N. Kalyvas

Technological Educational Institute of Athens, Athínai, Attica, Greece

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Publications (44)48.13 Total impact

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    ABSTRACT: The aim of the present study was to investigate the absolute luminescence efficiency (AE) of mixed oxyorthosilicate (Lu,Gd)2SiO5:Ce (LGSO:Ce) single crystals, under X-ray irradiation. Six (Lu,Gd)2SiO5:Ce crystal samples, with dimensions of 3 × 3 × 5, 3 × 3 × 6, 3 × 3 × 10, 3 × 3 × 15, 10 × 10 × 10 and 10 × 10 × 20 mm3 were examined. The light emitted by the crystals, was evaluated by performing measurements of the AE under X-ray exposure conditions, with tube voltages ranging from 50 to 130 kV. Results were compared with previously published data for GSO:Ce and LSO:Ce crystals. The spectral compatibility of the (Lu,Gd)2SiO5:Ce crystal, with various existing optical detectors, was investigated after emission spectra measurements. Absolute efficiency was found maximum at 130 kVp for the 3 × 3 × 15 mm3 (Lu,Gd)2SiO5:Ce crystal (25.40 E.U). AE of the 10 × 10 × 10 mm3 (Lu,Gd)2SiO5:Ce crystal was found higher than both GSO:Ce and LSO:Ce crystals, in the whole X-ray tube range. The emission spectrum of (Lu,Gd)2SiO5:Ce is excellent matched with the spectral sensitivities of photocathodes and silicon photomultipliers often employed in radiation detectors. Considering the high luminescence efficiency values and the spectral compatibility with the various photodetectors, (Lu,Gd)2SiO5:Ce crystal could be considered for use in combined medical imaging detectors i.e. integrated PET/CT detectors.
    No preview · Article · Sep 2015 · Radiation Measurements
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    Preview · Article · Sep 2015 · Journal of Physics Conference Series
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    ABSTRACT: The aim of the present study was to assess image quality of PET scanners through a thin layer chromatography (TLC) plane source. The source was simulated using a previously validated Monte Carlo model. The model was developed by using the GATE MC package and reconstructed images obtained with the STIR software for tomographic image reconstruction, with cluster computing. The PET scanner simulated in this study was the GE DiscoveryST. A plane source consisted of a TLC plate, was simulated by a layer of silica gel on aluminum (Al) foil substrates, immersed in 18F-FDG bath solution (1MBq). Image quality was assessed in terms of the Modulation Transfer Function (MTF). MTF curves were estimated from transverse reconstructed images of the plane source. Images were reconstructed by the maximum likelihood estimation (MLE)-OSMAPOSL algorithm. OSMAPOSL reconstruction was assessed by using various subsets (3 to 21) and iterations (1 to 20), as well as by using various beta (hyper) parameter values. MTF values were found to increase up to the 12th iteration whereas remain almost constant thereafter. MTF improves by using lower beta values. The simulated PET evaluation method based on the TLC plane source can be also useful in research for the further development of PET and SPECT scanners though GATE simulations.
    No preview · Article · Sep 2015 · Journal of Physics Conference Series
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    ABSTRACT: Dual Energy imaging is a promising method for visualizing masses and microcalcifications in digital mammography. Currently commercially available detectors may be suitable for dual energy mammographic applications. The scope of this work was to theoretically examine the performance of the Radeye CMOS digital indirect detector under three low- and high-energy spectral pairs. The detector was modeled through the linear system theory. The pixel size was equal to 22.5μm and the phosphor material of the detector was a 33.9 mg/cm2 Gd2O2S:Tb phosphor screen. The examined spectral pairs were (i) a 40kV W/Ag (0.01cm) and a 70kV W/Cu (0.1cm) target/filter combinations, (ii) a 40kV W/Cd (0.013cm) and a 70kV W/Cu (0.1cm) target/filter combinations and (iii) a 40kV W/Pd (0.008cm) and a 70kV W/Cu (0.1cm) target/filter combinations. For each combination the Detective Quantum Efficiency (DQE), showing the signal to noise ratio transfer, the detector optical gain (DOG), showing the sensitivity of the detector and the coefficient of variation (CV) of the detector output signal were calculated. The second combination exhibited slightly higher DOG (326 photons per X-ray) and lower CV (0.755%) values. In terms of electron output from the RadEye CMOS, the first two combinations demonstrated comparable DQE values; however the second combination provided an increase of 6.5% in the electron output.
    No preview · Article · Sep 2015 · Journal of Physics Conference Series
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    ABSTRACT: Breast microcalcifications are mainly composed of calcite (CaCO3), calcium oxalate (CaC2O4) and apatite (a calcium-phosphate mineral form). Any pathologic alteration (carcinogenesis) of the breast may produce apatite. In the present simulation study, an analytical model was implemented in order to distinguish malignant and non-malignant lesions. The Calcium/Phosphorus (Ca/P) mass ratio and the standard deviation (SD) of the calcifications were calculated. The size of the calcifications ranged from 100 to 1000 μm, in 50 μm increments. The simulation was performed for hydroxyapatite, calcite and calcium oxalate calcifications. The optimum pair of energies for all calcifications was 22keV and 50keV. Hydroxyapatite and calcite calcifications were sufficiently characterized through their distinct confidence interval (99.7%, 3SD) values for calcifications sizes above 500 μm, while the corresponding sizes for hydroxyapatite and calcium oxalate characterization were found above 250 μm. Initial computer simulation results indicate that the proposed method can be used in breast cancer diagnosis, reducing the need for invasive methods, such as biopsies.
