Optical absorption characteristics in the assessment of powder phosphor-based x-ray detectors: from nano- to micro-scale.

X-ray phosphor-based detectors have enormously improved the quality of medical imaging examinations through the optimization of optical diffusion. In recent years, with the development of science and technology in the field of materials, improved powder phosphors require structural and optical properties that contribute to better optical signal propagation. The purpose of this paper was to provide a quantitative and qualitative understanding of the optical absorption characteristics in the assessment of powder phosphor-based detectors (from nano- scale up to micro-scale). Variations on the optical absorption parameters (i.e. the light extinction coefficient [Formula: see text] and the percentage probability of light absorption p%) were evaluated based on Mie calculations examining a wide range of light wavelengths, particle refractive indices and sizes. To model and assess the effects of the aforementioned parameters on optical diffusion, Monte Carlo simulation techniques were employed considering: (i) phosphors of different layer thickness, 100 µm (thin layer) and 300 µm (thick layer), respectively, (ii) light extinction coefficient values, 1, 3 and 6 µm(-1), and (iii) percentage probability of light absorption p% in the range 10(-4)-10(-2). Results showed that the [Formula: see text] coefficient is high for phosphor grains in the submicron scale and for low light wavelengths. At higher wavelengths (above 650 nm), optical quanta follow approximately similar depths until interaction for grain diameter 500 nm and 1 µm. Regarding the variability of the refractive index, high variations of the [Formula: see text] coefficient occurred above 1.6. Furthermore, results derived from Monte Carlo modeling showed that high spatial resolution phosphors can be accomplished by increasing the [Formula: see text] parameter. More specifically, the FWHM was found to decrease (i.e. higher resolution): (i) 4.8% at 100 µm and (ii) 9.5%, at 300 µm layer thickness. This study attempted to examine the role of the optical absorption parameters on optical diffusion studies. A significant outcome of the present investigation was that the improvement of phosphor spatial resolution without decreasing the light collection efficiency too much can be better achieved by increasing the parameter [Formula: see text] rather than the parameter p%.

Light wavelength effects in submicrometer phosphor materials using Mie scattering and Monte Carlo simulation.

PURPOSE: Phosphor materials provide challenges to both fundamental research and breakthrough development of technologies in research areas. In recent years, with the development of science and technology in the field of materials, a number of physical or chemical synthesis methods have been developed and successfully used for the preparation of submicrometer-sized phosphors. The present paper provides a rigorous analysis of light diffusion capabilities of phosphor materials in submicrometer-scale investigating the effect of light wavelength. METHODS: Mie scattering theory and Monte Carlo simulation techniques were used for the optical diffusion performance providing numerical calculations. The Monte Carlo model included: (i) phosphor layers composed of different thickness (200, 500, 1000 µm) and (ii) different light wavelength values (420, 545, 610 nm) corresponding to different types of activators, such as Ce, Tb, and Eu activators, respectively. RESULTS: Based on Mie calculations, it was found that for low values of refractive index (e.g., 1.4) and for particle radius from 250 up to 500 nm no significant variations occurred on optical parameters. Monte Carlo simulations showed that the resolution increases as light wavelength decreases, respectively, however, this increase is more obvious at lower thickness values (i.e., at 200 µm). In particular, as light wavelength decreases from 610 down to 545 and 420 nm, the resolution increases 4.4% and 13.9%, respectively (at 200 µm layer thickness). In addition, as layer thickness increases from 200 up to 500 µm the resolution decreases 50.2% while an increase up to 1000 µm causes a decrease of 70.2% (at 420 nm light wavelength). CONCLUSIONS: The goal of the author's study was to investigate the optical diffusion characteristics of submicrometer phosphor materials using Mie scattering theory and Monte Carlo simulation. The present investigation indicated that a key parameter on resolution improvement was the amount of light loss which depends on the choice of activator and affects the lateral spreading.

Monte Carlo performance on the x-ray converter thickness in digital mammography using software breast models.

