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J Vrba

Publications and source records attributed to J Vrba.

At least 19 recordsLinked to original sources
PubMed2001

Signal processing in magnetoencephalography.

The subject of this article is detection of brain magnetic fields, or magnetoencephalography (MEG). The brain fields are many orders of magnitude smaller than the environmental magnetic noise and their measurement represent a significant metrological challenge. The only detectors capable of resolving such small fields and at the same time handling the large dynamic range of the environmental noise are superconducting quantum interference devices (or SQUIDs). The SQUIDs are coupled to the brain magnetic fields using combinations of superconducting coils called flux transformers (primary sensors). The environmental noise is attenuated by a combination of shielding, primary sensor geometry, and synthetic methods. One of the most successful synthetic methods for noise elimination is synthetic higher-order gradiometers. How the gradiometers can be synthesized is shown and examples of their noise cancellation effectiveness are given. The MEG signals measured on the scalp surface must be interpreted and converted into information about the distribution of currents within the brain. This task is complicated by the fact that such inversion is nonunique. Additional mathematical simplifications, constraints, or assumptions must be employed to obtain useful source images. Methods for the interpretation of the MEG signals include the popular point current dipole, minimum norm methods, spatial filtering, beamformers, MUSIC, and Bayesian techniques. The use of synthetic aperture magnetometry (a class of beamformers) is illustrated in examples of interictal epileptic spiking and voluntary hand-motor activity.

Brain↗
PubMed2001

Impact of ionic aluminium on extracellular phosphatases in acidified lakes.

We studied direct inhibiting effects of aluminium (Al) on extracellular phosphatases produced by the plankton of acidified lakes in the Bohemian Forest. In laboratory experiments we tested the effect of different Al concentrations (0-1000 microg l(-1)) on kinetic parameters of acid phosphatases (pH optimum approximately 5.0) at pH between 4.5 and 5.2. We observed a significant reduction of an apparent substrate affinity at Al concentrations between 300 and 1000 microg l(-1) at pH 4.5 and 4.8 (but not at 5.2). In contrast, maximum acid phosphatase activity (AcPA) remained unchanged. Such behaviour of saturation kinetics is compatible with the assumption that ionic Al acts as a competitive inhibitor of acid phosphatases. To decide whether the observed Al effects could be explained alternatively by complexation of Al with substrate, we tested statistically the best fits of data with both possible models (competitive versus complexation). Experimental results supported the competitive hypothesis rather than the complexation model suggested originally by some authors. Furthermore, we tested the Al effect within a wide range of pH from 4.0 to 6.0. For pH values < 5.2, the results of an Al-pH matrix experiment gave a more detailed picture: the higher the Al concentration, the wider the pH range in which Al could negatively affect AcPA. The ecological ramifications of this effect were evaluated in the context of field AcPA data on three strongly acidified lakes.

Acid Phosphatase↗
PubMed2001

Predator-specific enrichment of actinobacteria from a cosmopolitan freshwater clade in mixed continuous culture.

We investigated whether individual populations of freshwater bacteria in mixed experimental communities may exhibit specific responses to the presence of different bacterivorous protists. In two successive experiments, a two-stage continuous cultivation system was inoculated with nonaxenic batch cultures of the cryptophyte Cryptomonas sp. Algal exudates provided the sole source of organic carbon for growth of the accompanying microflora. The dynamics of several 16S rRNA-defined bacterial populations were followed in the experimental communities. Although the composition and stability of the two microbial communities differed, numerous members of the first assemblage could again be detected during the second experiment. The introduction of a size-selectively feeding mixotrophic nanoflagellate (Ochromonas sp.) always resulted in an immediate bloom of a single phylotype population of members of the class Actinobacteria (Ac1). These bacteria were phylogenetically affiliated with an uncultured lineage of gram-positive bacteria that have been found in freshwater habitats only. The Ac1 cells were close to the average size of freshwater bacterioplankton and significantly smaller than any of the other experimental community members. In contrast, no increase of the Ac1 population was observed in vessels exposed to the bacterivorous ciliate Cyclidium glaucoma. However, when the Ochromonas sp. was added after the establishment of C. glaucoma, the proportion of population Ac1 within the microbial community rapidly increased. Populations of a beta proteobacterial phylotype related to an Aquabacterium sp. decreased relative to the total bacterial communities following the addition of either predator, albeit to different extents. The community structure of pelagic microbial assemblages can therefore be influenced by the taxonomic composition of the predator community.

