Biomedical subjects

D Van Dyck

Publications and source records attributed to D Van Dyck.

35 records · Page 2Linked to original sources

PubMed2001

Does a monochromator improve the precision in quantitative HRTEM?

This paper addresses the question as to what extent the incorporation of a monochromator in an electron microscope can enhance the performance of high resolution transmission electron microscopy (HRTEM). The monochromator will reduce the chromatic aberration, and hence the information limit, at the expense of beam current, leading to a decrease in signal intensity and a corresponding decrease in signal-to-noise ratio (SNR). Both aspects, information limit and SNR, have been included in a quantitative evaluation based on the statistical precision with which the position of an atom column can be estimated. It is shown that the effect of a monochromator on the attainable precision depends on the microscope and monochromator parameters, as well as on the characteristics of the object.

Journal Article↗

PubMed2001

Otolith function after acoustic neuroma surgery.

The response of the vestibular system after acoustic neuroma surgery was investigated in nine patients. The otolith system was studied by means of ocular counterrolling, assessed by video oculography. Horizontal vestibulo-ocular reflex (VOR) function was tested by the sinusoidal harmonic acceleration test using electronystagmography. The results were compared with those obtained from a normal control population. The response to slow rotation tests was symmetric, but the gain was significantly reduced when compared to the normal population. Phase lag was significantly increased. No difference in ocular torsion was observed with lateroflexion of the head to the ipsilateral side in comparison with lateroflexion to the contralateral side. Moreover, the overall ocular counterrolling was well within normal limits. We conclude that the semicircular canal response differs from the otolith response. The component of the torsional VOR mediated by otolith stimulation appears to be more robust than the horizontal VOR mediated mainly by the horizontal semicircular canal system. Ocular counterrolling induced by lateroflexion does not reveal abnormalities in patients with surgically produced unilateral peripheral loss.

Adult↗

PubMed2000

Exit wave reconstructions using through focus series of HREM images.

The through focus exit wave reconstruction technique uses a series of high resolution electron microscopy images to reconstruct the complex electron wavefunction at the exit plane of the specimen. The feasibility of the through focus exit wave reconstruction method and its most important limitations are discussed. It is shown that-provided all aberrations of the microscope are well corrected for-a large improvement in the interpretability of the information can be obtained.

Biophysical Phenomena↗

PubMed2000

Automatic localization of EEG electrode markers within 3D MR data.

The electrical activity of the brain can be monitored using ElectroEncephaloGraphy (EEG). From the positions of the EEG electrodes, it is possible to localize focal brain activity. Thereby, the accuracy of the localization strongly depends on the accuracy with which the positions of the electrodes can be determined. In this work, we present an automatic, simple, and accurate scheme that detects EEG electrode markers from 3D MR data of the human head.

Electrodes↗

PubMed2000

Reduction of ECG and gradient related artifacts in simultaneously recorded human EEG/MRI data.

Nowadays, electroencephalography signals can be acquired from a patient lying in a magnetic resonance imaging system. It is even possible to acquire EEG signals during an MR imaging sequence. However, such EEG signals are severely distorted by artifacts originating from various effects (e.g., MR gradients, ECG). In this paper, a simple method is presented to reduce such artifacts. Thereby, special attention is focused on artifacts related to the patient's electrocardiogram. The method is shown to be effective, adaptive, and automatic.

Artifacts↗

PubMed2000

Wavelets as chromatin texture descriptors for the automated identification of neoplastic nuclei.

Chromatin distribution reflects the organization of the DNA of a nucleus and contains important cellular diagnostic and prognostic information. Feulgen staining of breast tissue enables the chromatin distribution of the nucleus to be visualized in the form of texture. Describing texture in an objective and quantitative way by means of a set of texture parameters, combined with the study of the relationship of such parameters to the pathobiological cell properties, is useful both for reduction of the subjectivity inherently coupled to visual observation and for more accurate prognosis or diagnosis. We have presented an automated classification scheme for the diagnosis and grading of invasive breast cancer. The input to this scheme was a digitized microscopical image, from which nuclei were segmented. Chromatin texture was described using a set of textural parameters that include first- and second-order statistics of the image grey levels. The more recently developed wavelet energy parameters were also included in our study. Classification was performed by a Knn-classifier, which is a versatile multivariate statistical classification technique. We investigated the role of the tissue preparation technique and found that parameters derived from cytospins were better texture descriptors than those from sections. A 100% correct classification was achieved in a patient diagnosis experiment and 82% in a nuclear grading experiment.

