMTF is seen at low spatial frequencies. Measurements of the magnitude of X-ray backscatter from the array substrate, along with the results of a theoretical model for K-fluorescence X-ray scatter, indicate that a significant fraction of this low-frequency drop is due to K-fluorescence from heavy elements in the glass substrate of the array. This K-fluorescence may be excited directly by primary X-rays that penetrate the overlying phosphor and interact in the glass, or by gadolinium K-fluorescence X-rays that escape from the phosphor into the glass. The measurements indicate that the spatial resolution of such an X-ray imaging system may be improved by the use of a substrate containing as low a concentration of heavy elements as possible.

Understanding the relative sensitivity of radiographic screens to scattered radiation

This study compared the relative response of various screen-film and computed radiography (CR) systems to diagnostic radiation exposure. An analytic model was developed to calculate the total energy deposition within the depth of screen and the readout signal generated from this energy for the x-ray detection system. The model was used to predict the relative sensitivity of several screen-film and CR systems to scattered radiation as a function of various parameters, such as x-ray spectra, phantom thickness, phosphor composition, screen thickness, screen configuration (single front screen, single back screen, screen pair), and readout conditions. In addition, measurements of the scatter degradation factor (SDF) for different screen systems by using the beam stop technique with water phantoms were made to verify the model results. Theoretically calculated values of SDF were in good agreement with experimental data. These results are consistent with the common observation that rare-earth screens generally produce better image quality than calcium tungstate screens and the CR screen.

Conversion noise measurement for front and back x-ray intensifying screens

Two years ago in these proceedings1 we reported on a new method for measuring the noise associated with the variation in light output of x-ray intensifying screens caused by absorption of x-ray quanta of equal energy, together with data for the Kodak Lanex intensifying screens. We have extended our measurements to screens in the back-screen configuration, in addition to the front-screen configuration previously reported. By back-screen configuration we mean that the x-rays are incident from the same side as that from which the emission will be measured. This has been realized by means of an integrating sphere, which allows screens to be mounted as back or front screens; or even as a pair. The light emission statistics (including the mean light output and the Swank I factor) for some Kodak Lanex intensifying screens in the front and back-screen configurations are given and compared. These data can provide a basis for understanding the depth dependent emission probability which in turn provides a useful test of theories of light propagation within the screen.

Micro-jet nozzle array for precise droplet metering and steering having increased droplet deflection

We present the architecture and fabrication method of a fluidic device with increased droplet deflection. The device is capable of producing picoliter size droplets precisely and steering them. The precision is a consequence of the reproducibility of the nozzles that are made using VLSI technology and tools. In addition, the droplet size is determined by the precise timing of applied heat pulses. We present both experimental and modeling results.

Screen design for flat-panel imagers in diagnostic radiology

The image quality of Gd2O2S:Tb phosphor screens used in a flat-panel photodiode array system is examined. The presampled Modulation Transfer Function (MTF), the Normalized Noise Power Spectrum surface (NNPS) and the resulting Detective Quantum Efficiency (DQE) are discussed for a variety of screens used in such a system. A technique for extracting the limiting DQE for a system with significant electronic noise is described. This allows for the examination of the imaging performance of the X-ray converter (or phosphor screen) and removes issues of photodiode array and readout electronics performance. It is shown that depending on the metric being used to judge the imaging performance (i.e., MTF or DQE), it is possible to design a more optimal screen than those currently available for use in screen-film imaging. Evidence is also presented for a significant degradation of the DQE at higher spatial frequencies due to the variation in the light spread MTF through the depth of the screen.

Sensitivity of radiographic screens to scattered radiation

This study compares the relative response of various screen-film and computed radiography (CR) systems to diagnostic radiation exposure. An analytic model was developed to calculate the total energy deposition within the depth of screen and the readout signal generated from this energy for the x-ray detection system. The model was used to predict the relative sensitivity of several screen-film and CR systems to scattered radiation as a function of selected parameters, such as x-ray spectra, phantom thickness, phosphor composition, screen thickness, screen configuration (single front screen, single back screen, screen pair), and readout conditions. Measurements of scatter degradation factor (SDF) for different screen systems were made by using the beam stop technique with water phantoms. Calculated results were found to be consistent with experimental observations, namely, both the BaFBr screen used in a CR system and the CaWO4 screen pair have higher scatter sensitivity than the rare earth Gd2O2S screen pair; the BaFBr screen in the CR front-screen configuration is less sensitive to scatter radiation than in the normal back-screen configuration; and these screens have higher scatter sensitivity as x-ray tube voltage increases.

Substrate effect on indirect digital radiography system performance

In a typical indirect flat-panel digital radiography detector, a phosphor screen is coupled to an a-Si:H imaging array, whose pixels comprise an a-Si:H photodiode and an a-Si:H TFT switch. This two-dimensional array is fabricated on a thin glass substrate that usually contains a rather high concentration of heavy elements such as barium. In previous system performance analyses, only the effect of K-fluorescence reabsorption in the phosphor screen was included. The effect of K-fluorescence from heavy elements in the glass substrate of the array was not taken into account. This K-fluorescence may be excited directly by primary x-rays that penetrate the overlying phosphor and interact in the glass, or by K-fluorescence x-rays that escape from the phosphor into the glass. In this paper, we extend the parallel-cascaded linear systems model to include the effect of K-fluorescence from heavy elements in the glass substrate. As an example, the MTF, NPS, and DQE of an indirect flat-panel imager consisting of a Gd2O2S:Tb phosphor screen and an a-Si:H photodiode/TFT array fabricated on a glass substrate containing barium, are calculated. Degradations in MTF and DQE as a result of the K-fluorescence from the substrate are presented and discussed.