I. E. Seferis

Light emission efficiency of Lu2O3:Eu nanophosphor scintillating screen under x-ray radiographic conditions

Powder phosphors scintillators are used in indirect digital radiography as x-ray to light converters coupled to electronic optical sensors (photodiodes, CCDs, CMOS). Recently, nanophosphors have been reported to have enhanced luminescence efficiency. The purpose of the present study was to evaluate Lu2O3:Eu nanophosphor as a candidate for digital medical imaging applications. Lu2O3:Eu was employed in the form of a 30.2 mg/cm2 powder screen with 50 nm grain size and 5% Eu concentration. Both the nanophosphor material and the screen were prepared in our laboratories. Parameters such as the Absolute Efficiency-AE (light energy flux over exposure rate), the Luminescence Efficiency- XLE (Light energy flux over incident x-ray energy flux), Detector Quantum Gain-DQG (optical quanta emitted per incident x-ray quantum) and the light spectral compatibility to electronic optical sensors (Effective Efficiency) were investigated under x-ray excitation in the radiographic energy range. Results were compared with previously published data for a 33.1 mg/cm2 Gd2O2S:Eu conventional phosphor screen. It was found that Lu2O3:Eu nanophosphor has higher AE and XLE by a factor of 1.32 and 1.37 on average, respectively, in the whole radiographic energy range. DQG was also found higher in the energy range from 50 kVp to 100 kVp and comparable thereafter. Effective efficiency was found with high values for electronic optical sensors such as CCDs and CMOS, due to the high spectral compatibility with the upper visible wavelength range. These results indicate that Lu2O3:Eu nanophosphor could potentially be considered for applications in digital x-ray radiography detectors.

X-ray imaging resolution of phosphor screens prepared with different grains size and shape of granular Lu2O3:Eu

The influence of the grain shape and size on spatial resolution (ranging from nano to micro scale) of various Lu2O3:Eu phosphor screens was investigated. All screens were prepared using the sedimentation method. Three screens were prepared with spherical grains and sizes 50 nm, 200 nm and 5 μm, whilst two screens with rod-like shape grains and sizes 500 nm and 1-8 μm. All screens were coupled to a high resolution CMOS digital imaging sensor (Remote RadEye HR) consisting of 1200 x 1600 pixels with 22.5 μm pixel pitch. Experiments were performed under radiographic conditions, using 70 kVp tube voltage and 63 mAs tube load. Spatial resolution was assessed utilizing the Modulation Transfer Function (MTF). It was found that the influence of the grains shape on imaging performance was more crucial than the grain size. The rod-like grains showed very poor spatial resolution. The influence of grains size between 50 nm 200 nm and 5 μm was negligible on MTF values.

Imaging performance of a thin Lu2O3:Eu nanophosphor scintillating screen coupled to a high resolution CMOS sensor under X-ray radiographic conditions: comparison with Gd2O2S:Eu conventional phosphor screen

The purpose of the present study was to experimentally evaluate the imaging characteristics of the Lu2O3:Eu nanophosphor thin screen coupled to a high resolution CMOS sensor under radiographic conditions. Parameters such as the Modulation Transfer Function (MTF), the Normalized Noise Power Spectrum (NNPS) and the Detective Quantum Efficiency (DQE) were investigated at 70 kVp under three exposure levels (20 mAs, 63 mAs and 90 mAs). Since Lu2O3:Eu emits light in the red wavelength range, the imaging characteristics of a 33.3 mg/cm2 Gd2O2S:Eu conventional phosphor screen were also evaluated for comparison purposes. The Lu2O3:Eu nanophosphor powder was produced by the combustion synthesis, using urea as fuel. A scintillating screen of 30.2 mg/cm2 was prepared by sedimentation of the nanophosphor powder on a fused silica substrate. The CMOS/Lu2O3:Eu detector`s imaging characteristics were evaluated using an experimental method proposed by the International Electrotechnical Commission (IEC) guidelines. It was found that the CMOS/Lu2O3:Eu nanophosphor system has higher MTF values compared to the CMOS/Gd2O2S:Eu sensor/screen combination in the whole frequency range examined. For low frequencies (0 to 2 cycles/mm) NNPS values of the CMOS/Gd2O2S:Eu system were found 90% higher compared to the NNPS values of the CMOS/Lu2O3:Eu nanophosphor system, whereas from medium to high frequencies (2 to 13 cycles/mm) were found 40% higher. In contrast with the CMOS/ Gd2O2S:Eu system, CMOS/Lu2O3:Eu nanophosphor system appears to retain high DQE values in the whole frequency range examined. Our results indicate that Lu2O3:Eu nanophosphor is a promising scintillator for further research in digital X-ray radiography.

Light Emission Efficiency of Gd3Al2Ga3O12:Ce (GAGG:Ce) Single Crystal Under X-ray Radiographic Conditions

Inorganic scintillators, in either powder or crystal form, are used in indirect digital radiography as X-ray to light converters coupled to electronic optical sensors (photodiodes, CCDs, CMOS). A recent developed single crystal scintillator with high density, fast scintillation response and high light yield is the Ce doped Gadolinium Aluminum Gallium Garnet (Gd3Al2Ga3O12:Ce or GAGG:Ce). The purpose of the present study was to evaluate GAGG:Ce single crystal scintillator under general radiographic imaging conditions. The two GAGG:Ce crystals used in this study were supplied by Furukawa Co Ltd with dimensions 10×10×10 mm3 and 10×10×20 mm3. Parameters such as the Absolute Luminescence Efficiency-AE (light energy flux over exposure rate) and the light spectral compatibility to electronic optical sensors (Effective Efficiency) were investigated under X-ray excitation in the radiographic energy range from 50 to 125 kVp. Results showed that the absolute efficiency of GAGG:Ce scintillator crystals increased with X-ray tube voltage. The emission spectrum of scintillator examined in the present study is well matched with the spectral sensitivities of the optical photon detectors often employed in radiation detectors. Taking these results into account GAGG:Ce crystal scintillator could potentially be considered for further research for use in X-ray medical imaging.

Modeling noise properties of a high resolution CMOS detector for X-ray digital mammography

A theoretical model based on Linear Cascaded Systems (LCS) theory was developed in order to study the noise properties of a commercially available high resolution CMOS sensor (RadEye CMOS). The parameters studied were the Normalized Noise Power Spectrum (NNPS) and the Noise Transfer Function (NTF). The modeling was applied to digital mammography conditions. It considered the physical properties of the scintillator incorporated into RadEye CMOS as well as the probability of X-ray absorption directly in the photoreceptor. Furthermore it took into account the pixel pitch and the thickness of the photoreceptor. The model was compared with experimental results obtained from literature. The NTF curves were I good agreement with the experimentally determined. NNPS curves deviations were attributed to the effect of the digitization procedure as well as the non-linear behavior of CMOS detector.