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At least 811 records · Page 45Linked to original sources
PubMed2000

Theoretical model for an MRI radio frequency resonator.

A theoretical model is described for a magnetic resonance imaging (MRI) radio-frequency resonator (an MRI "coil") that is useful at ultrahigh frequencies. The device is a "TEM resonator," which is based on a concept originally proposed by Röschmann (1988). The coil comprises a circular cavity-like structure containing several coaxial transmission lines operating in a transverse electromagnetic (TEM) mode. The model developed herein treats the empty coil and is based on multiconductor transmission line theory. This work generalizes and extends similar analyses of the device by Röschmann (1995) and Chingas and Zhang (1996). The model employs explicit calculation of per-unit-length parameters for TEM lines having arbitrary geometries. Calculations of the resonator's frequency response are found to compare well with measurements. Fields produced by linear (single-point) and quadrature drive are also computed and compared to images of low-permittivity phantoms.

Electromagnetic Fields↗
PubMed2002

Radio-frequency heating of the cornea: theoretical model and in vitro experiments.

We present a theoretical model for the study of cornea heating with radio-frequency currents. This technique is used to reshape the cornea to correct refractive disorders. Our numerical model has allowed the study of the temperature distributions in the cornea and to estimate the dimensions of the lesion. The model incorporates a fragment of cornea, aqueous humor, and the active electrode placed on the cornea surface. The finite element method has been used to calculate the temperature distribution in the cornea by solving a coupled electric-thermal problem. We analyzed by means of computer simulations the effect of: a) temperature influence on the tissue electrical conductivity; b) the dispersion of the biological characteristics; c) the anisotropy of the cornea thermal conductivity; d) the presence of the tear film; and e) the insertion depth of the active electrode in the cornea, and the results suggest that these effects have a significant influence on the temperature distributions and thereby on the lesion dimensions. However, the cooling of the aqueous humor in the endothelium or the realistic value of the cornea curvature did not have a significant effect on the temperature distributions. An experimental model based on the lesions created in rabbit eyes has been used in order to compare the theoretical and experimental results. There is a tendency toward the agreement between experimental and theoretical results, although we have observed that the theoretical model overestimates the lesion dimension.

Animals↗
PubMed2003

Assessment of myocardial lesion size during in vitro radio frequency catheter ablation.

We report our experience with a system that utilizes changes in several biophysical characteristics of cardiac tissue to determine lesion formation and to estimate lesion size both on and off-line in vitro during radio frequency (RF) energy delivery. We analyzed the reactive and resistive components of tissue impedance and tracked the change of phase angle during RF ablation. We correlated the amount of tissue damage with these and other biophysical parameters and compared them with off-line analysis. We found that there are irreversible changes in the reactive and resistive components of impedance that occurred during tissue ablation. The irreversible changes of these components are greater in magnitude, and correlate better with the size of lesions than that of impedance alone that is currently used. Numerically, the best single on-line and off-line correlation for combined perpendicular and parallel electrode orientation was with phase angle. On-line and off-line capacitance and susceptance correlations were essentially similar suggesting that they may be useful as lesion size predictors, given these parameter's persistent change without temperature sensitivity. This study indicates that it is technically feasible to assess lesion formation using biophysical parameters.

Animals↗
PubMed2003

Radio-wave exposure of the human head: analytical study based on a versatile eccentric spheres model including a brain core and a pair of eyeballs.

A versatile eccentric-spheres model of the human head is used in this paper to investigate radio-wave absorption. Numerical results, obtained by use of an exact analytical solution, are presented for the total, percentage, and gram-specific absorption. Interest is mainly in the brain and in the eyes of an adult or an infant head. Our model comprises a host sphere and several spherical inclusions, all concentrically stratified with respect to their own center. Any number of inclusions and any number of concentric layers for the host sphere and each one of the inclusions can be considered. Excitation is provided either by a plane-wave or by a nearby electric dipole. The analytical solution is obtained by use of the indirect-mode matching method. The theory of this paper and the accompanying computer code constitute a versatile tool for analytical studies of cellular-phone interactions with the human head. Specific absorption rate maps in a horizontal cross section of the head model manifest the existence of hot spots in the eyes and near the center of the brain.

