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Biomedical subjects

A M Vossepoel

Publications and source records attributed to A M Vossepoel.

At least 19 recordsLinked to original sources
PubMed2005

Automated detection of wedge-shaped defects in polarimetric images of the retinal nerve fibre layer.

PURPOSE: Automated glaucoma detection in images obtained by scanning laser polarimetry is currently insensitive to local abnormalities, impairing its performance. The purpose of this investigation was to test and validate a recently proposed algorithm for detecting wedge-shaped defects. METHODS: In all, 31 eyes of healthy subjects and 37 eyes of glaucoma patients were imaged with a GDx. Each image was classified by two experts in one of four classes, depending on how clear any wedge could be identified. The detection algorithm itself aimed at detecting and combining the edges of the wedge. The performance of both the experts and the algorithm were evaluated. RESULTS: The interobserver correlation, expressed as ICC(3,1), was 0.77. For the clearest cases, the algorithm yielded a sensitivity of 80% at a specificity of 93%, with an area under the ROC of 0.95. Including less obvious cases by the experts resulted in a sensitivity of 55% at a specificity of 95%, with an area under the ROC of 0.89. CONCLUSIONS: It is possible to automatically detect many wedge-shaped defects at a fairly low rate of false-positives. Any detected wedge defect is presented in a user-friendly way, which may assist the clinician in making a diagnosis.

Algorithms↗
PubMed2005

Inter-frame motion correction for MR thermometry.

Noninvasive temperature measurement is feasible with MRI to monitor changes in thermal therapy. Phase shift based MR thermometry gives an estimate of the relative temperature variation between thermal and baseline images. This technique is limited, however, when applied on targets under inter-frame motion. Simple image registration and subtraction are not adequate to recover the temperature properly since the phase shift due to temperature changes is corrupted by an unwanted phase shift. In this work, the unwanted phase shift is predicted from the raw registered phase shift map itself. To estimate the unwanted phase shift, a thin plate smoothing spline is fitted to the values outside the heated region. The spline value in the heated area serves as an estimate for the offset. The estimation result is applied to correct errors in the temperature maps of an ex-vivo experiment.

Algorithms↗
PubMed2005

Interactive 3D segmentation using connected orthogonal contours.

This paper describes a new method for interactive segmentation that is based on cross-sectional design and 3D modelling. The method represents a 3D model by a set of connected contours that are planar and orthogonal. Planar contours overlayed on image data are easily manipulated and linked contours reduce the amount of user interaction.1 This method solves the contour-to-contour correspondence problem and can capture extrema of objects in a more flexible way than manual segmentation of a stack of 2D images. The resulting 3D model is guaranteed to be free of geometric and topological errors. We show that manual segmentation using connected orthogonal contours has great advantages over conventional manual segmentation. Furthermore, the method provides effective feedback and control for creating an initial model for, and control and steering of, (semi-)automatic segmentation methods.

Algorithms↗
PubMed2004

A model based method for retinal blood vessel detection.

Retinal blood vessels are important structures in ophthalmological images. Many detection methods are available, but the results are not always satisfactory. In this paper, we present a novel model based method for blood vessel detection in retinal images. It is based on a Laplace and thresholding segmentation step, followed by a classification step to improve performance. The last step assures incorporation of the inner part of large vessels with specular reflection. The method gives a sensitivity of 92% with a specificity of 91%. The method can be optimized for the specific properties of the blood vessels in the image and it allows for detection of vessels that appear to be split due to specular reflection.

Algorithms↗
PubMed2003

Detecting glaucomatous wedge shaped defects in polarimetric images.

Wedge shaped defects of the retinal nerve fiber layer (RNFL) may occur in glaucoma. Currently, automatic detection of wedge shaped defects in Scanning Laser Polarimetry (SLP) images of the RNFL is not available. An automatic classification is currently based only on global parameters, thereby ignoring important local information. Our method works by a modified dynamic programming technique that searches for locally strong edges with a preference for straight edges. These edges are initially classified based on their strength and next combined into wedge shaped defects. Our method yields a sensitivity of 73% and a specificity of 90% on a limited set of 65 images.

Diagnosis, Computer-Assisted↗
PubMed1996

Largest contour segmentation: a tool for the localization of spots in confocal images.

An accurate determination of the 3-D positions of multiple spots in images obtained by confocal microscopy is essential for the investigation of the spatial distribution of specific components or processes in biological specimens. The position of the centroid, as an estimator for the position of a spot, can be calculated on the basis of all voxels that belong to the domain of the spot. For this calculation a domain that defines which voxels belong to the spot must be delimited. To create a boundary for a domain we developed a 3-D image segmentation procedure: the largest contour segmentation (LCS). This procedure is based on an iterative region-growing procedure around each local maximum of intensity. By means of this procedure the position of each spot was determined accurately and automatically. Qualities of the procedure were evaluated by means of simulated test-images as well as 3-D images of real biological specimens.

