Indirect flat-panel detector with avalanche gain: Fundamental feasibility investigation for SHARP-AMFPI (scintillator HARP active matrix flat panel imager)

Feasibility of internal-source tracking with C-arm CT/SPECT imaging with limited-angle projection data for online in vivo dose verification in brachytherapy: A Monte Carlo simulation study

PURPOSE

The current protocol for use of the image-guided adaptive brachytherapy (IGABT) procedure entails transport of a patient between the treatment room and the 3-D tomographic imaging room after implantation of the applicators in the body, which movement can cause position displacement of the applicator. Moreover, it is not possible to track 3-D radioactive source movement inside the body, even though there can be significant inter- and intra-fractional patient-setup changes. In this paper, therefore, we propose an online single-photon emission computed tomography (SPECT) imaging technique with a combined C-arm fluoroscopy X-ray system and attachable parallel-hole collimator for internal radioactive source tracking of every source position in the applicator.

METHODS AND MATERIALS

In the present study, using Geant4 Monte Carlo (MC) simulation, the feasibility of high-energy gamma detection with a flat-panel detector for X-ray imaging was assessed. Further, a parallel-hole collimator geometry was designed based on an evaluation of projection image quality for a 192Ir point source, and 3-D limited-angle SPECT-image-based source-tracking performances were evaluated for various source intensities and positions.

RESULTS

The detector module attached to the collimator could discriminate the 192Ir point source with about 3.4% detection efficiency when including the total counts in the entire deposited energy region. As the result of collimator optimization, hole size, thickness, and length were determined to be 0.5, 0.2, and 45 mm, respectively. Accordingly, the source intensities and positions also were successfully tracked with the 3-D SPECT imaging system when the C-arm was rotated within 110° in 2 seconds.

CONCLUSIONS

We expect that this system can be effectively implemented for online IGABT and in vivo patient dose verification.

Biogenic Selenium Nanoparticles in Biomedical Sciences: Properties, Current Trends, Novel Opportunities and Emerging Challenges in Theranostic Nanomedicine

Selenium is an important dietary supplement and an essential trace element incorporated into selenoproteins with growth-modulating properties and cytotoxic mechanisms of action. However, different compounds of selenium usually possess a narrow nutritional or therapeutic window with a low degree of absorption and delicate safety margins, depending on the dose and the chemical form in which they are provided to the organism. Hence, selenium nanoparticles (SeNPs) are emerging as a novel therapeutic and diagnostic platform with decreased toxicity and the capacity to enhance the biological properties of Se-based compounds. Consistent with the exciting possibilities offered by nanotechnology in the diagnosis, treatment, and prevention of diseases, SeNPs are useful tools in current biomedical research with exceptional benefits as potential therapeutics, with enhanced bioavailability, improved targeting, and effectiveness against oxidative stress and inflammation-mediated disorders. In view of the need for developing eco-friendly, inexpensive, simple, and high-throughput biomedical agents that can also ally with theranostic purposes and exhibit negligible side effects, biogenic SeNPs are receiving special attention. The present manuscript aims to be a reference in its kind by providing the readership with a thorough and comprehensive review that emphasizes the current, yet expanding, possibilities offered by biogenic SeNPs in the biomedical field and the promise they hold among selenium-derived products to, eventually, elicit future developments. First, the present review recalls the physiological importance of selenium as an oligo-element and introduces the unique biological, physicochemical, optoelectronic, and catalytic properties of Se nanomaterials. Then, it addresses the significance of nanosizing on pharmacological activity (pharmacokinetics and pharmacodynamics) and cellular interactions of SeNPs. Importantly, it discusses in detail the role of biosynthesized SeNPs as innovative theranostic agents for personalized nanomedicine-based therapies. Finally, this review explores the role of biogenic SeNPs in the ongoing context of the SARS-CoV-2 pandemic and presents key prospects in translational nanomedicine.