Actinobacteria↗
PubMed2000

Comment to Sherr and Sherr (1999): "Is there any appropriate way to distinguish different beta-N-acetylhexosaminidase activities in aquatic environments?"

The recent paper of Sherr and Sherr on detecting low-affinity beta-glucosaminidase activity in several marine microbes extends current knowledge about hydrolytic enzyme activities in natural aquatic systems. However, their conclusions regarding the whole-cell assay with MUF-N-acetyl-beta-D-glucosaminide (MUF-[GlcNAc]) cannot be accepted. First, we explicitly demonstrate a strong correlation between extracellular activities of the high-affinity enzymes and grazing rates of bacterivorous protists. Therefore, the assay can still be recommended for the estimation of total protistan grazing on prokaryotic picoplankton. Second, the ability of many aquatic organisms to produce enzymes which cleave fluorogenic substrates, such as MUF-[GlcNAc] and/or MUF-beta-D-N,N',N"-triacetylchitotriose (MUF-[GlcNAc](3)), has been well-documented during the last decade. Thus, neither of the two substrates may be considered as exclusively specific for targeting either lysozymes or beta-N-acetylhexosaminidases.

Journal Article↗
PubMed1999

Technical aspects of microwave thermotherapy.

We describe our new technical results dealing with microwave thermotherapy (hyperthermia) in cancer treatment, see Refs. [S.B. Field, C. Franconi (Eds.), Physics and technology of hyperthermia, NATO Seminar Proceedings, Urbino, Italy, 1986; J. Hand, J.R. James (Eds.), Physical Techniques in Clinical Hyperthermia, Wiley, New York, 1986; J. Vrba, M. Lapes, Microwave Applicators for Medical Purposes, CTU Press, 1996, in Czech; J. Vrba, C. Franconi, M. Lapes, Theoretical limits for the penetration depth of the intracavitary applicators, International Journal of Hyperthermia, 12:6 (1996) 737-742; C. Franconi, J. Vrba, F. Montecchia, 27 MHz hybrid evanescent-mode applicators with flexible heating field for deep and safe subcutaneous hyperthermia, International Journal of Hyperthermia, 9:5 (1993) 655-674.]. Our research interest is to develop applicators for deep local heating and for intracavitary cancer and/or prostate treatment as well. Further, a system for 3D SAR distribution measurements in water phantom is explained. Basic evaluation of clinical results is given.

Equipment Design↗
PubMed1996

Evaluation of Extracellular, High-affinity beta-N-acetylglucosaminidase Measurements from Freshwater Lakes: An Enzyme Assay to Estimate Protistan Grazing on Bacteria and Picocyanobacteria

