George Zentai

Multiple-gain-ranging readout method to extend the dynamic range of amorphous silicon flat-panel imagers

The dynamic range of many flat panel imaging systems are fundamentally limited by the dynamic range of the charge amplifier and readout signal processing. We developed two new flat panel readout methods that achieve extended dynamic range by changing the read out charge amplifier feedback capacitance dynamically and on a real-time basis. In one method, the feedback capacitor is selected automatically by a level sensing circuit, pixel-by-pixel, based on its exposure level. Alternatively, capacitor selection is driven externally, such that each pixel is read out two (or more) times, each time with increased feedback capacitance. Both methods allow the acquisition of X-ray image data with a dynamic range approaching the fundamental limits of flat panel pixels. Data with an equivalent bit depth of better than 16 bits are made available for further image processing. Successful implementation of these methods requires careful matching of selectable capacitor values and switching thresholds, with the imager noise and sensitivity characteristics, to insure X-ray quantum limited operation over the whole extended dynamic range. Successful implementation also depends on the use of new calibration methods and image reconstruction algorithms, to insure artifact free rebuilding of linear image data by the downstream image processing systems. The multiple gain ranging flat panel readout method extends the utility of flat panel imagers and paves the way to new flat panel applications, such as cone beam CT. We believe that this method will provide a valuable extension to the clinical application of flat panel imagers.

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).

Dark current and DQE improvements of mercuric iodide medical imagers

A new TFT array has been developed specifically for mercuric iodide (HgI2) deposition. This new TFT array combined with a modified HgI2 Physical Vapor Deposition (PVD) process provides less than 10 pA/mm2 dark current at room temperature (22 °C) measured at 1 V/&mgr;m electrical field. This photoconductor (direct) imager was run at 10 fr/s framerate and gave a measured sensitivity of 19 μC/(R*cm2) using a RQA5 radiation quality x-ray beam (70kVp x-ray with 21 mm Al filtering). This sensitivity value is higher than the sensitivity reported by our group for any previous HgI2 imagers. MTF, NPS and DQE values were also evaluated on this 13 cm x 13 cm size imager with 127 μm pixel pitch. The MTF value is higher than 40% at the Nyquist frequency (3.9 lp/mm). This is much better than the MTF of a 600 μm CsI scintillator/photodiode (indirect) imager, which is only 16% (Varian internal data) and it is similar to the MTF value of the a-Se (another photoconductor) imagers. The first frame image lag is less than 8% when the imager was run at a 10 fr/s framerate. The low dark current and some noise reduction in the detector electronics, made it possible for the DQE to be measured down to low fluoroscopic dose levels (< 4 μR/fr). The DQE(0) value is over 50% at a dose of 35 μR/fr and still about 40% at 3.76 μR/fr. The 270 μm thick PVD HgI2 layer only absorbs less than 75% of the ~51 keV mean energy X-ray photons (70 kVp RQA5 filtered beam). This means that if the thickness of the HgI2 layer is increased to 500 μm (increasing the absorption up to over 90%) the DQE(0) should then increase to about 60- 65% (assuming everything else remains unchanged). This value is close to the 65 - 70 % DQE(0), measured for the indirect (CsI) imagers at higher doses. Such a high DQE value makes this material competitive both for fluoroscopic and for radiographic applications.

Comparison of CMOS and a-Si flat panel imagers for X-ray imaging

CMOS X-ray imagers are gaining importance in the field of small area X-ray imaging. The paper gives a comparison of CMOS and a-Si flat panel imagers for X-ray imaging applications. Advantages of the CMOS imagers include the higher readout speed and lower noise due to the much higher electrical charge mobility in crystalline versus amorphous silicon. The lower noise provides a wider dynamic range in CMOS even when the total pixel storage capacitance is the same as in a-Si. Also, because the noise floor is lower, the low dose Detective Quantum Efficiency (DQE) is significantly higher and the X-ray detection is quantum noise limited down to very low dose levels. The higher readout speed provides faster CT scanning. This is important when the patient has to hold their breath during a scan such as during breast CT exams. Besides the lower noise and higher speed, the pixel size can be much smaller because active components made of crystalline silicon are smaller than active components made of a-Si. Small pixels are advantageous for mammography, dental and other very high resolution X-ray imaging applications.

