PurposeActive matrix flat panel imagers (AMFPIs) with thin-film transistor arrays experience image quality degradation by electronic noise in low-dose radiography and fluoroscopy. One potential solution is to overcome electronic noise using avalanche gain in an amorphous selenium (a-Se) (HARP) photoconductor in indirect AMFPI. In this work, we aim to improve temporal performance of HARP using a novel composite hole blocking layer (HBL) structure and increase optical quantum efficiency (OQE) to CsI:Tl scintillators by tellurium (Te) doping.ApproachTwo different HARP structures were fabricated: Composite HBL samples and Te-doped samples. Dark current and optical sensitivity measurements were performed on the composite HBL samples to evaluate avalanche gain and temporal performance. The OQE and temporal performance of the Te-doped samples were characterized by optical sensitivity measurements. A charge transport model was used to investigate the hole mobility and lifetime of the Te-doped samples in combination with time-of-flight measurements.ResultsThe composite HBL has excellent temporal performance, with ghosting below 3% at 10 mR equivalent exposure. Furthermore, the composite HBL samples have dark current <10−10 A/cm2 and achieved an avalanche gain of 16. Te-doped samples increased OQE from 0.018 to 0.43 for 532 nm light. The addition of Te resulted in 2.1% first-frame lag, attributed to hole trapping within the layer.ConclusionsThe composite HBL and Te-doping can be utilized to improve upon the limitations of previously developed indirect HARP imagers, showing excellent temporal performance and increased OQE, respectively.
Active matrix flat panel imagers (AMFPIs) with thin-film transistor (TFT) arrays have become the dominant technology for digital x-ray imaging. However, their performance is degraded by electronic noise in low dose imaging applications. One potential solution is to overcome electronic noise using avalanche gain in an amorphous selenium (a-Se) photoconductor in indirect AMFPI, known as the scintillating high-gain avalanche rushing photoconductor AMFPI (SHARP-AMFPI). We previously developed two SHARP-AMFPI prototypes, however both have several areas of desired improvement. In this work, we fabricate and characterize HARP samples with a composite hole blocking layer (HBL) structure to reliably maintain avalanche fields while reducing temporal effects, as well as samples with tellurium (Te) alloyed a-Se to increase the optical quantum efficiency (OQE) to thallium doped cesium iodide (CsI:Tl) columnar scintillators. Our measurements show that the composite HBL has improved temporal performance over the original prototype, with ghosting below 3% at 10 mR equivalent exposure and no noticeable lag observed. We also show that the layer has comparable dark current to the previously used organic HBL and can reach an avalanche gain of 16. We aim to further reduce the dark current by improving the formulation of the n-type metal oxide layer using different deposition methods. Introducing Te-alloying to HARP samples shows an increase in OQE from 0.018 to 0.43 for 532 nm light. The addition of Te resulted in increased lag, attributed to charge trapping within the layer. Future work will investigate arsenic and chlorine co-doping to restore charge transport in this layer.
Indirect flat panel imager (I-FPI) performance is limited by noise in the scintillator’s response to single x-rays, i.e. random variation of gain and blur. Single x-ray imaging (SXI) is an experimental methodology to record the response of scintillators to single x-rays using an image intensifier that is lens coupled to an EMCCD camera. In this work we developed a framework for using ensembles of SXI images to compute the spatial-frequency-dependent pulse height spectrum, Pr[g(f)], a novel detector performance metric whose moments may be used to compute scintillator MTF(f), qNNPS(f), and DQE(f). The approach is demonstrated using cesium iodide (CsI:Tl) samples of various thicknesses and screen-optical properties. SXI was conducted on each sample using ~60 keV γ-rays emitted by Am-241. Each image in an SXI ensemble was summed in 1D, and Fourier transformed to form an ensemble of frequencydependent responses. The histogram at each frequency f forms the Pr[g(f)]. To validate this approach against standard methods, each sample was also evaluated using the standard method by coupling it to a photodiode-TFT array as in a conventional I-FPI. MTF(f) and qNNPS(f) were measured using a slanted edge and flood field images, respectively, using an 85 kVp x-ray spectrum filtered to achieve an average energy of 60 keV. MTF(f) and qNNPS(f) results agreed well between both methods, and discrepancies were consistent with the differing experimental conditions. The Pr[g(f)] may be used to help optimize CsI:Tl in I-FPIs, and to improve the computational efficiency of Monte Carlo simulations of I-FPI performance.
