I. Valais

Experimental and Theoretical Evaluation of a High Resolution CMOS Based Detector Under X-Ray Imaging Conditions

Fundamental imaging performance in terms of Modulation Transfer Function (MTF), Noise Power Spectrum (NPS) and Detective Quantum Efficiency (DQE) was investigated for a high resolution CMOS based imaging sensor. The device consists of a 33.91 mg/cm2 Gd2O2S:Tb scintillator screen, placed in direct contact with a CMOS photodiode array. The CMOS photodiode array, featuring 1200×1600 pixels with a pixel pitch of 22.5 μm, was used as an optical photon detector. In addition to the conventional frequency dependent parameters characterizing image quality, image information content was assessed through the application of information capacity (IC). The MTF was measured using the slanted-edge method to avoid aliasing while the Normalized NPS (NNPS) was determined by two-dimensional (2D) Fourier transforming of uniformly exposed images. Both measurements were performed under the representative radiation quality (RQA) settings, RQA-5 (70 kVp digital-radiography) and RQA-M2 (28 kVp digital-mammography) recommended by the International Electrotechnical Commission Reports 62220-1 and 62220-1-2 respectively. The DQE was assessed from the measured MTF, NPS and the direct entrance surface air-Kerma (ESAK) obtained from X-ray spectra measurement with a portable cadmium telluride (CdTe) detector. The ESAK values ranged between 11-87 μGy for RQA-5 and 6-40 μGy for RQA-M2. Additionally the output electrons per X-ray photon of the detector and its signal transfer characteristics were assessed via an analytical model, within the framework of the linear cascaded systems (LCS) theory. It was found that the detector response function was linear for the exposure ranges under investigation. Additionally our results showed that for the same RQA quality the output electrons per X-ray photon, as well as the measured and analytically predicted MTF, were not significantly affected by ESAK. MTF and DQE where found better compared to previously published data for other CCD and CMOS sensors, while the NNPS appeared to be comparable in the frequency range under investigation (0-10 cycles/mm).

Luminescence efficiency of Gd/sub 2/SiO/sub 5/:Ce scintillator under X-ray excitation

Gd/sub 2/SiO/sub 5/:Ce (GSO:Ce) is a high-Z, nonhygroscopic fast emitting scintillator used in detectors of positron emission tomography (PET) systems. The purpose of this study was to evaluate the luminescence efficiency of GSO:Ce scintillator under irradiation conditions employed in X-ray medical imaging, since findings may be of value in novel applications such as computed tomography breast imaging or modern fast image producing X-ray computed tomography. To this aim the absolute luminescence efficiency (emitted light flux over X-ray exposure), the spectral matching factor (compatibility to optical detectors), and the effective efficiency (the product of absolute efficiency and spectral compatibility) were determined for X-ray tube voltages ranging from 20 to 140 kV. The efficiency of GSO:Ce was found to increase with increasing X-ray tube voltage, while the GSO:Ce spectrum, peaking at 440 and 490 nm, was found compatible to most optical detectors (photodiodes, photocathodes, and charge-coupled devices).

Luminescence Emission Properties of

LYSO:Ce and LuYAP:Ce are single crystal non-hygroscopic scintillators of high density, high light yield and short decay time, which have been successfully used in small animal PET imagers. In the present study, the luminescence emission properties of (Lu0.9, Y0.1)2SiO5:Ce (LYSO:Ce) and (Lu0.7, Y0.3)AIO3:Ce (LuYAP:Ce) crystals were investigated for use in X-ray medical imaging. Both crystals had dimensions of 2 times 2 times 8 mm3, with all surfaces polished. Evaluation was performed by determining the X-ray luminescence efficiency (XLE) (emitted light energy flux over incident X-ray energy flux) and the detector optical gain (DOG) (emitted light photons per incident x-ray photon) in a wide range of X-ray energies employed in mammography (22-49 kVp) and in general X-ray imaging (50-140 kVp). Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. The emission spectrum under X-ray excitation was measured using an optical grating monochromator to determine the spectral compatibility to various optical photon detectors incorporated in medical imaging detectors. Optical characteristics such as transmission and absorption spectra were investigated in addition to the scintillation properties. The light emission performance of the two scintillation materials studied was found adequately high for X-ray imaging.