Breast Neoplasms↗

PubMed1999

Adaptive anisotropic noise filtering for magnitude MR data.

Conventional noise filtering schemes applied to magnitude magnetic resonance (MR) images tacitly assume Gauss distributed noise. Magnitude MR data, however, are Rice distributed. Not incorporating this knowledge leads inevitably to biased results, in particular when applying such filters in regions with low signal-to-noise ratio. In this work, we show how the Rice data probability distribution can be incorporated so as to construct a noise filter that is far less biased.

Anisotropy↗

PubMed1999

Restoration of MR-induced artifacts in simultaneously recorded MR/EEG data.

During a Magnetic Resonance sequence, simultaneously acquired ElectroEncephaloGraphy (EEG) data are compromised by severe pollution due to artifacts originating from the switching of the magnetic field gradients. In this work, it is shown how these artifacts can be strongly reduced or even removed through application of an adaptive artifact restoration scheme. The method has proved to be fully automatic and to retain high frequency EEG information, which is indispensable for many EEG applications.

Animals↗

PubMed1999

A patchy horizontal organization of the somatosensory activation of the rat cerebellum demonstrated by functional MRI.

Blood oxygenation level dependent contrast (BOLD) functional MRI (fMRI) responses, in a 7-T magnet, were observed in the cerebellum of alpha-chloralose anaesthetized rats in response to innocuous electrical stimulation of a forepaw or hindpaw. The responses were imaged in both coronal and sagittal slices which allowed for a clear delineation and localization of the observed activations. We demonstrate the validity of our fMRI protocol by imaging the responses in somatosensory cortex to the same stimuli and by showing reproducibility of the cerebellar responses. Widespread bilateral activations were found with mainly a patchy and mediolateral band organization, more pronounced ipsilaterally. Possible parasagittal bands were observed only in contralateral lobule VI. There was no overlap between the cerebellar activations caused by forepaw and hindpaw stimuli. The overall horizontal organization of these responses was quite remarkable. For both stimulation paradigms most of the activation patches were positioned in either a rostral or caudal broad plane running anteroposteriorly through both anterior and posterior cerebellum. The rostral planes were completely separated, with the forepaw activation closer to the surface, while the caudal plane was common to both stimulation protocols. We relate our findings to the known projection patterns of spinocerebellar and cuneocerebellar mossy fibres, and to human fMRI studies.

Animals↗

PubMed1998

Estimation of the noise in magnitude MR images.

Magnitude magnetic resonance data are Rician distributed. In this note a new method is proposed to estimate the image noise variance for this type of data distribution. The method is based on a double image acquisition, thereby exploiting the knowledge of the Rice distribution moments.

Magnetic Resonance Imaging↗

PubMed1998

Maximum-likelihood estimation of Rician distribution parameters.

The problem of parameter estimation from Rician distributed data (e.g., magnitude magnetic resonance images) is addressed. The properties of conventional estimation methods are discussed and compared to maximum-likelihood (ML) estimation which is known to yield optimal results asymptotically. In contrast to previously proposed methods, ML estimation is demonstrated to be unbiased for high signal-to-noise ratio (SNR) and to yield physical relevant results for low SNR.

Computer Simulation↗

PubMed1998

Desktop X-ray microscopy and microtomography.

Recent developments in X-ray microtomography have made it possible to miniaturize a CT scanner into a versatile and cost-effective desktop system that fits into any laboratory environment. The possibilities of the technique are demonstrated for a range of applications. It is also shown how an existing scanning electron microscope with an X-ray detector can, with a specially developed attachment, be transformed into an X-ray microscope and microtomograph.

Animals↗

PubMed1997

Computer analysis of two-dimensional electrophoresis gels: a new segmentation and modeling algorithm.

The complexity of the spot patterns of two-dimensional electrophoresis gels made it necessary to use image processing techniques to analyze the gels. An important issue in the analysis is the detection and quantification of the protein spots. In this paper we describe a new technique to segment and model the different spots. For the segmentation of a gel into its different spot regions we apply a watershed technique, which is robust and efficient. For the quantification of the spots, a new spot model is constructed, based on diffusion principles. Besides the advantage of having a physical interpretation, the model is demonstrated to be superior to the commonly used Gaussian models.