PURPOSE: In x-ray mammography, some of the components that play significant role to early diagnosis are the x-ray source, the breast composition as well as the composition of the x-ray converter. Various studies have previously investigated separately the influence of breast characteristics and detector configuration on the optimization of mammographic imaging systems. However, it is important to examine the combined effect of both components in improving the signal transfer properties in mammography systems of the mammograms. In the present study, the authors compared and evaluated x-ray converters using software breast models and realistic mammographic spectra in terms of: (a) zero-frequency detective quantum efficiency (DQE) and (b) sensitivity. The impact of x-ray converter thickness on contrast threshold (C(TH)) for observer assessment, based on the Rose model, was demonstrated as well. METHODS: Monte Carlo techniques were applied to simulate the x-ray interactions within the software breast phantoms and thereafter within the detective medium. Simulations involved: (a) two mammographic x-ray spectra: 28 kV Mo, 0.030 mm Mo, and 32 kV W, 0.050 mm Rh of different entrance surface air kerma (ESAK: 3-7 mGy), (b) realistic breast models (dense and fatty) and (c) x-ray converter materials most frequently considered in investigations on energy integrating digital mammography detectors: the Gd(2)O(2)S:Tb granular phosphor, the CsI:Tl structured phosphor, and the a-Se photoconductive layer. Detector material thickness was considered to vary in the range from 50 mg∕cm(2) up to 150 mg∕cm(2). RESULTS: The Monte Carlo study showed that: (a) the x-ray beam becomes less penetrating after passing through dense breasts leading to higher values of zero-frequency DQE of the x-ray imaging converters and improved C(TH) values in all cases considered, (b) W∕Rh target∕filter combination results in improved C(TH) values at higher ESAK values, and (c) a-Se shows higher zero-frequency DQE values than the phosphor-based converters, Gd(2)O(2)S:Tb and CsI:Tl. However, thicker layers of CsI:Tl could be comparable to a-Se layers achieving approximately 27.6% C(TH) improvement at a thickness of 150 mg∕cm(2). CONCLUSIONS: The present Monte Carlo investigation indicates that in the energy range employed in mammography, an upper limit, approximately 100 mg∕cm(2), should be considered in the development of thicker a-Se converters. On the other hand, above this thickness value, CsI:Tl converter could improve its imaging performance.

Overestimations in zero frequency DQE of x-ray imaging converters assessed by Monte Carlo techniques based on the study of energy impartation events.

PURPOSE: The performance of various x-ray converters, employed in medical imaging systems, has been widely examined by several methodologies (experimental, analytical, and Monte Carlo techniques). The x-ray converters most frequently employed in energy integrating digital radiology detectors are the Gd2O2S:Tb granular phosphor, the CsI:TI structured phosphor, and the a-Se photoconductor. The imaging characteristics of an x-ray converter are affected by its x-ray detection properties. However, various definitions of x-ray detection have been used in the literature, leading to different results for the quantum detection efficiency (QDE) for the same type of x-ray converter. For this reason, there is a need for accurate determination of the x-ray detection and, in particular, its relation to detector response. METHODS: The present article reports on the performance of the three aforementioned x-ray converters in terms of the QDE and the x-ray statistical factor Ix, and examines the effect of the x-ray detection, directly related to converter output signal, on the zero-frequency DQE. For the purposes of this study, Monte Carlo simulation was used to model the x-ray interactions within the x-ray converter. Simulations were carried out in the energy range from 10 keV up to 80 keV and considering two layers of different coating weights (50 and 100 mg/cm2). The prediction and comparison of zero-frequency DQE were based on two different approaches for x-ray detection, i.e., (a) fraction of interacting photons and (b) fraction of photons leading to energy deposition. In addition, the effect of energy deposition through Compton scattering events on the DQE values was estimated. RESULTS: Our results showed discrepancies between Monte Carlo techniques (based on energy deposition events) and analytical calculations (based on x-ray attenuation) on QDE. Discrepancies were found to range up to 10% for Gd2O2S:Tb (100 mg/cm2), 7.7% for CsI:T1 (50 mg/cm2), and 8.2% for a-Se (50 mg/cm2). Discrepancies were analyzed by examining the scattering effects (elastic and inelastic) within the converters and led to further analysis of scattering events on Ix, as well. Significant overestimations were found for both factors (QDE and I,) on the zero-frequency DQE. CONCLUSIONS: Considering that the highest overestimation was found in the thin layer (50 mg/cm2), Monte Carlo evaluation showed that the overestimation (%) between DQE values (based on either x-ray interacting events or energy impartation events) was more significant at 20 keV for CsI:T1 (approximately 2.1%), at 40 keV for Gd2O2S:Tb (approximately 8.1%), and finally at 60 and 80 keV for a-Se converter (approximately 4.8 and 8.2%, respectively). Finally, the overestimation due to the effect of Compton scattering on DQE was found more significant for the a-Se converter and especially above 40 keV reaching approximately the value of 8.9% at 80 keV.

