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Purpose: Scintillators and photoconductors used in energy integrating detectors (EIDs) have inherent variations in their imaging response to single‐detected X‐rays due to variations in X‐ray energy deposition and secondary quanta generation and transport, which degrades DQE(f). The imaging response of X‐ray scintillators to single X‐rays may be recorded and studied using single X‐ray imaging (SXI) experiments; however, no method currently exists for relating SXI experimental results to EID DQE(f). This work proposes a general analytical framework for computing and analyzing the DQE(f) performance of EIDs from single X‐ray image ensembles using a spatial frequency‐dependent pulse‐height spectrum. Methods: A spatial frequency (f)‐dependent gain, g∼(f)$\tilde{g}(f)$, is defined as the Fourier transform of the imaging response of an EID to a single‐detected X‐ray. A f‐dependent pulse‐height spectrum, Pr[g∼(f)]$\Pr [\tilde{g}(f)]$, is defined as the 2D probability density function of g∼(f)$\tilde{g}(f)$ over the complex plane. Pr[g∼(f)]$\Pr [\tilde{g}(f)]$ is used to define a f‐dependent Swank factor, AS(f), which fully characterizes the DQE(f) degradation due to single X‐ray noise. AS(f) is analyzed in terms of its degradation due to Swank noise, variations in the frequency‐dependent attenuation of |g∼(f)|$| {\tilde{g}(f)} |$, and noise in argg∼(f)$\arg \tilde{g}(f)$ which occurs due to variations in the asymmetry in each single X‐ray's imaging response. Three example imaging systems are simulated to demonstrate the impact of depth‐dependent variation in g∼(f)$\tilde{g}(f)$, remote energy deposition, and a finite number of secondary quanta, on Pr[g∼(f)]$\Pr [\tilde{g}(f)]$, AS(f), MTF(f), and NPS(f)/NPS(0), which are computed from ensembles of single X‐ray images. The same is also demonstrated by simulating a realistic imaging system; that is, a Gd2O2S‐based EID. Using the latter imaging system, the convergence of AS(f) estimates is investigated as a function of the number of detected X‐rays per ensemble. Results: Depth‐dependent g∼(f)$\tilde{g}(f)$ variation resulted in AS(f) degradation exclusively due to depth‐dependent optical Swank noise and the Lubberts effect. Conversely, the majority of AS(f) degradation caused by remote energy deposition and finite secondary quanta occurred due to variations in argg∼(f)$\arg \tilde{g}(f)$. When using input X‐ray energies below the K‐edge of Gd, variations in the frequency‐dependent attenuation of |g∼(f)|$| {\tilde{g}(f)} |$ accounted for the majority of AS(f) degradation in the GOS‐based EID, and very little Swank noise and variations in argg∼(f)$\arg \tilde{g}(f)$ were observed. Above the K‐edge, however, AS(f) degradation due to Swank noise and variations in argg∼(f)$\arg \tilde{g}(f)$ greatly increased. The convergence of AS(f) was limited by variation in argg∼(f)$\arg \tilde{g}(f)$; imaging systems with more variation in argg∼(f)$\arg \tilde{g}(f)$ required more detected X‐rays per ensemble. Conclusions: An analytical framework is proposed that generalizes the pulse‐height spectrum and Swank factor to arbitrary f. The impact of single X‐ray noise sources, such as the Lubberts effect, remote energy deposition, and finite secondary quanta on detector performance, may be represented using Pr[g∼(f)]$\Pr [\tilde{g}(f)]$, and quantified using AS(f). The approach may be used to compute MTF(f), NPS(f), and DQE(f) from ensembles of single X‐ray images and provides an additional tool to analyze proposed EID designs. [ABSTRACT FROM AUTHOR]

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 (ESe) 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 ESe = 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Purpose: This study investigates the fundamental signal and noise performance limitations imposed by the stochastic nature of x-ray interactions in selected photoconductor materials, such as Si, a-Se, CdZnTe, HgI2, PbI2, PbO, and TlBr, for x-ray spectra typically used in mammography. Methods: It is shown how Monte Carlo simulations can be combined with a cascaded model to determine the absorbed energy distribution for each combination of photoconductor and x-ray spectrum. The model is used to determine the quantum efficiency, mean energy absorption per interaction, Swank noise factor, secondary quantum noise, and zero-frequency detective quantum efficiency (DQE). Results: The quantum efficiency of materials with higher atomic number and density demonstrates a larger dependence on convertor thickness than those with lower atomic number and density with the exception of a-Se. The mean deposited energy increases with increasing average energy of the incident x-ray spectrum. HgI2, PbI2, and CdZnTe demonstrate the largest increase in deposited energy with increasing mass loading and a-Se and Si the smallest. The best DQE performances are achieved with PbO and TlBr. For mass loading greater than 100 mg cm-2, a-Se, HgI2, and PbI2 provide similar DQE values to PbO and TlBr. Conclusions: The quantum absorption efficiency, average deposited energy per interacting x-ray, Swank noise factor, and detective quantum efficiency are tabulated by means of graphs which may help with the design and selection of materials for photoconductor-based mammography detectors. Neglecting the electrical characteristics of photoconductor materials and taking into account only x-ray interactions, it is concluded that PbO shows the strongest signal-to-noise ratio performance of the materials investigated in this study. [ABSTRACT FROM AUTHOR]