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K R Foster

Publications and source records attributed to K R Foster.

70 records · Page 4Linked to original sources
PubMed1980

Microwave dielectric relaxation in muscle. A second look.

The dielectric permittivity and conductivity of muscle fibers from the giant barnacle, Balanus nubilus, have been measured at 1, 25, and 37 degrees C, between 10 MHz and 17 GHz. The dominant microwave dielectric relaxation process in these fibers is due to dipolar relaxation of the tissue water, which shows a characteristic relaxation frequency equal to that of pure water, ranging from 9 GHz (1 degree C) to 25 GHz (37 degree C). The total permittivity decrease, epsilon 0 -- epsilon infinity, due to this process accounts for approximately 95% of the water content of the tissue; thus, the major fraction of tissue water is dielectrically identical to the pure fluid on a picosecond time scale. A second dielectric process contributes significantly to the tissue dielectric properties between 0.1 and 1--5 GHz, and arises in part form Maxwell-Wagner effects due to the electrolyte content of the tissue, and in part from dielectric relaxation of the tissue proteins themselves.

Animals↗
PubMed1980

The UHF and microwave dielectric properties of normal and tumour tissues: variation in dielectric properties with tissue water content.

Dielectric measurements have been made on various soft tumour and normal tissues between 0.01 and 17 GHz at body temperature. At microwave frequencies above 1-5 GHz, the tissue dielectric properties can be fitted to Debye equations with the same relaxation frequency (25 GHz) as found for pure water at 37 degrees C. The tissue dielectric properties correlate well with their water contents. The conductivity of the tissue at 0.1 GHz (which is close to that of the cytoplasm itself) increases with the volume fraction of water in the tissue, in a manner consistent with that previously observed in proteins suspended in electrolyte solution. The contribution of the tissue water to the tissue dielectric permittivity at frequencies below 1 GHz is fitted by a function of water content different to that describing the conductivity data. Empirical equations that may be used to predict the dielectric properties of other soft tissues within this wide frequency range are suggested.

Animals↗
PubMed1979

Dielectric properties of brain tissue between 0.01 and 10 GHz.

Dielectric permittivity and conductivity are reported for grey and white matter from dog brain tissue between 0.01 and 10 GHz. Between 0.01 and approximately 1 GHz, the permittivity decreases and conductivity increases as a power law of frequency. Above 1 GHz, the conductivity increases quadratically with frequency due to dipolar reorientation of free water molecules in tissue; the apparent rotational relaxation frequency at 37 degrees C is 21--25 GHz, slightly below the 25 GHz characteristic frequency of pure water at that temperature. The microwave data are analysed using the Maxwell mixture theory applicable for a suspension of nonconducting, low permittivity spheres in bulk water. From the increase in conductivity above 1 GHz, and the tissue permittivity at 2--4 GHz, the apparent volume fraction of water is approximately 0.70 and 0.55 for grey and white matter, respectively, about 10--15% lower than respective values from the literature. This discrepancy is apparently due to a small fraction of water which does not contribute to the tissue permittivity above 1 GHz. Empirical equations are given to summarise the dielectric properties of 'average' brain tissue at 37 degrees C for future theoretical studies of microwave absorption in the head.

Animals↗
PubMed1978

Auditory responses in cats produced by pulsed ultrasound.

Auditory-nerve responses and cochlear microphonics are produced in cats by pulsed 5-MHz ultrasonic energy from a transducer placed against the dura mater. The pulses must be relatively intense (approximately 30 W/cm2) to produce a response, but can be sufficiently brief (less than 70 microsecond) that the brain tissue is not observably heated. The cats apparently respond to radiation pressure transients accompanying the absorption of the ultrasound in the brain tissue. Both the amplitude and latency of the N1 neural responses to the ultrasound can be matched to those produced by relatively weak tone pips or clicks from an external source. The cochlear microphonic (CM) produced by a pulse shows a prominent ringing at 5-10 kHz in different cats; the amplitued of the N1 response exhibits broad maximum, for constant amplitude pulses, a pulse widths of 20-60 microsecond. This variation of N1 response amplitude with pulse width is similar to that of a high-pass filter with a cutoff frequency at the dominent frequency of the CM, which is tentatively identified with a ringing frequency of the skull.

