Experimental investigation of the optical signal, gain, signal-to-noise ratio and information content characteristics of X-ray phosphor screens

In this study, the quality of medical images produced by X-ray phosphor screens is described by a model based on the light generation and emission properties of the phosphor material. Combined detector gain (CDG), modulation transfer function (MTF), detective quantum efficiency (DQE) and information capacity (IC) of the screens were expressed and evaluated as functions of emitted optical fluence, incident X-ray fluence and emitted optical spectrum. Phosphor screens with coating thickness ranging from 21 mg/cm² to 137 mg/cm² were prepared in the laboratory and were irradiated by X-rays with X-ray voltages from 50 to 140 kVp. Experimental data were obtained in both transmission and reflection modes of measurement (light emission from both screen sides). Results showed that most image quality parameters (CDG, MTF, DQE) depend strongly on phosphor screen thickness. CDG and DQE also depend on X-ray tube voltage. However, the total quantity of information (IC), which depends on both the incident X-ray fluence and phosphor material type, was not found to be significantly affected by phosphor thickness. Received: 7 September 2000 / Revised version: 8 January 2001 / Published online: 27 April 2001     

Measurement of the luminescence properties of Gd2O2S:Pr,Ce,F powder scintillators under X-ray radiation

The aim of the present study was to investigate the absolute luminescence efficiency of gadolinium oxysulfide powder scintillators, doped with praseodymium, cerium and fluorine (Gd₂O₂S:Pr,Ce,F). Gd₂O₂S:Pr,Ce,F is a non-hygroscopic material, emitting green light with short decay time. For the purposes of this study, two scintillating screens with coating thicknesses 35.7 and 71.2 mg/cm², were prepared in our laboratory from Gd₂O₂S:Pr,Ce,F powder (Phosphor Technology, Ltd) by sedimentation on silica substrates. The light emitted by the phosphors under investigation was evaluated by performing measurements of the absolute efficiency under X-ray exposure conditions with tube voltages ranging from 50 to 130 kV. Results were compared with previously published data for Gd₂O₂S:Eu screens. The spectral compatibility of Gd₂O₂S:Pr,Ce,F with various existing optical detectors, was investigated after emission spectra measurements. Particle size and morphology parameters of the Gd₂O₂S:Pr,Ce,F powder phosphor were verified via scanning electron microscope (SEM) micrographs. Absolute efficiency was found to maximize at 60 kVp for the 71.2 mg/cm² and at 90 kVp for the 35.7 mg/cm² Gd₂O₂S:Pr,Ce,F screens. Absolute efficiency of the 71.2 mg/cm² Gd₂O₂S:Pr,Ce,F screen was higher in the whole X-ray tube voltage range showing potential interest for non-CT medical imaging (i.e. dental radiology) or industrial digital radiography.     

Light emission efficiency and imaging performance of Y3Al5O12: Ce (YAG: Ce) powder screens under diagnostic radiology conditions

In this study Y₃Al₅O₁₂: 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. Y₃Al₅O₁₂: Ce powder was used to prepare various test screens (33–166 mg/cm²). 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/cm² 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/cm² screens. The light emission efficiency and imaging performance of Y₃Al₅O₁₂: 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.     

Imaging performance and light emission efficiency of Lu<Subscript>2</Subscript> SiO<Subscript>5</Subscript>:Ce (LSO:Ce) powder scintillator under X-ray mammographic conditions

The aim of the present study was to measure the imaging transfer characteristics and the luminescence efficiency (XLE) of a Lu₂SiO₅:Ce (LSO:Ce) powder scintillator for use in X-ray mammography detectors. An LSO:Ce powder scintillating screen, with a coating thickness of 25 mg/cm², was prepared in our laboratory. The imaging performance of the screen was assessed by experimental determination of the modulation transfer function (MTF) and the detective quantum efficiency (DQE) as well as single index image quality parameters such as noise equivalent pass band (Ne) and informational efficiency (n _I). A theoretical model, describing radiation and light transfer, was fitted to experimental MTF values in order to estimate optical properties of the scintillator. Screen irradiation was performed under exposure conditions employed in mammographic applications (27 kVp, 63 mAs). MTF was determined by the square wave response function (SWRF) method. Results showed that LSO:Ce exhibits high MTF and DQE values, which are comparable to those of the commercially used Gd₂O₂S:Tb. Considering our image quality parameters and luminescence efficiency results as well as the fast response of the LSO:Ce scintillator screen (40 ns), this material can be considered for use in X-ray mammographic detectors.     