Minimization of image lag in polycrystalline mercuric iodide converters through incorporation of Frisch grid structures for digital breast tomosynthesis

Objective. Polycrystalline mercuric iodide photoconductive converters fabricated using particle-in-binder techniques (PIB HgI₂) provide significantly more detected charge per x-ray interaction than from a-Se and CsI:Tl converters commonly used with active matrix flat-panel imagers (AMFPIs). This enhanced sensitivity makes PIB HgI₂an interesting candidate for applications involving low x-ray exposures-since the relatively high levels of additive electronic noise exhibited by AMFPIs incorporating a-Se and CsI:Tl reduce detective quantum efficiency (DQE) performance under such conditions. A theoretical study is reported on an approach for addressing a major challenge impeding practical use of PIB HgI₂converters-the high lag exhibited by the material (over 10%) which would lead to undesirable image artifacts in applications involving acquisition of consecutive images such as digital breast tomosynthesis.Approach. Charge transport modeling accounting for the trapping and release of holes (thought to be the primary contributor to lag) was used to examine signal properties, including lag, of pillar-supported Frisch grids embedded in the photoconductor for 100μm pitch AMFPI pixels. Performance was examined as a function of electrode voltage, grid pitch (center-to-center distance between neighboring grid wires) and the ratio of grid wire width to grid pitch.Main results. Optimum grid designs maximizing suppression of signal generated by hole transport, without significantly affecting the total signal due to electron and hole transport, were identified and MTF was determined. For the most favorable designs, additional modeling was used to determine DQE. The results indicate that, through judicious choice of grid design and operational conditions, first frame lag can be significantly reduced to below 1%-less than the low levels exhibited by a-Se. DQE performance is shown to be largely maintained as exposure decreases-which should help to maintain good image quality.Significance. Substantial reduction of lag in PIB HgI₂converters via incorporation of Frisch grids has been demonstrated through modeling.

Surface and Optoelectronic Properties of Ultrathin Trigonal Selenium: A Density Functional Theory Study with van der Waals Correction

Selenium is an important earth-abundant and nontoxic semiconductor with numerous applications across semiconductor industries. Selenium has drawn attention from scientific communities for photovoltaics and imaging devices. Its usage as a photosensitive material largely involves the synthesis of the amorphous phase (a-Se) via various experimental techniques. However, the ground state crystalline phase of this material, the trigonal selenium (t-Se), a layered van der Waals solid, has not been extensively studied for its optimum electronic and optical properties. In this work, we present systematic studies based on density functional theory (DFT) for ultrathin (101̅0) surface slabs of t-Se. We report the surface energy as well as work function and electronic and optical properties as a function of number of layers for (101̅0) surface slabs to access its suitability for applications as a photosensitive material and compare these calculations to historical data.

Theoretical investigation of the signal performance of HgI2x-ray converters incorporating a Frisch grid structure at mammographic energies

Active matrix, flat-panel imagers (AMFPIs) suffer from decreased detective quantum efficiency under conditions of low dose per image frame (such as for digital breast tomosynthesis, fluoroscopy and cone-beam CT) due to low signal compared to the additive electronic noise. One way to address this challenge is to introduce a high-gain x-ray converter called particle-in-binder mercuric iodide (PIB HgI₂) which exhibits 3-10 times higher x-ray sensitivity compared to that of a-Se and CsI:Tl converters employed in commercial AMFPI systems. However, a remaining challenge for practical implementation of PIB HgI₂is the high level of image lag, which is believed to largely originate from the trapping of holes. Towards addressing this challenge, this paper reports a theoretical investigation of the use of a Frisch grid structure embedded in the converter to suppress hole signal-which would be expected to reduce image lag. The grid acts as a third electrode sandwiched between a continuous top electrode and pixelated bottom electrodes having a 100μm pitch. Signal properties of such a detector are investigated as a function of VDR (the ratio of the voltage difference between the electrodes in the region below the grid to that above the grid), grid pitch (the center-to-center distance between two neighboring grid wires) andR_GRID(the ratio of grid wire width to grid pitch) for mammographic x-ray energies. The results show that smaller grid pitch suppresses hole signal to a higher degree (up to ∼96%) while a larger gap between grid wires and higher VDR provide minimally impeded electron transport. Examination of the tradeoff between maximizing electron signal and minimizing hole signal indicates that a grid design having a grid pitch of 20μm withR_GRIDof 50% and 65% provides hole signal suppression of ∼93% and ∼95% for VDR of 1 and 3, respectively.

Enhanced image sensing with avalanche multiplication in hybrid structure of crystalline selenium photoconversion layer and CMOSFETs

The recent improvements of complementary metal–oxide–semiconductor (CMOS) image sensors are playing an essential role in emerging high-definition video cameras, which provide viewers with a stronger sensation of reality. However, the devices suffer from decreasing sensitivity due to the shrinkage of pixels. We herein address this problem by introducing a hybrid structure comprising crystalline-selenium (c-Se)-based photoconversion layers and 8 K resolution (7472 × 4320 pixels) CMOS field-effect transistors (FETs) to amplify signals using the avalanche multiplication of photogenerated carriers. Using low-defect-level NiO as an electric field buffer and an electron blocking layer, we confirmed signal amplification by a factor of approximately 1.4 while the dark current remained low at 2.6 nA/cm² at a reverse bias voltage of 22.6 V. Furthermore, we successfully obtained a brighter image based on the amplified signals without any notable noise degradation.

