Biomedical subjects

G Panayiotakis

Publications and source records attributed to G Panayiotakis.

At least 37 records · Page 2Linked to original sources

PubMed2000

A digital density equalization technique to improve visualization of breast periphery in mammography.

In mammographic imaging, the film area corresponding to the breast periphery is overexposed, resulting in high optical density and degraded contrast in this region. A digital, model-driven density equalization technique was designed and developed to overcome this overexposure problem, taking into account the non-linear characteristic curve of the film-digitizer system. The method is based on several image processing and analysis techniques, such as thresholding, which is used to segment the pixels of the mammogram belonging to the breast region from the background, and wavelet-based fusion, which is used to equalize the pixels of breast periphery selectively while leaving the remaining breast region unaffected. Initial application of the method resulted in density-equalized mammographic images, characterized by improved contrast at the breast periphery.

Breast Neoplasms↗

PubMed1999

Computer simulations for the optimization of magnetic resonance phase imaging applied in the study of trabecular bone.

A new technique for the evaluation of bone trabeculation using magnetic resonance (MR) phase images has been recently presented. This technique calculates the phase variance in a region of interest (ROI) on the phase images of a gradient echo sequence. In this study, a computer program was developed which simulates the phase distribution in gradient echo acquired phase images of a structure that mimics trabecular bone, consisting of a three-dimensional connected network of orthogonal bone struts. Several tests were performed in order to assess the influence of imaging parameters such as the echo time, the pixel size and the slice width on phase variance. The results from this work show that with selection of appropriate imaging parameters, phase variance strongly reflects variations in trabecular bone density. Representative MR experiments were performed in the distal radius to verify the simulation results.

Bone and Bones↗

PubMed1999

Calculating shielding requirements in diagnostic X-ray departments.

Structural radiation protection for diagnostic X-ray facilities is most commonly performed following the recommendations of the National Council on Radiation Protection and Measurements Report No. 49. A number of analytical methods have already been developed to improve the design of these facilities. Specifically, these methods reassess shielding calculations in X-ray areas with respect to the methodology of the calculation of the barrier thickness and the number of sources considered in the area. Thus, they generate an overall solution for the cases met at the medical radiation structural design. This paper presents an extension of an existing method for calculating shielding requirements, for multiple X-ray tubes in a room operated at various beam qualities. The methodology computes the required shielding thickness such that the exposure behind it stays below a desired value. The presented method eliminates the overestimation of added shielding thickness which may occur using the other methods already mentioned. A user-friendly windows-based program has also been developed to assist shielding computations.

Algorithms↗

PubMed1999

A phantom-based evaluation of an exposure equalization technique in mammography.

An anatomical filter based exposure equalization technique in mammography is evaluated quantitatively using a phantom. The evaluation is carried out by a comparative observer performance study, comparing the equalization technique with a conventional one based on visualization of low contrast, 6 mm circular details and high contrast, 0.5 mm and 0.25 mm small size details. These details are situated at the phantom edge, simulating the breast periphery. Visualization of these details is studied with respect to the parameters of tube voltage, optical density, detail location and phantom thickness. Phantom images are interpreted independently by three observers using a four-point grading scale. Use of the Wilcoxon signed ranks test for paired data shows statistically highly significant improvement (p < 0.0001) in the visualization of details for the equalization technique for all values of the parameters studied. The improvement is independent of tube voltage but dependent on optical density, detail location and phantom thickness. Optimal performance is obtained for detail location closer to the outer border of the simulated breast periphery and/or further away from the film, as well as for a greater phantom thickness simulating both thick and dense breast.

Breast Neoplasms↗

PubMed1998

Comparison of computer simulated and phantom measured phase variance in the study of trabecular bone.

A new method using magnetic resonance phase images for the assessment of trabecular bone structure has recently been proposed. To test this method, a mathematical model is developed which calculates the phase distribution in gradient echo acquired phase images of a structure of Pyrex glass rods immersed in a copper sulfate solution. Several experiments were performed using a phantom built in the same way as the structure used in the mathematical model. The results from the model are compared with those from the phantom tests, and the influence of resolution and bone area fraction on the phase dispersion is studied. The good correlation between theoretical and experimental results shows that phase variance increases with increasing resolution and bone density. However, the dependence of variance on bone density is less prominent for large pixel sizes.

Bone Density↗

PubMed1998

Monte Carlo generated mammograms: development and validation.

We have developed a model using Monte Carlo methods to simulate x-ray mammography. All possible physical processes of interaction of x-rays with matter have been taken into account. A simplified geometry of the mammographic apparatus has been considered along with a software phantom of compressed breast. The phantom may contain inhomogeneities of various compositions and sizes. We have used this model to produce Monte Carlo mammograms under realistic conditions. The validation of the simulation includes both the modelling of physical processes and the production of Monte Carlo mammograms. The first part is accomplished by the demonstration of the coincidence between Monte Carlo and theoretical data, whereas the second is accomplished by the comparison of real mammograms, taken from irradiation of a simplified breast phantom that we have constructed, and Monte Carlo mammograms taken from simulation of the above phantom under the corresponding exposure conditions. The limitations of the model as well as the future use of Monte Carlo mammograms are discussed.

Biophysical Phenomena↗

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

Patient radiation doses during cardiac catheterization procedures.

The objective of the present project was the determination of the dose received by patients during cardiac procedures, such as coronary angiography, percutaneous transluminal coronary angioplasty (PTCA) and stent implantation. Thermoluminescent dosemeters (TLDs), suitably calibrated, were used for the measurement of the dose received at four anatomical locations on the patient's skin. A dose-area product (DAP) meter was also used. The contribution of cinefluorography to the total DAP was higher than that of fluoroscopy. A DAP to effective dose conversion factor equal to 0.183 mSv Gy-1 cm-2 was estimated with the help of a Rando phantom. Thus, the effective dose received by the patients could be assessed. Mean values of effective dose equal to 5.6 mSv, 6.9 mSv, 9.3 mSv, 9.0 mSv and 13.0 mSv were estimated for coronary angiography, PTCA, coronary angiography and ad hoc PTCA, PTCA followed by stent implantation and coronary angiography and ad hoc PTCA followed by stent implantation, respectively.