    No preview · Article · Sep 2015 · Journal of Physics Conference Series
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    ABSTRACT: Dual energy methods can suppress the contrast between adipose and glandular tissues in the breast and therefore enhance the visibility of calcifications. In this study, a dual energy method based on analytical modeling was developed for the detection of minimum microcalcification thickness. To this aim, a modified radiographic X-ray unit was considered, in order to overcome the limited kVp range of mammographic units used in previous DE studies, combined with a high resolution CMOS sensor (pixel size of 22.5 μm) for improved resolution. Various filter materials were examined based on their K-absorption edge. Hydroxyapatite (HAp) was used to simulate microcalcifications. The contrast to noise ratio (CNR tc ) of the subtracted images was calculated for both monoenergetic and polyenergetic X-ray beams. The optimum monoenergetic pair was 23/58 keV for the low and high energy, respectively, resulting in a minimum detectable microcalcification thickness of 100 μm. In the polyenergetic X-ray study, the optimal spectral combination was 40/70 kVp filtered with 100 μm cadmium and 1000 μm copper, respectively. In this case, the minimum detectable microcalcification thickness was 150 μm. The proposed dual energy method provides improved microcalcification detectability in breast imaging with mean glandular dose values within acceptable levels.
    Full-text · Article · Aug 2015 · Computational and Mathematical Methods in Medicine
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    ABSTRACT: During the last decades, radiation protection and dosimetry in medical X-ray imaging practice has been extensively studied. The purpose of this study was to measure secondary radiation in a conventional radiographic room, in terms of ambient dose rate equivalent H*(10) and its dependence on the radiographic exposure parameters such as X-ray tube voltage, tube current and distance. With some exceptions, the results indicated that the scattered radiation was uniform in the space around the water cylindrical phantom. The results also showed that the tube voltage and filtration affect the dose rate due to the scatter radiation. Finally, the scattered X-ray energy distribution was experimentally calculated. © The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.
    No preview · Article · Apr 2015 · Radiation Protection Dosimetry
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    ABSTRACT: Medical X-ray digital imaging systems such as mammography, radiography and computed tomography (CT), are composed from efficient radiation detectors, which can transform the X-rays to electron signal. Scintillators are materials that emit light when excited by X-rays and incorporated in X-ray medical imaging detectors. Columnar scintillator, like CsI:T1 is very often used for X-ray detection due to its higher performance. The columnar form limits the lateral spread of the optical photons to the scintillator output, thus it demonstrates superior spatial resolution compared to granular scintillators. The aim of this work is to provide an analytical model for calculating the MTF, the DQE and the emission efficiency of a columnar scintillator. The model parameters were validated against published Monte Carlo data. The model was able to predict the overall performance of CsI:Tl scintillators and suggested an optimum thickness of 300 μm for radiography applications.
    No preview · Article · Apr 2015 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
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    ABSTRACT: The purpose of the present study was to assess the information content of structured CsI:Tl scintillating screens, specially treated to be compatible to a CMOS digital imaging optical sensor, in terms of the information capacity (IC), based on Shannon’s mathematical communication theory. IC was assessed after the experimental determination of the Modulation Transfer Function (MTF) and the Normalized Noise Power Spectrum (NNPS) in the mammography and general radiography energy range. The CMOS sensor was coupled to three columnar CsI:Tl scintillator screens obtained from the same manufacturer with thicknesses of 130, 140 and 170 μm respectively, which were placed in direct contact with the optical sensor. The MTF was measured using the slanted-edge method while NNPS was determined by 2D Fourier transforming of uniformly exposed images. Both parameters were assessed by irradiation under the mammographic W/Rh (130, 140 and 170 μm CsI screens) and the RQA-5 (140 and 170 μm CsI screens) (IEC 62220-1) beam qualities. The detector response function was linear for the exposure range under investigation. At 70 kVp, under the RQA-5 conditions IC values were found to range between 2229 and 2340 bits/mm2. At 28 kVp the corresponding IC values were found to range between 2262 and 2968 bits/mm2. The information content of CsI:Tl scintillating screens in combination to the high resolution CMOS sensor, investigated in the present study, where found optimized for use in digital mammography imaging systems.
    No preview · Article · Feb 2015 · Radiation Measurements
