Biomedical subjects

D Cavouras

Publications and source records attributed to D Cavouras.

At least 19 recordsLinked to original sources

PubMed2000

Assessment of the gain transfer function of phosphors for application in medical imaging radiation detectors.

OBJECTIVE: to study various phosphors used in detectors of medical imaging systems by the gain transfer function (GTF), defined in terms of X-ray luminescence efficiency, light spectrum and modulation transfer function. MATERIALS AND METHODS: four phosphor materials, La(2)O(2)S:Tb, Y(2)O(2)S:Tb, Y(2)O(2)S:Eu and Y(2)O(3):Eu were used in the form of fluorescent layers prepared in the laboratory. The GTF was determined at 30 kVp and 80 kVp X-ray tube voltages for various phosphor coating weights. RESULTS: La(2)O(2)S:Tb, which was the highest density and effective atomic number phosphor used, was found to exhibit the best GTF performance at 80 kVp. At 30 kVp, the yttrium based phosphors were found of increased performance. This is mainly due to the proximity of the X-ray energy to the K-absorption edge of yttrium at 17 keV. Europium activated phosphors were found to perform very well when combined with the red sensitive film and the silicon photodiode. CONCLUSION: The GTF may be a useful method for comparing and selecting phosphor materials for use in various medical imaging applications.

Diagnostic Imaging↗

PubMed2000

Assessing the information content of phosphor produced medical images: application to Zn2SiO4:Mn phosphor.

In this study a method to assess the information content of medical images produced by phosphors is described. The optical signal emitted by the phosphor after X-ray excitation, the detective quantum efficiency (DQE), expressing the signal-to-noise ratio (SNR) transfer efficiency, and the information capacity were experimentally determined. The method was based on light flux and modulation transfer function (MTF) measurements and was used to assess the imaging performance of the Zn2SiO4:Mn phosphor. The latter was employed in the form of laboratory prepared phosphor layers (test screens). Results showed that high values for optical signal emission and DQE were obtained for medium thickness phosphor layers (56 and 89 mg/cm2) at 20 kVp X-ray tube voltage. The information capacity was found to decrease continuously with phosphor coating weight.

Biophysical Phenomena↗

PubMed1999

A method for determining the information capacity of x-ray imaging scintillator detectors by means of luminescence and modulation transfer function measurements.

A method to determine the information capacity of x-ray phosphor screens used in the detectors of medical imaging systems is described. Information capacity was determined via x-ray luminescence efficiency (XLE), modulation transfer function (MTF) and emission spectrum measurements. The method was applied to laboratory prepared screens from commonly employed phosphor materials. The screen coating weight varied from 50 mg cm-2 to 140 mg cm-2. Results indicated that information capacity decreased with screen coating thickness but also depended on intrinsic phosphor properties (density, effective atomic number, intrinsic conversion efficiency, light wavelength). The Gd2O2S:Tb phosphor, exhibiting high density and effective atomic number, was found to be superior to La2O2S:Tb and Y2O2S:Tb.

Electronic Data Processing↗

PubMed1999

Evaluating scintillators used in radiation detectors of medical imaging systems by the effective fidelity index method.

OBJECTIVE: The performance of medical X-ray image receptors depends: (1) on the scintillator light emission efficiency; and (2) on the compatibility of the scintillator light spectrum with the spectral sensitivity of the light detector (film, photocathode, or photodiode), employed in conjunction with the scintillator. In this study, a scintillator performance measure, the effective fidelity index (EFI), is defined as function of both the scintillator light emission efficiency and spectral compatibility. MATERIALS AND METHOD: CsI:Na, Gd2O2S:Tb and La2O2S:Tb scintillators were employed in the form of phosphor screens prepared in our laboratory with various coating thicknesses. The screens were irradiated with X-rays employing tube voltages ranging between 50-120 kVp. RESULTS: The EFI performance of CsI:Na was found to increase with screen coating thickness and it was best when combined with the orthochromatic film or the ES/20 photocathode. Gd2O2S:Tb showed peak EFI performance at 70 mg/cm2 coating thickness and it was well combined with the light detectors considered. CONCLUSION: In accordance with our results, CsI:Na may be employed in radiography when adequately protected against humidity. Gd2O2S:Tb suitability for conventional imaging was verified and it was found that it may be useful in all types of digital imaging. La2O2S:Tb could also be used in digital detectors of imaging applications demanding medium X-ray tube voltages.