Adult↗
PubMed2006

A gradual noisy chaotic neural network for solving the broadcast scheduling problem in packet radio networks.

In this paper, we propose a gradual noisy chaotic neural network (G-NCNN) to solve the NP-complete broadcast scheduling problem (BSP) in packet radio networks. The objective of the BSP is to design an optimal time-division multiple-access (TDMA) frame structure with minimal TDMA frame length and maximal channel utilization. A two-phase optimization is adopted to achieve the two objectives with two different energy functions, so that the G-NCNN not only finds the minimum TDMA frame length but also maximizes the total node transmissions. In the first phase, we propose a G-NCNN which combines the noisy chaotic neural network (NCNN) and the gradual expansion scheme to find a minimal TDMA frame length. In the second phase, the NCNN is used to find maximal node transmissions in the TDMA frame obtained in the first phase. The performance is evaluated through several benchmark examples and 600 randomly generated instances. The results show that the G-NCNN outperforms previous approaches, such as mean field annealing, a hybrid Hopfield network-genetic algorithm, the sequential vertex coloring algorithm, and the gradual neural network.

Electricity↗
PubMed2002

Processing radio frequency ultrasound images: a robust method for local spectral features estimation by a spatially constrained parametric approach.

Spectral estimation is a major component in studies aiming at characterizing biological tissues through the analysis of backscattered radio frequency (RF) ultrasonic signals and images. However, conventional spectral estimation techniques yield a well-known trade-off between spatial resolution and variance. The backscattered signals are stochastic by nature, so short-term local analysis results in a high variance of the estimates, which cannot efficiently be reduced through conventional spatial averaging. We address this issue by describing a spectral estimation technique that reduces the variance of the estimates (by smoothing the local estimates in spectrally homogeneous regions) while preserving spectral discontinuities (i.e., the smoothing is not performed across regions with different spectral contents). The proposed approach is set in a Bayesian framework and is based on local autoregressive (AR) estimation, constrained by smoothness priors. These smoothness priors are introduced through a Markov random field in which the associated potential functions are nonquadratic, allowing thereby to preserve discontinuity. The method is validated on simulated RF images and tested on echocardiographic images acquired in vivo. The results are compared to the estimates provided by the conventional Burg technique. These results clearly demonstrate the ability of the proposed approach to improve spectral estimation in terms of variance reduction and discontinuity detection.

Algorithms↗
PubMed2006

Statistics of the radio-frequency signal based on K distribution with application to echocardiography.

We study in this paper the statistics of the radio frequency (RF) signal in the case of partially developed speckle. Using the K distribution framework, we give the probability density function of the associated distribution, the corresponding moments, and estimators for the parameters of the distribution. The consistency of the proposed estimators is evaluated in terms of their bias and variance through numerical simulations. The ability of the proposed distribution to model RF echographic signals from cardiac tissues is evaluated from data acquired in vivo.

Algorithms↗
PubMed2005

Impact of radio frequency electromagnetic radiation on DNA integrity in the male germline.

Concern has arisen over human exposures to radio frequency electromagnetic radiation (RFEMR), including a recent report indicating that regular mobile phone use can negatively impact upon human semen quality. These effects would be particularly serious if the biological effects of RFEMR included the induction of DNA damage in male germ cells. In this study, mice were exposed to 900 MHz RFEMR at a specific absorption rate of approximately 90 mW/kg inside a waveguide for 7 days at 12 h per day. Following exposure, DNA damage to caudal epididymal spermatozoa was assessed by quantitative PCR (QPCR) as well as alkaline and pulsed-field gel electrophoresis. The treated mice were overtly normal and all assessment criteria, including sperm number, morphology and vitality were not significantly affected. Gel electrophoresis revealed no gross evidence of increased single- or double-DNA strand breakage in spermatozoa taken from treated animals. However, a detailed analysis of DNA integrity using QPCR revealed statistically significant damage to both the mitochondrial genome (p < 0.05) and the nuclear beta-globin locus (p < 0.01). This study suggests that while RFEMR does not have a dramatic impact on male germ cell development, a significant genotoxic effect on epididymal spermatozoa is evident and deserves further investigation.

Animals↗
PubMed2005

Exposure to pulse-modulated radio frequency electromagnetic fields affects regional cerebral blood flow.

