Search

Phosphor materials are used in medical imaging combined with radiographic film or other photodetectors. Cerium (Ce3+) -doped scintillators are of particular interest for medical imaging, because of their very fast response. YAP:Ce scintillator-based image detectors have already been evaluated in single-crystal form and under conditions of positron emission tomography and synchrotron or γ-ray irradiation. Furthermore, YAP:Ce phosphor has been evaluated in conjunction with radiographic films. The present work reports experimental and theoretical data concerning the light output absolute luminescence efficiency (AE) of the YAP:Ce screens under irradiation conditions employed in medical X-ray projection imaging (i.e., in diagnostic radiology). projection imaging (i.e., in diagnostic radiology). YAP:Ce phosphor screens with surface densities ranging between 53 and 110 mg/cm2 were prepared by sedimentation on fused silica substates in our laboratory. The resulted surface density of the screens was determined by dividing the phosphor mass deposited on the screen surface with the area of the surface. Additionally this work addresses the imaging performance of YAP:Ce by estimation of the detective quantum efficiency (DQE), i.e., the square of the signal to noise ratio transfer. Absolute efficiency was found to decrease with X-ray tube voltage for for YAP:Ce phosphor. The highest experimental efficiency was obtained for the 53.7 mg/cm2 and 88.0 mg/cm2 YAP:Ce screens. The highest DQE value was found for the 88.0 mg/cm2 screen irradiated at 60 kVp. [ABSTRACT FROM AUTHOR]

Abstract: X-ray absorption and X-ray fluorescence properties of medical imaging scintillating screens were studied by Monte Carlo methods as a function of the incident photon energy and screen-coating thickness. The scintillating materials examined were Gd2O2S, (GOS) Gd2SiO5 (GSO) YAlO3 (YAP), Y3Al5O12 (YAG), LuSiO5 (LSO), LuAlO3 (LuAP) and ZnS. Monoenergetic photon exposures were modeled in the range from 10 to 100keV. The corresponding ranges of coating thicknesses of the investigated scintillating screens ranged up to 200mgcm−2. Results indicated that X-ray absorption and X-ray fluorescence are affected by the incident photon energy and the screen''s coating thickness. Regarding incident photon energy, this X-ray absorption and fluorescence was found to exhibit very intense changes near the corresponding K edge of the heaviest element in the screen''s scintillating material. Regarding coating thickness, thicker screens exhibited higher X-ray absorption and X-ray fluorescence. Results also indicated that a significant fraction of the generated X-ray fluorescent quanta escape from the scintillating screen. This fraction was found to increase with screen''s coating thickness. At the energy range studied, most of the incident photons were found to be absorbed via one-hit photoelectric effect. As a result, the reabsorption of scattered radiation was found to be of rather minor importance; nevertheless this was found to increase with the screen''s coating thickness. Differences in X-ray absorption and X-ray fluorescence were found among the various scintillators studied. LSO scintillator was found to be the most attractive material for use in many X-ray imaging applications, exhibiting the best absorption properties in the largest part of the energy range studied. Y-based scintillators were also found to be of significant absorption performance within the low energy ranges. [Copyright &y& Elsevier]

Abstract: The purpose of the present study was to evaluate, under X-ray medical imaging conditions, the X-ray luminescence efficiency (XLE) and the optical quantum gain (OQG) of the Gd2SiO5:Ce scintillator in single crystal form, suitable for tomographic applications. Intrinsic physical properties and light emission characteristics of the Gd2SiO5:Ce scintillator, were also studied. Both experimental and Monte Carlo techniques were used. Various X-ray tube voltages (40–140kV), currently employed in X-ray imaging applications, were used. XLE was found to vary slowly with X-ray tube voltage from (0.021±0.003) to (0.017±0.003). OQG varied from (317±18) to (466±23) light photons per incident X-ray. These values were adequately high for imaging applications using the particular energy range. Additionally, it was found by Monte Carlo simulations that for crystal thicknesses higher than 0.5cm both XLE and OQG reached saturation levels, indicating that higher thickness crystals are of no practical use in X-ray medical imaging. [Copyright &y& Elsevier]

