Biomedical subjects

J M Thijssen

Publications and source records attributed to J M Thijssen.

At least 19 recordsLinked to original sources

PubMed1995

Improving the ensemble average of visual evoked potentials. I. Theory of correction by spectral phase difference methods.

The considered causes of variability of visual evoked potentials (VEPs) are additive noise and time variability. The time variability can be modelled in three different ways: 1. time invariant VEP waveform and delay-time "jitter"; 2. time variant VEP waveform, due to 2a. jitter of individual components or 2b. constant VEP waveform and "wow", i.e. expansion or compression of the time scale. The wow model has not been considered so far in the literature. Correction of jitter and/or wow is based on a frequency domain method: the "spectral phase difference" method (SPD). When jitter is present the SPD method is applied in the frequency domain on the single responses, or on a subensemble of a small number of responses. For the wow a new method is proposed where the time scale is logarithmically compressed prior to the calculation of the spectrum. It is shown that the SPD method can also be applied then to estimate the compression factor yielding the wow. The single responses, or the averaged sub-ensembles, can subsequently be corrected by this factor and finally be averaged. Improvement of the precision and accuracy of estimates of both the peak latencies and the peak amplitudes, as well as a comparison between the results of the jitter and wow models, have been reported in a companion paper.

Computer Simulation↗

PubMed1995

Improving the ensemble average of visual evoked potentials. II. Simulations and experiments.

Ensemble averaging is generally used for the estimation of Evoked Potentials. This paper deals with the assessment of correction procedures for the time variability of the ensemble components, this time variability reduces the improvement of the signal-to-noise ratio (SNR) by averaging. Evoked potentials were estimated by ensemble averaging, synchronized to a periodic stimulus. It is assumed that VEP-instability is partly caused by time-variability of the evoked potentials. Two time-variate models were used, from which procedures were derived to correct the single VEP-responses prior to ensemble averaging. The models are: (1) variation in response delay (jitter), (2) variable compression/expansion of the time scale of the response (wow). The Spectral Phase Difference method was applied to estimate both the delay time jitter and the wow factor of single responses with respect to a template (conventional ensemble average). The effects of the devised correction on the average VEP waveform and on the SNR of the ensemble were investigated by using data from realistic simulations and from experiments (n = 23) with a number of healthy human volunteers (n = 17). Jitter- and wow-corrections were effective on simulations with time variability due to delay time jitter and time scale distortion (wow), respectively. Both wow- and jitter correction of the single responses improved the SNR of the VEP measurements significantly and to the same amount. A combined wow-jitter approach resulted in significantly better results than the exclusive application of jitter- or wow correction.

Computer Simulation↗

PubMed1994

Lesion detection in simulated elastographic and echographic images: a psychophysical study.

The image quality of two ultrasonic imaging techniques was studied: conventional echography and the recently introduced elastography. The image quality was assessed by estimating the detectability of disc-shaped lesions of various sizes and contrast levels. The study was designed to verify the hypothesis that elastograms could show lesions at a higher subjective and objective level of detectability than echograms of the same object contrast. This hypothesis was adopted because homogeneous elastograms can present a higher point signal-to-noise ratio than uniform echograms. Both elastograms and echograms were generated by two-dimensional (2D) simulations. The subjective assessment was performed by psychophysical experiments using the staircase up-down method. The threshold contrast of detection for both modalities was determined at different diameters of the disc-shaped lesion. These values were used to construct the contrast-detail curves for both techniques. For identical object contrasts, elastography was found to have significantly higher detectability at all lesion diameters considered. The contrast thresholds were also used for an objective evaluation with the lesion signal-to-noise ratio. The objective measure evaluated at the subjective threshold of detection for both modalities was not found to be identical, nor constant over the range of lesion diameters as expected.

Computer Simulation↗

PubMed1994

Ultrasonic spectroscopy of the porcine eye lens.

