A novel inverse-geometry CT based on multi-pixel thermionic emission x-ray (MPTEX) sources is under development. Inverse-geometry CT is capable of modulating x-ray flux based on region of interest (ROI) location and thus minimizing x-ray exposure to the patients. The variations of emission currents of the MPTEX source at each projection angle are predetermined based on ROI position and realized by varying filament heating powers. To power and control filament electron emissions, a 50-channel prototype MPTEX control unit was developed based on GaN switching power technology. The filament heating powers of electron sources are controlled individually by a microprocessor while electron emission is turned on and off by switching the bias voltages during inverse-geometry CT scans. The emission current was measured as a function of filament heating voltage. The result shows that magnitude of emission current can be controlled dynamically by varying filament heating voltages. Compared with pulse width modulation, this approach can effectively prolong the lifetime of the filaments by minimizing their working temperature. A simulation of electron emission current sinogram was conducted as well. This work is supported by NIH grant R42EB026401.
KEYWORDS: Modulation transfer functions, Electrodes, Electrons, Sensors, Digital breast tomosynthesis, X-rays, Imaging systems, Signal detection, X-ray imaging, Spatial resolution
While active matrix flat-panel imagers (AMFPIs) are commonly used in low dose-per-frame applications such as digital breast tomosynthesis, DQE performance under such irradiation conditions is constrained by the large electronic additive noise relative to imaging signal. Replacement of the CsI:Tl and a-Se converters typically used in AMFPIs with polycrystalline mercuric iodide fabricated using particle-in-binder techniques (PIB-HgI2) would largely overcome this limitation due to the 3 to 10 times higher signal per interacting x-ray offered by this photoconductive material. However, practical implementation of PIB-HgI2 converters in clinical systems requires significant reduction of the high levels of image lag associated with the material. A promising strategy for significantly reducing lag is to diminish the contribution of signal induced by hole transport (which is believed to be a principal source of lag) through incorporation of a grid structure (referred to as a Frisch grid) into the converter material – with previous theoretical studies indicating hole signal reductions of up to ~95%. In this paper, an initial examination of the direct influence of the grid on image lag is reported. This involved modeling the trapping and release of holes in the converter for grids supported by insulating pillars. Signal properties, including line spread function, MTF, and image lag, were examined as a function of grid design parameters and operational conditions. The modeling shows that, when charge trapping and release are accounted for, favorably high suppression of image lag as well as good MTF is possible for some grid designs.
The DQE performance of active matrix flat-panel imagers (AMFPIs) degrades under low dose conditions – such as those encountered in digital breast tomosynthesis where electronic additive noise becomes comparable to imaging signal. Compared to commercially available x-ray converter materials such as CsI:Tl and a-Se, particle-in-binder polycrystalline mercuric iodide (PIB HgI2) offers imaging signal 3 to 10 times larger. However, PIB HgI2 exhibits an unacceptably high degree of image lag, believed to originate from the trapping of holes. To suppress hole signal contribution (with the expectation of decreasing image lag), a Frisch grid structure embedded within a PIB HgI2 detector is under investigation. In this theoretical study involving finite element analysis modeling, the effects of grid design and charge carrier lifetimes on the line spread function and modulation transfer function (MTF) were investigated. Two design parameters were considered: grid pitch (defined as the center-to-center distance between adjacent grid elements) and RGRID (defined as the ratio of grid element width to grid pitch). Results show that for a grid pitch comparable to the pixel pitch (i.e., 100 μm) and high RGRID, MTF is significantly degraded compared to a detector without a grid while, for a grid pitch of 20 μm, MTF is largely maintained and is almost independent of RGRID. The best identified design is a grid pitch of 20 μm and RGRID of 45% – providing an MTF similar to that of a detector without a grid while suppressing hole signal by 78%. This work is supported by NIH grant R01-EB022028.
KEYWORDS: Electrodes, X-rays, Electrochemical etching, Digital breast tomosynthesis, X-ray imaging, Imaging systems, Signal to noise ratio, Dielectrics, Sensors, Interference (communication), Finite element methods
For active matrix flat-panel imagers (AMFPIs), relatively high levels of additive noise compared to the average imaging signal per interacting x-ray limits DQE under conditions of low dose per frame – degrading imaging performance in applications such as digital breast tomosynthesis. The development of particle-in-binder, polycrystalline mercuric iodide (PIB HgI2) as an alternative converter is of interest since the material can provide significantly larger imaging signal per x-ray than converters based on CsI:Tl or a-Se. In order to address the high degree of charge trapping exhibited by HgI2 (leading to unacceptably large image lag), incorporation of a Frisch grid structure into the bulk of the converter is under investigation. Earlier theoretical studies demonstrated that such a grid can greatly reduce the contribution of hole charge to imaging signal – which should reduce the effects of charge trapping. In those studies, the grid was assumed to be positioned between two polycrystalline HgI2 layers – a configuration referred to as a floating grid. In this paper, early results from a theoretical examination of the performance of converters containing a grid supported on insulating pillars over an underlying AMFPI array and a single layer of PIB HgI2 material, are reported. The simulation results indicate that, while charge accumulating on the pillars changes the shape of electric fields that allow collection of electron signal, high electron collection efficiency accompanied by substantial suppression of hole signal can nevertheless be maintained through judicious selection of grid design and operational conditions. This work is supported by NIH grant R01- EB022028.
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