Scattered radiation remains one of the primary challenges for digital mammography, resulting in decreased image contrast and visualization of key features. While anti-scatter grids are commonly used to reduce scattered radiation in digital mammography, they are an incomplete solution that can add radiation dose, cost, and complexity. Instead, a software-based scatter correction method utilizing asymmetric scatter kernels is developed and evaluated in this work, which improves upon conventional symmetric kernels by adapting to local variations in object thickness and attenuation that result from the heterogeneous nature of breast tissue. This fast adaptive scatter kernel superposition (fASKS) method was applied to mammography by generating scatter kernels specific to the object size, x-ray energy, and system geometry of the projection data. The method was first validated with Monte Carlo simulation of a statistically-defined digital breast phantom, which was followed by initial validation on phantom studies conducted on a clinical mammography system. Results from the Monte Carlo simulation demonstrate excellent agreement between the estimated and true scatter signal, resulting in accurate scatter correction and recovery of 87% of the image contrast originally lost to scatter. Additionally, the asymmetric kernel provided more accurate scatter correction than the conventional symmetric kernel, especially at the edge of the breast. Results from the phantom studies on a clinical system further validate the ability of the asymmetric kernel correction method to accurately subtract the scatter signal and improve image quality. In conclusion, software-based scatter correction for mammography is a promising alternative to hardware-based approaches such as anti-scatter grids.

Mercuric iodide (HgI2) and. lead iodide (PbI2) materials have been investigated for several years as direct converter layers for digital x-ray imaging applications. A difficult challenge of both lead iodide and mercuric iodide is the higher than desired leakage currents. These currents are influenced by different factors such as applied electrical field, layer thickness, layer density, elec trode structure, material purity and by the deposition parameters. Minimizing the leakage current must also be achieved without adversely affecting charge transport, which plays a large role in gain and is influenced by these parameters. Other challenges relate to increasing film thickness without degrading electrical properties. This paper compares some imagers as the result of optimization process.We deposited the above materials on flat panel thin film transistor (TFT) arrays with 127 um pixel pitch. The imagers were evaluated for both radiographic and fluoroscopic imaging. Modulation Transfer Function (MTF) was measured as a function of the spatial frequency. The MTF data were compared to values published in the literature for indirect detector (CsI). Image lag characteristics of mercuric iodide appear adequate for fluoroscopic rates. The structure and x-ray diffraction data of the two materials were compared to explain the difference in image lag between them.

Mercuric iodide (HgI2) and lead iodide (PbI2) thin polycrystalline films have been under development for several years as direct converter layers for digital X-ray imaging. In this paper, we cover the basic electrical and physical characteristics of these materials and compare to other X-ray sensitive photoconductive materials. Both lead iodide and mercuric iodide were vacuum deposited on a-Si TFT arrays with 127μm pixel pitch. This coating technology is scalable to sizes required in common X-ray imaging applications, as proved by the recent 10cm×10cm and 20cm×25cm imager results. A difficult challenge of both lead iodide and mercuric iodide detectors is higher than the desired leakage current. Minimizing the leakage current must also be achieved without adversely affecting charge transport, which plays a large role in gain and is also influenced by these parameters. New deposition technologies have been developed through which the leakage current has now decreased by more than an order of magnitude while showing no negative effects on gain. The improvement in dark current correlates with more perfect (single crystalline like) structures as shown by X-ray diffraction data in HgI2 films. The imagers were evaluated for both radiographic and fluoroscopic imaging. MTF was measured as a function of the spatial frequency and results were compared to values for indirect detectors (CsI). The ability to operate at moderate voltages (∼0.2–1.0V/μm) provides adequate dark current for most applications and allows low voltage electronics design. Image lag characteristics of mercuric iodide appear adequate for fluoroscopic rates.

Photoconductive polycrystalline mercuric iodide deposited on flat panel thin film transistor (TFT) arrays is being developed for direct digital X-ray detectors that can perform both radiographic and fluoroscopic medical imaging. The mercuric iodide is either vacuum deposited by Physical Vapor Deposition (PVD) or coated onto the array by a wet Particle-In-Binder (PIB) process. The PVD deposition technology has been scaled up to the 20 cm x 25 cm size required in common medical imaging applications. A TFT array with a pixel pitch of 127 microns is used for these imagers. Arrays of 10 cm x 10 cm size have been used to evaluate performance of mercuric iodide imagers. Radiographic and fluoroscopic images of diagnostic quality at up to 15 pulses per second were demonstrated. As we previously reported, the resolution is limited to the TFT array Nyquist frequency of similar to3.9 lp/mm (127 micron pixel pitch). Detective Quantum Efficiency (DQE) has been measured as a function of spatial frequency for these imagers. The DQE is lower than the theoretically calculated value due to some additional noise sources of the electronics and the array. We will retest the DQE after eliminating these noise sources.Reliability and stress testing was also began for polycrystalline mercuric iodide PVD and PIB detectors. These are simplified detectors based upon a stripe electrode or circular electrode structure. The detectors were stressed under various voltage bias, temperature and time conditions. The effects of the stress tests on the detector dark current and sensitivity were determined.