The purpose of the work is to measure and study the acoustic characteristics of the porcine eye lens and find correlations with chemical and optical parameters, obtained from literature. Ultrasonic spectroscopy was performed by using a scanning acoustic macroscope (frequency 20 MHz, resolution 150 microns). The transducer performed a two-dimensional scan over a central slice (1 mm thickness) of porcine lens (number of lenses = 10). A double transmission pulse-echo method was used to acquire the ultrasonic data from the lens. Two-dimensional images were reconstructed of the local ultrasound velocity and the frequency-dependent ultrasound attenuation. Axial and equatorial profiles of these parameters were calculated from the images. The acoustic parameters are not constant, but show a systematic dependence on the location within the lens. The profiles of the acoustic parameters are similar in shape to profiles of the protein and water contents of eye lens and to the profiles of the optical refractive index. A thorough quantitative correlation study is indicated, which should be based on detailed protein content data in porcine lenses.

Animals↗

PubMed1994

Correlation of histology and acoustic parameters of liver tissue on a microscopic scale.

The correlation of several acoustic parameters with histological features was investigated in healthy White New Zealander rabbit liver (n = 10). Thin sections (10 microns) were studied by means of a light microscope in combination with a digital image processing system. Adjacent thick sections (250 microns) were studied by means of a custom-designed acoustic microscope. Markers of black silk suture material, that could be identified both optically and acoustically, enabled the spatial correlation of both imaging modalities. Acoustic images were reconstructed from the velocity of ultrasound, the attenuation at the central frequency (30 MHz), the attenuation spectral slope, the backscatter spectral slope and the backscatter at the central frequency. The measurements comprised the frequency range from 10 to 50 MHz, yielding a resolution of approximately 50 microns. From the thin sections (10 microns) the local histological composition was obtained by digital segmentation techniques. The features that were segmented are: the collagen rich fibrous tissue content, the area lumina, the area interstitial spaces, the number density of nuclei and the area parenchymal tissue. Correlation techniques revealed that the main feature responsible for attenuation is collagen. There was a fair correlation between area lumina and attenuation, but this was caused by a high correlation between the collagen that surrounds the lumina, and attenuation. No correlation was found between any histological feature and backscatter parameters or velocity.

Animals↗

PubMed1994

Acoustic velocity and attenuation of eye tissues at 20 MHz.

The ultrasound velocity and frequency-dependent attenuation of human and porcine eye tissues (cornea, lens, retina, choroid, sclera, vitreous body) were measured in the frequency range from 17 to 23 MHz. The results for the ultrasound velocity were compared to values taken from the literature and appeared to be in the same range. A comparison made between the acoustic parameters of human and porcine eyes showed that the porcine eye can serve as an animal model for the human eye. A mathematical operation is proposed to extrapolate the attenuation to the lower frequencies that are commonly used in clinical equipment. Finally, a first attempt was made to investigate the age dependence of the acoustic parameters of human tissues: some tissues showed a significant age effect.

Acoustics↗

PubMed1994

Echographic classification of intraocular tumours. A 15-year retrospective analysis.

The data on echographic routine diagnosis of intraocular tumours (prominence equal to, or larger than 2 mm) during the years 1976-1990 in the authors' department, were analyzed by multivariate statistical methods, using the final diagnosis either from pathology after enucleation or from the confirmed clinical diagnosis. The material consisted of melanomas (n = 325), metastases (n = 44), haemangiomas (n = 19) and other intraocular tumours (n = 16). The best set of echographic parameters in descending order of significance was: reflectivity (A-mode), choroidal excavation (B-mode), shape (B-mode), and regularity (A/B mode). Echographic differentiation by computer analysis of data on the three main kinds of tumours (melanoma/metastasis/haemangioma) separate from all the remaining tumours was expressed by the correct fraction: melanoma 89%, metastasis 80% and haemangioma 97%. The clinical echographic classification for these cases was 89%, 93% and 99.5%, respectively. The simultaneous differentiation between the three classes was found to yield a correct fraction of 85% by computer statistics and 95% by routine echography. The results of the present study might be used for prospective classification through the use of the parameter 'knowledge base' contained in the statistical classification procedure.

Choroid Neoplasms↗

PubMed1994

Fast scan conversion algorithms for displaying ultrasound sector images.

