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At least 487 records · Page 27Linked to original sources

PubMed2005

Radial frequency stimuli and sine-wave gratings seem to be processed by distinct contrast brain mechanisms.

An assumption commonly made in the study of visual perception is that the lower the contrast threshold for a given stimulus, the more sensitive and selective will be the mechanism that processes it. On the basis of this consideration, we investigated contrast thresholds for two classes of stimuli: sine-wave gratings and radial frequency stimuli (i.e., j0 targets or stimuli modulated by spherical Bessel functions). Employing a suprathreshold summation method, we measured the selectivity of spatial and radial frequency filters using either sine-wave gratings or j0 target contrast profiles at either 1 or 4 cycles per degree of visual angle (cpd), as the test frequencies. Thus, in a forced-choice trial, observers chose between a background spatial (or radial) frequency alone and the given background stimulus plus the test frequency (1 or 4 cpd sine-wave grating or radial frequency). Contrary to our expectations, the results showed elevated thresholds (i.e., inhibition) for sine-wave gratings and decreased thresholds (i.e., summation) for radial frequencies when background and test frequencies were identical. This was true for both 1- and 4-cpd test frequencies. This finding suggests that sine-wave gratings and radial frequency stimuli are processed by different quasi-linear systems, one working at low luminance and contrast level (sine-wave gratings) and the other at high luminance and contrast levels (radial frequency stimuli). We think that this interpretation is consistent with distinct foveal only and foveal-parafoveal mechanisms involving striate and/or other higher visual areas (i.e., V2 and V4).

Adult↗

PubMed2003

Stochastic excitation and Hadamard correlation spectroscopy with bandwidth extension in RF FT-EPR.

The application of correlation spectroscopy employing stochastic excitation and the Hadamard transform to time-domain Fourier transform electron paramagnetic resonance (FT-EPR) spectroscopy in the radiofrequency (RF) band is described. An existing, time-domain FT-EPR spectrometer system with a Larmor frequency (L(f)) of 300 MHz was used to develop this technique by incorporating a pseudo-random pulse sequence generator to output the maximum length binary sequence (MLBS, 10- and 11-bit). Software developed to control the EPR system setup, acquire the signals, and post process the data, is outlined. The software incorporates the Hadamard transform algorithm to perform the required cross-correlation of the acquired signal and the MLBS after stochastic excitation. To accommodate the EPR signals, bandwidth extension was accomplished by sampling at a rate many times faster than the RF pulse repetition rate, and subsequent digital signal processing of the data. The results of these experiments showed that there was a decrease in the total acquisition time, and an improved free induction decay (FID) signal-to-noise (S/N) ratio compared to the conventional coherent averaging approach. These techniques have the potential to reduce the RF pulse power to the levels used in continuous wave (CW) EPR while retaining the advantage of time-domain EPR methods. These methods have the potential to facilitate the progression to in vivo FT-EPR imaging of larger volumes.

Animals↗

PubMed2001

Numerical and experimental analysis of e-field scattering from the ground for base station antennae.

Based on the numerical determination of the complete irradiation volume of a commercial RBS antenna--performed using the FDTD method and the Kirchhoff integral formula for near to far field transformation--open site estimations of the electric field are made and compared with experimentally measured values. To describe the actual behaviour of the radiation field, the inherently complex phasic nature of plane waves is taken into account, together with their two independent states of polarisation. This information is contained in the radiation pattern previously deduced. Moreover, a reflected contribution from flat ground is introduced, along with the line-of-sight ray. Amplitude and phase of the reflected wave are calculated using Fresnel formulae for stratified ground and two polarisation states, i.e. normal and parallel to the plane of incidence. Good agreement with measured values is achieved only by using such assumptions.

Electromagnetic Fields↗

PubMed2001

Effects of radiofrequency exposure on the GABAergic system in the rat cerebellum: clues from semi-quantitative immunohistochemistry.