Algorithms↗
PubMed1989

Automatic assignment of elliptical ROIs: first results in planar scintigrams of the left ventricle.

Several algorithms for quantitative analysis of 201Tl scintigrams require operator assigned elliptical regions. Therefore these algorithms are hampered by inter- and intra-observer errors. In this paper we present a robust algorithm to determine automatically contours with elliptical shapes. The algorithm is based on the Hough transformation and can determine the parameters describing an ellipse even in low signal-to-noise ratio images and when the contour is only partially visualized such as in non-perfused regions of the left ventricular myocardium. As long as the number of parameters to describe the shape of the contour is not too large, the same kind of algorithm can also be used for differently shaped contours, and for 3D elliptical contours as in SPECT.

Algorithms↗
PubMed1986

Flow karyotyping of human melanoma cell lines.

Single-laser flow cytometry has been used to study the feasibility of flow karyotyping of human solid tumors. As a model, seven human melanoma cell lines have been used with varying numerical chromosome composition as verified by FCM DNA content measurements and chromosome countings. For all seven cell lines, flow karyotypes that showed a variety of consistent deviations from the normal diploid flow karyotype could be obtained although the resolution of the flow system and varying debris continuum limited the number of resolvable peaks. The predominant changes observed involved the regions normally representing chromosomes 3-8, 9-12, and 13-16. It is concluded that at present the preparation procedure is the main limiting factor for exploring the full potential of flow karyotyping for cytogenetic analysis of solid-tumor cell lines.

Cell Line↗
PubMed1986

Methodological aspects of flow cytometric analysis of DNA polyploidy in human heart tissue.

The purpose of the study was to investigate the possibilities of flow cytometry (FCM) for the analysis of DNA polyploidy in human heart tissue. Suspensions of single nuclei were prepared with the detergent-trypsin procedure and stained with propidium iodide. A mathematical correction procedure was developed to correct for background and clumping. For diploid model populations of chicken and trout red blood cells this correction reduced artifactual fractions in the FCM DNA profile to less than 0.5%, indicating that interference by background and clumping was almost completely overcome by this correction procedure. FCM DNA profiles were obtained from 12 hypertrophic and 7 normal human adult hearts. Clear differences were found between DNA profiles from the normal and the hypertrophic hearts, the latter showing a higher degree of polyploidization. From the corrected DNA profiles, six different polyploidization parameters were computed, all of which showed a significant correlation with at least three out of four different parameters for heart hypertrophy. FCM appears to be a reliable method for the measurement of polyploidization in heart tissue, provided background and clumping are corrected for.

Adult↗
PubMed1986

Thallium myocardial perfusion scintigraphy: influence of perfusion, scatter, and photon energy on the detection of lesions.

Experimental studies have been made of lesion detectability in myocardial perfusion studies using thallium-201. A series of images (AP view) was generated mathematically. Images were simulated with 100k, 200k, and 300k counts for the complete image and either non- or half-perfused lesions (lesion volume 1-6 1/2% of total myocardial volume). In addition thallium images were generated excluding scatter; images were also generated with a radiopharmaceutical using 140, 184 and 296 keV, assuming the biological distribution of thallium. All images were interpreted by seven observers. Count density did not affect the true-positive fraction of the interpretations. However, the false-positive fraction decreased significantly when count density increased. Lesions located far from the camera were detected worst. The intraobserved variability was largest for interpretation of these regions. Small lesions were poorly detected. Perfusion of lesions significantly affected their detection, independent of the count density, lesion volume, and lesion location. Scatter did not affect the detection of lesions. No significant difference was found when the performance of the observers was compared for 80 keV and 140 keV. The use of 184-keV and 296-keV photons resulted in a lower detection rate probably because of the use of a high energy collimator.

Coronary Circulation↗
PubMed1983

Quantitative assessment of wall motion in multiple-gated studies using temporal Fourier analysis.

We described a method for quantification of left-ventricular wall motion in multiple-gated studies, based on approximation of local time-activity curves by the first harmonic of the corresponding Fourier spectrum. The Fourier transform is adapted to the global left-ventricular time-activity curve by shortening the length of the base period. The degree of shortening depends on the corresponding power in the second and higher harmonics; the base period that resulted in minimal power in the higher harmonics was selected for a Fourier transform of the regional curves. This usually results in exclusion of both the diastasis phase and the left-atrial filling phase from the calculations. With a model of the left-ventricular volume curve it is shown that important advantages of this adaptation are (a) a better approximation of the maximum difference in the curves, and (b) less spread in the phase determination. Wall motion was related to the Fourier parameters. The combination of the mean amplitude and the standard deviation of the phase histogram gives the best results for global analysis of the motion of the wall.

Diastole↗
PubMed1982

Detection of lesions in thallium-201 myocardial perfusion scintigraphy.