Protistan community grazing rates upon both bacterioplankton and autotrophic picoplankton were estimated using fluorescently-labeled prey and by measurement of extracellular hydrolysis of 4-methylumbelliferyl (MUF) beta-N-acetylglucosaminide in a eutrophic reservoir and an oligo-mesotrophic lake during phytoplankton blooms. In addition, enzyme methods were optimized in bacterivorous flagellate cultures by two enzyme assays, based on fluorometric detection of protistan digestive activity, which were compared and calibrated independently against flagellate bacterivory. Enzymatic hydrolyses of MUF beta-N,N',N''-triacetylchitotriose and MUF beta-N-acetylglucosaminide were measured in cell-free (sonicated) and whole-cell (unsonicated) samples. The hydrolysis of both substrates, using the whole-cell enzyme assay at in situ pH, was correlated significantly with total grazing rate of Bodo saltans. Thus the whole-cell enzyme assay with MUF beta-N-acetylglucosaminide was used for freshwater samples. High-affinity (Km < 1 μmol l-1) and low-affinity (Km > 100μmol l-1) enzymes were distinguished kinetically in most samples from both systems studied. Activities (Vmax) of the high-affinity enzyme varied from 0.24 to 1.43 nmol l-1 h-1. Protistan community grazing on bacterioplankton was in the range of 0.15-1.36 μg C l-1 h-1 both for lake and reservoir, the differences being observed in grazing on picocyanobacteria (lake, 0.03-0.22 μg C l-1 h-1; reservoir, 0.35-1.56 μg C l-1 h-1). The enzyme activities were correlated significantly with the protistan grazing both on bacterioplakton (rs=0.62, P<0.001) and total procaryotic picoplankton (the sum of organic carbon grazed from bacteria and picocyanobacteria, rs=0.73, P<0.001) in the eutrophic reservoir. Weaker relationships (rs=0.42) with a lower slope were found for the oligo-mesotrophic lake. Ingestion rate studies are time-consuming and the digestive enzyme assay with MUF beta-N-acetylglucosaminide presents a rapid alternative for estimating total protistan prokaryotic picoplanktivory in freshwaters.

Journal Article↗
PubMed1995

Modulation of somatosensory evoked magnetic fields by sensory and motor interferences.

Modulatory influences of skilled exploratory finger movements on somatosensory evoked magnetic fields evoked by median nerve stimulation were investigated in six healthy subjects using a whole head magnetometer (MEG) system. The exploratory finger movements caused major changes in the somatosensory evoked fields. The most prominent effect was a reversal of the dipolar magnetic field around 30 ms after median nerve stimulation. Similar but less pronounced effects were exerted by repetitive finger movements and tactile stimulation of the hand. A dipole analysis and super-imposition of resulting sources on individual MRI scans showed that all somatosensory evoked fields up to 60 ms after stimulation, and the modulation of these responses were located in the primary somatosensory cortex (SI).

Adult↗
PubMed1993

Evanescent-mode applicators (EMA) for superficial and subcutaneous hyperthermia.

Evanescent-mode waveguide aperture applicators are proposed for hyperthermic treatments of superficial and subcutaneous tissues. They consist of air-filled waveguide segments which are working below the cutoff frequency, and therefore support only evanescent transverse modes. These are excited by radiators of suitable symmetry and configuration to produce modal heating fields of selected cross-section. This field is emerging from the waveguide active aperture and enters the tissue to be heated through an air gap on non-critical length. These devices work in a very large range of frequencies, and are extremely simple to manufacture even with a variety of cross-section size and shape, on account of their air-filled feature. This enables to obtain good heating field flexibility with improved penetration. Their operation is safe and practical also on irregular and curved tissue surfaces.

Electromagnetic Fields↗
PubMed1991

Biomagnetometers for unshielded and well shielded environments.

The paper describes magnetometers for operation in well shielded and unshielded environments. For unshielded environments, the noise cancellation is accomplished by spatial filtering using higher-order gradiometers which are formed in software. The theory of the software formation of high-order gradiometers has been successfully tested experimentally to the second-order gradient level, using a seven-channel prototype system. An unshielded, open environment system, designed to operate with third-order noise cancellation is presently under construction. It is a whole head system, consisting of a minimum of 64 sensing channels, and is expected to be operational by late 1991. In the opposite extreme, a magnetometer is being developed for operation in a well shielded environment of a whole body superconducting shield. The high temperature superconducting shield will operate in liquid nitrogen, while the magnetometer detection system will operate in liquid helium. The high level of shielding will allow detection of biomagnetic signals using magnetometers. The whole body high temperature superconducting shielded system is presently under development and a half scale working shield already exists. The complete whole body shielded system is expected to be operational in 1994/95.