In this study YAlO: Ce powder scintillator was evaluated for use in X-ray imaging detectors. This phosphor, also known as YAG: Ce scintillator or P-46 phosphor, is a non-hygroscopic, emitting green light with very short decay time. These properties are very attractive for X-ray imaging. YAlO: Ce powder was used to prepare various test screens (33-166 mg/cm). Absolute luminescence efficiency measurements were performed for various X-ray tube voltages (50-130 kVp). In addition parameters related to image quality such as the modulation transfer function and the detective quantum efficiency were examined. A theoretical model, describing radiation and light transfer, was employed to fit experimental data and to estimate values of optical parameters. Absolute efficiency was found to decrease with X-ray tube voltage. Highest efficiency was obtained for the 107 mg/cm screen. Light attenuation coefficients were close to those of green emitting rare earth scintillators. At low spatial frequencies the detective quantum efficiency was high for the 107-166 mg/cm screens. The light emission efficiency and imaging performance of YAlO: Ce was not better than currently employed scintillators. However due to its very fast response and high spectral compatibility to optical sensors it may be considered for use in digital imaging detectors. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The aim of the present study was to evaluate the imaging performance of Lu2SiO5:Ce (LSO:Ce) powder scintillator for use in X-ray detectors used in mammography. LSO:Ce scintillator is a high efficiency, fast emitting material, which in single-crystal form is used in positron emission tomography detectors. A scintillating screen, with a coating thickness of 25mg/cm2, was prepared in our laboratory from commercially available LSO:Ce powder (Phosphor Technology Ltd.). The imaging performance of the screen was assessed by experimental determination of the modulation transfer function (MTF) and the noise transfer function (NTF). Experimental MTF values were compared to data obtained by a custom Monte Carlo simulation program. Screen irradiation was performed under exposure conditions employed in mammographic applications (27kVp, 63mAs). MTF was determined by the Square Wave Response Function (SWRF) method whereas NTF was estimated by Noise Power Spectrum (NPS) measurements, under uniform screen irradiation. Our results showed that LSO:Ce exhibits high MTF, which is comparable to that of the commercially used Gd2O2S:Tb powder scintillator. Considering our MTF results and the fast response of LSO:Ce scintillator screen (40ns), this material can be considered for use in X-ray mammographic detectors. [Copyright &y& Elsevier]

Abstract: This work investigates the effect of the energy-weighting technique on the signal to noise ratio (SNR) response of X-ray imaging detectors. So far in the literature all scintillation-detector characteristics (detection efficiency, conversion efficiency, light-attenuation effects, etc) that degrade image quality have been ignored. A theoretical evaluation of the scintillator''s SNR output was carried out. An algorithm was produced to describe the variation of the weighting factor, and SNR, with respect to the anode material (Mo or W), in a particular energy range (25–40keV), typical for mammography, using two different phantoms. Results show that under non-ideal conditions the ratio of the weighted SNR to the original SNR appears to be increasing from values that are close to unity, and under specific conditions, can reach values up to 30. For the further investigation of this method, a more complex, simulated computed tomography breast imaging system was modeled and studied for various parameters such as breast software phantoms, scintillation materials and reconstruction filters. [Copyright &y& Elsevier]

Abstract: The aim of the present study was to evaluate the imaging performance of YAP:Ce powder scintillating screens under exposure conditions employed in diagnostic radiology (50–140kV). Various screens were prepared in our laboratory from YAP: Ce powder (Phosphor Technology, Ltd.), with coating thickness ranging from 53 to 110mg/cm2. The imaging performance of the screens was assessed by experimental determination of the modulation transfer function (MTF) and the noise transfer function (NTF). MTF was determined by the edge spread function (ESF) method while NTF was estimated by noise power spectrum (NPS) measurements after uniform screen irradiation. In addition, parameters related to overall image quality, such as the signal-to-noise ratio transfer (MTF/NTF), were estimated. MTF curves were affected by the beam hardening effects caused by the patient simulating 20mm thick aluminum phantom. Under these conditions MTF values were found to increase with the mean X-ray photon energy. A similar effect was observed for NTF curves. Results were compared with data obtained on CsI:Tl scintillator. Taking into consideration the very fast response of YAP:Ce, these data may be of interest in designing X-ray imaging detectors. [Copyright &y& Elsevier]

Abstract: The aim of this study was to examine Y3Al5O12:Ce (also known as YAG:Ce) powder scintillator under X-ray imaging conditions. This material shows a very fast scintillation decay time and it has never been used in X-ray medical imaging. In the present study various scintillator layers (screens) with coating thickness ranging from 13 to 166mg/cm2 were prepared in our laboratory by sedimentation of Y3Al5O12: Ce powder. Optical emission spectra and light emission efficiency (spectrum area over X-ray exposure) of the layers were measured under X-ray excitation using X-ray tube voltages (80–120kVp) often employed in general medical radiography and fluoroscopy. Spectral compatibility with various optical photon detectors (photodiodes, photocathodes, charge coupled devices, films) and intrinsic conversion efficiency values were determined using emission spectrum data. In addition, parameters related to X-ray detection, energy absorption efficiency and K-fluorescence characteristic emission were calculated. A theoretical model describing radiation and light transfer through scattering media was used to fit experimental data. Intrinsic conversion efficiency and light attenuation coefficients were derived through this fitting. Y3Al5O12:Ce showed peak emission in the wavelength range 530–550nm. The light emission efficiency was found to be maximum for the 107mg/cm2 layer. Due to its “green” emission spectrum, Y3Al5O12:Ce showed excellent compatibility (of the order of 0.9) with the sensitivity of many currently used photodetectors. Taking into account its very fast response Y3Al5O12:Ce could be considered for application in X-ray imaging especially in various digital detectors. [Copyright &y& Elsevier]