The widespread use of cellular phones raises the problem of interaction of electromagnetic fields with the central nervous system (CNS). In order to measure these effects on neurotransmitter content in the CNS, we developed a protocol of neurotransmitter detection based on immunohistochemistry and image analysis. Gamma-vinyl-GABA (GVG), an inhibitor of the GABA-transaminase was injected in rats to increase GABA concentration in the CNS. The cellular GABA contents were then revealed by immunohistochemistry and semi-quantified by image analysis thanks to three parameters: optical density (O.D.), staining area, and number of positive cells. The increase in cerebellar GABA content induced by GVG 1200 mg/kg was reflected in these three parameters in the molecular and the granular layers. Therefore, control of immunohistochemistry parameters, together with appropriate image analysis, allowed both the location and the detection of variations in cellular neurotransmitter content. This protocol was used to investigate the effects of exposure to 900 MHz radiofrequencies on cerebellar GABA content. Both pulsed emission with a specific absorption rate (SAR) of 4 W/kg and continuous emission with high SAR (32 W/kg) were tested. We observed a selective diminution of the stained processes area in the Purkinje cell layer after exposure to pulsed radiofrequency and, in addition, a decrease in O.D. in the three cell layers after exposure to continuous waves. Whether this effect is, at least partly, due to a local heating of the tissues is not known. Overall, it appears that high energetic radiofrequency exposure induces a diminution in cellular GABA content in the cerebellum.

4-Aminobutyrate Transaminase↗

PubMed1994

[Effect of ultra high frequency electromagnetic waves and lead on the workers' health; phytotherapy of the disorders].

The article deals with the study of combined exposure to ulta-high frequency electromagnetic waves and lead. Neurocirculatory asthenia was diagnosed in almost half of the examinees, cerebrovascular disorders, erythronormoblastic anemia and impaired porphyric metabolism were seen in some cases. Vegetable drugs improved the health status of examinees and corrected the changed parameters.

Electromagnetic Phenomena↗

PubMed1993

Present and future status of noninvasive selective deep heating using RF in hyperthermia.

To achieve hyperthermia using electromagnetic energy, RF of under 100 MHz is basically suitable for the external heating of the deep portions of the body. For applicators using such RF, the following types are considered: capacitive, inductive, radiative and hybrid. With radiative applicators, the intensity of the EM waves radiated from the applicator decreases with propagation into the material to be heated, but the phased annular array of radiative applicators potentially increases the intensity of the EM energy in the deep portion owing to the interference of the waves. Using this method, the focusing of EM energy depends on the dielectric properties of the material to be heated. With respect to RF heating at a lower frequency than the RF used for the annular phased array, some devices have been said to concentrate EM energy in the deep portions, where the characteristics of 'wave' are not utilised. To this end, some methods using capacitive electrodes, an inductive coil, or a combination of both, are being designed. The results of using such methods have shown that it is possible to supply sufficient EM energy to the muscle layers deep in the material to be heated, without heating the fat layers excessively.

Humans↗

PubMed1984

Biological studies with continuous-wave radiofrequency (28 MHz) radiation.

Effects of high-frequency (28 MHz) continuous-wave radiation have been studied in the rat and monkey. No histopathological (rat-125 mW cm-2 for 28 days) or hematological (monkey-25 mW cm-2 for 24 days) changes could be attributed to the radiation. In the monkey (125 mW cm-2 for 11 days) there was an increase in urinary calcium concentration which was most likely due to restricted movement. In the rat (220 mW cm-2 for 13 days) there was reduced uptake of iodine by the thyroid, lower levels of plasma thyroid-stimulating hormone, and reduced ratio of protein bound to nonprotein bound iodine. Food consumption was also decreased. The changes are likely to have arisen as a compensatory response to an induced heat load. A nonthermal effect of continuous-wave high-frequency radiation has not been shown in this study. The effects were likely to be associated with either physiological compensation for induced heating or restriction of movement.

Animals↗

PubMed1987

Mitogen responsiveness after exposure of influenza virus-infected human mononuclear leukocytes to continuous or pulse-modulated radiofrequency radiation.

Data are available regarding interactions of radiofrequency radiation (RFR) with normal human mononuclear leukocytes. However, no data have emerged regarding effects of RFR on human leukocytes already challenged by a commonly encountered alternate agent, such as a virus. Therefore, in these studies, uninfected (control) and in vitro influenza virus-infected human mononuclear leukocytes were exposed to 2450 MHz RFR as continuous waves or pulse-modulated at 60 or 16 Hz, at a specific absorption rate of 4 mW/ml. Such exposures produced no significant effects on leukocyte viability or on mitogen-stimulated DNA synthesis by either uninfected or influenza virus-infected leukocytes when compared to sham-RFR-exposed of cells.

Humans↗

PubMedDate not supplied

[Specificity of the effect of a super high frequency pulse-modulated electromagnetic field on evoked potentials of the cat visual, auditory,and sensomotor cortex upon stimulation with light and sound].

