Ioannis Kandarakis

Bio: Ioannis Kandarakis is an academic researcher from University of the West. The author has contributed to research in topics: Scintillator & Light emission. The author has an hindex of 27, co-authored 206 publications receiving 2416 citations. Previous affiliations of Ioannis Kandarakis include American Hotel & Lodging Educational Institute & Technological Educational Institute of Athens.

Evaluating x-ray detectors for radiographic applications: a comparison of ZnSCdS:Ag with Gd2O2S:Tb and Y2O2S:Tb screens.

Abstract: ZnSCdS:Ag was evaluated as a radiographic image receptor and was compared with Gd2O2S:Tb and Y2O2S:Tb phosphors often used in radiography. The evaluation of a radiographic receptor was modelled as a three-step process: (i) determination of light output intensity as related to the input radiation dose, (ii) determination of visible light characteristics with respect to radiographic optical detectors, and (iii) determination of image information transfer efficiency. The light intensity emitted per unit of x-ray exposure rate was measured and theoretically calculated for laboratory prepared screens with coating thicknesses from 20 to 220 mg cm-2 and tube voltages from 50 to 250 kVp. ZnSCdS:Ag light intensity was higher than that of Gd2O2S:Tb or Y2O2S:Tb for tube voltages less than 70 and 80 kVp respectively. ZnSCdS:Ag displayed the highest x-ray to light conversion efficiency (0.207) and had optical properties close to those of Gd2O2S:Tb and Y2O2S:Tb, and its emission spectrum was well matched to optical detectors. The image information transfer properties described by the modulation transfer function, the quantum noise transfer function, and the detective quantum efficiency were calculated for both general radiographic and mammographic conditions and were found to be intermediate between those of Gd2O2S:Tb and Y2O2S:Tb screens. Conclusively, ZnSCdS:Ag is an efficient phosphor well suited to radiography.

Modeling granular phosphor screens by Monte Carlo methods.

Abstract: The intrinsic phosphor properties are of significant importance for the performance of phosphor screens used in medical imaging systems. In previous analytical-theoretical and Monte Carlo studies on granular phosphor materials, values of optical properties, and light interaction cross sections were found by fitting to experimental data. These values were then employed for the assessment of phosphor screen imaging performance. However, it was found that, depending on the experimental technique and fitting methodology, the optical parameters of a specific phosphor material varied within a wide range of values, i.e., variations of light scattering with respect to light absorption coefficients were often observed for the same phosphor material. In this study, x-ray and light transport within granular phosphor materials was studied by developing a computational model using Monte Carlo methods. The model was based on the intrinsic physical characteristics of the phosphor. Input values required to feed the model can be easily obtained from tabulated data. The complex refractive index was introduced and microscopic probabilities for light interactions were produced, using Mie scattering theory. Model validation was carried out by comparing model results on x-ray and light parameters (x-ray absorption, statistical fluctuations in the x-ray to light conversion process, number of emittedmore » light photons, output light spatial distribution) with previous published experimental data on Gd{sub 2}O{sub 2}S:Tb phosphor material (Kodak Min-R screen). Results showed the dependence of the modulation transfer function (MTF) on phosphor grain size and material packing density. It was predicted that granular Gd{sub 2}O{sub 2}S:Tb screens of high packing density and small grain size may exhibit considerably better resolution and light emission properties than the conventional Gd{sub 2}O{sub 2}S:Tb screens, under similar conditions (x-ray incident energy, screen thickness)« less

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

Abstract: 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).

Validation of a GATE model for the simulation of the Siemens biograph™ 6 PET scanner

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.

Light emission efficiency and imaging performance of Gd2O2S: Eu powder scintillator under x-ray radiography conditions.

Abstract: Purpose: To evaluate Gd 2 O 2 S : Eu powder phosphor as a radiographic image receptor and to compare it to phosphors often used in radiography. Gd 2 O 2 S : Eu is nonhygroscopic, emitting red light with decay time close to that of Gd 2 O 2 S : Tb . Methods: The light intensity emitted per unit of x-ray exposure rate (absolute luminescence efficiency) was measured for laboratory prepared screens with coating thicknesses of 33.1, 46.4, 63.1, 78.3, and 139.8 mg / cm 2 and tube voltages ranging from 50 to 140 kVp. Parameters related to image quality such as the modulation transfer function(MTF) and the detective quantum efficiency (DQE) were also experimentally examined. In addition, a previously validated Monte Carlo code was used to estimate intrinsic x-ray absorption and optical properties, as well as the MTF and the Swank factor ( I ) of the Gd 2 O 2 S : Eu scintillators. Results: Gd 2 O 2 S : Eu light intensity was found higher than that of single CsI:Tl crystal for tube voltages up to 100 kVp. The MTF and the DQE were found to be comparable with those of Gd 2 O 2 S : Tb and CsI:Tl screens. MTF estimated by the Monte Carlo code was found very close to the experimental MTF values. Gd 2 O 2 S : Eu showed peak emission in the wavelength range 620–630 nm. Its emission spectrum was excellently matched to various optical detectors (photodiodes, photocathodes, CCDs, and CMOS) employed in flat panel detectors. Conclusions: Gd 2 O 2 S : Eu is an efficient phosphor potentially well suited to radiography and especially to some digital detectors sensitive to red light.