Acoustic Stimulation↗
PubMed1977

Microwave dielectric properties of tissue. Some comments on the rotational mobility of tissue water.

Dielectric permitivity and conductivity data are reviewed for tissue over the frequency range of 0.1-10 GHz. The conductivity of muscle increases quadratically with frequency above 1 GHz, suggesting a Debye relaxation for tissue water centered at 20 GHz at room temperature, the same as for bulk water. Approximate mixture equations suggest that this "free" water accounts for about 70% of the tissue weight, showing that most of the tissue water has rotational mobilities similar to those in the bulk fluid.

Animals↗
PubMed1976

Bounds on "bound water": transverse nuclear magnetic resonance relaxation in barnacle muscle.

Relatively mobile protons that do not exchange with D2O exist in barnacle muscle cells. These are not part of the nonfreezing "bound water" that does exchange. Ninety-seven percent of the muscle water exhibits a single transverse relaxation time of 35 milliseconds: one water molecule per thousand, which is briefly and irrotationally bound, will produce the observed relaxation properties.

Animals↗
PubMed1976

The electrical resistivity of cytoplasm.

The apparent cytoplasmic resistivity of two different giant cells has been measured using an extension of a previously developed single microelectrode technique. Each cell is penetrated by a metal microelectrode whose complex impedance is measured as a function of frequency between 500 kHz and 5.7 MHz. By plotting the measured impedance data on the complex Z plane and extrapolating the data to infinite frequency, the substantial effects of electrode polarization can be overcome. For Aplysia giant neurons and muscle fibers of the giant barnacle, the extrapolated cytoplasmic specific resistivities are 40 and 74 omega-cm, respectively, at infinite frequency. The barnacle data are in excellent agreement with sarcoplasmic resistivity values derived from the measured cable properties of other marine organisms, and from high frequency conductivity cell measurements in intact barnacle muscle tissue. In the Aplysia neurons, the frequency-dependent part of the electrode impedance is larger when the electrode is in a cell than when it is in an electrolyte solution with the same specific resistivity as the aqueous cytoplasm; however, the phase angle of the frequency-dependent component of the electrode impedance is the same in both cases. This suggests that the high apparent values of cytoplasmic resistivity found using the single microelectrode technique at lower frequencies probably reflect an artifact caused by reduction of the effective surface area of the electrode by intracellular membranes, with a corresponding increase in its polarization impedance.

Animals↗
PubMedDate not supplied

Electrical impedance properties of the body and the problem of alternate-site burns during electrosurgery.

The radiofrequency (RF) impedance at 500 kHz was measured between multiple contact points on the bodies of six healthy volunteers by a tetrapolar measurement technique. The impedance between various contact points was predictable and could increase by a factor of two, depending on the sites selected for electrode application. These results were related to a simple resistive model of RF current distribution through the body. Based on the model and on data from human subjects, a hierarchy of optimal locations was developed to minimize the impedance between a surgical site and a dispersive electrode site and to reduce the potential for alternate-site burns from electrosurgery.

Adolescent↗
PubMedDate not supplied

Radiofrequency field surveys in hospitals.

The authors surveyed levels of radiofrequency (RF) fields in the frequency range 0.1-1,000 MHz in four hospitals in the Philadelphia area, to obtain background information related to the possible interference of radiofrequency fields with medical equipment. Two large center-city hospitals, a regional county hospital, and two suburban hospitals were surveyed. Measurements were made at six to 12 sites in each hospital, in each of the three frequency bands. More limited additional measurements were conducted in a fifth hospital as well. Sites were selected to include areas where strong RF signals from transmitting antennas might be expected to be present (e.g., locations close to windows in upper stories of buildings near paging antennas) as well as other representative sites in the hospital. The median RF field strengths were quite low (0.1-0.5 V/m), but at specific locations the RF signals from broadcast sources exceeded 1 V/m. Much stronger fields were recorded close to electrosurgical units and hand-held transmitters (cellular telephones and UHF transceivers).

Electromagnetic Fields↗