Frequency-dependent signal and noise in spectroscopic x-ray imaging

Purpose

We present a new framework for theoretical analysis of the noise power spectrum (NPS) of photon‐counting x‐ray detectors, including simple photon‐counting detectors (SPCDs) and spectroscopic x‐ray detectors (SXDs), the latter of which use multiple energy thresholds to discriminate photon energies.

Methods

We show that the NPS of SPCDs and SXDs, including spatio‐energetic noise correlations, is determined by the joint probability density function (PDF) of deposited photon energies, which describes the probability of recording two photons of two different energies in two different elements following a single‐photon interaction. We present an analytic expression for this joint PDF and calculate the presampling and digital NPS of CdTe SPCDs and SXDs. We calibrate our charge sharing model using the energy response of a cadmium zinc telluride (CZT) spectroscopic x‐ray detector and compare theoretical results with Monte Carlo simulations.

Results

Our analysis shows that charge sharing increases pixel signal‐to‐noise ratio (SNR), but degrades the zero‐frequency signal‐to‐noise performance of SPCDs and SXDs. In all cases considered, this degradation was greater than 10%. Comparing the presampling NPS with the sampled NPS showed that degradation in zero‐frequency performance is due to zero‐frequency noise aliasing induced by charge sharing.

Conclusions

Noise performance, including spatial and energy correlations between elements and energy bins, are described by the joint PDF of deposited energies which provides a method of determining the photon‐counting NPS, including noise‐aliasing effects and spatio‐energetic effects in spectral imaging. Our approach enables separating noise due to x‐ray interactions from that associated with sampling, consistent with cascaded systems analysis of energy‐integrating systems. Our methods can be incorporated into task‐based assessment of image quality for the design and optimization of spectroscopic x‐ray detectors.

Count rate capabilities of polycrystalline silicon photon counting detectors for CBCT applications-a theoretical study

The signal-to-noise properties of active matrix, flat-panel imagers (AMFPIs) limit the imaging performance of this x-ray imaging technology under conditions of low dose per image frame. This limitation can affect cone-beam computed tomography (CBCT) procedures where an AMFPI is used to acquire hundreds of image frames to form a single volumetric data set. An approach for overcoming this limitation is to replace the energy-integrating pixel circuits of AMFPI arrays with photon counting pixel circuits which examine the energy of each x-ray interaction and count those events whose signals exceed user-defined energy thresholds. A promising material for fabricating the circuits of such photon-counting detectors (PCDs) is polycrystalline silicon (poly-Si)-a semiconductor that facilitates economic manufacture of large area, monolithic arrays of the size presently provided by AMFPIs as well as provides good radiation damage resistance. In this paper, results are reported from a theoretical investigation of the potential for poly-Si PCDs to satisfy the count rate needs, while maintaining good energy resolution, of two CBCT applications-CBCT used for breast imaging and kilo-voltage CBCT used for providing localization information in image guided radiotherapy (referred to as BCT and kV-CBCT, respectively). The study focused on the performance of the critical first component of a PCD pixel circuit, the amplifier, under conditions relevant to the two applications. The study determined that, compared to the average input fluxes associated with BCT and kV-CBCT, a promising amplifier design employing poly-Si thin-film transistors can provide count rates two and four times in excess of those levels, respectively, assuming a dead time loss of 10%. In addition, calculational estimates based on foreseeable poly-Si circuit densities suggest that it should be possible to include sufficient circuitry to support 2 and 3 energy thresholds per pixel, respectively. Finally, prospects for further improvements are discussed.

Comparison of CsI:Tl and Gd2 O2 S:Tb indirect flat panel detector x-ray imaging performance in front- and back-irradiation geometries

PURPOSE

The detective quantum efficiency (DQE) of indirect flat panel detectors (I-FPDs) is limited at higher x-ray energies (e.g., 100-140 kVp) by low absorption in their scintillating x-ray conversion layer. While increasing the thickness of the scintillator can improve its x-ray absorption efficiency, this approach is potentially limited by reduced spatial resolution and increased noise due to depth dependence in the scintillator's response to x rays. One strategy proposed to mitigate these deleterious effects is to irradiate the scintillator through the pixel sensor in a "back-irradiation" geometry. This work directly evaluates the impact of irradiation geometry on the inherent imaging performance of I-FPDs composed with columnar CsI:Tl and powder Gd2 O2 S:Tb (GOS) scintillators.

