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At least 451 records · Page 25Linked to original sources
PubMed1987

Hippocampal electrical activity following local tetanization. I. Afterdischarges.

Following a short (1-10 s) train of repetitive stimulation delivered to the hippocampal CA1 region, the following sequelae of afterdischarges (ADs) was seen: (1) a silent period of 2-4 s, (2) a large primary (1 degree) AD usually alvear-surface negative and deep positive, (3) a period of suppressed hippocampal EEG, (4) a secondary (2 degrees) hippocampal AD, and after 3-6 min, (5) 15-25 min of enhanced (up to 10 times normal) fast (30-70 Hz) waves. The 2 degrees hippocampal AD was preceded by or simultaneous with large AD at the amygdaloid electrodes. Electrolytic lesions (n = 7) or large heat lesions of the amygdala (n = 5) or electrolytic lesions of the medial septum (n = 10) were not successful in suppressing the 2 degrees hippocampal AD. However, 4 rats with radiofrequency lesion and 3 rats with bilateral aspiration lesion of the entorhinal cortex had diminished or no 2 degrees hippocampal AD. The fast waves after tetanization were reversed 180 degrees across surface and deep CA1 electrodes. The fast wave increase was blocked by atropine sulfate (25-50 mg/kg i.p.), scopolamine hydrochloride (5 mg/kg i.p.) and medial septal lesions. It was concluded that the 2 degrees hippocampal AD may depend on a reverberation of neural circuitry involving the entorhinal cortex. The 2 degrees AD recorded from amygdala electrodes may partly reflect spreading of activities from the entorhinal cortex. On the other hand, the increase in fast waves after tetanization requires an intact septohippocampal, muscarinic cholinergic input, and may depend on an enhanced cholinergic input or an increased response.

Action Potentials↗
PubMed2000

Primary DNA damage in human blood lymphocytes exposed in vitro to 2450 MHz radiofrequency radiation.

Human peripheral blood samples collected from three healthy human volunteers were exposed in vitro to pulsed-wave 2450 MHz radiofrequency (RF) radiation for 2 h. The RF radiation was generated with a net forward power of 21 W and transmitted from a standard gain rectangular antenna horn in a vertically downward direction. The average power density at the position of the cells in the flask was 5 mW/cm(2). The mean specific absorption rate, calculated by finite difference time domain analysis, was 2.135 (+/-0.005 SE) W/kg. Aliquots of whole blood that were sham-exposed or exposed in vitro to 50 cGy of ionizing radiation from a (137)Cs gamma-ray source were used as controls. The lymphocytes were examined to determine the extent of primary DNA damage (single-strand breaks and alkali-labile lesions) using the alkaline comet assay with three different slide-processing schedules. The assay was performed on the cells immediately after the exposures and at 4 h after incubation of the exposed blood at 37 +/- 1 degrees C to allow time for rejoining of any strand breaks present immediately after exposure, i.e. to assess the capacity of the lymphocytes to repair this type of DNA damage. At either time, the data indicated no significant differences between RF-radiation- and sham-exposed lymphocytes with respect to the comet tail length, fluorescence intensity of the migrated DNA in the tail, and tail moment. The conclusions were similar for each of the three different comet assay slide-processing schedules examined. In contrast, the response of lymphocytes exposed to ionizing radiation was significantly different from RF-radiation- and sham-exposed cells. Thus, under the experimental conditions tested, there is no evidence for induction of DNA single-strand breaks and alkali-labile lesions in human blood lymphocytes exposed in vitro to pulsed-wave 2450 MHz radiofrequency radiation, either immediately or at 4 h after exposure.

Comet Assay↗
PubMed2003

[Medical implantable devices and electromagnetic compatibility].

Continuing progress in medicine has led to a corresponding population growth among the elderly population resulting in an increase in the number of patients with active implanted medical devices. At the same time, there continues to be a proliferation of electromagnetic wave sources within our technological environment. The coexistence of implanted active medical devices and environmental electromagnetic waves requires particular attention in order to avoid electromagnetic interference. For this reason, experts are more and more involved in writing specific manufacturing standards.

Defibrillators, Implantable↗
PubMed2001

Predicted SAR in Sprague-Dawley rat as a function of permittivity values.