We investigated the effects of radio frequency electromagnetic fields (RF EMF) similar to those emitted by mobile phones on waking regional cerebral blood flow (rCBF) in 12 healthy young men. Two types of RF EMF exposure were applied: a 'base-station-like' and a 'handset-like' signal. Positron emission tomography scans were taken after 30 min unilateral head exposure to pulse-modulated 900 MHz RF EMF (10 g tissue-averaged spatial peak-specific absorption rate of 1 W/kg for both conditions) and sham control. We observed an increase in relative rCBF in the dorsolateral prefrontal cortex on the side of exposure. The effect depended on the spectral power in the amplitude modulation of the RF carrier such that only 'handset-like' RF EMF exposure with its stronger low-frequency components but not the 'base-station-like' RF EMF exposure affected rCBF. This finding supports our previous observation that pulse modulation of RF EMF is necessary to induce changes in the waking and sleep EEG, and substantiates the notion that pulse modulation is crucial for RF EMF-induced alterations in brain physiology.

Adult↗
PubMed2004

Effects of histamine receptor blockade on cardiovascular changes induced by 35 GHz radio frequency radiation heating.

1. The role of histamine in heat-induced cardiovascular changes is uncertain. The purpose of this study was to examine effects of histamine H-1- and H-2-antagonism on heart rate, mean arterial blood pressure (MAP), localized body temperature changes, survival times, and lethal body temperatures that occur during the exposure of anaesthetized rats to 35 GHz radio frequency radiation (RFR). 2. Forty-eight ketamine-anaesthetized Sprague-Dawley rats were exposed, in several different treatment groups (n = 8 in each), to 35 GHz RFR at a level that resulted in significant body heating and subsequent death. During irradiation, a continuous increase in heart rate and a biphasic response in blood pressure (initial increase followed by a decrease) were observed in all groups of animals. 3. An H-1-antagonist, diphenhydramine (1 mg kg(-1) body wt) and an H-2-antagonist, cimetidine (5 mg kg(-1)), administered after sustained RFR exposure, failed to reverse the RFR-induced hypotension. High doses of the drugs (5 and 10 mg kg(-1), respectively) also did not alter the response. Post-RFR survival time was significantly decreased in the high-dose drug-treated group, compared with vehicle-treated (0.9% NaCl, 50% ethanol and 50% D5W) controls. 4. In experiments in which the two drugs were administered prior to RFR exposure, MAP in animals receiving high-dose antihistamines was significantly depressed compared with that of vehicle-treated animals during the first 35 min of RFR exposure. Antihistamine pretreatment, however, did not alter the total RFR exposure time required for death to occur. 5. In summary, pharmacological blockade of H-1 and H-2 receptors is not beneficial in anaesthetized rats made hypotensive by RFR exposure. This indicates that activation of H-1 and H-2 receptors by histamine does not occur to any significant extent and does not mediate the hypotensive response developed in this model of hyperthermia.

Animals↗
PubMed2006

The role of autacoids and the autonomic nervous system in cardiovascular responses to radio-frequency energy heating.

Among the potential effects of exposure to high levels of radio-frequency energy (RFE) (which includes microwaves), an increase in body temperature is the primary consequence. Release of autacoids and activity of the autonomic nervous system may influence (or be directly responsible for) some of the physiological changes that occur in conjunction with this hyperthermia. The main focus of this review is the interaction of autacoids and the autonomic nervous system with cardiovascular changes during heating. Differences between environmental and RFE-induced heating (such as rate of temperature change and degree of skin vs. core heating) may be important when considering these effects. Antihistamines exhibited no beneficial effect on circulatory collapse during RFE-induced heating. The serotonergic blocker methysergide decreased survival time in rats during terminal RFE exposure, despite no effects on heart rate (HR) or blood pressure. Although blockade of platelet-activating factor resulted in lower HR before RFE exposure, there was a lack of effect on the subsequent increase in HR during heating. Nitric oxide did not contribute to the hypotension that occurs due to rapid heating by RFE exposure. There have been either no or very limited studies of effects of prostaglandins, bradykinin, or angiotensin on RFE-induced heating responses. beta-Adrenoceptor antagonism with propranolol resulted in significantly decreased survival times and lower final colonic temperatures during RFE exposure. A lack of effects of nadolol on survival time and temperature, coupled with its poor ability to traverse the blood-brain barrier, suggests that central beta-adrenergic stimulation rather than peripheral stimulation may alter thermoregulation. Effects of the autonomic nervous system (as studied by adrenoceptor blockade) on potassium changes during heating have not been fully investigated. Such changes could be important in animals' responses to RFE and other modalities of heating, and should be studied in future.