Scintillation detectors for x-rays

Abstract: Recent research in the field of phosphor and scintillator materials and related detectors is reviewed. After a historical introduction the fundamental issues are explained regarding the interaction of x-ray radiation with a solid state. Crucial parameters and characteristics important for the performance of these materials in applications, including the employed measurement methods, are described. Extended description of the materials currently in use or under intense study is given. Scintillation detector configurations are further briefly overviewed and selected applications are mentioned in more detail to provide an illustration.

GATE V6: a major enhancement of the GATE simulation platform enabling modelling of CT and radiotherapy

Abstract: GATE (Geant4 Application for Emission Tomography) is a Monte Carlo simulation platform developed by the OpenGATE collaboration since 2001 and first publicly released in 2004. Dedicated to the modelling of planar scintigraphy, single photon emission computed tomography (SPECT) and positron emission tomography (PET) acquisitions, this platform is widely used to assist PET and SPECT research. A recent extension of this platform, released by the OpenGATE collaboration as GATE V6, now also enables modelling of x-ray computed tomography and radiation therapy experiments. This paper presents an overview of the main additions and improvements implemented in GATE since the publication of the initial GATE paper (Jan et al 2004 Phys. Med. Biol. 49 4543–61). This includes new models available in GATE to simulate optical and hadronic processes, novelties in modelling tracer, organ or detector motion, new options for speeding up GATE simulations, examples illustrating the use of GATE V6 in radiotherapy applications and 0031-9155/11/040881+21$33.00 © 2011 Institute of Physics and Engineering in Medicine Printed in the UK 881

The Essential Physics Of Medical Imaging

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Fatty infiltration of liver in hyperlipidemic patients.

Abstract: Hyperlipidemia is a known risk factor for fatty infiltration of the liver, a condition that can progress to cirrhosis and liver failure. The objectives of this study were to document the prevalence of fatty infiltration in the livers of hyperlipidemic patients and to identify the predictor variables associated with this condition. Over an 18-month recruitment period, clinical, biochemical, and radiologic assessments were performed in a cross-sectional manner in 95 adult patients referred to an urban hospital-based lipid clinic for evaluation and management of hyperlipidemia. The mean (±sd) age of the patients was 55 ± 13 years. Forty-eight (51%) were male. Fifty-two patients (55%) had hypercholesterolemia, 25 (26%) severe hypertriglyceridemia, 14 (15%) mixed hyperlipidemia, and 4 (4%) moderate hypertriglyceridemia. Obesity and diabetes were present in 36 (38%) and 12 (12%) of cases, respectively. A total of 61 (64%) patients had elevated liver enzyme tests. The most common enzyme abnormalities were an elevated serum ALT in 45 (47%) and GGT in 43 (45%) of patients. Ultrasound findings revealed diffuse fatty liver in 47 patients (50%), of which 21 cases (22%) were mild, 18 (19%) moderate, and 8 (9%) severe. The majority of patients with hypercholesterolemia [35/52 (67%)] had normal ultrasounds, whereas severe hypertriglyceridemia and mixed hyperlipidemia were frequently associated with radiologic evidence of fatty liver (odds ratios 5.9 and 5.1 respectively, P < 0.01). Independent predictors of fatty liver were; AST (P = 0.001), hyperglycemia (P = 0.02), and age (P = 0.04). In a model incorporating known risk factors for fatty liver, diabetes was the only risk factor other than hypertriglyceridemia that was significantly associated with fatty infiltration. No such effect was seen with age, gender, obesity, or alcohol consumption. In conclusions, the results of this study indicate that ultrasonographic evidence of fatty infiltration of the liver is evident in approximately 50% of patients with hyperlipidemia. Hypertriglyceridemia is the lipid profile most often associated with this condition. Serum AST values, hyperglycemia, and age independently predict the presence of fatty infiltration, while hypertriglyceridemia and diabetes are the only risk factors that significantly increase the risk of fatty infiltration in hyperlipidemic patients.