({\rm Lu},{\rm Y})_{2}{\rm SiO}_{5}

The present study is a comparative investigation of the luminescence properties of (Lu,Y)(2)SiO(5):Ce (LYSO:Ce), Lu(2)SiO(5):Ce (LSO:Ce), Gd(2)SiO(5):Ce (GSO:Ce) and (Bi(4)Ge(3)O(12)) BGO single crystal scintillators under medical X-ray excitation. All scintillating crystals have dimensions of 10 x 10 x 10 mm(3) are non-hygroscopic exhibiting high radiation absorption efficiency in the energy range used in medical imaging applications. The comparative investigation was performed by determining the absolute luminescence efficiency (emitted light flux over incident X-ray exposure) in X-ray energies employed in general X-ray imaging (40-140 kV) and in mammographic X-ray imaging (22-49 kV). Additionally, light emission spectra of crystals at various X-ray energies were measured, in order to determine the spectral compatibility to optical photon detectors incorporated in medical imaging systems and the overall efficiency (effective efficiency) of a scintillator-optical detector combination. The light emission performance of LYSO:Ce and LSO:Ce scintillators studied was found very high for X-ray imaging.

:Ce (LYSO:Ce) and

The aim of the present study was to investigate the imaging transfer characteristics and the luminescence efficiency (XLE) of Gd2O2S:Eu powder scintillator for use in X-ray mammography detectors. Gd2O2S:Eu emits in the red part of the visible spectrum, having very good spectral compatibility with optical sensors employed in digital imaging systems. Three Gd2O2S:Eu powder scintillating screens, with coating thicknesses 33.1, 46.4 and 63.1 mg/cm2 , were prepared in our laboratory. The imaging performance of these screens was assessed by experimental determination of the modulation transfer function (MTF), the noise transfer function (NTF) and the detective quantum efficiency (DQE) as well as a single index image quality parameter such as the information capacity (IC). A theoretical model, describing radiation and light transfer, was used to fit experimental MTF data. This has allowed the estimation of optical attenuation coefficients of the scintillator. In addition, a previously validated Monte Carlo code, based on the X-ray attenuation properties and on the Mie light scattering theory, was used to estimate the X-ray detection efficiency, the Swank factor and the zero frequency DQE of the Gd2O2S:Eu scintillator. Results showed that Gd2O2S:Eu exhibits high MTF and DQE values, which are comparable to those of a commercially employed Gd2O2S:Tb screen. In addition Gd2O2S:Eu shows high compatibility (effective gain) to CCDs and to recently introduced CMOS based detectors. Considering our image quality parameters and luminescence efficiency results, this material can potentially be considered for use in digital X-ray mammography detectors.

({\rm Lu},{\rm Y}){\rm AlO}_{3}

The aim of this study was to examine Gd2O2S:Eu (also known as GOS:Eu) powder scintillator under X-ray mammography imaging conditions. For this purpose, three scintillator screens with coating thicknesses of 33.1, 46.4 and 63.1 mg/cm2 were prepared in our laboratory by sedimentation of Gd2O2S:Eu powder. Light emission efficiency and optical emission spectra of the screens were measured under X-ray excitation using X-ray tube voltages (22-49 kVp) employed in mammography. Spectral compatibility with various optical photon detectors (photodiodes, photocathodes, charge coupled devices, films) and intrinsic conversion efficiency value (0.12) were determined by using emission spectrum data. In addition, parameters related to X-ray detection and energy absorption efficiency were calculated. Gd2O2S:Eu showed peak emission in the wavelength range 620-630 nm. The 63.1 mg/cm2 phosphor screen appeared with the maximum light emission efficiency. Due to its reddish emission spectrum, Gd2O2S:Eu showed excellent compatibility with the sensitivity of many currently used photodetectors and could be considered for application in X-ray imaging especially in various digital detectors.

:Ce (LuYAP:Ce) Single Crystal Scintillators Under Medical Imaging Conditions

The aim of the present work is to study the performance of scintillators currently used in PET and animal PET systems, under conditions met in radiation therapy and PET/CT imaging. The results of this study will be useful in applications where both CT and PET photons as well as megavoltage cone beam CT (MV CBCT) photons could be detected using a common detector unit. To this aim crystal samples of GSO, LSO, LYSO, LuYAP and YAP scintillators, doped with cerium (Ce+3) were examined under a wide energy range of photon energies. Evaluation was performed by determining the absolute luminescence efficiency (emitted light flux over incident X-ray exposure) in the energy range employed in X-ray CT, in Nuclear Medicine (70 keV up to 662 keV) and in radiotherapy 6 MV (approx. 2.0 MeV mean energy)-18 MV (approx. 4.5 MeV mean energy). Measurements were performed using an experimental set-up based on a photomultiplier coupled to a light integration sphere. The emission spectrum under X-ray excitation was measured, using an optical grating monochromator, to determine the spectral compatibility to optical photon detectors incorporated in medical imaging systems. Maximum absolute luminescence efficiency values were observed at 70 keV for YAP:Ce and LuYAP:Ce and at 140 keV for LSO:Ce, LYSO:Ce and GSO:Ce. Highest absolute efficiency between the scintillators examined was observed for LSO:Ce, followed by LYSO:Ce. The detector optical gain (DOG) exhibited a significant variation with the increase of energy between 70 keV to 2.0 MeV. All scintillators exhibited low compatibility when combined with GaAsP (G5645) photodetector.