Specific absorption rate (SAR) value is dependent on permittivity value. However, variability in the published permittivity values for human and animal tissue and the development of sophisticated 3-dimensional digital anatomical models to predict SAR values has resulted in the need to understand how model parameters (permittivity value) affect the predicted whole body and localized SAR values. In this paper, we establish the partial derivative of whole body SARs and localized SAR values (defined as SAR for individual organs with respect to a change in the permittivity values of all tissue types, as well as for those tissues with the most variable permittivity values. Variations in the published permittivity values may substantially influence whole body and localized SAR values, but only under special conditions. Orientation of the exposed object to the incident electromagnetic wave is one of the most crucial factors. Published 2001 Wiley-Liss, Inc.

Animals↗
PubMed2004

The RF-powered surface wave sensor oscillator--a successful alternative to passive wireless sensing.

A novel, passive wireless surface acoustic wave (SAW) sensor providing a highly coherent measurand proportional frequency, frequency modulated (FM) with identification (ID) data and immune to interference with multiple-path signals is described. The sensor is appropriate for bandwidth-limited applications requiring high-frequency accuracy. It comprises a low-power oscillator, stabilized with the sensing SAW resonator and powered by the rectified radio frequency (RF) power of the interrogating signal received by an antenna on the sensor part. A few hundred microwatts of direct current (DC) power are enough to power the sensor oscillator and ID modulation circuit and achieve stable operation at 1.0 and 2.49 GHz. Reliable sensor interrogation was achieved over a distance of 0.45 m from a SAW-based interrogation unit providing 50 mW of continuous RF power at 915 MHz. The -30 to -35 dBm of returned sensor power was enough to receive the sensor signal over a long distance and through several walls with a simple superheterodyne FM receiver converting the sensor signal to a low measurand proportional intermediate frequency and retrieving the ID data through FM detection. Different sensor implementations, including continuous and pulsed power versions and the possibility of transmitting data from several measurands with a single sensor, are discussed.

Electronics↗
PubMed2001

Monitoring of acidified milk gel formation by ultrasonic shear wave measurements. High-frequency viscoelastic moduli of milk and acidified milk gel.

In the present paper we describe the application of the thickness shear mode resonator technique for the measurement of viscoelastic parameters of milk and acidified milk gels in the megahertz frequency range. The technique provides information on the viscoelasticity of milk and milk gels in the time scale of 10(-8) to 10(-7) s. The length scale of the measurements, determined by the depth of penetration and the wavelength of the shear wave, falls between the submicron and micron range. In milk acidified by glucono-delta-lactone we observed an increase in the high- and low-frequency loss modulus, G", below pH 5.1, indicating aggregation of casein particles into clusters. There was a sharp rise in high- and low-frequency storage (G') and loss (G") moduli between pH 4.85 and 4.7, as a result of gel network formation in acidified milk. Both G" and G' of milk gel in the megahertz frequency range are several orders higher than those we obtained at low frequencies (0.02 to 10 Hz) using dynamic rheology. The high-frequency (5 to 25 MHz) viscosity of milk was found to be the same as at low frequencies. Overall, our results demonstrate the high sensitivity of the ultrasonic shear wave measurements to the changes in the rheological parameters in acidified milk during gelation.

Animals↗
PubMedDate not supplied

[Possible mechanism of specific action of pulsed UHF-fields].

Experimental data were obtained for conditions of stimulation of mechanical vibrations in the liquid models of microwave fields. Possible role of different elastic waves modes in the formation of specific effects in pulsed HF-fields is discussed. Mathematical estimation suggests that excited shearing waves may have biological action. The results obtained suggest an acoustic nature of the action of pulsed HF-fields on biological objects.

Action Potentials↗
PubMed1985

Energy deposition in a model of man in the near field.

The spatial distribution of the specific absorption rate (SAR) was measured in a full-scale model of man using implantable electric field probes. The model was exposed in the near-field of linear and aperture antennas at 350 MHz. Effects of the wave polarization, antenna position and antenna gain on the SAR distribution and the average SAR in the whole-body and body parts are reported.

Humans↗
PubMed1990

Cholinergic neurons of the feline pontomesencephalon. I. Essential role in 'Wave A' generation.

Wave A in the cat appears to be analogous to P1 in the human. Both are positive middle-latency auditory-evoked potentials, present at slow click rates during wakefulness and REM sleep but absent during slow-wave sleep. Wave A has been recorded in the parabrachial and medial tegmental areas of the midbrain and in thalamic target projections of the reticular activating system. Two nuclei in this system, the pedunculopontine tegmental (PPT) and laterodorsal tegmental (LDT) nuclei, contain cholinergic cells; the cholinergic antagonist scopolamine eliminates Wave A. To test whether PPT and LDT were important in Wave A generation, we attempted to lesion these nuclei bilaterally in 11 cats. Wave A was markedly diminished or absent in all but 2 cats, in which the lesions did not include PPT. Loss of choline acetyltransferase-positive cells in PPT, but not LDT, was correlated with effects on Wave A, i.e. greatest cell loss occurred in cats in which Wave A disappeared, and least cell loss in cats with no change in Wave A. We conclude that the PPT nucleus, and particularly its cholinergic cell component, is essential for Wave A generation and suggests that a similar substrate may be significant for generation of the human P1.