Algorithms↗

PubMed1996

Quantification and improvement of the signal-to-noise ratio in a magnetic resonance image acquisition procedure.

A procedure is developed to quantify and improve the signal-to-noise ratio (SNR) of magnetic resonance images. The image SNR is quantified using the correlation function of two independent acquisitions of an image. To test the performance of the quantification, SNR measurement data are fitted to theoretically expected curves. The proposed correlation technique is also used to improve the SNR by estimating the amplitude of the signal spectrum. The technique is applied to a set of MR images, and its performance in terms of gain in SNR, contrast-to-noise ratio (CNR), and resolution loss is compared to that of classical noise filters. The SNR as well as the CNR is improved significantly with minor loss of resolution. Finally, it is shown that the correlation technique can be implemented in a highly efficient way in almost any acquisition procedure of a magnetic resonance imaging system.

Image Processing, Computer-Assisted↗

PubMed1977

Ophthalmoplegic migraine: amelioration by Flufenamic acid, a prostaglandin inhibitor.

It was recently suggested that prostaglandins' release (especially PGE1 and PGF2alpha) play a key role in the development of the migrainous attack. Based on this hypothesis a therapeutic trial with flufenamic acid (a prostaglandin inhibitor of the fenamates group) was conducted during 1 year in 5 patients suffering from recurrent ophthalmoplegic migraine. We treated 25 migrainous attacks; in 22 of them the patients reported marked alleviation of the headaches and only in two occasions a partial third nerve palsy accompanied the attack.

Adolescent↗

PubMedDate not supplied

High-resolution electron microscopy and electron tomography: resolution versus precision.

The performance of high-resolution electron microscopy and electron tomography is usually discussed in terms of two-point resolution, expressing the possibility of perceiving separately two image points of an object. However, the concept resolution obtains another meaning if one uses prior knowledge about the object and the imaging procedure in the form of a parametric model describing the expectations of the observations. The unknown parameters, such as the positions of the components in an object, can be measured quantitatively by fitting this model to the observations. Due to the statistical nature of the experiment, the resulting solutions for the positions of the components and therefore for the distance between the components will never be exact. An alternative to resolution is then the precision with which the distance can be measured. In the present paper, it is shown that the precision depends on the size of the components, the distance between the components, the resolution of the instrument, and the number of electron counts. For electron tomography, it also depends on the orientation of the object with respect to the rotation axis.

Image Processing, Computer-Assisted↗

PubMedDate not supplied

Biotransformation of sufentanil in liver microsomes of rats, dogs, and humans.

The biotransformation of sufentanil (SUF), an analog of the synthetic opioids fentanyl and alfentanil, was investigated in liver microsomes of rats, dogs, and humans. The drug was extensively metabolized and the metabolism was found to be very similar, both kinetically and metabolically, in the three species. The initial metabolism of SUF occurred monophasically in man and dog and biphasically in the rat over a concentration range of 0.13-20.1 microM. The apparent Vm values were 7.30 and 6.15 nmol metabolized.min-1.mg protein-1, and the apparent Km values were 4.98 microM and 15.2 microM for dog and human microsomes, respectively. In rat microsomes, apparent Km values were 0.10 and 20.8 microM, and the apparent Vm values were 0.10 and 7.32 nmol metabolized.min-1.mg protein-1 for the high and low affinity site, respectively. The major metabolic pathways were similar in the three species and included oxidative N-dealkylation at the piperidine nitrogen, oxidative N-dealkylation of the piperidine ring from the phenylpropanamide nitrogen, oxidative O-demethylation, and aromatic hydroxylation. Desmethyl-SUF was formed at the shorter incubation times but quickly metabolized into secondary metabolites. The major metabolites which could be detected at the end of the incubation were N-[4-(methoxymethyl)-4-piperidinyl]-N-phenylpropanamide, N-[4-(hydroxymethyl)-4-piperidinyl]-N-phenylpropanamide, and N-phenylpropanamide. The relevance of the in vitro results is discussed in relation to previous in vivo studies of the metabolism of SUF in rats, dogs, and humans.

Analgesics↗