A comparative study of the luminescence properties of LYSO:Ce, LSO:Ce, GSO:Ce and BGO single crystal scintillators for use in medical X-ray imaging.

The luminescence of (Lu,Y)2SiO5 : Ce (LYSO:Ce) and Gd2SiO5:Ce (GSO:Ce) crystals was studied for use in tomographic medical x-ray imaging. LYSO:Ce and GSO:Ce are high density (7.1 g/cm3 and 6.71 g/cm3 respectively), high atomic number (71 for Lu and 64 for Gd), non-hydroscopic, and short decay time (40 ns and 60 ns respectively) scintillators. Evaluation was performed by determining: 1) the luminescence efficiency (LE) (emitted light energy flux over incident x-ray energy flux) in x-ray energies employed in general x-ray imaging (40-140 kVp) and in mammographic x-ray imaging (22-49 kVp), 2) the light emission spectrum, determined at various x-ray energies (22-140 kVp), and 3) the spectral compatibility to optical photon detectors, incorporated in medical imaging systems. Both scintillation materials exhibited adequately high LE in the x-ray diagnostic energy range, with LYSO:Ces LE being distinctively higher. LYSO:Ce and GSO:Ce were found most compatible with the S-20 photocathode (0.9 for both materials) and adequately compatible to the amorphous silicon photodiode (0.74 for both materials), incorporated in many digital x-ray detectors

Evaluation of the Red Emitting

Gd2SiO5:Ce (GSO:Ce) is a high-Z, non-hygroscopic fast emitting scintillator used in detectors of some positron tomography systems. The purpose of this study was to evaluate the luminescence of GSO:Ce scintillator at lower photon energies employed in other fields of medical imaging (e.g. x-ray computed tomography, general x-ray imaging, low energy gamma ray imaging). To this aim the absolute luminescence efficiency (emitted light flux over x-ray exposure), the matching factor (spectral compatibility to optical detectors) and the effective efficiency (combination of absolute efficiency with spectral compatibility) were experimentally determined. Various x-ray tube voltages ranging from 20 to 140 kV (employed in x-ray mammography, general radiography, and computed tomography) were used. In addition the behaviour of GSO:Ce under low energy gamma irradiation could be estimated from these data. Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. The GSO:Ce optical emission spectrum was measured under x-ray excitation using an optical grating monochromator. The efficiency of GSO:Ce was found to increase with increasing x-ray tube voltage, while the GSO:Ce spectrum, peaking at 440 and 490 nm, was found compatible with most optical detectors (photodiodes, photocathodes, charge coupled devices). However the spectral compatibility (0.77) and the effective efficiency were found highest for GSO coupled to photodiode

{\rm Gd}_{2}{\rm O}_{2}{\rm S}\!\!:\!\!{\rm Eu}

Objectives. In this work, a simple technique to assess the image quality characteristics of the postprocessed image is developed and an easy to use figure of image quality (FIQ) is introduced. This FIQ characterizes images in terms of resolution and noise. In addition information capacity, defined within the context of Shannons information theory, was used as an overall image quality index. Materials and Methods. A digital mammographic image was postprocessed with three digital filters. Resolution and noise were calculated via the Modulation Transfer Function (MTF), the coefficient of variation, and the figure of image quality. In addition, frequency dependent parameters such as the noise power spectrum (NPS) and noise equivalent quanta (NEQ) were estimated and used to assess information capacity. Results. FIQs for the “raw image” data and the image processed with the “sharpen edges” filter were found 907.3 and 1906.1, correspondingly. The information capacity values were 60.86 × 103 and 78.96 × 103 bits/mm2. Conclusion. It was found that, after the application of the postprocessing techniques (even commercial nondedicated software) on the raw digital mammograms, MTF, NPS, and NEQ are improved for medium to high spatial frequencies leading to resolving smaller structures in the final image.