Gamma Cameras↗

PubMed1999

A method to evaluate the performance of X-ray imaging scintillators by means of the brightness-sharpness index (BSI).

PURPOSE: To propose an image quality index, the brightness-sharpness index (BSI), for assessing the quality of the image produced by phosphors of medical imaging detectors. MATERIAL AND METHODS: BSI was evaluated by experimental X-ray luminescence and modulation transfer function measurements. BSI was determined for a number of test phosphor screens prepared from Gd2O2S:Tb, La2O2S:Tb, and Y2O2S:Tb phosphor materials. The screens covered a wide range of coating thicknesses from 50 to 150 mg/cm2 and measurements were performed for X-ray tube voltages between 50 and 120 kVp. RESULTS: Gd2O2S:Tb phosphor exhibited higher brightness and sharpness, as compared to the other phosphor materials, for all screens and X-ray tube voltages used. Best Gd2O2S:Tb performance was observed for thin screens and high tube voltages. La2O2S:Tb exhibited higher BSI values than Y2O2S:Tb for medium and high tube voltages. CONCLUSION: Results showed that phosphor materials of high X-ray detection and X-ray-to-light conversion properties exhibit high BSI values indicating that BSI may provide a means of phosphor performance evaluation for imaging applications.

Humans↗

PubMed1999

Evaluating phosphors for use in X-ray image detectors by the effective performance index (EPI) method: application to Eu3+ activated yttrium based materials.

Phosphor materials used in X-ray image receptors were evaluated by the effective performance index (EPI). EPI describes the dependence of image quality on X-ray to light energy conversion and light diffusion processes. EPI was experimentally determined by means of X-ray luminescence (XLE) and MTF measurements performed on Y2O2S:Eu, Y2O3:Eu, and YVO4:Eu phosphors. The spectral compatibility of these materials with optical detectors (films, photocathodes, photodiodes) was also determined. Highest EPI values were obtained for Y2O2S:Eu-GaAs combination at mammographic energies. All phosphors could be of use in digital X-ray imaging being adequately compatible to silicon photodiodes employed in digital detectors.

Europium↗

PubMed1998

Europium-activated phosphors for use in X-ray detectors of medical imaging systems.

Three europium-activated phosphors Y2O2S:Eu, Y2O3:Eu, and YVO4:Eu emitting red light were studied to investigate their suitability for radiographic cassettes or digital imaging systems. Screens were prepared from phosphor powders with various coating thicknesses by sedimentation. To assess phosphor light producing efficiency in relation to patient dose, each screen was X-rayed using 40-120 kVp and the number of light photons emitted per X-ray incident was experimentally and theoretically evaluated. Additionally, the capability of the emitted light to sensitize films or to generate electrons in silicon photodiodes used in digital imaging systems was examined. Y2O2S:Eu screens were most efficient in light emission, and when combined with either red sensitive films or Si photodiodes, they were found superior to Y2O3:Eu or YVO4:Eu screens in film grain or electron signal generation. In many cases they were also found superior to terbium-activated phosphors. Provided that several problems related to industrial production (special dyes, reflective backing, crossover effects) are dealt with, those europium-activated screens could be employed in low-tube-voltage radiographic applications.

Europium↗

PubMed1998

An investigation of the imaging characteristics of the Y2O2S:Eu3+ phosphor for application in X-ray detectors of digital mammography.