Equipment Design↗
PubMed1989

Impact of diffuse nitrate pollution sources on groundwater quality--some examples from Czechoslovakia.

In several regions of Czechoslovakia with intensive agricultural production, the correlation between the amount of nitrogen fertilizer applied and the nitrate content in groundwater has been recognized. Nitrate pollution of groundwater is considered to be the most serious source of nonpoint pollution in Czechoslovakia. A program of research into the effects of farming activities on groundwater quality in Czechoslovakia is under way on experimental fields (20 to 30 hectares) and, simultaneously, in regions in which shallow, vulnerable aquifers occur. The importance of the soil organic matter's stability for maintaining the groundwater quality is emphasized. Research based on nitrogen and organic carbon balance has shown that the restoration of a soil-groundwater system is a complicated process that usually requires changes in the extent and intensity of agricultural activities and consistent attention to the effects produced by natural conditions. Regional investigation of the impact of farming on shallow aquifers in the fluvial deposits of the Elbe River in Bohemia has proved the hydrochemical instability and vertical hydrochemical heterogeneity of these aquifers. The WASTEN deterministic model was used for modeling the transport and transformation of various types of inorganic fertilizers. The input data is based on laboratory and field measurements. Special topics are the verification of model calculations and the time and spatial variability of input data with respect to the unsaturated zone. The research results are being used for making regional and national agro-groundwater managerial schemes more precise, as well as for decision-making.

Agriculture↗
PubMed1988

Effects of cysteamine on blood pressure: possible mediation through vasopressin release.

Cysteamine (beta-mercaptoethylamine, CSH) has been reported to have various effects on the neuroendocrine system. Reports indicate CSH decreases pituitary oxytocin (OT) without affecting pituitary vasopressin (VP). However, preliminary studies from our laboratory strongly indicate that CSH has an effect on VP release. Experiments were conducted with dibenzyline-treated, urethane-anesthetized, male Sprague-Dawley (SD) rats. Rats were injected with 4 mU of standard VP and 4 mg/100 g of CSH. Administration of VP resulted in an increase in mean arterial pressure (MAP) of 23.5 +/- 3.2 mm Hg. Administration of CSH resulted in a consistent, immediate decrease in MAP of 13.0 +/- 2.0 mm Hg prior to an increase of 21.0 +/- 2.6 mm Hg. The effects due to VP and CSH were strikingly different; the CSH-induced MAP rise took longer to peak and to return to baseline. Both the VP- and CSH-induced MAP rise were markedly inhibited by a prior administration of a specific VP antagonist d(CH2)5[Tyr(Me)]AVP. In addition, the typical increase in MAP observed in SD rats following CSH administration was substantially reduced when the same dose was administered in homozygous diabetes insipidus (HODI) rats. The data presented here strongly suggest that CSH-induced MAP elevation is due to the release of VP from the pituitary gland.

Animals↗
PubMed1984

The use of a squid third order spatial gradiometer to measure magnetic fields of the brain.

It appears to be clear from the results that the third order gradiometer is able to detect small biomagnetic signals from the brain which are related to evoked potentials and spontaneous electrical activity. The instrument operates reasonably well within a noisy environment, however further development is necessary to balance the first gradient. We intend to pursue this direction with software systems. Some of the data presented suggest that components of MEG evoked activity may change independently of EEG. One interpretation which may derive from this is that the same current dipoles are probably not responsible for the entire configuration of evoked fields. This interpretation is consistent with EEG evidence which indicates that analogous components in the evoked potential may vary independently as a function of stimulus parameters and information processing. Perhaps a model of magnetic dipoles due to small current loops would be more compatible with the electrophysiological data.

Alpha Rhythm↗