Adrenergic alpha-Antagonists↗
PubMed1991

Effects of airway luminal concentration of albumin on histamine-induced mucosal exudation of radio-iodine labelled albumin.

In an in vivo study of guinea-pig airway barriers we have examined the effects of the luminal concentration of albumin on exudation (outward) and absorption (inward) permeabilities to radio-iodine labelled albumin. Previously validated techniques for superfusion of solutes onto the tracheobronchial mucosal surface and for subsequent tracheal lavage were employed. [125I]albumin was administered intravenously as plasma tracer and [131I]albumin was superfused topically as absorption tracer. Histamine (5.0 nmol) was superfused onto the mucosal surface together with the absorption tracer and in the presence of different albumin concentrations (0.3, 3.0 and 30 mg ml-1). The experiment was terminated 10 minutes after the tracheal mucosal superfusion and samples of plasma and tracheal lavage fluid were collected. The mucosal exudation response was calculated from the detection of [125I]albumin in the airway lumen. The absorption ability of the mucosa was determined by the detection of [131I]albumin in circulating plasma. Histamine induced a significant mucosal exudation of [125I]albumin. This effect was unaffected by the level of albumin on the mucosal surface. There was a small but significant absorption of [131I]albumin in the presence of 0.3 and 3.0 mg ml-1 of albumin in the luminal liquid. An albumin concentration of 30 mg ml-1 markedly increased the rate of absorption of [131I]albumin. However, the absorption rate was not affected by the histamine-induced mucosal exudation process at any level of luminal albumin. The present data further demonstrates that plasma exudation and mucosal absorption are independent processes. The data are in keeping with the view that an increased subepithelial hydrostatic pressure moves the plasma exudate across the mucosa as a distinct outward process.

Absorption↗
PubMed1975

Discussion paper: Strong dependence of whole animal absorption on polarization and frequency or radio-frequency energy.

A two-plate stripline is used to determine wide-band radio-frequency (285-4000 MHz) absorption characteristics of 96-390-g rats and brain-phantom prolate spheroidal bodies. The results compare well to those for free space irradiation. At resonance, for E along the long dimension (â), a power deposition nine times higher than that for the H parallel â orientation is observed. For rats in the k parallel â configuration, the frequencies of peak absorption and the maximum absorption at these values demonstrate W-1/3 and W 2/3 dependencies, respectively, upon the weight W of the animal. This finding implies that whole animal absorption is a size- and shape-dependent phenomenon.

Animals↗
PubMed1999

Evaluation of the effectiveness of transurethral radio frequency hyperthermia in the canine prostate: temperature distribution analysis.

The heating pattern of a transurethral radio frequency (RF) applicator and its induced steady-state temperature field in the prostate during transurethral hyperthermia treatment were investigated in this study. The specific absorption rate (SAR) of the electromagnetic energy was first quantified in a tissue-equivalent gel phantom. It was used in conjunction with the Pennes bioheat transfer equation to model the steady-state temperature field in prostate during the treatment. Theoretical predictions were compared to in vivo temperature measurements in the canine prostate and good agreement was found to validate the model. The prostatic tissue temperature rise and its relation to the effect of blood perfusion were analyzed. Blood perfusion is found to be an important factor for removal of heat especially at the higher RF heating level. To achieve a temperature above 44 degrees C within 10 percent of the prostatic tissue volume, the minimum RF power required ranges from 5.5 W to 36.4 W depending on the local blood perfusion rate (omega = 0.2-1.5 ml/gm/min). The corresponding histological results from the treatment suggest that to obtain better treatment results, either higher RF power level or longer treatment time (> 180 minutes) is necessary. This is consistent with the predictions from the theoretical model developed in this study.

Animals↗
PubMed2006

Strategies for inner volume 3D fast spin echo magnetic resonance imaging using nonselective refocusing radio frequency pulses.