Animals↗
PubMed2004

The role of viscosity in the impulse diffraction field of elastic waves induced by the acoustic radiation force.

Several ultrasound-based techniques for the estimation of soft tissue elasticity are currently being investigated. Most of them study the medium response to dynamic excitations. Such responses are usually modeled in a purely elastic medium using a Green's function solution of the motion equation. However, elasticity by itself is not necessarily a discriminant parameter for malignancy diagnosis. Modeling viscous properties of tissues could also be of great interest for tumor characterization. We report in this paper an explicit derivation of the Green's function in a viscous and elastic medium taking into account shear, bulk, and coupling waves. From this theoretical calculation, 3D simulations of mechanical waves in viscoelastic soft tissues are presented. The relevance of the viscoelastic Green's function is validated by comparing simulations with experimental data. The experiments were conducted using the supersonic shear imaging (SSI) technique which dynamically and remotely excites tissues using acoustic radiation force. We show that transient shear waves generated with SSI are modeled very precisely by the Green's function formalism. The combined influences of out-of-plane diffraction, beam shape, and shear viscosity on the shape of transient waves are carefully studied as they represent a major issue in ultrasound-based viscoelasticity imaging techniques.

Acoustics↗
PubMed2002

A fast Green's function method for the analysis of IDT's for acousto-optical devices.

Surface acoustic wave (SAW) interdigital transducers are key components in X-Y LiNbO3 acousto-optical (A-O) devices. SAW interdigital transducers (IDT's) on this substrate exhibit a high spurious resonance that may reduce the A-O efficiency. In this paper we present a detailed analysis of X-Y LiNbO3 IDT's based on a fast Green's function method (GFM). In order to correctly evaluate the spurious effects on IDT's performance, we also considered bulk terms of the Green's function. When the GFM is applied to IDT's with general topology and over a wide frequency range, the required computation time can reach quickly unacceptable values for long IDT structures. We developed a new model order reduction technique based on the singular value decomposition (SVD) for the fast generation of the IDT's frequency response. Numerical results for different configurations of X-Y LiNbO3 IDT's are in good agreement with measured data and a correct interpretation of the spurious resonance is reported. It is pointed out that bulk wave excitation may be a serious limitation in the design of efficient, wide band IDT's for A-O devices.

Acoustics↗
PubMedDate not supplied

[Visualization of the effects of millimeter radiation on blood plasma].

Experiments on millimeter wave radiation (range 37.5-78.5 Ghz incident radiation power density 1-10 mVt/cm2) upon index refraction of 2-10% water solution of human blood plasma are presented. Investigations were carried out by holographic interferometry with sufficient sensitivity for estimating the index refraction -10(-6). Index refraction variations in 2% water solution of human blood plasma are detected to be equal to 10(-4). Such index refraction variations upon millimeter wave is one order greater than that of the temperature effect and two order greater than the sensitivity of settlement. The mechanism of these phenomena is discussed.

Blood↗
PubMed2000

Heating around intravascular guidewires by resonating RF waves.

We examined the unwanted radiofrequency (RF) heating of an endovascular guidewire frequently used in interventional magnetic resonance imaging (MRI). A Terumo guidewire was partly immersed in an oblong saline bath to simulate an endovascular intervention. The temperature rise of the guidewire tip during an FFE sequence [average specific absorption rate (SAR) = 3.9 W/kg] was measured with a Luxtron fluoroscopic fiber. Starting from 26 degrees C, the guidewire tip reached temperatures up to 74 degrees C after 30 seconds of scanning. Touching the guidewire may cause sudden heating at the point of contact, which in one instance caused a skin burn. The excessive heating of a linear conductor like the guidewire can only be explained by resonating RF waves. The capricious dependencies of this resonance phenomenon on environmental factors have severe consequences for predictability and safety guidelines.

Blood Vessels↗
PubMed1992

Assessment of the arterial distension waveform using Doppler signal processing.