Light emission efficiency and imaging performance of Gd2O2S: Eu powder scintillator under x-ray radiography conditions.

PURPOSE: To evaluate Gd2O2S:Eu powder phosphor as a radiographic image receptor and to compare it to phosphors often used in radiography. Gd2O2S:Eu is nonhygroscopic, emitting red light with decay time close to that of Gd2O2S:Tb. METHODS: The light intensity emitted per unit of x-ray exposure rate (absolute luminescence efficiency) was measured for laboratory prepared screens with coating thicknesses of 33.1, 46.4, 63.1, 78.3, and 139.8 mg/cm2 and tube voltages ranging from 50 to 140 kVp. Parameters related to image quality such as the modulation transfer function (MTF) and the detective quantum efficiency (DQE) were also experimentally examined. In addition, a previously validated Monte Carlo code was used to estimate intrinsic x-ray absorption and optical properties, as well as the MTF and the Swank factor (I) of the Gd2O2S:Eu scintillators. RESULTS: Gd2O2S:Eu light intensity was found higher than that of single CsI:T1 crystal for tube voltages up to 100 kVp. The MTF and the DQE were found to be comparable with those of Gd2O2S:Tb and CsI:T1 screens. MTF estimated by the Monte Carlo code was found very close to the experimental MTF values. Gd2O2S:Eu showed peak emission in the wavelength range 620-630 nm. Its emission spectrum was excellently matched to various optical detectors (photodiodes, photocathodes, CCDs, and CMOS) employed in flat panel detectors. CONCLUSIONS: Gd2O2S:Eu is an efficient phosphor potentially well suited to radiography and especially to some digital detectors sensitive to red light.

Experimental validation of a radiographic simulation code using breast phantom for X-ray imaging.

Computer models and simulations of X-ray imaging systems are becoming a very precious tool during the development and evaluation of new X-ray imaging techniques. To provide, however, a faithful simulation of a system, all components must be accurately modelled and tested, followed by verification through experimental measurements. This paper presents a validation study of the XRayImagingSimulator, an in-house developed X-ray imaging simulator, which is extensively used as a basic tool in carrying out complex breast imaging simulations. The approach followed compares results obtained via an experimental setup for breast phantom (CIRS 011A) imaging, using synchrotron radiation (SYRMEP beamline at ELETTRA), with those from its simulated setup under the same conditions. The study demonstrated a very good agreement between experimental and simulated images compared both in terms of subjective and objective criteria. The combination of the XRayImagingSimulator with our BreastSimulator provides a powerful tool for in silico testing of new X-ray breast imaging approaches.

The imaging performance of compact Lu2O3:Eu powdered phosphor screens: Monte Carlo simulation for applications in mammography.