Fast spin echo (FSE) trains elicited by nonselective "hard" refocusing radio frequency (RF) pulses have been proposed as a means to enable application of FSE methods for high-resolution 3D magnetic resonance imaging (MRI). Hard-pulse FSE (HPFSE) trains offer short (3-4 ms) echo spacings, but are unfortunately limited to imaging the entire sample within the coil sensitivity thus requiring lengthy imaging times, consequently limiting clinical application. In this work we formulate and analyze two general-purpose combinations of 3D HPFSE with inner volume (IV) MR imaging to circumvent this limitation. The first method employs a 2D selective RF excitation followed by the HPFSE train and focuses on required properties of the spatial excitation profile with respect to limiting RF pulse duration in the 5-6 ms range. The second method employs two orthogonally selective 1D RF excitations (a 90x degrees - 180y degrees pair) to generate an echo from magnetization within the volume defined by their intersection. Subsequent echoes are formed via the HPFSE train, placing the focus of the method on (a) avoiding spurious echoes that may arise from transverse magnetization located outside the slab intersection when it is unavoidably affected by the nonselective refocusing pulses and (b) avoiding signal losses due to the necessarily different spacing (in time) of the RF pulse applications. The performance of each method is experimentally measured using Carr-Purcell-Meiboom-Gill (CPMG) multi-echo imaging, enabling examination of the magnetization evolution throughout the echo train. The methods as implemented achieve 95% to 97% outer volume signal suppression, and higher suppression appears to be well within reach, by further refinement of the selective RF excitations. Example images of the human brain and spine are presented with each technique. We conclude that the SNR efficiency of volume imaging in conjunction with the short echo spacing afforded by hard pulse trains enables high-resolution 3D HPFSE MRI of a small field-of-view (FOV) with minimal aliasing artifact.

Algorithms↗
PubMedDate not supplied

Prediction of transient temperature fields and cumulative tissue destruction for radio frequency heating of a tumor.

A therapeutic hyperthermia protocol using a radio frequency (rf) electrode placed adjacent to a bronchial wall tumor has been modeled using the finite element technique. Variable physical properties and variable blood perfusion have been assigned to the tumor and to the surrounding normal lung tissue. The Laplace equation was solved on a curvilinear grid for a single rf source electrode to determine the steady-state electric field, which in turn governs the energy deposition function. The heat generation in the tumor and in the lung tissue is then calculated from the energy deposition profile, and the bioheat equation is solved on the same finite element mesh to determine the transient temperature history. The temperatures are displayed as isothermal contours at designated times during the protocol and as temperature histories at selected points. In addition, an Arrhenius-type injury model has been implemented to predict thermally induced damage, from which equal total amounts of energy are deposited into the tissue using a constant power density for an appropriate time or using a cyclic heating pattern. The cyclic heating pattern consisted of a series of equal duration time periods during which the rf current source is alternately turned on and off (50% duty cycle). This study illustrates how a finite element model could be used to evaluate alternative protocols for heating a tumor of a specific geometry and to evaluate thermally induced damage to surrounding normal tissue.

Humans↗
PubMedDate not supplied

Radio frequency eddy current losses for an annular conductor in MRI: theory and applications.

A theoretical derivation of the equivalent loading resistance of an annular volume conductor within a magnetic resonance imaging radio frequency (RF) coil is presented. Theoretical predictions of the magnitude of the load resistance agree well with measurement over a range of frequencies. The loading resistance is proportional to the sample conductivity, the frequency and coil sensitivity squared, and depends strongly upon the sample dimensions. The orientation of the magnetic vector potential for the specific coil geometry is also important. An experimental comparison of loading by annular and cylindrical objects with the human head over the imaging frequency range is made. Cylindrical test objects are adversely affected by the skin effect. The optimum means of simulating RF induced losses for quality assurance and performance evaluation is discussed in the light of these results.

Biophysical Phenomena↗
PubMed1967

Radio tracking of homing bats.

Neotropical bats, Phyllostomas hastatus, were released 10 kilometers from their home roost, and their homeward flights were tracked by radio. Flights of bats with unimpeded vision were strongly oriented in the homeward direction, while the flights of blindfolded bats did not show this marked orientation.

Animals↗