METHODS

A "bidirectional" FPD was constructed which allows scintillator samples to be interchangeably coupled with the detector's active matrix to compose an I-FPD. Radio-translucent windows in the detector's housing permit imaging in both "front-irradiation" (FI) and "back-irradiation" (BI) geometries. This test device was used to evaluate the impact of irradiation geometry on the x-ray sensitivity, modulation transfer function (MTF), noise power spectrum (NPS), and DQE of four I-FPDs composed using columnar CsI:Tl scintillators of varying thickness (600-1000 µm) and optical backing, and a Fast Back GOS screen. All experiments used an RQA9 x-ray beam.

RESULTS

Each I-FPD's x-ray sensitivity, MTF, and DQE was greater or equal in BI geometry than in FI. The I-FPD composed with CsI:Tl (1 mm) and an optically absorptive backing had the largest variation in sensitivity (17%) between FI and BI geometries. The detector composed with GOS had the largest improvement in limiting resolution (31%). Irradiation geometry had little impact on MTF(f) and DQE(f) measurements near zero frequency, however, the difference between FI and BI measurements generally increased with spatial frequency. The CsI:Tl scintillator with optically absorptive backing (1 mm) in BI geometry had the highest spatial resolution and DQE over all frequencies.

CONCLUSIONS

Back irradiation may improve the inherent x-ray imaging performance of I-FPDs composed with CsI:Tl and GOS scintillators. This approach can be leveraged to improve tradeoffs between detector dose efficiency, spatial resolution and noise for higher energy x-ray imaging.

Objective assessment of task performance: a comparison of two FFDM detectors using an anthropomorphic breast phantom

Current digital mammography systems primarily employ one of two types of detectors: indirect conversion, typically using a cesium-iodine scintillator integrated with an amorphous silicon photodiode matrix, or direct conversion, using a photoconductive layer of amorphous selenium (a-Se) combined with thin-film transistor array. The goal of this study was to evaluate a methodology for objectively assessing image quality to compare human observer task performance in detecting microcalcification clusters and extended mass-like lesions achieved with different detector types. The proposed assessment methodology uses a novel anthropomorphic breast phantom fabricated with ink-jet printing. In addition to human observer detection performance, standard linear metrics such as modulation transfer function, noise power spectrum, and detective quantum efficiency (DQE) were also measured to assess image quality. An Analogic Anrad AXS-2430 a-Se detector used in a commercial FFDM/DBT system and a Teledyne Dalsa Xineos-2329 with CMOS pixel readout were evaluated and compared. The DQE of each detector was similar over a range of exposures. Similar task performance in detecting microcalcifications and masses was observed between the two detectors over a range of clinically applicable dose levels, with some perplexing differences in the detection of microcalcifications at the lowest dose measurement. The evaluation approach presented seems promising as a new technique for objective assessment of breast imaging technology.

Toward Scintillator High-Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager (SHARP-AMFPI): Initial fabrication and characterization

PURPOSE

We present the first prototype Scintillator High-Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager (SHARP-AMFPI). This detector includes a layer of avalanche amorphous Selenium (a-Se) (HARP) as the photoconductor in an indirect detector to amplify the signal and reduce the effects of electronic noise to obtain quantum noise-limited images for low-dose applications. It is the first time avalanche a-Se has been used in a solid-state imaging device and poses as a possible solution to eliminate the effects of electronic noise, which is crucial for low-dose imaging performance of AMFPI.

METHODS

We successfully deposited a solid-state HARP structure onto a 24 × 30 cm² array of thin-film transistors (TFT array) with a pixel pitch of 85 μm. The HARP layer consists of 16 μm of a-Se with a hole-blocking and electron-blocking layer to prevent charge injection from the high-voltage bias and pixel electrodes, respectively. An electric field (E_Se ) up to 105 V μm^-1 was applied across the a-Se layer without breakdown. A 150 μm thick-structured CsI:Tl scintillator was used to form SHARP-AMFPI. The x-ray imaging performance is characterized using a 30 kVp Mo/Mo beam. We evaluate the spatial resolution, noise power, and detective quantum efficiency at zero frequency of the system with and without avalanche gain. The results are analyzed using cascaded linear system model (CLSM).