Y2O2S:Eu laboratory prepared screens were evaluated as mammographic image receptors and were compared to similarly prepared screens of Gd2O2S:Tb and Y2O2S:Tb phosphor materials, often used in X-ray imaging detectors. The evaluation was performed by determining the Modulation Transfer Function (MTF) and the spatial frequency dependent Detective Quantum Efficiency (DQE). Y2O2S:Eu exhibited higher DQE values at low frequencies and given its good spectral matching with digital optical detectors, it may be appropriate for use in X-ray digital mammography.

Europium↗

PubMed1998

Distance learning in mammographic digital image processing.

The potential of interactive multimedia and Internet technologies is investigated with respect to the implementation of a distance learning system in medical imaging. The system is built according to a client-server architecture, based on the Internet infrastructure, composed of server nodes conceptually modelled as World Wide Web (WWW) sites. Sites are implemented by integration and customization of available components. The system evolves around network-delivered interactive multimedia courses and network-based tutoring, which constitute its main learning features. This potential has been demonstrated by means of an implemented system, validated with digital image processing content, specifically image enhancement. Image enhancement methods are theoretically described and applied on mammograms. Emphasis is given in the interactive presentation of the effects of algorithm parameters on images. The system end-user access depends on available bandwidth, so high speed access can be achieved via LAN or local ISDN connections.

Biomedical Engineering↗

PubMed1998

An experimental method to determine the effective luminescence efficiency of scintillator-photodetector combinations used in X-ray medical imaging systems.

The scintillator effective luminescence efficiency, which may be defined in terms of the scintillator's X-ray luminescence efficiency and the scintillator-photodetector spectral matching and geometrical configuration, was studied for various X-ray imaging applications. Four scintillator materials Gd2O2S:Tb, Y2O2S:Tb, ZnSCdS:Ag and CsI:Na were used to prepare test screens. They were evaluated in relation to various photodetectors used in X-ray imaging, such as radiographic films, photocathodes, and photodiodes. Effective luminescence efficiency was determined for a range of X-ray tube voltages (50-140 kVp) by measuring the light flux emitted per unit of incident exposure rate and the spectra of the light emitted by the four scintillators. Scintillator-photodetector combinations resulting in higher image brightness level were determined for different X-ray imaging systems. Findings indicate that CsI:Na is very efficient with orthochromatic radiographic films, Gd2O2S:Tb could be useful in conventional or digital fluoroscopy and in CT and ZnSCdS:Ag could be employed in some medium to low voltage digital radiography applications.

Humans↗

PubMed1997

Brain atrophy in children undergoing systemic chemotherapy for extracranial solid tumors.

It has been shown that intrathecal chemotherapy may cause brain damage, which can be depicted in neuroimaging studies. The aim of this work was to examine possible morphologic alterations in the brain of children with extracranial solid tumors, without CNS complications, treated with systemic chemotherapy. Brain CT images of 69 children with extracranial malignancies were reviewed and the extent of 12 CSF compartments was measured in 49 CT examinations performed during intravenously given chemotherapy and in 20 after therapy completion. Measurements were compared with corresponding normative data. About half of the children undergoing chemotherapy and half of the patients examined after treatment were found to have diffuse brain atrophy. Focal lesions that might be associated with therapy toxicity were not observed. Chemotherapy, even when administered via the systemic route, may cause brain damage, which is observed long after the end of treatment.

Adolescent↗

PubMed1997

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

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.

Cadmium Compounds↗

PubMed1997

PRONET services for distance learning in mammographic image processing.

The potential of telematics services is investigated with respect to learning needs of medical physicists and biomedical engineers. Telematics services are integrated into a system, the PRONET, which evolves around multimedia computer based courses and distance tutoring support. In addition, information database access and special interest group support are offered. System architecture is based on a component integration approach. The services are delivered in three modes: LAN, ISDN and Internet. Mammographic image processing is selected as an example content area.

Biomedical Engineering↗

PubMed1997

Use of wavelet analysis for contrast enhancement in mammography.