Vapor deposited lead iodide films show a wide range of physical attributes dependant upon fabrication conditions. High density is most readily achieved with films less than 100 μm. Thicker films, with lessening density, often show lower response (gain) as charge collection becomes less efficient. Lack of consistency in density throughout a deposition invariably leads to non-uniform electronic properties, which is challenging to both model and predict. To overcome this, tighter control of deposition parameters is required during the slow growth process (<10 μm/hour). Lead iodide films are characterized in forms of planar devices deposited onto conductive glass and active pixel arrays deposited onto a-Si TFT arrays1. Electronic properties (e.g. leakage current, gain) show little variation that can be traced to substrate choice. Films generally provide less than 100 pA/mm2 leakage current as they show saturation in gain (at approximate fields of 1 V/μm). We recently modified our readout electronics to accept positive bias. Using positive bias on the top electrode provides better charge collection for the lower mobility electrons and (despite process variability) better quality films can provide sensitivities greater than 6 μC/R*cm2, with only partial x-ray absorption, and show less than 20 pA/mm2 dark current.

Mercuric iodide (HgI2) and lead iodide (PbI2) materials as direct converter layers for digital x-ray imaging have been studied for several years. This paper present results of basic imaging parameters by comparing dark current, sensitivity and image lag properties of these materials.A difficult challenge of both lead iodide and mercuric iodide photon detectors is higher than desired leakage currents. These currents are influenced by factors such as applied electrical field, layer thickness, layer density, electrode structure and material purity. Mininuzing the leakage current must also be achieved without adversely affecting charge transport, which plays a large role in gain and is also influenced by these parameters. New deposition technologies have been developed through which the leakage current has now decreased by more than an order of magnitude while showing no negative affects on gain. Other challenges relate to increasing film thickness without degrading electrical properties.The image lag of the polycrystalline PbI2 is much larger than that of the polycrystalline HgI2 material, however, no significant image lag is observed for single crystal PbI2. Optical microscopy and SEM studies showed that the polycrystalline PbI2 has a low density, randomly oriented morphology with small crystallites while the best HgI2 has a much better oriented (single crystal-like) structure. We believe that the long image lag can be attributed to the large number of deep defect states generated on the surface of the small PbI2 crystallites.The imagers were evaluated for both radiographic and fluoroscopic imaging modes. MTF was measured as a function of the spatial frequency. The MTF data were compared to values published in the literature for indirect detectors (CsI) and direct detectors (a-Se). Resolution tests on resolution target phantoms showed that for both materials resolution is mostly limited by the TFT array Nyquist frequency.

Sensor fill factor is one of the key pixel design requirements for high performance imaging arrays. In our conventional imaging pixel architecture with a TFT and a photodiode deposited in the same plane, the maximum area that the photodiode can occupy is limited by the size of the TFT and the surrounding metal lines. A full fill factor array design was previously proposed using a continuous sensor layer1. Despite the benefits of 100% fill factor, when applied to large-area applications, this array design suffers from high parasitic line capacitances and, thus, high line noise. We have designed and fabricated an alternative pixel structure in which the photodiode is deposited and patterned over the TFT, but does not overlap with the lines underneath. Separating the diode from the TFT plane allows extra space for an additional TFT which can be used for pixel reset and clipping excessive charge in the photodiode developed under high illumination. This reduces memory effect by 250%. The yield and the reliability are expected to improve as well since the TFTs and lines are buried underneath the diode. With the increased fill factor, we collect 56% more electrons per pixel, thereby improving the signal to noise ratio. The maximum signal to noise ratio is achieved when the increased signal and the undesirable parasitic capacitance on the data line are best optimized. Linearity, sensitivity, leakage, and MTF characteristics of a prototype X-ray imager based on this architecture are presented.