RESULTS

An avalanche gain of 76 ± 5 was measured at E_Se = 105 V μm^-1 . We demonstrate that avalanche gain can amplify the signal to overcome electronic noise. As avalanche gain is increased, image quality improves for a constant (0.76 mR) exposure until electronic noise is overcome. Our system is currently limited by poor optical transparency of our high-voltage electrode and long integrating time which results in dark current noise. These two effects cause high-spatial frequency noise to dominate imaging performance.

CONCLUSIONS

We demonstrate the feasibility of a solid-state HARP x-ray imager and have fabricated the largest active area HARP sensor to date. Procedures to reduce secondary quantum and dark noise are outlined. Future work will improve optical coupling and charge transport which will allow for frequency DQE and temporal metrics to be obtained.

Theoretical investigation of the noise performance of active pixel imaging arrays based on polycrystalline silicon thin film transistors

PURPOSE

Active matrix flat-panel imagers, which typically incorporate a pixelated array with one a-Si:H thin-film transistor (TFT) per pixel, have become ubiquitous by virtue of many advantages, including large monolithic construction, radiation tolerance, and high DQE. However, at low exposures such as those encountered in fluoroscopy, digital breast tomosynthesis and breast computed tomography, DQE is degraded due to the modest average signal generated per interacting x-ray relative to electronic additive noise levels of ~1000 e, or greater. A promising strategy for overcoming this limitation is to introduce an amplifier into each pixel, referred to as the active pixel (AP) concept. Such circuits provide in-pixel amplification prior to readout as well as facilitate correlated multiple sampling, enhancing signal-to-noise and restoring DQE at low exposures. In this study, a methodology for theoretically investigating the signal and noise performance of imaging array designs is introduced and applied to the case of AP circuits based on low-temperature polycrystalline silicon (poly-Si), a semiconductor suited to manufacture of large area, radiation tolerant arrays.

METHODS

Computer simulations employing an analog circuit simulator and performed in the temporal domain were used to investigate signal characteristics and major sources of electronic additive noise for various pixel amplifier designs. The noise sources include photodiode shot noise and resistor thermal noise, as well as TFT thermal and flicker noise. TFT signal behavior and flicker noise were parameterized from fits to measurements performed on individual poly-Si test TFTs. The performance of three single-stage and three two-stage pixel amplifier designs were investigated under conditions relevant to fluoroscopy. The study assumes a 20 × 20 cm² , 150 μm pitch array operated at 30 fps and coupled to a CsI:Tl x-ray converter. Noise simulations were performed as a function of operating conditions, including sampling mode, of the designs. The total electronic additive noise included noise contributions from each circuit component.

RESULTS

The total noise results were found to exhibit a strong dependence on circuit design and operating conditions, with TFT flicker noise generally found to be the dominant noise contributor. For the single-stage designs, significantly increasing the size of the source-follower TFT substantially reduced flicker noise - with the lowest total noise found to be ~574 e [rms]. For the two-stage designs, in addition to tuning TFT sizes and introducing a low-pass filter, replacing a p-type TFT with a resistor (under the assumption in the study that resistors make no flicker noise contribution) resulted in significant noise reduction - with the lowest total noise found to be ~336 e [rms].

CONCLUSIONS

A methodology based on circuit simulations which facilitates comprehensive explorations of signal and noise characteristics has been developed and applied to the case of poly-Si AP arrays. The encouraging results suggest that the electronic additive noise of such devices can be substantially reduced through judicious circuit design, signal amplification, and multiple sampling. This methodology could be extended to explore the noise performance of arrays employing other pixel circuitry such as that for photon counting as well as other semiconductor materials such as a-Si:H and a-IGZO.

Scintillator high-gain avalanche rushing photoconductor active-matrix flat panel imager: zero-spatial frequency x-ray imaging properties of the solid-state SHARP sensor structure

PURPOSE

The authors are investigating the feasibility of a new type of solid-state x-ray imaging sensor with programmable avalanche gain: scintillator high-gain avalanche rushing photoconductor active matrix flat panel imager (SHARP-AMFPI). The purpose of the present work is to investigate the inherent x-ray detection properties of SHARP and demonstrate its wide dynamic range through programmable gain.