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    ABSTRACT: Phosphor materials are widely used in X-ray medical imaging detector applications, coupled with suitable photoreceptors. Upon the most demanding imaging modality is X-ray mammography, since the best defense against breast cancer is its early detection. A material suitable as a mammographic detector should efficiently absorb X-ray photons and transforms them to optical photons, so as to minimize breast dose. The aim of the present study was to investigate the X-ray absorption efficiency and the absolute efficiency (AE), defined as the output optical photon power divided by the incident exposure, of Gd2O2S:Pr powder scintillator. For the purposes of this study, three scintillating screens with coating thicknesses, 34.1, 46.0 and 81.5 mg/cm2 respectively, were prepared in our laboratory from Gd2O2S:Pr powder (Phosphor Technology, Ltd) by sedimentation on silica substrates. The quantum detection efficiency (QDE), the energy absorption efficiency (EAE), the spectral matching factor and the absolute efficiency (AE) were evaluated for X-ray mammographic conditions. Furthermore theoretical models were utilized to investigate the optical photon transmission properties through the phosphor mass. Gd2O2S:Pr presented high X-ray absorption properties and good spectral compatibility with several photoreceptors. It may be utilized for X-ray mammographic imaging if is put in conjunction with a sensitive photoreceptor, so as to enhance Gd2O2S:Pr light emission properties.
    No preview · Article · Jan 2015 · Journal of Luminescence
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    ABSTRACT: Dual Energy imaging is a promising method for visualizing masses and microcalcifications in digital mammography. The advent of two X-ray energies (low and high) requires a suitable detector. The scope of this work is to determine optimum detector parameters for dual energy applications. The detector was modeled through the linear cascaded (LCS) theory. It was assumed that a phosphor material was coupled to a CMOS photodetector (indirect detection). The pixel size was 22.5 μm. The phosphor thickness was allowed to vary between 20mg/cm2 and 160mg/cm2 The phosphor materials examined where Gd2O2S:Tb and Gd2O2S:Eu. Two Tungsten (W) anode X-ray spectra at 35 kV (filtered with 100 μm Palladium (Pd)) and 70 kV (filtered with 800 pm Ytterbium (Yb)), corresponding to low and high energy respectively, were considered to be incident on the detector. For each combination the contrast- to-noise ratio (CNR) and the detector optical gain (DOG), showing the sensitivity of the detector, were calculated. The 40 mg/cm2 and 70 mg/cm2 Gd2O2S:Tb exhibited the higher DOG values for the low and high energy correspondingly. Higher CNR between microcalcification and mammary gland exhibited the 70mg/cm2 and the 100mg/cm2 Gd2O2S:Tb for the low and the high energy correspondingly.
    No preview · Article · Jan 2015 · Journal of Physics Conference Series
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    ABSTRACT: Dual energy mammography has the ability to improve the detection of microcalcifications leading to early diagnosis of breast cancer. In this simulation study, a prototype dual energy mammography system, using a CMOS based imaging detector with different X-ray spectra, was modeled. The device consists of a 33.91 mg/cm2 Gd2O2S:Tb scintillator screen, placed in direct contact with the sensor, with a pixel size of 22.5 μm. Various filter materials and tube voltages of a Tungsten (W) anode for both the low and high energy were examined. The selection of the filters applied to W spectra was based on their K- edges (K-edge filtering). Hydroxyapatite (HAp) was used to simulate microcalcifications. Calcification signal-to-noise ratio (SNRtc) was calculated for entrance surface dose within the acceptable levels of conventional mammography. Optimization was based on the maximization of SNRtc while minimizing the entrance dose. The best compromise between SNRtc value and dose was provided by a 35kVp X-ray spectrum with added beam filtration of 100μm Pd and a 70kVp Yb filtered spectrum of 800 μm for the low and high energy, respectively. Computer simulation results show that a SNRtc value of 3.6 can be achieved for a calcification size of 200 μm. Compared with previous studies, this method can improve detectability of microcalcifications.
    No preview · Article · Jan 2015 · Journal of Physics Conference Series
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    ABSTRACT: The aim of the present study was to obtain modified X-ray spectra, by using appropriate filter materials for use in applications such as dual energy X-ray imaging. K-edge filtering technique was implemented in order to obtain narrow energy bands for both dual-and single-kVp techniques. Three lanthanide filters (cerium, holmium, and ytterbium) and a filter outside lanthanides (barium), with low K-edge, were used to modify the X-ray spectra. The X-ray energies that were used in this work ranged from 60 to 100 kVp. Relative root mean square error (RMSE) and the coefficient of variation were used for filter selection. The increasing filter thicknesses led to narrower energy bands. For the dual-kVp technique, 0.7916 g/cm(2) Ho, 0.9422 g/cm(2) Yb, and 1.0095 g/cm(2) Yb were selected for 70, 80, and 90 kVp, respectively. For the single-kVp technique 0.5991 g/cm(2) Ce, 0.8750 g/cm(2) Ba, and 0.8654 g/cm(2) Ce were selected for 80, 90, and 100 kVp, respectively. The filtered X-ray spectra of this work, after appropriate modification, could be used in various X-ray applications, such as dual-energy mammography, bone absorptiometry, and digital tomosynthesis.
    Full-text · Article · Jan 2015 · Journal of Spectroscopy

