Haim Gilboa

Mercuric iodide medical imagers for low exposure radiography and fluoroscopy

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 ~3.9 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.

Mercuric iodide and lead iodide x-ray detectors for radiographic and fluoroscopic medical imaging

Mercuric iodide (HgI2) and lead iodide (PbI2) have been under development for several years as direct converter layers in digital x-ray imaging. Previous reports have covered the basic electrical and physical characteristics of these and several other materials. We earlier reported on 5cm x 5cm and 10cm x 10cm 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, NDT, and security applications. This paper, presents results and comparison of both lead iodide and mercuric iodide imagers scaled up to 20cm x 25cm sizes. Both the mercuric iodide and lead iodide direct conversion layers are vacuum deposited onto TFT array by Physical Vapor Deposition (PVD). This process has been successfully scaled up to 20cm x 25cm -- the size required in common medical imaging applications. A TFT array with a pixel pitch of 127 microns was used for this imager. In addition to increasing detector size, more sophisticated, non-TFT based small area detectors were developed in order to improve analysis methods of the mercuric and lead iodide photoconductors. These small area detectors were evaluated in radiographic mode, continuous fluoroscopic mode and pulsed fluoroscopic mode. Mercuric iodide coating thickness ranging between 140 microns and 300 microns and lead iodide coating thickness ranging between 100 microns and 180 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 fluoroscopic images have been generated at up to 15 frames per second. Mercuric iodide image lag appears adequate for fluoroscopic imaging. The longer image lag characteristics of lead iodide make it only suitable for radiographic imaging. For both material the MTF is determined primarily by the aperture and pitch of the TFT array (Nyquist frequency of ~3.93 mm-1 (127 micron pixel pitch).

Characterization of mercuric iodide photoconductor for radiographic and fluoroscopic medical imagers

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 ~3.9 lp/mm is shown (127 micron pixel pitch).

Medical imaging with mercuric iodide direct digital radiography flat-panel x-ray detectors

Photoconductive polycrystalline mercuric iodide coated on amorphous silicon flat panel thin film transistor (TFT) arrays is the best candidate for direct digital X-ray detectors for radiographic and fluoroscopic applications in medical imaging. The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This coating technology is capable of being scaled up to sizes required in common medical imaging applications. Coatings were deposited on 2”×2” and 4”×4” TFT arrays for imaging performance evaluation and also on conductive-coated glass substrates for measurements of X-ray sensitivity and dark current. TFT arrays used included pixel pitch dimensions of 100, 127 and 139 microns. Coating thickness between 150 microns and 250 microns were tested with beam energy between 25 kVP and 100kVP utilizing exposure ranges typical for both fluoroscopic, and radiographic imaging. X-ray sensitivities measured for the mercuric iodide samples and coated TFT detectors were superior to any published results for competitive materials (up to 7100 ke/mR/pixel for 100 micron pixels). It is believed that this higher sensitivity can result in fluoroscopic imaging signal levels high enough to overshadow electronic noise. Diagnostic quality of radiographic and fluoroscopic images of up to 15 pulses per second were demonstrated. Image lag characteristics appear adequate for fluoroscopic rates. Resolution tests on resolution target phantoms showed that resolution is limited to the TFT array Nyquist frequency including detectors with pixel size of 139 microns resolution ~3.6 lp/mm) and 127 microns (resolution~3.9 lp/mm). The ability to operate at low voltages (~0.5 volt/micron) gives adequate dark currents for most applications and allows low voltage electronics designs.

Large-area mercuric iodide x-ray imager

Single crystals of mercuric iodide have been studied for many years for nuclear detectors. We have investigated the use of x-ray photoconductive polycrystalline mercuric iodide coatings on amorphous silicon flat panel thin film transistor (TFT) arrays as x-ray detectors for radiographic and fluoroscopic applications in medical imaging. The mercuric iodide coatings were vacuum deposited by Physical Vapor Deposition (PVD). This coating technology is capable of being scaled up to sizes required in common medical imaging applications. Coatings were deposited on 4 inches X 4 inches TFT arrays for imaging performance evaluation and also on conductive-coated glass substrates for measurements of x-ray sensitivity, dark current and image lag. The TFT arrays used included pixel pitch dimensions of both 100 and 139 microns. Coating thickness between 150 microns and 250 microns were tested in the 25 kVp-100 kVp x-ray energy range utilizing exposures typical for both fluoroscopic, and radiographic imaging. X-ray sensitivities measured for the mercuric iodide samples and coated TFT detectors were superior to any published results for competitive materials (up to 7100 ke/mR/pixel for 100 micron pixels). It is believed that this higher sensitivity, can result in fluoroscopic imaging signal levels high enough to overshadow electronic noise. Image lag characteristics appear adequate for fluoroscopic rates. Resolution tests on resolution target phantoms showed that resolution is limited to the Nyquist frequency for the 139 micron pixel detectors. The ability to operate at low voltages gives adequate dark currents for most applications and allows low voltage electronics designs. Mercuric Iodide coated TFT arrays were found to be outstanding 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 provide a unique combination of desirable imaging performance parameters.

Microstructure and energy resolution of 59.6 keV /sup 241/Am gamma absorption in polycrystalline HgI/sub 2/ detectors

Polycrystalline HgI/sub 2/ layers prepared by different modifications of physical vapor deposition (PVD) exhibit different microstructure. Under some fabrication procedures, the samples exhibit a columnar structure, with columns highly oriented in the [001] direction (c-axis) normal to the layer surface. Differences in manufacturing procedures manifest themselves in different average column length, different porosity, and different average material density. The most nonporous, dense, thick HgI/sub 2/ layers are obtained by activating the preferential growth along the c-axis perpendicularly to the substrate plane. The microstructure correlates to the material electrical conduction properties: dark current, mobility, and trapping time. For a sufficiently pure starting material, and grain length approaching the layer thickness, the layer may exhibit electron mobility as high as /spl mu//sub n/=87 cm/sup 2//V/spl middot/s, electron trapping time as long as /spl tau//sub n/=18 /spl mu/s, hole mobility /spl mu//sub p/=4.1 cm/sup 2//V/spl middot/s, and hole trapping time of /spl tau//sub p/=3.5 /spl mu/s. These values are quite close to those of a single crystal. Nuclear detectors fabricated using such layers exhibit energy resolution of gamma absorption, as demonstrated for the 59.6 keV emission of /sup 241/Am.