Two fast algorithms for interpolation of ultrasonic sector-scans were developed. Both algorithms are based on line-drawing algorithms and are free from multiplications in the innermost loops. The algorithms were compared to the following conventional interpolators: 2-D windowed sinc, bicubic spline, 4 x 4 point bicubic spline, bilinear, and nearest neighbor. The most accurate of the two new algorithms is about eight times faster than nearest neighbor interpolation. The quantitative errors are of the same order as the errors of the nearest neighbor interpolator. The subjective image quality is between nearest neighbor and bilinear interpolation.

Algorithms↗

PubMed1993

The history of ultrasound techniques in ophthalmology.

The history of 50 years of applications of ultrasound in ophthalmology is described. This period started in 1938 with a study of the possible effects of high intensity ultrasound on eyes. The measurement of biological effects for the assessment of potential hazards characterizes the first decades. More recently, the therapeutic use of ultrasound by hyperthermia has gained much interest in ophthalmology. Further topics are: the measurement of acoustic characteristics of ocular tissues; the biometry of the eye ball, the results of which are used to calculate the optical power of artificial lens implants; the development of diagnostic instruments in various parts of the world; and ultrasonic tissue characterization for the differential diagnosis of pathology. The final topic is concerned with some recent developments which present a look into the future.

Biometry↗

PubMed1993

Detection and differentiation of diffuse liver disease by quantitative echography. A retrospective assessment.

RATIONALE AND OBJECTIVES: The detectability of diffuse liver diseases by quantitative echography was retrospectively investigated using scans of patients with known pathologic findings (n = 103) and of normal subjects (n = 129). The authors determined the best set of quantitative parameters for this task. METHODS: Quantitative echography was comprised of acoustospectrographic parameters (frequency dependence of attenuation and backscattering) and image texture parameters. The disease processes studied included: acute hepatitis, hepatitis/cirrhosis, alcoholic hepatitis/cirrhosis, primary biliary cirrhosis, and steatosis. RESULTS: Correct differentiation of these diseases ranged from 88% to 97%. Correlations between histologic grading and echographic parameters were poor. With only one exception, the differentiation between any two of the diseases could be made in 60% to 99% of cases. Different parameters better differentiated abnormal from normal scans than among diseases. CONCLUSIONS: The detection of diffuse liver diseases can be based on echographic parameters, related to a diffuse scattering model, whereas the differentiation among diseases needs additional parameters derived from a structural scattering model. Further studies are indicated to assess the prospective potential of the devised methods.

Discriminant Analysis↗

PubMed1993

Potential of fractal analysis for lesion detection in echographic images.

The application of fractal analysis to parametric imaging of B-mode echograms and to differentiation of echographic speckle textures was investigated. Echograms were obtained from realistic simulations and from a clinical study on diffuse liver disease. The simulations comprised tissue models with randomly positioned scatterers in a 3-D volume in which the number density was varied over a range from 0.5 to 25 mm-3. The clinical echograms comprised both normals and patients with liver cirrhosis. Three methods of estimating the fractal dimension were investigated, two in the spatial image domain and one in the spatial frequency domain. The results of these methods are compared and the applicability and the limitations of texture differentiation using fractal analysis is discussed. The main conclusion is that fractal analysis offers no obvious advantage over statistical analysis of the texture of echographic images. Its use for parametric imaging is further limited by the need to use relatively large windows for local estimation of the fractal dimension.

Fractals↗

PubMed1992

Effects of tissue processing techniques in acoustical (1.2 GHz) and light microscopy.

In this study the influence of various tissue processing and staining techniques on the acoustical properties of liver tissue was investigated. A qualitative study was performed using ultrasound attenuation as the imaged parameter of a combined optical/acoustical microscope with a 1.2 GHz transducer. Images were made of three sets of adjacent liver sections (6 microns in thickness) which were prepared in ten different ways: fixed by alcohol or formalin; stained by hematoxylin-eosin (HE), toluidine blue (TB) or non-stained; sectioned by a cryostat or by a paraffin microtome. It was concluded that the images obtained from cryostat sections were of much higher quality than those from paraffin sections. Images obtained from sections that were sectioned while embedded in paraffin displayed no detail at all. No consistent effect was noticed with respect to staining by HE or TB. Alcohol fixed sections gave more detailed images than formalin fixed sections. Formalin fixation in combination with cryostat sectioning yielded many cytoplasmic vacuoles.