METHODS

A distributed resistive layer (DRL) was developed to maintain stable avalanche gain operation in a solid-state HARP. The signal and noise properties of the HARP-DRL for optical photon detection were investigated as a function of avalanche gain both theoretically and experimentally, and the results were compared with HARP tube (with electron beam readout) used in previous investigations of zero spatial frequency performance of SHARP. For this new investigation, a solid-state SHARP x-ray image sensor was formed by direct optical coupling of the HARP-DRL with a structured cesium iodide (CsI) scintillator. The x-ray sensitivity of this sensor was measured as a function of avalanche gain and the results were compared with the sensitivity of HARP-DRL measured optically. The dynamic range of HARP-DRL with variable avalanche gain was investigated for the entire exposure range encountered in radiography∕fluoroscopy (R∕F) applications.

RESULTS

The signal from HARP-DRL as a function of electric field showed stable avalanche gain, and the noise associated with the avalanche process agrees well with theory and previous measurements from a HARP tube. This result indicates that when coupled with CsI for x-ray detection, the additional noise associated with avalanche gain in HARP-DRL is negligible. The x-ray sensitivity measurements using the SHARP sensor produced identical avalanche gain dependence on electric field as the optical measurements with HARP-DRL. Adjusting the avalanche multiplication gain in HARP-DRL enabled a very wide dynamic range which encompassed all clinically relevant medical x-ray exposures.

CONCLUSIONS

This work demonstrates that the HARP-DRL sensor enables the practical implementation of a SHARP solid-state x-ray sensor capable of quantum noise limited operation throughout the entire range of clinically relevant x-ray exposures. This is an important step toward the realization of a SHARP-AMFPI x-ray flat-panel imager.

A solid-state amorphous selenium avalanche technology for low photon flux imaging applications

PURPOSE

The feasibility of a practical solid-state technology for low photon flux imaging applications was investigated. The technology is based on an amorphous selenium photoreceptor with a voltage-controlled avalanche multiplication gain. If this photoreceptor can provide sufficient internal gain, it will be useful for an extensive range of diagnostic imaging systems.

METHODS

The avalanche photoreceptor under investigation is referred to as HARP-DRL. This is a novel concept in which a high-gain avalanche rushing photoconductor (HARP) is integrated with a distributed resistance layer (DRL) and sandwiched between two electrodes. The avalanche gain and leakage current characteristics of this photoreceptor were measured.

RESULTS

HARP-DRL has been found to sustain very high electric field strengths without electrical breakdown. It has shown avalanche multiplication gains as high as 10(4) and a very low leakage current (< or = 20 pA/mm2).

CONCLUSIONS

This is the first experimental demonstration of a solid-state amorphous photoreceptor which provides sufficient internal avalanche gain for photon counting and photon starved imaging applications.

Active pixel imagers incorporating pixel-level amplifiers based on polycrystalline-silicon thin-film transistors

Active matrix, flat-panel imagers (AMFPIs) employing a 2D matrix of a-Si addressing TFTs have become ubiquitous in many x-ray imaging applications due to their numerous advantages. However, under conditions of low exposures and/or high spatial resolution, their signal-to-noise performance is constrained by the modest system gain relative to the electronic additive noise. In this article, a strategy for overcoming this limitation through the incorporation of in-pixel amplification circuits, referred to as active pixel (AP) architectures, using polycrystalline-silicon (poly-Si) TFTs is reported. Compared to a-Si, poly-Si offers substantially higher mobilities, enabling higher TFT currents and the possibility of sophisticated AP designs based on both n- and p-channel TFTs. Three prototype indirect detection arrays employing poly-Si TFTs and a continuous a-Si photodiode structure were characterized. The prototypes consist of an array (PSI-1) that employs a pixel architecture with a single TFT, as well as two arrays (PSI-2 and PSI-3) that employ AP architectures based on three and five TFTs, respectively. While PSI-1 serves as a reference with a design similar to that of conventional AMFPI arrays, PSI-2 and PSI-3 incorporate additional in-pixel amplification circuitry. Compared to PSI-1, results of x-ray sensitivity demonstrate signal gains of approximately 10.7 and 20.9 for PSI-2 and PSI-3, respectively. These values are in reasonable agreement with design expectations, demonstrating that poly-Si AP circuits can be tailored to provide a desired level of signal gain. PSI-2 exhibits the same high levels of charge trapping as those observed for PSI-1 and other conventional arrays employing a continuous photodiode structure. For PSI-3, charge trapping was found to be significantly lower and largely independent of the bias voltage applied across the photodiode. MTF results indicate that the use of a continuous photodiode structure in PSI-1, PSI-2, and PSI-3 results in optical fill factors that are close to unity. In addition, the greater complexity of PSI-2 and PSI-3 pixel circuits, compared to that of PSI-1, has no observable effect on spatial resolution. Both PSI-2 and PSI-3 exhibit high levels of additive noise, resulting in no net improvement in the signal-to-noise performance of these early prototypes compared to conventional AMFPIs. However, faster readout rates, coupled with implementation of multiple sampling protocols allowed by the nondestructive nature of pixel readout, resulted in a significantly lower noise level of approximately 560 e (rms) for PSI-3.