Evaluating Zn2SiO4:Mn phosphor for use in medical imaging radiation detectors

2 SiO₄:Mn phosphor was evaluated for use in radiation detectors of medical imaging systems. Zn₂SiO₄:Mn was used in the form of laboratory-prepared fluorescent layers (screens) with coating weights from 18 to 150 mg/cm². The phosphor was excited to luminescence by low-energy X-raysusing X-raytube voltages ranging from 15 to 50 kVp. The number of emitted optical photons per incident X-rayquantum was thus determined for various X-rayenergies and phosphor coating weights. The optical emission spectrum was also measured and it was used to evaluate the spectral compatibility of Zn₂SiO₄:Mn with radiographic films, photocathodes and the Si photodiode. Finally, phosphor optical properties were estimated by fitting a theoretical model to experimental data. Results showed that Zn₂SiO₄:Mn is more efficient for low-energy X-rays. Its intrinsic conversion efficiency was found equal to 0.08, which is comparable to that of actually used phosphors. Zn₂SiO₄:Mn was also adequately compatible with orthochromatic films and the ES-20 photocathode, thus being appropriate for low-voltage radiography and fluoroscopy. Received: 31 July 1998/Accepted: 3 August 1998     

A study of X-ray luminescence and spectral compatibility of europium-activated yttrium-vanadate (YVO4: Eu) screens for medical imaging applications

The X-ray luminescence efficiency of laboratory-prepared YVO₄: Eu screens and their spectral compatibility to common optical detectors were studied under medical fluoroscopy conditions. YVO₄: Eu screens were prepared by sedimentation and with different coating thickness. Luminescence efficiency of the YVO₄: Eu screens was measured at various X-ray tube voltages (50–250 kVp) and for screens of different coating thicknesses (20–180 mg/cm²). Spectral response was also measured and spectral matching factors between the YVO₄: Eu screens and some common optical detectors (photocathodes, photodiodes, photographic emulsion) were calculated. Experimental results on efficiency were fitted by formulas of the theoretical model developed by Hamaker and Ludwig in order to determine phosphor intrinsic X-ray-to-light conversion efficiency and intrinsic optical characteristics, such as coefficients related to light scattering and absorption. Although the luminescence efficiency of YVO₄: Eu screens was found to be relatively low (3–11 M s/mR m²), the matching factor of YVO₄ : Eu screens with some red sensitive optical detectors was excellent, of the order of 0.96. High spectral compatibility may indicate that YVO₄: Eu scintillators could be used in medical image detectors.     

Degradation of Y2SiO5:Ce phosphor powders

The degradation of the cathodoluminescence (CL) intensity of cerium-doped yttrium silicate (Y₂SiO₅:Ce) phosphor powders was investigated for possible application in low voltage field emission displays (FEDs). Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and CL spectroscopy were used to monitor changes in the surface chemical composition and luminous efficiency of commercially available Y₂SiO₅:Ce phosphor powders. The degradation of the CL intensity for the powders is consistent with a well-known electron-stimulated surface chemical reaction (ESSCR) model. It was shown with XPS and CL that the electron stimulated reaction led to the formation of a luminescent silicon dioxide (SiO₂) layer on the surface of the Y₂SiO₅:Ce phosphor powder. XPS also indicated that the Ce concentration in the surface layer increased during the degradation process and the formation of CeO₂ and CeH₃ were also part of the degradation process. The CL intensity first decreased until about 300 C cm⁻² and then increased due to an extra peak arising at a wavelength of 650 nm.     