Alcohols↗

PubMed1992

In vitro classification of gallstones by quantitative echography.

Gallstones (n = 20) were classified by dual energy Computer Tomography (CT) into three main classes: pure cholesterol stones (I), combination stones (II) and calcium stones (III). Further subclassification was possible by using morphological criteria. The acoustic measurements that were performed were measurements of the velocity of sound, the attenuation coefficient slope with frequency and intercept at 4.5 MHz, the attenuation coefficient slope per unit of time, the backscattering characteristics, and the appearance of B-mode echograms. The velocity of sound in calcium stones (c = 1695 +/- 107 m/s) was distinctly lower than in those containing some cholesterol (c > 2000 m/s). The attenuation coefficient slope ranged from 4.3 to 16.2 dB/cm MHz, the 4.5 MHz intercept from 21 to 66.2 dB/cm. The lowest values were found for the pure cholesterol stones (class IA), the highest values for subclass IIB (combination stones with shell). The attenuation coefficient slope per unit of time was distinctly lower (< 0.50 dB/microseconds MHz) for the cholesterol stones than for the combination and calcium stones (>0.64 dB/microseconds MHz). The backscattering spectrum was approximated by a straight line fit, and the slope for the cholesterol stones was lower than for the combination and calcium stones (<0.75 dB/MHz vs. >1.0 dB/MHz, respectively). The latter two parameters were assessed by in vivo applicable methods. The front echo level was found to be more than 5 dB higher for class IIB as compared to the other classes, while the spectral backscatter level at 4.5 MHz was considerably higher for both classes IIB and III. The B-mode echograms showed that a strong front echo in combination with a strong attenuation of the remaining echo signals was mainly found for stones of classes IIB and III. A significant difference between the group of stones that are suitable for lithotripsy and dissolution treatment (classes IA, IB and IIA) and the nontreatable calcium-containing stones (classes IIB and III) was found for the velocity of sound (p <0.01), the attenuation coefficient slope per time unit (p <0.10), the slope of the backscattering spectrum (p <0.05) and the 4.5 MHz intercept (p <0.01). B-mode classification yielded no complete distinction of these two groups of stones. It can be concluded that in vivo assessment of quantitative characteristics (front reflection, backscattering characteristics, attenuation coefficient slope per unit of time) in combination with the B-mode characteristics might be useful for in vivo gallstone classification.

Cholelithiasis↗

PubMed1992

Echographic differentiation of intraocular melanomas by computer analysis.

The aim of this study was to assess the differentiation of histologically different types of choroidal melanomas by means of clinical and quantitative acoustic/texture parameters of echograms. Clinical parameters were graded by morphological, kinetic and quantitative statements about the A- and B-mode echograms by a skilled diagnostician. Acoustic/texture parameters were obtained by processing and analysing radio frequency, AM-demodulated and FM-demodulated echograms. The latter data were preprocessed to remove influence induced by beam diffraction and focusing. The best set of four clinical echographic parameters enabled a retrospective classification of spindle type melanomas vs. mixed+epithelioid type melanomas with an accuracy of 77% (area under the ROC-curve of 86%). The discriminant analysis performed with the best four acoustic/texture parameters (n = 30) yielded a sensitivity of 89%, a specificity of 92%, and an area under the ROC-curve which corresponds to a probability of correct classification of 96.6%. When using these data prospectively to classify tumours of unknown cell type (n = 21), this classification could be performed in 86% of cases.

Algorithms↗

PubMed1991

B-mode echography of choroidal melanoma; echographic and histological aspects of choroidal excavation.