Effects of a pulsed-modulated superhigh frequency field (950 MHz, peak power of 300 mkW/cm2 and 1.5 mW/cm2, pulse duration of 1 ms, single pulse irradiation mode, a pause between an electromagnetic pulse and an optic or auditory stimulus--20 ms) on the cat's brain evoked potentials were investigated in the optic, auditory and sensomotor cortex using acoustic and visual stimulation. It was established that during an exposure to electromagnetic waves with peak power of 300 mkW/cm2 there were no reliable changes of the amplitude characteristics; but the amplitude of the auditory evoked potentials was reliably higher and the amplitude of the optic evoked potentials was reliably lower when they were affected by an electromagnetic field with peak power of 1.5 mW/cm2.

Acoustic Stimulation↗

PubMed2004

Development of numerical phantoms by MRI for RF electromagnetic dosimetry: a female model.

Numerical human models for electromagnetic dosimetry are commonly obtained by segmentation of CT or MRI images and complex permittivity values are ascribed to each issue according to literature values. The aim of this study is to provide an alternative semi-automatic method by which non-segmented images, obtained by a MRI tomographer, can be automatically related to the complex permittivity values through two frequency dependent transfer functions. In this way permittivity and conductivity vary with continuity--even in the same tissue--reflecting the intrinsic realistic spatial dispersion of such parameters. A female human model impinged by a plane wave is tested using finite-difference time-domain algorithm and the results of the total body and layer-averaged specific absorption rate are reported.

Adult↗

PubMedDate not supplied

Measurement of hertzian waves capture at body surface in patients with traumatic paraplegia.

Using an original method--the Biohertzmetry--the level of hertzian waves capture at body surface was measured in 20 patients with traumatic paraplegia. A statistically significant decrease of the hertzian waves capture level in the toes in comparison with the proximal area has been noted in 12 patients with complete spinal cord lesion. In patients presenting an open or cicatricial decubitus ulcer, at scar level, in the surrounding skin area with modified colour--and even in a contiguous few millimeters large skin strip normally coloured,--a marked decrease of the level of hertzian waves capture in comparison with the healthy skin ares situated off the decubitus ulcers has been pointed out. Thus, the method used permitted to reveal the alterations of the tissue trophicity--including those undetectable with the naked eye--and to make an estimation of the degree of mortification of the tissues and a delimitation of the reduced trophicity area.

Absorption↗

PubMed1983

31P NMR spectroscopy of rat organs, in situ, using chronically implanted radiofrequency coils.

A technique for making 31P NMR spectroscopic measurements in rat kidney, heart, and liver in vivo is presented. Two-turn solenoid coils were surgically implanted around the organ sufficiently in advance of NMR experiments to allow recovery of the animal. These chronically implanted coils allowed acquisition of high-resolution spectra at 40.5 and 97.3 MHz. No resolution improvement occurred at the higher field. Spectra were stable for up to 24 hr, during which time a variety of experiments could be performed. By accumulating spectra at 10-min intervals, the effects of intraperitoneal fructose injections were monitored; in kidney and liver, a rapid increase in sugar phosphates at the expense of Pi and ATP resulted. Fructose had no effect on heart metabolite levels. Spectra from the heart in vivo were obtained at systole and diastole by gating the spectrometer to the aortic pressure wave; no differences in phosphate metabolites were detected. Finally, saturation transfer techniques were used to monitor the rate of ATP synthesis in the kidney. The unidirectional rate constant for the conversion of Pi to ATP was 0.12 +/- 0.03 sec-1.

Adenosine Diphosphate↗

PubMed2002

The origin of ultrasound-induced friction reduction in microscopic mechanical contacts.

We present a study of the origin of ultrasound-induced friction reduction in microscopic mechanical contacts. The effect of friction reduction caused by Rayleigh-type surface acoustic waves (SAWs) is demonstrated for propagating and two-dimensional, standing wave fields using lateral force microscopy (LFM). It is shown that with increasing wave amplitude, friction is completely suppressed. To detect and distinguish between the effect of lateral and vertical surface oscillation components on the cantilever movement, we employed multimode scanning acoustic force microscopy (SAFM). We found that the friction reduction effect is only due to the vertical oscillation component. Because this effect does not appear for purely in-plane polarized Love waves, we concluded that the mechanical diode effect is most probably responsible for the SAW-induced lubrication. This explanation is also supported by vertical and longitudinal SAFM measurements, which show that, in areas where friction is completely suppressed, low frequency vertical cantilever oscillations can still be observed, whereas lateral or torsional oscillations are no longer excited.

