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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]

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: The aim of the present study was to examine the light emission efficiency of Lu2SiO5:Ce (LSO) powder scintillator under X-ray mammographic imaging conditions. Powder LSO scintillator has never been used in X-ray imaging. For the purposes of the present study, a 25mg/cm2 thick scintillating screen was prepared in our laboratory, by sedimentation of Lu2SiO5:Ce powder. Absolute luminescence efficiency measurements were performed within the range of X-ray tube voltages (22–49kVp) used in mammographic applications. Parameters related to X-ray detection, i.e. the energy absorption efficiency (EAE) and the quantum detection efficiency (QDE) were calculated. A theoretical model, describing radiation and light transfer, was employed to fit experimental data and to estimate values of the intrinsic conversion efficiency and the light attenuation coefficients of the screen. The spectral compatibility of the LSO powder scintillator to mammographic X-ray films and to various electronic optical detectors was determined by performing light emission spectrum measurements and by taking into account the spectral sensitivity of the optical detectors. Results in the voltage range used in mammography showed that Lu2SiO5:Ce powder scintillator has approximately 10% higher values of QDE and 4.5% higher values of EAE than Gd2O2S:Tb. [Copyright &y& Elsevier]

Abstract: The purpose of the present work was to model a modern X-ray detection system and to investigate the effect of the scintillator material on the detector''s output signal. The scintillators were used in the form of screens. The parameters investigated were the Modulation Transfer Function (MTF), the Detective Quantum Efficiency (DQE) and the Energy Absorption Efficiency (EAE). The results for some well-known scintillators (Y3Al5O12:Ce, Y2O3:Eu, ZnSCdS:Ag, Lu3Al5O7, CdWO4) are presented. Typical radiographic conditions were considered as input parameters. For simulation purposes, the intrinsic conversion efficiency (η c), the total number of optical photons produced per incident X-ray (m 0), the attenuation coefficients and other optical parameters of the scintillator materials, were taken as input data. The complete simulation procedure was performed in a specially designed Graphical User Interface (GUI). The results showed that the Y2O3:Eu scintillator presented similar behavior to that of ZnSCdS:Ag, exhibiting higher DQE at zero spatial frequencies. For higher frequencies, however, the DQE values of Lu3Al5O7 and CdWO4 prevailed. [Copyright &y& Elsevier]

Abstract: The recently developed Geant4 Application for Tomographic Emission (GATE) toolkit is a Monte Carlo simulation platform developed for PET and SPECT simulations and is freely distributed by the OpenGATE collaboration. GATE provides the ability of modelling time-dependent phenomena, such as geometry element movements and source decay kinetics, allowing the simulation of time curves under realistic acquisition conditions. The purpose of this paper was to validate a GATE model for the simulation of the Siemens PET Biograph™ 6 scanner. This three-dimensional GATE model simulated 24336 LSO (Lutetium Oxyorthosilicate) detectors grouped into 144 blocks in accordance with the vendor''s specifications. GATE results were compared with experimental data, obtained in accordance with the NEMA NU 2-2001 performance measurement protocol. The scatter phantom used by this protocol was also modelled within GATE, allowing us to simulate scatter fraction and count rate performance measurements. [Copyright &y& Elsevier]

Abstract: Geant4 application for tomographic emission (GATE) is a generic Monte Carlo simulation platform based on a general-purpose code GEANT4 and designed to simulate positron emission tomography (PET) and single photon emission tomography systems. Monte Carlo simulations are used in nuclear medicine to model imaging systems and develop and assess tomographic reconstruction algorithms and correction methods for improved image quantification. The purpose of this study is to validate two GATE models of the commercial available PET scanner HR+ and the PET/CT Biograph 2. The geometry of the system components has been described in GATE, including detector ring, crystal blocks, PMTs etc. The energy and spatial resolution of the scanners as given by the manufacturers have been taken into account. The GATE simulated results are compared directly to experimental data obtained using a number of NEMA NU-2-2001 performance protocols, including spatial resolution, sensitivity and scatter fraction. All the respective phantoms are precisely modeled. Furthermore, an approximate dead-time model both at the level of single and coincidence events was developed so that the simulated count rate curve can satisfactorily match the experimental count rate performance curve for each scanner In addition a software tool was developed to build the sinograms from the simulated data and import them into the software for tomographic image reconstruction where the reconstruction algorithm of FBP3DRP was applied. An agreement of less than 0.8mm was obtained between the spatial resolution of the simulated system and the experimental results. Also the simulated scatter fraction for the NEMA NU 2-2001 scatter phantom matched the experimental results to within 3% of measured values. Finally the ratio of the simulated sensitivities with sources radially offset 0 and 10cm from the central axis of each of the two scanners reaches an agreement of less than 1% between the simulated and experimental values. This simulation code will be used in a second phase in order to study scatter phenomena and motion artifacts. The simulation results will be used to optimize image reconstruction algorithms, with emphasis on dynamic PET studies. [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]

Abstract: The luminescence properties of Lu0.7Y0.3AlO3:Ce (LuYAP:Ce) and (Lu0.9,Y0.1)2SiO5:Ce (LYSO:Ce) crystals were studied for use in medical X-ray imaging. LuYAP:Ce and LYSO:Ce are single-crystal scintillators of high density, non-hygroscopic, high light yield and short decay time. The two cerium doped crystals have dimensions of 2×2×8mm3 with all surfaces polished. Evaluation was performed by determining the absolute luminescence efficiency (emitted light flux over incident X-ray exposure) in X-ray energies employed in general medical X-ray imaging (40–140kVp) and in mammographic X-ray imaging (22–49kV). Additionally, the light emission spectrum at various X-ray energies was measured in order to determine the spectral compatibility to optical photon detectors, incorporated in medical imaging systems. The optical characteristics, such as emission spectra, were investigated and important correlations with the scintillation properties were pointed out. The light emission performance of the two scintillation materials studied was found adequately high for medical X-ray imaging. [Copyright &y& Elsevier]