A wavelet analysis approach to contrast enhancement in mammography is presented. The approach consists of application of non-linear enhancement of multiscale gradient magnitudes utilising edge location information. The performance of wavelet analysis is evaluated by means of a breast phantom. Preliminary results indicate the value of the approach in contrast enhancement and edge preservation for both simulated tumours and microcalcifications.

Breast Neoplasms↗

PubMed1997

A model for image formation and image quality prediction in diagnostic radiology.

A computer program modelling the formation of radiological images and predicting values of physical quantities that determine diagnostic image quality has been developed. The quantities are the modulation transfer function (MTF), the noise power spectrum (NPS), and the detective quantum efficiency (DQE) associated with signal to noise ratio (SNR). The program is based on mathematical models that describe the effects of x-ray interactions with the imaged object and the image detector as well as phenomena concerning the optical signal propagation within the detector. All data on x-ray effects necessary for computer calculations were derived from published work whereas optical data were determined in our laboratory using experimental techniques. Model predictions were compared with direct quality measurements performed after image formation, on the image itself.

Computer Simulation↗

PubMed1997

Computer image analysis of ultrasound images for discriminating and grading liver parenchyma disease employing a hierarchical decision tree scheme and the multilayer perceptron neural network classifier.

Differential diagnosis of liver parenchyma disease and grading of the hepatic disease on ultrasound is a common radiological problem that influences patient management. The aim of this study was to apply image analysis methods on ultrasound images for discriminating liver cirrhosis from fatty liver infiltration and for grading hepatic disease, which is important in the management of the patients. Ultrasound images of histologically confirmed 18 livers with cirrhosis, 37 livers with fatty infiltration, and 24 normal livers of healthy volunteers were selected and were digitized for further computer processing. Twenty two textural features were calculated from small matrix samples selected from the ultrasound image matrix of the liver parenchyma. These features were used in the design a three level hierarchical decision tree classification scheme, employing the multilayer perceptron neural network classifier at each hierarchical tree level. At the first tree level, classification accuracy for distinguishing normal from abnormal livers was 93.7%, at the second level the accuracy for discriminating cirrhosis from fatty infiltration was 90.9%, and at the third level the accuracy in distinguishing between low and high grading liver cirrhosis or fatty infiltration was 94.1% and 84.9% respectively. The proposed computer software system may be of value to the radiologists in assessing liver parenchyma disease.

Decision Trees↗

PubMed1996

Developmental changes in the posterior cranial fossa of children studied by CT.

We reviewed 181 brain CT examinations, with no abnormal findings, to determine normative data for the posterior cranial fossa (PF) in children. The volume of the PF and the supratentorial cranial cavity (SC) was assessed by summing consecutive CT cross-sectional areas. Linear measurements of the PF structures-cerebellum, vermis, brain stem and fourth ventricle were also performed. The PF grows rapidly during the first 3 years of life and thereafter a smaller increase in size is observed. All the PF structures showed a positive correlation with the PF volume. The PF volume increased in parallel with that of the SC and the PF/SC volume ratio remained relatively constant during childhood. The establishment of normative data for the volume of the PF and its contents may be of value in the CT study of diseases associated with morphological alterations in the PF.

Adolescent↗

PubMed1996

X-ray induced luminescence and spatial resolution of La2O2S:Tb phosphor screens.

Absolute efficiency and modulation transfer function of laboratory prepared La2O2S:Tb phosphor screens of various coating thickness were studied. Detailed experimental data on the variation of absolute efficiency with x-ray tube voltage up to 200 kVp and screen coating thickness in transmission and reflection observation mode are given. Data were compared with similar results from other rare earth phosphor materials. Theoretical calculations were in good agreement with experimental data to permit estimation of intrinsic efficiency and coefficients related to light scattering and absorption within the phosphor material. The MTF of La2O2S:Tb screens was also experimentally and theoretically evaluated.

Dose-Response Relationship, Radiation↗