Angioplasty, Balloon, Coronary↗

PubMed1998

Evaluation of an anatomical filter-based exposure equalization technique in mammography.

An anatomical filter-based exposure equalization technique in mammography is clinically evaluated. An observer performance comparative study, between this technique and the conventional one, is carried out on the basis of the visualization of six anatomical features situated at the breast periphery. The effect of parenchymal breast patterns and compressed breast thickness in the visualization of these features is studied. 533 craniocaudal mammograms were interpreted by two radiologists independently, using a five-point rating scale. Use of the Wilcoxon ranking test for unpaired data shows statistically highly significant improvement (p < 0.0001) in the visualization of the nipple, areola, skin and subcutaneous fat in all cases, for the equalization technique. The improvement is dependent on parenchymal breast patterns and compressed breast thickness for peripheral Cooper's ligaments and peripheral surface veins. In the case of total fatty replacement and/or thin breasts neither technique seems to be superior (p > 0.05) with respect to these two features. Clinical results indicate the value of this technique in clinical routine.

Adult↗

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↗

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↗

PubMed1996

Accidental embryo irradiation during barium enema examinations. An estimation of absorbed dose.

RATIONALE AND OBJECTIVES: To investigate the possibility of an embryo to receive a dose of more than 10 cGy, the threshold of malformations induction in embryos reported by the International Commission on Radiological Protection, during barium enema examinations. METHODS: Thermoluminescent dosimeters were placed in a phantom to calculate the depth-to-skin conversion coefficient needed for dose estimation at the average embryo depth in patients. Barium enema examinations were performed in 20 women of childbearing age with diagnostic problems demanding longer fluoroscopy times. Doses at 6 cm, the average embryo depth, were determined by measurements at the patients' skin followed by dose calculation at the site of interest. RESULTS: The range of doses estimated at embryo depth for patients was 1.9 to 8.1 cGy. The dose always exceeded 5 cGy when fluoroscopy time was longer than 7 minutes. CONCLUSION: The dose at the embryo depth never exceeded 10 cGy. This study indicates that fluoroscopy time should not exceed 7 minutes in childbearing-age female patients undergoing barium enema examinations.

Abnormalities, Radiation-Induced↗

PubMed1996

Clinical evaluation of manual and automatic exposure control techniques in film-based chest radiography.

Chest radiographs obtained with either automatic exposure control (AEC) technique, or manual exposure control (MEC) technique, were compared in a sample of 329 cardiological and cardiosurgery patients. Parameters evaluated, were various anatomical regions, according to their appearance in both posteroanterior (PA) and lateral radiographs. Lateral radiographs obtained with the AEC technique demonstrated better image quality than those obtained with the manual technique. There was no definite advantage of the AEC technique in PA radiographs. The AEC technique has the potential to improve the quality of lateral chest radiographs of cardiological and cardiosurgery patients. It is considered to be a useful tool in chest radiography.

Adolescent↗

PubMed1996

Radiographic skills learning: procedure simulation using adaptive hypermedia.

The design and development of a simulation tool supporting learning of radiographic skills is reported. This tool has by textual, graphical and iconic resources, organized according to a building-block, adaptive hypermedia approach, which is described and supported by an image base of radiographs. It offers interactive user-controlled simulation of radiographic imaging procedures. The development is based on a commercially available environment (Toolbook 3.0, Asymetrix Corporation). The core of the system is an attributed precedence (priority) graph, which represents a task outline (concept and resources structure), which is dynamically adjusted to selected procedures. The user interface imitates a conventional radiography system, i.e. operating console, tube, table, patient and cassette. System parameters, such as patient positioning, focus-to-patient distance, magnification, field dimensions, tube voltage and mAs are under user control. Their effects on image quality are presented, by means of an image base acquired under controlled exposure conditions. Innovative use of hypermedia, computer based learning and simulation principles and technology in the development of this tool resulted in an enhanced interactive environment providing radiographic parameter control and visualization of parameter effects on image quality.

Computer Simulation↗

PubMed1996

Improved determination of breast cancer size in mammography.

Breast cancer size determination in a preoperative stage is of great importance, as it is taking into account the classification of the tumour. In this paper a simple method of the improved determination of the actual tumour size is presented. The method is based on the calculation of correction factors taking into account the magnification of the tumour image during the mammographic examination. The method was applied for evaluation on forty cases of breast cancer. In all cases the final approximation of tumour maximum diameter with the presented method is almost equal to this obtained from the pathologists' reports. The possibility of wrong stage characterisation of a tumour, which is a problem arising from the measurement of the maximum diameter directly from the mammogram, is also minimised, especially in case that tumour size is between two stages.

Bias↗

PubMedDate not supplied

An anatomical filter for exposure equalization in mammography.

An anatomical filter has been designed, tested and subsequently used in clinical practice to equalize optical density over the entire mammographic image, thus eliminating the need for dual exposure whenever both the mammary gland and the peripheral structures of the breast must be clearly visualized. The use of the filter results in a reduction of the optical density, at the peripheral structures of the breast, with a simultaneous increase of the contrast, without causing deterioration of the mammary gland image. Improved visualization of the connective tissue, the skin, the nipple and the subcutaneous adipose tissue is achieved, as well as minimization of the radiation dose to the breast for patients with an indication of peripheral breast lesions.

Female↗