Measurement of the (Gd,La)2O2S:Tb phosphor efficiency for x-ray imaging applications

(Gd,La)₂O₂S:Tb phosphor performance was tested under x-ray excitation for medical imaging applications with tube voltages from 70 to 180 kVp and phosphor coating weights from 20 to 240 mg/cm². The emission spectrum of (Gd,La)₂O₂S:Tb was also measured and it was centered at 549 nm. (Gd,La)₂O₂S:Tb was found efficient in a wide range of x-ray tube voltages and adequately compatible with photocathodes, photodiodes, and orthochromatic films currently used in x-ray medical imaging.     

Effect of excitation level on the photoluminescence of barium thiogallate activated with europium and cerium ions

The photoluminescence of barium thiogallates doped with Eu²⁺, Ce³⁺, and Eu²⁺ + Ce³⁺ ions is studied over a wide range of excitation levels (10^–3–10⁶ W/cm²). Introduction of 3 at.% Eu and 3 at.% Ce instead of 5 at.% Eu into a BaGa₂S₄ matrix doubles the luminescence quantum yield of the phosphor. Doped BaGa₂S₄ exhibits a high linearity in its luminescence intensity as a function of excitation level (a constant efficiency) up to 2·10⁴ W/cm² for excitation pulse durations of 8 ns, which corresponds to cw pumping at a power density of about 5·10² W/cm² in terms of the concentration of excited ions. It is shown that using BaGa₂S₄:Eu,Ce along with the “yellow” phosphor of a Nichia NS6L083 LED may increase its color rendering index from 0.64 to 0.80 with no reduction in its luminous efficiency.     

Combustion synthesis of YAG:Ce and related phosphors

YAG:Ce is an important phosphor having applications in various fields ranging from solid state lighting to scintillation detectors. YAG phosphors doped with activators are mainly synthesized by solid state reaction techniques that require high sintering temperatures (above 1500°C) to eliminate YAM and YAP phases. Though several soft chemical routes have been explored for synthesis of YAG, most of these methods are complex and phase pure materials are not obtained in one step, but prolonged annealing at temperatures around 1000°C or above become necessary. One step combustion synthesis of YAG:Ce³⁺ and related phosphors carried out at 500°C furnace temperature is reported here. Activation with Ce³⁺ could be achieved during the synthesis without taking recourse to any post-combustion thermal treatment. LEDs prepared from the combustion synthesized YAG:Ce³⁺, exhibited properties comparable to those produced from the commercial phosphor.     

Growth and luminescent properties of (Lu–Y)AlO3:Ce single crystalline films

The work is dedicated to the development of scintillating screens based on single crystalline films (SCF) of Ce doped Lu–Y–Al perovskites grown by the liquid phase epitaxy method. We show in this work that Lu_1−xY_xAP:Ce SCF with very good structural and optical perfection can be grown from the PbO-based flux onto YAP substrates in all range of x-values. We also found the Lu_1−xY_xAP:Ce SCF can be crystallized by LPE method from the lead free BaO–B₂O₃–BaF₂ flux onto YAP substrates in the range of x values up to 0.4 formula units.     

Studying the luminescence efficiency of Lu<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu nanophosphor material for digital X-ray imaging applications