AIM: To determine the distension waveform of an artery. METHODS: Ultrasound signals returned by moving structures were processed by pulsed Doppler in order to detect the displacement of the signals with respect to the transducer as a continuous function of time. The results of two sample volumes combined, one located at the anterior and the other at the posterior wall of an artery, yielded the distension waveform of the artery. The distance between the sample volumes in combination with the distension from diastole to systole provided the local distensibility and compliance coefficients. To prevent any part of the structure under observation from moving outside the sample volume, the position of the sample volume was adjusted according to the displacement detected. A disadvantage with this method is that the phase interference of closely spaced structures, i.e. layers of the arterial wall, and the readjustment of the sample volume position may cause artifacts but these can be effectively suppressed by proper processing. CONCLUSIONS: The phase delay between two distension waveforms assessed simultaneously at a known distance can provide the local pulse wave velocity. Subsequently, the Moens-Korteweg relationship, expressing the relationship between pulse wave velocity and the distensibility coefficient, can be used to estimate the distensibility coefficient without measuring the local pulse pressure. The techniques, however, need further evaluation.

Arteries↗
PubMed2001

Micronuclei in the peripheral blood and bone marrow cells of rats exposed to 2450 MHz radiofrequency radiation.

PURPOSE: To determine the incidence of micronuclei in peripheral blood and bone marrow cells of rats exposed continuously for 24h to 2450 MHz continuous wave radiofrequency radiation (RFR) at an average whole-body specific absorption rate (SAR) of 12W/kg. MATERIALS AND METHODS: Eight adult male Sprague-Dawley rats were exposed to 2450 MHz RFR in circularly polarized waveguides. Eight sham-exposed rats were kept in similar waveguides without the transmission of RFR. Four rats were treated with mitomycin-C (MMC) and used as positive controls. All rats were necropsied 24h after the end of RFR and sham exposures, and after the 24h treatment with MMC. Peripheral blood and bone marrow smears were examined to determine the frequency of micronuclei (MN) in polychromatic erythrocytes (PCE). RESULTS: The results indicated that the incidence of MN/2000 PCE were not significantly different between RFR- and sham-exposed rats. The group mean frequencies of MN in the peripheral blood were 2.3+/-0.7 in RFR-exposed rats and 2.1+/-0.6 in sham-exposed rats. In bone marrow cells, the average MN incidence was 3.8+/-1.0 in RFR-exposed rats and 3.4+/-0.7 in sham-exposed rats. The corresponding values in positive control rats treated with MMC were 23.5+/-4.7 in the peripheral blood and 33.8+/-7.4 in bone marrow cells. CONCLUSION: There was no evidence for the induction of MN in peripheral blood and bone marrow cells of rats exposed for 24h to 2450 MHz continuous wave RFR at a whole body average SAR of 12 W/kg.

Animals↗
PubMed1997

FDTD calculations of the whole-body averaged SAR in an anatomically realistic voxel model of the human body from 1 MHz to 1 GHz.

This paper presents finite-difference time-domain (FDTD) calculations of the whole-body averaged SAR in an anatomically realistic voxel model of the human body. This model, NORMAN, consists of approximately 9 million voxels, of 2 mm dimension in the adult phantom, segmented into 37 tissue types. SAR values are presented for an adult phantom and for scaled 10, 5 and 1 year old models, grounded and isolated in air from 1 MHz to 1 GHz for plane wave exposure. External electric field values corresponding to a whole-body averaged SAR of 0.4 W kg-1 are also presented.

Absorption↗
PubMed2006

Observations of polarized seismoacoustic T waves at and beneath the seafloor in the abyssal Pacific ocean.

Combined seismic and hydrophone observations show that the traditional T wave propagates as a seismoacoustic polarized interface wave (Ti) coupled to the seafloor. Seismoacoustic Ti waves propagating at the sound speed of water are routinely observed over megameter distances at the deep (4979 m) seafloor Hawaii-2 Observatory (H2O) between Hawaii and California, even though the seafloor site is within a shadow zone for acoustic wave propagation. Ti has also been observed on seismometers 225 km SSW of Oahu at the OSN1 site at the seafloor and within an ODP borehole into the basalt basement. Analyses of timing, apparent velocity, energy, and polarization of these interface waves are presented. At low frequency (< approximately 5 Hz) Ti propagates dominantly in the sediments and is consistent with higher-mode Rayleigh waves. At higher frequencies the observed Ti waves dominantly propagate acoustically with characteristics suggesting local scattering. The observation of Ti from an earthquake in Guatemala at OSN1, whose path is blocked by the Island of Hawaii, is consistent with scattering from the vicinity of the Cross Seamount.

Acoustics↗