Purpose: Monte Carlo simulations were used to evaluate the imaging properties of a composite direct–indirect active matrix flat-panel imager (AMFPI) with potentially more favorable tradeoffs between x-ray quantum efficiency and spatial resolution than direct or indirect AMFPIs alone. This configuration, referred to as a hybrid AMFPI, comprises a scintillator that is optically coupled to an a-Se direct AMFPI through a transparent electrode and hole blocking layer, such that a-Se acts as both a direct x-ray converter and an optical sensor.
Approach: GEANT4 was used to simulate x-ray energy deposition, optical transport, and charge signal generation processes in various hybrid AMPFI configurations under RQA5 and RQA9 x-ray beam conditions. The Fujita–Lubberts–Swank method was used to quantify the impact of irradiation geometry, x-ray converter thicknesses, conversion gain of each layer, and x-ray cross talk between layers on detective quantum efficiency (DQE).
Results: Each hybrid configuration had a greater DQE than its direct AMFPI layer alone. The DQE improvement was largest at low spatial frequencies in both front- and back-irradiation (BI) geometries due to increased x-ray quantum efficiency provided by the scintillator. DQE improvements persisted at higher frequencies in BI geometry due to preferential x-ray absorption in a-Se. Matching the x-ray-to-charge conversion gains of a hybrid AMFPI’s direct and indirect detection layers affects its Swank factor and, thus, DQE(0). X-ray cross talk has a negligible impact on the DQE ( f ) of hybrid AMFPIs with sufficiently high optical quantum efficiency.
Conclusion: An optimized hybrid AMFPI can achieve greater DQE performance than current direct or indirect AMFPIs.
Direct active matrix flat panel imagers (AMFPIs) using amorphous selenium (a-Se) offer high intrinsic spatial resolution but have limited x-ray quantum efficiency at general radiographic energies due to selenium’s low atomic number. Conversely, indirect AMFPIs using inorganic scintillators typically have superior x-ray quantum efficiency at these energies, but inferior spatial resolution and increased noise due to optical effects in the scintillator. These inherent limitations motivate alternative AMFPI designs to further improve detector xray sensitivity and signal-to-noise performance. Towards this goal, this work constructs and experimentally investigates the x-ray imaging performance of a novel direct-indirect prototype imager referred to as Hybrid AMFPI. The imager comprises a direct conversion a-Se layer that may be coupled to an interchangeable scintillator screen through a transparent blocking layer and bias electrode. In this direct-indirect “hybrid” configuration, a-Se serves as both an x-ray and optical sensor. Readout is performed by a thin-film transistor array with 85 μm pixel pitch. The prototype imager’s x-ray sensitivity, modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) are measured in a direct AMFPI configuration (i.e. a-Se alone) and in a Hybrid configuration under identical x-ray exposure conditions and the results are compared. Contrast-detail and spatial resolution phantoms are also imaged using direct, Hybrid and indirect AMFPI configurations under identical exposure conditions to evaluate differences in their imaging performance.
Direct and indirect active matrix flat-panel imagers (AMFPI) have become the dominant technology in digital radiography and fluoroscopy, and further improvements in imaging performance are being sought through novel detector designs. Two novel multilayer x-ray detectors are proposed to improve the DQEs of existing AMFPI in R/F and CBCT applications that require high DQE and wide dynamic range. Both detectors utilize a back-irradiation (BI) geometry, and incorporate both a-Se and scintillators in their designs. The first design, the Hybrid-AMFPI is a composite direct/indirect detector that aims to improve the quantum efficiency of a-Se (with a maximum thickness of 1 mm due to carrier trapping) by adding a scintillator. The second design, the BI-SHARP-AMFPI (Back-Irradiated Scintillator HARPAMFPI), uses a High Gain Avalanche Rushing Photoconductor (HARP) a-Se layer to detect and amplify optical photons from an x-ray scintillator. This work uses the Fujita-Lubberts-Swank (FLS) Monte Carlo (MC) framework proposed by Star-Lack et al. to investigate the potential improvements in imaging performance of these detectors and the optimal detector configuration. Simulations were carried out at RQA5 and RQA9 standard beam qualities. Both front-irradiation (FI) and BI geometries were evaluated to demonstrate the advantage of BI. Our simulations confirm that the DQE of the Hybrid AMFPI is substantially improved at low spatial frequencies compared to an otherwise identical direct AMFPI. Additionally, the role of gain matching of direct and indirect signal (a consideration unique to multilayer AMFPI) is investigated in the imaging performance of both the Hybrid and BI-SHARP-AMFPI.
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