A novel EPID design for enhanced contrast and detective quantum efficiency

Beams-eye-view imaging applications such as real-time soft-tissue motion estimation are hindered by the inherently low image contrast of electronic portal imaging devices (EPID) currently available for clinical use. We introduce and characterize a novel EPID design that provides substantially increased detective quantum efficiency (DQE), contrast-to-noise ratio (CNR) and sensitivity without degradation in spatial resolution. The prototype design features a stack of four conventional EPID layers combined with low noise integrated readout electronics. Each layer consists of a copper plate, a scintillator ([Formula: see text]) and a photodiode/TFT-switch (aSi:H). We characterize the prototypes signal response to a 6 MV photon beam in terms of modulation transfer function (MTF), DQE and CNR. The presampled MTF is estimated using a slanted slit technique, the DQE is calculated from measured normalized noise power spectra (nNPS) and the MTF and CNR is estimated using a Las Vegas contrast phantom. The prototype has been designed and built to be interchangeable with the current clinical EPID on the Varian TrueBeam platform (AS-1200) in terms of size and data output specifications. Performance evaluation is conducted in absolute values as well as in relative terms using the Varian AS-1200 EPID as a reference detector. A fivefold increase of DQE(0) to about 6.7% was observed by using the four-layered design versus the AS-1200 reference detector. No substantial differences are observed between each layers individual MTF and the one for all four layers operating combined indicating that defocusing due to beam divergence is negligible. Also, using four layers instead of one increases the signal to noise ratio by a factor of 1.7.

Improved properties of PbI2 x-ray imagers with tighter process control and using positive bias voltage

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.

50 μm pixel size a-Se mammography imager with high DQE and increased temperature resistance

The imager presented in this paper has a special blocking structure that ensures very low dark current of less than 1 pA/mm2 even with a 20 V/μm electric field. Hence the electric field can be increased from the generally applied 10 V/μm to 20V/μm, this reduces the energy required to produce an electron hole (e-h) pair from 60 eV to about 36 eV at the given (19.3 keV mean) mammo energy. Furthermore, with special doping and manufacturing processes this a-Se layer is very stable in the 0-70 C° temperature range as demonstrated by Ogusu et al. [1]. A new 5 cm × 5 cm size TFT array was developed with 50 μm pixel size, specifically for testing the resolution of photoconductor based imagers. The first new imager of this type had a 200 μm thick a-Se layer evaporated onto the array. Its MTF, NPS, and DQE values were evaluated using 28kVp Mo anode x-ray source with a 0.03mm thick Mo and an additional 2 mm thick Al filters. The MTF value is about 40% and 50% in x-and y directions at the Nyquist frequency of 10 lp/mm. The low frequency DQE at 20 V/μm electrical field is ~70% at 151 μGy dose and drops only about 10% when going down to 23 μGy. This new array also has excellent lag properties. The measured first frame image lag at 20 V/μm is less than 1%. Such low lag provides opportunities to use this material not only for mammography but also for breast tomosynthesis applications. Breast phantom images demonstrate that even the smallest 0.13 mm calcifications are clearly visible with this high-resolution imager.

Photoconductor-based (direct) large-area x-ray imagers

— A brief overview of the present status of active-matrix flat-panel direct x-ray imagers (D-AMFPI) is given. The spatial resolutions of direct and indirect imagers are compared, and it is pointed out that the lack of light scattering greatly improves resolution. Furthermore, the resolution does not degrade as layers of the x-ray detector materials become thicker for better x-ray absorption at higher x-ray energies, opposite to that of indirect imagers. Different direct x-ray conversion materials are compared, how the physical properties influence the x-ray detection efficiency, and imager stability are discussed. Ghosting and image-lag properties are also weighted. A few x-ray-sensitive photoconductor materials produce very-high x-ray conversion efficiency, which could be advantageous for low-dose fluoroscopy to overcome the noise of the readout electronics. Last, but not least, the manufacturing advantage of the direct imagers is emphasized. The direct imagers do not need p-i-n photodiodes, so the a-Si TFT matrices for these arrays can be manufactured at any LCD manufacturing sites and not only at a few, very specialized companies where the p-layers for the photodiodes can be deposited.

Comparison of mercuric iodide and lead iodide X-ray detectors for X-ray imaging applications

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, electrode 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