Direct-conversion flat-panel imager with avalanche gain: feasibility investigation for HARP-AMFPI

The authors are investigating the concept of a direct-conversion flat-panel imager with avalanche gain for low-dose x-ray imaging. It consists of an amorphous selenium (a-Se) photoconductor partitioned into a thick drift region for x-ray-to-charge conversion and a relatively thin region called high-gain avalanche rushing photoconductor (HARP) in which the charge undergoes avalanche multiplication. An active matrix of thin film transistors is used to read out the electronic image. The authors call the proposed imager HARP active matrix flat panel imager (HARP-AMFPI). The key advantages of HARP-AMFPI are its high spatial resolution, owing to the direct-conversion a-Se layer, and its programmable avalanche gain, which can be enabled during low dose fluoroscopy to overcome electronic noise and disabled during high dose radiography to prevent saturation of the detector elements. This article investigates key design considerations for HARP-AMFPI. The effects of electronic noise on the imaging performance of HARP-AMFPI were modeled theoretically and system parameters were optimized for radiography and fluoroscopy. The following imager properties were determined as a function of avalanche gain: (1) the spatial frequency dependent detective quantum efficiency; (2) fill factor; (3) dynamic range and linearity; and (4) gain nonuniformities resulting from electric field strength nonuniformities. The authors results showed that avalanche gains of 5 and 20 enable x-ray quantum noise limited performance throughout the entire exposure range in radiography and fluoroscopy, respectively. It was shown that HARP-AMFPI can provide the required gain while maintaining a 100% effective fill factor and a piecewise dynamic range over five orders of magnitude (10(-7)-10(-2) R/frame). The authors have also shown that imaging performance is not significantly affected by the following: electric field strength nonuniformities, avalanche noise for x-ray energies above 1 keV and direct interaction of x rays in the gain region. Thus, HARP-AMFPI is a promising flat-panel imager structure that enables high-resolution fully quantum noise limited x-ray imaging over a wide exposure range.

Design and feasibility of active matrix flat panel detector using avalanche amorphous selenium for protein crystallography

Protein crystallography is the most important technique for resolving the three-dimensional atomic structure of protein by measuring the intensity of its x-ray diffraction pattern. This work proposes a large area flat panel detector for protein crystallography based on direct conversion x-ray detection technique using avalanche amorphous selenium (a-Se) as the high gain photoconductor, and active matrix readout using amorphous silicon (a-Si:H) thin film transistors. The detector employs avalanche multiplication phenomenon of a-Se to make the detector sensitive to each incident x ray. The advantages of the proposed detector over the existing imaging plate and charge coupled device detectors are large area, high dynamic range coupled to single x-ray detection capability, fast readout, high spatial resolution, and inexpensive manufacturing process. The optimal detector design parameters (such as detector size, pixel size, and thickness of a-Se layer), and operating parameters (such as electric field across the a-Se layer) are determined based on the requirements for protein crystallography application. The performance of the detector is evaluated in terms of readout time (<1 s), dynamic range (approximately 10(5)), and sensitivity (approximately 1 x-ray photon), thus validating the detector's efficacy for protein crystallography.

SAPHIRE (scintillator avalanche photoconductor with high resolution emitter readout) for low dose x-ray imaging: spatial resolution

An indirect flat panel imager (FPI) with programmable avalanche gain and field emitter array (FEA) readout is being investigated for low-dose and high resolution x-ray imaging. It is made by optically coupling a structured x-ray scintillator, e.g., thallium (Tl) doped cesium iodide (CsI), to an amorphous selenium (a-Se) avalanche photoconductor called high-gain avalanche rushing amorphous photoconductor (HARP). The charge image created by the scintillator/HARP (SHARP) combination is read out by the electron beams emitted from the FEA. The proposed detector is called scintillator avalanche photoconductor with high resolution emitter readout (SAPHIRE). The programmable avalanche gain of HARP can improve the low dose performance of indirect FPI while the FEA can be made with pixel sizes down to 50 microm. Because of the avalanche gain, a high resolution type of CsI (Tl), which has not been widely used in indirect FPI due to its lower light output, can be used to improve the high spatial frequency performance. The purpose of the present article is to investigate the factors affecting the spatial resolution of SAPHIRE. Since the resolution performance of the SHARP combination has been well studied, the focus of the present work is on the inherent resolution of the FEA readout method. The lateral spread of the electron beam emitted from a 50 microm x 50 microm pixel FEA was investigated with two different electron-optical designs: mesh-electrode-only and electrostatic focusing. Our results showed that electrostatic focusing can limit the lateral spread of electron beams to within the pixel size of down to 50 microm. Since electrostatic focusing is essentially independent of signal intensity, it will provide excellent spatial uniformity.