Experimental studies have been made of the lesion detectability in myocardial perfusion studies using thallium-201. A series of images (AP-view) was generated using a convolution of a mathematical model of the left ventricular myocardium and an experimentally determined point spread function. Background was added. Images were simulated with 100 k, 200 k, and 300 k counts for the complete image. Each image contained a lesion with either 0% or 50% of the normal tracer concentration. All images were interpreted by five experienced observers, independently of each other. Their interpretations were analysed using the Kolmogorov-Smirnov two sample test. The true positive fraction (TPF) was hardly affected by changes in count density. The TPF decreased significantly if the lesion tracer concentration changed from 0% to 50% of the normal myocardial tracer concentration. The decrease was independent of the count density. The false positive fraction (FPF) decreased significantly if the count density increased; no difference in FPF was found for a change in lesion tracer concentration. In addition, plots were generated with the TPF versus lesion volume. The TPF was lowest for locations far from the camera; FPF was high for these segments. Increase of the count density mainly improved the FPF in these segments. Small lesion were more difficult to detect.

Heart Diseases↗
PubMed1979

DIODA: delineation and feature extraction of microscopical objects.

A computer program is described for delineation and measurement of microscopical objects, such as cells and chromosomes, which may have been scanned using absorbance, fluorescence or reflectance microscopy. The quality of the object delineation is optimized through the controur ratio, which is simply computed from the object contour. Geometrical features, like the perimeter and area are computed. A new definition is introduced for the background region, especially suited for the analysis of closely packed objects. This definition is based upon a comparison between total staining material content values resulting from a variety of methods and circumstances. The program is principally intended for use in the on-line real-time environment of a small laboratory computer, but may be used off-line as well. It has facilities for displaying the results and the process by which they are produced. A program module is described for displaying scan data on a bilevel display using an adaptation of the sigma-delta method.

Computers↗
PubMed1977

HIDACSYS: computer programs for interactive scanning cytophotometry.

A description is given of a combination of three programs developed for computer-assisted stage scanning cytophotometry and cytofluorometry of isolated, close-lying, or touching objects in different types of microscopical preparations. The advantages and limitations of the individual programs are discussed, as well as the local specimen conditions determining the optimal application range of each of the programs. The applicability of these programs was investigated by determination of the integrated absorbance values of Feulgen or gallocyanin-chrome alum stained chicken erythrocytes, human leucocytes, and skin biopsy cells in imprint preparations, as well as of guinea pig peritoneal granulocytes which had been submitted to a simultaneous coupling azo dye incubation for alkaline phosphatase activity.

Alkaline Phosphatase↗
PubMed1977

High-resolution scanning-densitometry of photographic negatives of human chromosomes. III. Determination of fluorescence emission intensities.

Principles and techniques are discussed for measuring with high topological resolution local emission in fluorescing objects, using photographic negatives. Determination of fluorescence intensities is only possible when an unequivocal relation between the original local fluorescence emission intensities of the object, and the transmittances or densities recorded in the microfluorophotograph is known. This relation is formulated in the theoretical part. From this relation it can be concluded that the recorded intensities can be measured optimally when the optical density values produced by the fluorescence emission fall in the range of the linear portion of the Hurter and Driffield curve. In order to obtain this situation, a uniform low-level pre-exposure of the film emulsion to (white) light is carried out prior to the actual fluorescence emission exposure. This pre-exposure acts to elevate the signal exposure to the linear (steeper) part of the H.D. curve. Inhomogeneity of the excitation beam in the object field, or differences in film emulsion response to the light exposure, will result in erroneous optical densities recorded in the photographic negative. Correction for such artifacts could be obtained by addition of a low concentration of fluorophore to the mounting medium of the microscopic preparation. The overall fluorescent background produced in this way, enabled calibration of local fluorescence intensities in different parts of one fluorophotographic negative, and also of the intensities in different negatives taken from one microscopic preparation. The validity of this approach was checked by comparing data obtained from several photographic negatives of the same quinacrine-stained metaphase, taken with different exposure times to imitate fluctuations in excitation illumination, after conversion of the scanning data into emission intensity values with an alogarithm based on the proposed theoretical relation. In another experiment, fluorescence emission intensities of Feulgen-stained chromosomes which had been measured with a cytofluorometer, were compared with results obtained by conversion of the scanning data measured in the fluorophotographic negatives of the same metaphases. Both types of experiment confirmed the applicability of the procedure described.

Chromosomes↗
PubMed1977

A computer program for long term anticoagulation control.

A computer program is described which calculates suggested daily anticoagulant dose schedules for long term patients. This program is a part of the real-time Information System for Thrombosis Centres. The program requires only the most recent information on the patient from the Information System's data base. This information consists of the latest two measured coagulation times, the previous dose schedule, the target and limiting values set by the physician to the coagulation time, and a very limited number of parameters accounting for the patient's anticoagulation history. A simple dose-response model is used to simulate and predict a coagulation time. By comparing this prediction with the measured coagulation time the program computes the necessary adjustment in the average dose and the appointment period. It gives remarks, if necessary. The Information System is used successfully for the majority of patients of Leiden Thrombosis Centre, where it operates as a part of a real-time Hospital Information System.

Anticoagulants↗