Photoconductive polycrystalline mercuric iodide deposited on flat panel thin film transistor (TFT) arrays is one of the best candidates for direct digital X-ray detectors for radiographic and fluoroscopic medical imaging.The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This deposition technology has been scaled up to the 20cmX25cm size required in common medical imaging applications. A TFT array with a pixel pitch of 127 microns is used for these imagers.In addition to successful imager scale up, non-TFT based detectors were developed in order to improve analysis methods of the mercuric iodide photoconductor itself. These substrates consist of an array of palladium or ITO stripes on a glass substrate. Following deposit of the photoconductor, striped bias electrodes are deposited on top of the photoconductor at a 90 degree orientation to the bottom electrodes. These substrates provide more information than was previously available on the dark current and signal uniformity of the mercuric iodide photoconductor without the use of expensive TFT arrays. Mercuric iodide photoconductor thicknesses between 110 microns and 300 microns were tested with beam energy between 40 kVp and 120 kVp utilizing exposure ranges typical for both fluoroscopic and radiographic imaging.Diagnostic quality radiographic and fluoroscopic images at up to 15 pulses per second were demonstrated. Resolution tests on resolution target phantoms were performed and performance close to the theoretical sinc function up to the Nyquist frequency of similar to3.9 lp/mm is shown (127 micron pixel pitch).

A new method (patent pending) was developed at Varian Medical Systems and OHT Inc. for non-destructive evaluation of printed circuit boards (PCB's). The electrostatic imager uses a TFT array, where each pixel has a small storage capacitor connected to it and a separate top electrode. An insulator layer covers these top electrodes. When we place a PCB on top of this insulator layer and activate a trace of the PCB by an electrical pulse, that trace induces charges in all of the underlying pixels. By reading out the image of the charges with electronics, similar to ones used for digital x-ray imaging(1,) we can reconstruct the image of the electrical trace.Using the above technique we can test and detect defects in PCB's such as shorted traces, broken traces, etc. This method is also applicable to test other electrical and electronic circuits and components with electrical pulses. The paper gives a detailed descripton of this new imaging technique illustrated by real applications.

We previously reported on 2" x 2" and 4" x 4" size imagers. direct digital radiography X-ray detectors, based on photoconductive polycrystalline mercuric iodide deposited on a flat panel thin film transistor (TFT) array, as having great potential for use in medical imaging, NDE, and security applications. Recently we successfully upgraded our mercuric iodide deposition technique to 20 cm x 25 cm size, the size required in common NDE and security imaging applications. A TFT array with a pixel pitch of 127 microns was used for this imager. The mercuric iodide direct conversion layers were vacuum deposited onto TFT array by Physical Vapor Deposition (PVD).In addition to successful imager scale up, more sophisticated, non-TFT based detectors were developed in order to improve analysis methods of the mercuric iodide photoconductor. Measurements on mercuric iodide photoconductor were performed using a 36 x 6 electrode array on a 10 cm x 10 cm substrate (total of 216 measurement points). The array is formed by 36 palladium stripes on the glass substrate, upon which the mercuric iodide is deposited, and 6 palladium stripes that are deposited on top of the mercuric iodide layer. These two sets of electrodes are oriented at 90 degrees to each other to create the measurement matrix. These detectors were evaluated in radiographic mode, continuous fluoroscopic mode and pulsed fluoroscopic mode. Mercuric iodide coatings with thickness ranging between 140 microns and 300 microns were tested using beams with energies between 40 kVp and 100 kVp utilizing exposure ranges typical for both fluoroscopic and radiographic imaging.Diagnostic quality radiographic and fluroscopic images at up to 15 pulses per second were demonstrated. We evaluated the dark current, sensitivity and MTF characteristics. The MTF is determined primarily by the aperture and pitch of the TFT array with Nyquist frequency of similar to3.93 mm(-1) (127 micron pixel pitch). The MTF curve of a good quality HgI2 imager is very close to the theoretical sinc function. Image lag characteristics of mercuric iodide appear adequate for fluoroscopic rates.

Polycrystalline Mercuric Iodide coated TFT arrays are now the best candidates for direct digital radiographic detectors for both static and dynamic (fluoroscopic) applications. Their high X-ray sensitivity, high resolution, low dark current, low voltage operation, and good lag characteristics meet the required imaging performance parameters. Small area (2'x2') mercuric iodide x-ray detectors were already reported to have good resolution and high x-ray sensitivity. The present paper reports results obtained with good quality large active area (4'x4') HgI2 imagers deposited on an a-Si TFT matrix containing 768 x 768 pixels with pitch dimensions of 139 micrometers . Coating thickness of the HgI2 is between 150 micrometers and 250 micrometers and the imagers were tested in the 25 kVp-100 kVp x-ray energy range utilizing exposures typical for both fluoroscopic and radiographic imaging. The resolution of the imagers is practically limited only by the pixel size (139micrometers ). The imager works at 15 Hz fluoro rate and the image lag is very small. The imager can use exceptionally low dose-rate x-ray illumination because of the very high x-ray sensitivity, which exceeds any other known X-ray imager materials. The paper also discusses potential application fields of this new unique imager.