Investigation of the signal behavior at diagnostic energies of prototype, direct detection, active matrix, flat-panel imagers incorporating polycrystalline HgI2

Active matrix, flat-panel x-ray imagers based on a-Si:H thin-film transistors offer many advantages and are widely utilized in medical imaging applications. Unfortunately, the detective quantum efficiency (DQE) of conventional flat-panel imagers incorporating scintillators or a-Se photoconductors is significantly limited by their relatively modest signal-to-noise ratio, particularly in applications involving low x-ray exposures or high spatial resolution. For this reason, polycrystalline HgI2 is of considerable interest by virtue of its low effective work function, high atomic number and the possibility of large-area deposition. In this study, a detailed investigation of the properties of prototype, flat-panel arrays coated with two forms of this high-gain photoconductor are reported. Encouragingly, high x-ray sensitivity, low dark current and spatial resolution close to the theoretical limits were observed from a number of prototypes. In addition, input-quantum-limited DQE performance was measured from one of the prototypes at relatively low exposures. However, high levels of charge trapping, lag and polarization, as well as pixel-to-pixel variations in x-ray sensitivity are of concern. While the results of the current study are promising, further development will be required to realize prototypes exhibiting the characteristics necessary to allow practical implementation of this approach.

Flat-panel detectors: how much better are they?

Interventional and fluoroscopic imaging procedures for pediatric patients are becoming more prevalent because of the less-invasive nature of these procedures compared to alternatives such as surgery. Flat-panel X-ray detectors (FPD) for fluoroscopy are a new technology alternative to the image intensifier/TV (II/TV) digital system that has been in use for more than two decades. Two major FPD technologies have been implemented, based on indirect conversion of X-rays to light (using an X-ray scintillator) and then to proportional charge (using a photodiode), or direct conversion of X-rays into charge (using a semiconductor material) for signal acquisition and digitization. These detectors have proved very successful for high-exposure interventional procedures but lack the image quality of the II/TV system at the lowest exposure levels common in fluoroscopy. The benefits for FPD image quality include lack of geometric distortion, little or no veiling glare, a uniform response across the field-of-view, and improved ergonomics with better patient access. Better detective quantum efficiency indicates the possibility of reducing the patient dose in accordance with ALARA principles. However, first-generation FPD devices have been implemented with less than adequate acquisition flexibility (e.g., lack of tableside controls/information, inability to easily change protocols) and the presence of residual signals from previous exposures, and additional cost of equipment and long-term maintenance have been serious impediments to purchase and implementation. Technological advances of second generation and future hybrid FPD systems should solve many current issues. The answer to the question "how much better are they?" is "significantly better", and they are certainly worth consideration for replacement or new implementation of an imaging suite for pediatric fluoroscopy.

X-ray imaging with amorphous selenium: Pulse height measurements of avalanche gain fluctuations

Avalanche multiplication in amorphous selenium (a-Se) can provide a large, adjustable gain for active matrix flat panel imagers (AMFPI), enabling quantum noise limited x-ray imaging during both radiography and fluoroscopy. In the case of direct conversion AMFPI, the multiplication factor for each x ray is a function of its depth of interaction, and the resulting variations in gain can reduce the detective quantum efficiency (DQE) of the system. An experimental method was developed to measure gain fluctuations by analyzing images of individual x rays that were obtained using a video camera with an a-Se target operated in avalanche mode. Pulse height spectra (PHS) of the charge produced per x ray were recorded for monoenergetic 30.9, 49.4, and 73.8 keV x-ray sources. The rapid initial decay and long tail of each PHS can be explained by a model in which positive charge dominates the initiation of avalanche. The Swank information factor quantifies the effect of gain fluctuation on DQE and was calculated from the PHS. The information factor was found to be 0.5 for a 25 microm a-Se layer with a maximum gain of approximately 300. Changing the energy of the incident x ray influenced the range of the primary photoelectron and noticeably affected the tail of the experimental PHS, but did not significantly change the avalanche Swank factor.