Acoustics↗

PubMed1997

Radiofrequency ablation of tachyarrhythmias in patients with Ebstein's anomaly.

We performed radiofrequency catheter ablation in five patients associated with Ebstein's anomaly to cure their refractory tachyarrhythmias. The presenting arrhythmias were four cases of orthodromic circus movement tachycardia using accessory pathways as a requisite limb, including one case of a Mahaim fiber and one of atrial flutter of common variety. All accessory pathways, including the Mahaim fiber, were ablated by RF energy delivered through the catheter placed at the AV annulus rather than the displaced anatomical AV groove. Interestingly, the antegrade or retrograde conduction interval over these accessory pathways was relatively longer than that of usual accessory pathways, and the accessory pathway potential was fractionated in some cases. The location of the atrioventricular node was displaced from the usual position to the postero-inferior area of Koch's triangle in one case. The configuration of the flutter wave was larger than usual in height as well as in width. All tachyarrhythmias were cured by RF catheter ablation. In the case of RF catheter ablation for patients with Ebstein's anomaly, close attention is indispensable in order to accomplish it safely and successfully, because of the anatomical and functional differences peculiar to Ebstein's anomaly.

Cardiac Catheterization↗

PubMed1996

Single- and double-strand DNA breaks in rat brain cells after acute exposure to radiofrequency electromagnetic radiation.

We investigated the effects of acute (2-h) exposure to pulsed (2-micros pulse width, 500 pulses s(-1)) and continuous wave 2450-MHz radiofrequency electromagnetic radiation on DNA strand breaks in brain cells of rat. The spatial averaged power density of the radiation was 2mW/cm2, which produced a whole-body average-specific absorption rate of 1.2W/kg. Single- and double-strand DNA breaks in individual brain cells were measured at 4h post-exposure using a microgel electrophoresis assay. An increase in both types of DNA strand breaks was observed after exposure to either the pulsed or continuous-wave radiation, No significant difference was observed between the effects of the two forms of radiation. We speculate that these effects could result from a direct effect of radiofrequency electromagnetic energy on DNA molecules and/or impairment of DNA-damage repair mechanisms in brain cells. Our data further support the results of earlier in vitro and in vivo studies showing effects of radiofrequency electromagnetic radiation on DNA.

Animals↗

PubMed1992

Estimates of absorption of radiofrequency radiation by the embryo and fetus during pregnancy.

This paper reports that the specific absorption rate induced in the embryo or fetus can exceed that recommended for the general public when the mother is exposed to radiofrequency radiation at the occupational limits. This result applies to two-tiered radiofrequency radiation standards where a factor of 5 differentiates occupational and nonoccupational exposure limits. Using simple axisymmetric geometries for the pregnant worker, and assuming plane wave exposures, a finite element method provides estimates of prenatal exposure. Various layered shapes are used to model skin, fat, uterus, blood, embryonic, and fetal tissues. Applying current exposure limits given by IRPA, ANSI, and SAA, the results indicate that overexposures to the embryo or fetus can occur from early pregnancy at 80-100 MHz, and in late pregnancy across the range 300-1500 MHz.

Embryo, Mammalian↗

PubMedDate not supplied

[The morphofunctional status of the hypothalamo-hypophyseal neurosecretory system and the energy processes in the gastric tissues following electromagnetic irradiation in the millimeter wave range].

A single and double 15 and 30 min exposure to electromagnetic waves of millimetric range (60 GGz, 3 mW/cm2) of a biological active point (acupuncture point) EIII36 of intact male rats and those subjected to stress inhibits the functional activity of hypothalamo-hypophysis neurosecretory system, thyroid gland and cell elements of gastric glands the inhibition being mostly pronounced in animals under stress.

Acupuncture Points↗

PubMed1999

Reasons of poor replicability of nonthermal bioeffects by millimeter waves.

It is shown that the reasons for the poor replicability of the nonthermal bioeffects of radiofrequency have the fundamental bases. The efficiency of the living system process is determined by the conversion of electromagnetic energy into Helmholtz free energy. The efficiency dependence on the absorbed power of electromagnetic radiation is quite different in the Wien region (visible light) and in the Rayleigh-Jeans region (radiofrequency). In the Wien region the endergonic and exergonic processes could never be confused. In the Rayleigh-Jeans region the endergonic and exergonic processes could be confused. It gives the poor replicability.

Electromagnetic Fields↗