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J Vrba

Publications and source records attributed to J Vrba.

17 recordsLinked to original sources
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↗