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Biomedical subjects

K R Foster

Publications and source records attributed to K R Foster.

At least 19 recordsLinked to original sources
PubMed1992

Microwave radiometry in biomedicine: a reappraisal.

Nearly 20 years ago the first papers appeared on biomedical applications of microwave radiometry, and many other papers have since appeared. Yet, despite its unique capabilities, microwave radiometry has so far received only limited acceptance by the medical community, and little commercial success. The chief reasons, we suggest, are the shallow depth of sensing and the difficulty of extracting imaging information from radiometry signals emitted by electrically heterogeneous media. A secondary factor has been the difficulty of validating many proposed clinical applications for the method--in particular, cancer detection. We suggest that microwave radiometry is a viable method of thermal sensing, but its successful applications are likely to be quite different than those that were originally conceived for the technique.

Extravasation of Diagnostic and Therapeutic Materi↗
PubMed1992

Health effects of low-level electromagnetic fields: phantom or not-so-phantom risk?

The possibility of subtle hazards from weak nonionizing electromagnetic fields (EMFs) has been controversial for many years, shifting over a range of issues without resolution. The confusion arises partly from the weak evidence for hazard and partly from the inconsistent and speculative nature of some of the bioeffects literature. More attention is needed to define ways to resolve the issues.

Abnormalities, Radiation-Induced↗
PubMed1992

Microwave radiometry in living tissue: what does it measure?

The sensitivity of microwave radiometry for detecting subcutaneous targets was studied both experimentally and theoretically. The radiometer used a dielectric loaded rectangular waveguide antenna in contact with a lossy dielectric medium. A cylindrical target with dielectric properties and/or temperature different from that of the surrounding medium was located beneath this surface. For most of the studies, the target and the surrounding medium were maintained at constant, but unequal, temperatures (i.e., heat conduction effects were insignificant). The received radiometric signal was calculated as the location and dielectric properties of the target were varied. Finally, the radiometer signal was calculated for the situation with the target maintained at constant temperature but with the surrounding medium modeled by the bioheat equation. Experimental studies were performed using a radiometer operating at 4.7 GHz. The target was a thin walled tube through which a temperature controlled liquid was circulated, located in a temperature controlled fluid tank. The results indicate that microwave radiometry (as used in this study) responds to the temperature averaged over the field pattern of the antenna with very strong weighting of regions near the surface. A simple quasi-static analysis provides a good indication of the sensitivity of the technique for detecting cylindrical targets whose dielectric properties are different from those of the surrounding medium. A simple estimate of thermal conduction around the target suggest that thermal effects greatly increase the apparent size of the target.

Electromagnetic Phenomena↗
PubMed1989

Dielectric properties of tissues and biological materials: a critical review.

We critically review bulk electrical properties of tissues and other biological materials, from DC to 20 GHz, with emphasis on the underlying mechanisms responsible for the properties. We summarize the classical principles behind dielectric relaxation and critically review recent developments in this field. Special topics include a summary of the significant recent advances in theories of counterion polarization effects, dielectric properties of cancer vs. normal tissues, properties of low-water-content tissues, and macroscopic field-coupling considerations. Finally, the dielectric properties of tissues are summarized as empirical correlations with tissue water content in other compositional variables; in addition, a comprehensive table is presented of dielectric properties. The bulk electrical properties of tissues are needed for many bioengineering applications of electric fields or currents, and they provide insight into the basic mechanisms that govern the interaction of electric fields with tissue.

Adipose Tissue↗
PubMed1987

"Resonances" in the dielectric absorption of DNA?

An attempt was made to confirm previous reports of resonant-like dielectric absorption of plasmid DNA in aqueous solutions at 1-10 GHz. The dielectric properties of the sample were measured using an automatic network analyzer with two different techniques. One technique used an open-ended coaxial probe immersed in the sample; the other employed a coaxial transmission line. No resonances were observed that could be attributed to the sample; however, resonance-type artifacts were prominent in the probe measurements. The coaxial line technique appears to be less susceptible to such artifacts. We note two important sources of error in the calibration of the automatic network analyzer using the probe technique.

DNA, Bacterial↗
PubMed1986

Perfused phantom models of microwave irradiated tissue.

The theoretical basis, practical design considerations, and prototype testing of a perfused model suitable for simulation studies of microwave heated tissue are presented. A parallel tube heat exchanger configuration is used to simulate the internal convection effects of blood flow. The global thermal response of the phantom, on a scale of several tube spacings, is shown theoretically to be nearly identical to that predicted by Pennes' bioheat equation, which is known to give a reasonable representation of tissue under many conditions. A parametric study is provided for the relationships between the tube size, spacing and material properties and the simulated perfusion rate. A prototype with a physiologically reasonable perfusion rate was tested using a typical hyperthermia applicator. The measured thermal response of the phantom compares favorably with the numerical solution of the bioheat equation under the same irradiation conditions. This similarity sheds light on the unexpected success of the bioheat equation for modeling the thermal response of real tissue.

Blood Circulation↗
PubMed1986

Small-scale temperature fluctuations in perfused tissue during local hyperthermia.

We develop analytical expressions (scaling laws) for the local temperature fluctuations near isolated and countercurrent blood vessels during hyperthermia. These scaling laws relate the magnitude of such fluctuations to the size of the heated region and to the thermal equilibration length of the vessels. A new equilibration length is identified for countercurrent vessels. Significant temperature differences are predicted between the vessels and the immediately adjacent tissue when the equilibration length is comparable to or longer than the size of the heated tissue region. Countercurrent vessels are shown to have shorter equilibration lengths and produce smaller temperature fluctuations than isolated vessels of the same size.

Blood Vessels↗
PubMed1986

Heat transport mechanisms in vascular tissues: a model comparison.

We have conducted a parametric comparison of three different vascular models for describing heat transport in tissue. Analytical and numerical methods were used to predict the gross temperature distribution throughout the tissue and the small-scale temperature gradients associated with thermally significant blood vessels. The models are: an array of unidirectional vessels, an array of countercurrent vessels, and a set of large vessels feeding small vessels which then drain into large vessels. We show that three continuum formulations of bioheat transfer (directed perfusion, effective conductivity, and a temperature-dependent heat sink) are limiting cases of the vascular models with respect to the thermal equilibration length of the vessels. When this length is comparable to the width of the heated region of tissue, the local temperature changes near the vessels can be comparable to the gross temperature elevation. These results are important to the use of thermal techniques used to measure the blood perfusion rate and in the treatment of cancer with local hyperthermia.

Biomechanical Phenomena↗
PubMed1985

Dielectric properties of low-water-content tissues.

The dielectric properties of two low-water-content tissues, bone marrow and adipose tissue, were measured from 1 kHz to 1 GHz. From 1 kHz to 13 MHz, the measurements were performed using a parallel-plate capacitor method. From 10 MHz to 1 GHz, a reflection coefficient technique using an open-ended coaxial transmission line was employed. The tissue water contents ranged from 1 to almost 70% by weight. The dielectric properties correlate well with the values predicted by mixture theory. Comparison with previous results from high-water-content tissues suggests that bone marrow and adipose tissues contain less motionally altered water per unit dry volume than do the previously studied tissues with lower lipid fractions. The high degree of structural heterogeneity of these tissues was reflected in the large scatter of the data, a source of uncertainty that should be considered in practical applications of the present data.

Adipose Tissue↗
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↗