  • No preview · Article · Dec 2014 · Physica Medica

  • No preview · Article · Dec 2014 · Physica Medica

  • No preview · Article · Dec 2014 · Physica Medica

  • No preview · Article · Dec 2014 · Physica Medica

  • No preview · Article · Dec 2014 · Physica Medica
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    ABSTRACT: 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.
    No preview · Article · Nov 2014 · Radiation Measurements
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    ABSTRACT: Luminescent materials are used as X-ray detectors of medical imaging systems. Out of a large variety of materials, Terbium (Tb)-activated phosphors and needle-like columnar structured CsI:Tl phosphors are currently the most widely used ones. Parameters commonly used to assess the imaging performance of luminescent materials are: the Quantum Detection Efficiency, the Luminescence Efficiency (LE), the Optical Spectral Distribution (OSD), the Modulation Transfer Function (MTF), the Noise Power Spectrum (NPS), the Detective Quantum Efficiency (DQE) and the Information Capacity (IC). The scope of the MISCIRLU project is an in depth theoretical and experimental analysis of the performance of scintillator materials as X-ray detectors. The theoretical analysis is performed through Mie scattering theory, Monte Carlo simulation and analytical modeling. The experimental analysis is performed through LE, OSD, MTF and NPS measurements. The initial results of the MISCIRLU project are focused in the theoretical analysis of the luminescent materials like the effect of the grain size, detector thickness, activator importance and scintillator crystal intrinsic conversion efficiency non-proportionality. In addition MTF and NPS have been evaluated via free software tools.
    Full-text · Conference Paper · Sep 2014

Publication Stats

148 Citations
48.13 Total Impact Points

Institutions

  • 2009-2015
    • Technological Educational Institute of Athens
      • Department of Medical Instruments Technology
      Athínai, Attica, Greece
  • 2014
    • University of Wroclaw
      • Faculty of Chemistry
      Vrotslav, Lower Silesian Voivodeship, Poland
  • 2008-2009
    • University of Patras
      • Department of Medical Physics
      Rhion, West Greece, Greece