Scintillator materials are widely used in X-ray medical imaging detector applications, coupled with available photoreceptors like radiographic film or photoreceptors suitable for digital imaging like a-Si, charge-coupled devises (CCD), complementary metal-oxide-semiconductors (CMOS) and GaAs). In addition, scintillators can be utilized in non-medical imaging detectors such as industrial detectors for non-destructive testing (NDT) and detectors used for security purposes (i.e. airport luggage control). Image quality and dose burden in the above applications is associated with the amount of optical photons escaping the scintillator as well as the amount of optical photons captured by the photoreceptor. The former is characterized by the scintillator efficiency and the latter by the spectral matching between the emission spectrum of the scintillator and the spectral response of the photoreceptor. Recently, a scintillator material, europium-activated lutetium oxide (Lu₂O₃:Eu), has shown improved scintillating properties. Lu₂O₃:Eu samples of compact nanocrystalline non-agglomerated powder were developed in our laboratory using homogeneous precipitation from a water-toluene solution in the presence of polyvinyl alcohol as a surfactant. In order to test their light-emission properties, experimental measurements under the excitation of X-ray spectra with X-ray tube voltages between 50 kVp and 140 kVp were performed. This range of applied voltages is appropriate for X-ray radiology, NDT and security applications. Lu₂O₃:Eu was evaluated with respect to output yield and spectral compatibility of digital imaging photoreceptors (CCD-based, CMOS-based, amorphous silicon a:Si flat panels, ES20 and GaAs). High light yield and spectral compatibility increase the performance of the medical detector and reduce the dose burden to the personnel involved. In addition a theoretical model was used to determine the values for the Lu₂O₃:Eu optical photon light propagation parameters. The inverse diffusion length was found to be equal to 33 cm²/g. In addition Lu₂O₃:Eu was found to match well with several photoreceptors capable of digital imaging (i.e. GaAs).     

Degradation of phosphors under cathode-ray excitation

A systematic analysis of phosphor degradation is presented. An increase of the self-absorption of the luminescence, destruction of luminescence centres, a decreased energy flow to these centres, and increased energy losses in the phosphor host lattice - both in the bulk and at the surface - can be distinguished as causes of the deterioration of the external radiant efficiency of phosphors. Simple measurements are suggested to disentangle the contribution of each of these parameters to the observed degradation effects. As an illustration degradation effects of Zn₂SiO₄ : Mn, Y₂SiO₅ : Ce, Sr₂Al₆O₁₁ : Eu and a Tb³⁺ activated borate glass are analyzed.     

Shunting effect in explosive electron emission

An explanation is given to the results of an experiment on studying the explosive electron emission in a wire-cathode diode where a strongly nonuniform energy deposition into the wire material was observed using an X pinch as a radiation source for projection x-ray imaging. The specific input energy, contrary to the well-known observations, was not a maximum at the wire end, i.e., in the region of the strongest electric field, and the wire explosion occurred in the bulk, distant from the end. This is accounted for by the contribution of the wire side surface to explosive electron emission and by the gas desorption from the wire intensely heated by a current of density 10⁸ A/cm². Thus, the space between anode and cathode (wire end) is bridged by two plasmas: one generated due to the explosive electron emission from the wire side surface and the other produced from the desorbed gas.     

Growth and spectroscopic properties of RE, Mn:YAP (RE=Yb and Ce) photorefractive crystals

RE, Mn:YAP (RE=Yb and Ce) crystals with dimension of Φ 25×60 mm were successfully grown by the Czochralski method. The spectroscopic properties of RE, Mn:YAP (RE=Yb and Ce) crystals before and after γ-irradiation were investigated at room temperature. The results show that the content of Mn⁴⁺ ions was increased with the Yb³⁺ ions co-doping, but decreased by Ce³⁺ ions co-doping. Thermoluminescence (TL) spectra of the crystals indicate three steps of recombination, and the probable recombination processes were discussed.     

Luminescent and scintillation properties of YAG:Dy and YAG:Dy,Ce single crystalline films

The paper is devoted to investigation of the luminescent properties of Dy³⁺ and Dy³⁺-Ce³⁺ doped single crystalline films (SCF) grown by LPE method from PbO–B₂O₃ flux. We have found that the YAG:Dy and YAG:Dy,Ce SCFs possess bright cathodoluminescence in the visible range and good scintillation figure of merit. For this reason LPE grown YAG:Dy and YAG:Dy,Ce SCF are proposed for different applications, namely, as cathodoluminescence screens or screens for microimaging. The Dy³⁺ co-doping can be also proposed for improvement of the scintillation efficiency of the Ce³⁺ doped garnet compounds in the SCF form due to Dy³⁺→ Ce³⁺ energy transfer and removing the trap related centers in the above RT range.     

