Biomedical subjects

G Panayiotakis

Publications and source records attributed to G Panayiotakis.

At least 19 recordsLinked to original sources

PubMed2002

A Monte Carlo simulation model of mammographic imaging with x-ray sources of finite dimensions.

A simulation model of mammographic x-ray sources with finite size has been developed. The model is based on Monte Carlo methods and it takes into account the electron penetration inside the anode, the anode geometry and material, as well as the resulting heel effect and the spectral and spatial distribution of x-rays. This x-ray source simulation model has been embedded into an earlier developed simulation package of a mammography unit. The main outputs of this model are Monte Carlo generated images that correspond to the irradiation of properly designed phantoms. In this way it is possible to make studies of the influence of x-ray source characteristics on MTF. This paper presents the development of the mammographic x-ray source model, accompanied by a set of simulation studies concerning the influence of magnification effects as well as that of the x-ray spatial and spectral distribution on the mammographic spatial resolution for a certain magnification factor (m = 1.4). The validity level of the model, as well as its limitations and perspectives, rise through comparisons with experimental and theoretical data.

Algorithms↗

PubMed2002

Model-based technique for the measurement of skin thickness in mammography.

A model-based method is proposed for the measurement of breast skin thickness from digitised mammograms that takes into account both the geometric and radiographic properties of the skin region. The method initially identifies a salient feature that discriminates the skin from the other anatomical structures of the breast. Its identification is based on a multi-scale grey-level gradient estimation, using a wavelet decomposition of the image. The spatial distribution of this feature is organised as a graph, with each of its nodes associated with a binary set of interpretation labels. A Markov random field is defined on the set of labels, and the best graph labelling is finally determined with a maximum a posteriori (MAP) probability criterion. The method was applied on 11 mammograms with improved contrast characteristics at the breast periphery, obtained by an exposure equalisation technique during image acquisition. The validation of the approach was performed by calculating the root mean square (RMS) error between the detected skin thickness and manual measurements performed on each of the films. The resulting error values ranged from 0.1 mm to 0.2 mm for normal cases and reached a maximum of 0.5mm in pathological cases with advanced skin thickening.

Breast Neoplasms↗

PubMed2001

A protocol-based evaluation of medical image digitizers.

Medical film digitizers play an important transitory role as digital-to-analogue bridges in radiology. Their use requires performance evaluation to assure medical image quality. A complete quality control protocol is presented, based on a set of test objects adaptable to the specification of various digitizers. The protocol includes parameters such as uniformity, input-output response, noise, geometric distortion, spatial resolution, low contrast discrimination, film slippage and light leakage, as well as associated measurement methods. The applicability of the protocol is demonstrated with two types of medical film digitizers; a charge-coupled device (CCD) digitizer and a laser digitizer. The potential value of the protocol is also discussed.

Analog-Digital Conversion↗

PubMed2001

Quantifying image quality at breast periphery vs mammary gland in mammography using wavelet analysis.

Use of high contrast film-screen systems in mammography, in combination with the fact that exposure parameters are selected to ensure good visualization of the mammary gland, results in overexposure of the film area corresponding to the breast periphery, therefore decreasing image quality. The aim of this work was to provide a quantitative evaluation of image quality at the breast periphery compared with the mammary gland. To deal with the difficulties in quantification of image quality introduced by low contrast encountered at the breast periphery, wavelet analysis has been used for derivation of a contrast indicator (CI) and a noise indicator (NI), taking into account local grey level variations. Gradient magnitude coefficients corresponding to region of interest (ROI) grey level values are the basis of CI definition. Mammary gland and breast periphery were sampled by equally spaced ROIs, the quantity of which was determined by a heuristic method. For NI definition, the power values of gradient magnitude coefficients corresponding to the ROI were utilized. Image quality at the breast periphery compared with the mammary gland was evaluated using 150 craniocaudal images from the Digital Database for Screening Mammography. Measurements were carried out using a tool developed in our department. A 50% contrast decrease at the breast periphery was observed, while noise decreased by approximately 2%.

Breast Diseases↗

PubMed2000

Radiation exposure of patients and coronary arteries in the stent era: A prospective study.

Previous studies have investigated the radiation dose to doctors and patients during coronary angiography and angioplasty, but most of them were retrospective, conducted in the prestent era, and results have not been consistent. Effective dose of 57 patients undergoing coronary angiography and/or angioplasty was assessed by using a dose-area product (DAP) to effective dose conversion factor. Radiation exposure risks to patients were then calculated for each procedure. Thermoluminescent dosimeters, mounted on a specially designed catheter that was advanced to the left or right sinus of Valsalva, were used to measure the dose received by the coronary arteries. Mean effective dose received by patients were 5.0 +/- 0.5 mSv for coronary angiography, 6.6 +/- 1.0 mSv for angioplasty, 10.2 +/- 1.5 mSv for angioplasty followed by stent implantation, 13.6 +/- 2.5 mSv for angiography followed by ad hoc angioplasty, and 16.7 +/- 2.8 mSv for angiography followed by ad hoc angioplasty and stent implantation. Patient risk of developing cancer after each procedure was 0.025%, 0.033%, 0.051%, 0.068%, and 0.084%, respectively. Corresponding mean coronary irradiation doses were 24 +/- 2.5, 31.0 +/- 3.6, 43.6 +/- 7.2, 55.0 +/- 7.5, and 64.7 +/- 5.6 mGy, respectively. A linear relationship of the DAP and the dose at the coronary arteries was found: DAP = 1,273 (cm(2)) x coronary dose (mGy). Radiation exposure to coronary arteries and associated risk to patients are relatively low, even following complicated, multivessel angioplasty with stent implantation. Our method can be used for calculation of radiation risk to patients and radiation dose to coronary arteries by using external dosimeters. Cathet. Cardiovasc. Intervent. 51:259-264, 2000.

Adult↗

PubMed2000

MR phase imaging to quantify bone volume fraction: computer simulations and in vivo measurements.

Magnetic resonance phase images can be used to assess trabecular bone by measuring the standard deviation of the phases in a region of interest. The standard deviation of regional phase measurements reflects the degree of magnetic field inhomogeneity caused by susceptibility differences between bone and marrow. A 3D computer model of trabecular bone was developed and then used to explore the influence of bone volume fraction and imaging parameters such as pixel size and slice width on the standard deviation of regional phase measurements. The results from these tests show that with appropriate selection of these parameters, phase spread strongly reflects variations in trabecular bone density (a correlation of R(2) = 0.98 with bone volume fraction between 0 and 10%). The technique was then applied in vivo on the radius of 25 patients who already had a bone density scan with peripheral quantitative tomography and a correlation between phase standard deviation and trabecular bone density was found (R(2) = 0.46).

Bone Density↗

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↗