The B-mode images of 32 patients affected with a malignant melanoma of the choroid were studied, in particular with respect to choroidal excavation. The degree of visibility of choroidal excavation at the B-scan was compared with the thickness of the choroid, scleral invasion, vascularisation, retinal detachment and the reflectivity of the tumour. None of these histological and echographical features appeared to be significantly related to the degree of the visibility of choroidal excavation. Also the condition of the choroid was studied. At examination of all the histological sections no choroid was found between tumour and sclera. This implies that choroidal excavation should be visible at echograms in every case instead of only in 75% of the cases as was found for our series.

Choroid↗

PubMed1991

Variability of quantitative echographic parameters of the liver: intra- and interindividual spread, temporal- and age-related effects.

The values of acoustic and image texture parameters were estimated from liver scans of healthy subjects. The values were obtained after appropriate preprocessing of the radio frequency echograms by an on-line computerized system. The preprocessing comprised a correction for the Time Gain Compensation (TGC), the beam diffraction and the frequency dependent attenuation in the Region of Interest (ROI). The intra- and interindividual variability of the parameter values appeared to be of the same order of magnitude, but significantly larger than the variability assessed by measurements of a homogeneously scattering tissue mimicking phantom. Significant temporal effects were found for all the parameters, which consistently occurred during the morning. These results are discussed in relation to the circadian rhythm of the glycogen content and of the hepatic circulation. All the parameters appeared to be significantly correlated to age. The slope of the regression ranged from 3.6% per decade (attenuation coefficient) to 7.6% per decade (mean echo-level). A tentative explanation to these results is presented: the increased stiffness of hepatic vasculature with age.

Adult↗

PubMed1991

Influence of histochemical preparation on acoustic parameters of liver tissue: a 5-MHz study.

In this study the influence of various histological techniques on the acoustic parameters of liver tissue was investigated. Radiofrequency (RF) echographic data were obtained in vitro from 21 liver samples taken from 8 white New Zealander rabbits. The samples were measured in four different subsequent histological tissue processing conditions (freshly excised, 4% buffered formalin fixed, after it went through a paraffin cycle and after staining with hematoxylin and eosin). The acoustic parameters that were obtained from the rf data were velocity of sound, slope of the attenuation coefficient versus frequency between 1.9 and 6.9 MHz, attenuation coefficient at 4.4 MHz, slope of the backscattering spectrum between 1.9 and 6.9 MHz, and intercept of the backscattering spectrum. It was found that fixation by formalin preserves the acoustic properties of the tissue to a reasonable extent. Embedding in paraffin and deparaffinizing induces large changes in the acoustic properties of the tissue. As an alternative, freezing prior to cutting, rather than the paraffin cycle, was investigated also in 10 liver samples obtained from 4 New Zealander rabbits. This method produced no significant changes of the acoustic parameters and should therefore be preferred in acoustic microscopy.

Acoustics↗

PubMed1991

Echographic differentiation of histological types of intraocular melanoma.

The aim of this study was to assess the differentiation of histologically different types of choroidal melanomas by means of clinical and quantitative acoustic/texture parameters of echograms. Clinical parameters were graded by morphological, kinetic and quantitative statements about the A- and B-mode echograms by a skilled diagnostician. Acoustic/texture parameters were obtained by processing and analysing radio frequency, AM-demodulated and FM-demodulated echograms. The data were preprocessed to remove influences induced by beam diffraction and focusing. The correlations between the clinical parameters were lower than 0.5 but significant in a few cases. The best set of four clinical echographic parameters enabled a retrospective classification of spindle type melanomas vs. mixed + epithelioid type melanomas with an accuracy of 77% (area under the ROC-curve of 86%). These figures do not enable, however, a prospective diagnosis. The correlations of the clinical parameters with the acoustic/texture parameters were investigated. The mutual correlations between the latter parameters were also assessed, the most significant (0.70) being between the speckle size and the slope of the linearized backscattering spectrum. The discriminant analysis performed with the best four acoustic/texture parameters (n = 30) yielded a sensitivity of 89%, a specificity of 92% and an area under the ROC-curve corresponding to a probability of correct classification of 96.6%. When using these data prospectively to classify tumours of unknown cell type (n = 21), this classification could be performed in 86% of cases.

Algorithms↗