Ce3+,Mn2+激活氟磷酸钙的合成及其形成机理

合成了发光效率较高、光衰较小的黄色光致发光材料——Ce3+,Mn2+激活的氟磷酸钙(FAP:Ce3+,Mo2+).研究了各种工艺条件对该材料的发光效率、发光光谱及老化性能的影响.结果表明,最佳的灼烧温度在900℃左右;弱还原气氛或保护气氛有利于得到性能较好的材料;原料的化学剂量比M a/Mp,对材料的发光效率和发光光谱影响不大,但对老化速率影响较大,原料中F量应高于正常化学剂量比近一倍.通过对材料形成过程的热分析(差热分析和热失重分析),发现FAP:Ce3+,Mn2+的形成过程与普通卤粉差异很大,研究表明,FAP:Ce2+,Mn2+的形成主要是由650℃～800℃时发生的气相输运过程而引起的.     

Enhanced luminescence and degradation of SiO2:Ce,Tb powder phosphors prepared by a sol-gel process

Enhanced green photoluminescence and cathodoluminescence (CL) from Tb³⁺ ions due to co-doping with Ce³⁺ ions were observed from SiO₂:Ce,Tb powder phosphors prepared by a sol-gel technique. Blue emission from the Ce³⁺ ions was completely suppressed by Tb co-doping, presumably due to energy transfer from Ce³⁺ to Tb³⁺. In addition, the green CL intensity from SiO₂:Ce,Tb degraded by ∼50% when the powders were irradiated for 10 h with a 2 keV, 54 mA/cm² beam of electrons in an ultra-high vacuum chamber containing either 1×10⁻⁸ or 1×10⁻⁷ Torr O₂. Desorption of oxygen from the surface was observed during the decrease of CL intensity. The mechanisms for energy transfer from Ce³⁺ ions to Tb³⁺ ions to enhance the green luminescence, and mechanisms for desorption of oxygen from the phosphor surface that would result in decreased CL intensity are discussed.     

Packing density and optical properties of crt screens

Relations are derived which can be used to determine the thickness of crt screens and to determine the average size and packing density of the phosphor grains on the basis of light transmission by the screens. An experimental study was made for two phosphors: ZnS(Ag) and Al₂O₃(Ce).Translated from IzvestiyaVysshikh Uchebnykh Zavedenii Fizika, No. 10, pp. 98–103, October, 1970.     

Cathodoluminescence characteristics of particles and film of (Y, Zn)2O3:Eu phosphor prepared by spray pyrolysis

Eu-doped Y₂O₃ particles with spherical shape and fine size were prepared by spray pyrolysis. The cathodoluminescence of Y₂O₃:Eu³⁺ powder was optimized by substituting small amount of zinc atoms in place of yttrium sites. As a result, the optimized (Y, Zn)₂O₃:Eu³⁺ phosphor showed 60% improved cathodoluminescence compared with Y₂O₃:Eu³⁺ particles. The prepared (Y, Zn)₂O₃:Eu³⁺ phosphor had spherical shape and 0.726 μm in mean size. Using these particles, the thickness of the phosphor film was controlled by varying the phosphor loading. The brightness and luminous efficiency of phosphor films prepared were monitored with varying the accelerating voltage ranges from 4 to 14 kV. The dependency of the luminous efficiency on the accelerating voltage was very sensitive to the phosphor loading. As increasing the accelerating voltage from 4 to 14 kV, the brightness of phosphor films prepared was monotonically increased from 200 to 1085 cd/cm²_, but the saturation in the luminous efficiency appeared at 10 kV. The highest efficiency was achieved when the number of phosphor-particles layer was about 3. More details about the luminous efficiency and brightness were discussed with changing the phosphor loading.