In medical mammographic imaging systems, one type of detector configuration, often referred to as indirect detectors, is based on a scintillator layer (phosphor screen) that converts the x-ray radiation into optical signal. The indirect detector performance may be optimized either by improving the structural parameters of the screen or by employing new phosphor materials with improved physical characteristics (e.g., x-ray absorption efficiency, intrinsic conversion efficiency, emitted light spectrum). Lu2O3:Eu is a relatively new phosphor material that exhibits improved scintillating properties indicating a promising material for mammographic applications. In this article, a custom validated Monte Carlo program was used in order to examine the performance of compact Lu2O3:Eu powdered phosphor screens under diagnostic mammography conditions (x-ray spectra: 28 kV Mo, 0.030 mm Mo and 32 kV W, 0.050 mm Rh). Lu2O3:Eu screens of coating weight in the range between 20 and 40 mg/cm2 were examined. The Monte Carlo code was based on a model using Mie-scattering theory for the description of light propagation within the phosphor. The overall performance of Lu2O3:Eu powdered phosphor screens was investigated in terms of the (i) quantum detection efficiency, (ii) luminescence efficiency, (iii) compatibility with optical sensors, (iv) modulation transfer function, (v) the Swank factor, and (vi) zero-frequency detective quantum efficiency. Results were compared to the traditional rare-earth Gd2O2S:Tb phosphor material. The increased packing density and therefore the light extinction properties of Lu2O3:Eu phosphor were found to improve the x-ray absorption (approximately up to 21% and 16% at 40 mg/cm2 for Mo and W x-ray spectra, respectively), the spatial resolution (approximately 2.6 and 2.4 cycles/mm at 40 mg/cm2 for Mo and W x-ray spectra, respectively), as well as the zero-frequency detective quantum efficiency (approximately up to 8% and 18% at 20 mg/cm2 for Mo and W x-ray spectra, respectively) of the screens in comparison to the Gd2O2S: Tb screens. Data obtained by the simulations indicate that certain optical properties of Lu2O3:Eu make this material a promising phosphor which, under appropriate conditions, could be considered for use in x-ray mammography imagers.

The Monte Carlo evaluation of noise and resolution properties of granular phosphor screens.

The imaging performance of phosphor screens, used as x-ray detectors in diagnostic medical imaging systems, is affected by their both noise and resolution properties. Amplification and blurring processes are due to a sequence of conversion stages within the screen which contribute to fluctuations in the number and spatial distribution of the optical quanta recorded by the optical detector (e.g. film, television camera, CCD, etc). The purpose of this paper is to investigate the stochastic noise arising from granularity as well as the variation of spatial resolution of granular fluorescent screens in terms of the detector's structure. Using a custom-validated Monte Carlo model, the parameters of interest were evaluated for the widely used Gd(2)O(2)S:Tb phosphor material. We have studied the variations of (i) the modulation transfer function, (ii) the Swank factor and (iii) the zero-frequency detective quantum efficiency (DQE), under several conditions employed in conventional and digital mammography and radiology. Several evaluations are provided for the imaging metrics as a function of the x-ray energy (18 keV, 49 keV and 51 keV), phosphor coating weight (20 mg cm(-2), 34 mg cm(-2) and 60 mg cm(-2)), grain size (from 4 microm up to 13 microm) and packing density (from 50% up to 85%). It was found that screens of high packing density can combine high zero-frequency DQE with improved resolution properties. For a digital mammographic imaging system (34 mg cm(-2), 18 keV), a packing density of 85% can improve the spatial resolution of the screen by 1.6 cycles mm(-1) in comparison to that of 50% packing density. Similarly, for radiographic cases (60 mg cm(-2), 49 keV), the spatial resolution can be improved by 1.7 cycles mm(-1). The aforementioned findings provide the resolution benefits of using high packing density screens.

A theoretical model evaluating the angular distribution of luminescence emission in X-ray scintillating screens.

The aim of this study was to examine the angular distribution of the light emitted from radiation-excited scintillators in medical imaging detectors. This distribution diverges from Lambert's cosine law and affects the light emission efficiency of scintillators, hence it also affects the dose burden to the patient. In the present study, the angular distribution was theoretically modeled and was used to fit experimental data on various scintillator materials. Results of calculations revealed that the angular distribution is more directional than that predicted by Lambert's law. Divergence from this law is more pronounced for high values of light attenuation coefficient and thick scintillator layers (screens). This type of divergence reduces light emission efficiency and hence it increases the incident X-ray flux required for a given level of image brightness.