Dielectric properties of mouse MCA1 fibrosarcoma at different stages of development.

The in vitro bulk electrical properties of MCA1 fibrosarcoma induced in C57B1/6 male mice were measured at frequencies of 10 kHz to 100 MHz, with some tissues measured to 2 GHz. The properties of normal surrounding tissue also were measured. A comparison of the dielectric properties between three different stages of tumor development as well as that between various locations within the tumor is reported. Statistical analysis of the experimental results revealed statistically significant differences in the dielectric constant and conductivity of the tumor tissues at various stages of development as measured at frequencies below 10 MHz. Conductivity values at different stages also differ at a frequency of 100 MHz. At other frequencies these differences were found to be statistically insignificant.

Comparison of finite-difference time-domain SAR calculations with measurements in a heterogeneous model of man.

A finite-difference time-domain technique was used to calculate the specific absorption rate (SAR) at various sites in a heterogeneous block model of man. The block model represented a close approximation to a full-scale heterogeneous phantom model. Both models were comprised of a skeleton, brain, lungs, and muscle. Measurements were conducted in the phantom model using an implantable electric-field probe and a computer-controlled data acquisition system. The calculation and measurement of SAR distributions were compared primarily in the head (including the neck) and chest. To obtain the necessary spatial resolution with the computer model, the head and neck were modeled with approximately 105,000 cells, while 86,000 cells were used to configure the chest. Planewave fields, polarized in the E orientation, were utilized to irradiate the models at an exposure frequency of 350 MHz. Reasonable correlation existed between the calculations and measurements.

In vitro dielectric properties of human tissues at radiofrequencies.

In vitro permittivity measurements of excised human liver, spleen, kidney and cardiac muscle at frequencies from 10 kHz to 100 MHz are described. An end-of-the-line capacitive sensor and a computer-controlled network analyser HP 3577 were employed. The results were compared with human data reported by other investigators as well as with the animal (cat) data obtained earlier in this laboratory. It was found that the conductivity of most of the human tissues tested was significantly higher than that of the animal tissues for the test frequencies. The dielectric constants for human kidney and spleen are higher than the corresponding animal (cat) tissues at frequencies from 10 kHz to approximately 1 MHz and at around 100 MHz. However, the values for liver do not differ significantly between the two species in the same frequency range.

Specific absorption rate in humans in vivo at radio frequencies.

Specific absorption rates were measured in three locations of humans exposed in the far field at 160, 350, and 915 MHz. Values obtained for six males are compared with data for a full-scale homogeneous model of man.

Dielectric properties of beef, beef liver, chicken and salmon at frequencies from 100 to 2500 MHz.

The dielectric properties of uncooked beef, beef liver, chicken and salmon meat were measured at frequencies between 100 and 2500 MHz and at selected temperatures between 1.2 degrees C and 65 degrees C. Experimental method and the sample holder are briefly described. The results for the dielectric constant and the dielectric loss factor are presented as a function of frequency at selected temperatures and show a trend consistent with theoretical expectation.

Measurements of electromagnetic fields in biomedical applications.

Electromagnetic fields can interact with biological systems. Under some conditions, such interactions result in biological effects that may be hazardous, while in some other situations they may be beneficial and lead to some new medical applications. Examples of the latter are electromagnetically induced hyperthermia, bone healing, and nuclear magnetic resonance (NMR) imaging. To quantify and assist in understanding the interactions, it is necessary to know the intensities of electromagnetic fields. In many cases, information abut the field strengths inside exposed biological bodies is equally or even more important than that about the strength of the external exposure fields. In this context, measurements of electromagnetic fields at frequencies from a few hertz to about tens of gigahertz are very important. Some classical measurement techniques used in electromagnetics are directly, or with certain modifications, applicable to biomedical applications. However, due to special requirements, new specifically tailored techniques have had to be developed. A review of measurement methods and instrumentation for probing external and internal electric and magnetic fields as relevant to biomedical applications is given. Specific requirements, limitations of existing methods, and future needs are discussed.

In vivo and in vitro dielectric properties of feline tissues at low radiofrequencies.

The dielectric constant and conductivity of muscle, liver, spleen and kidney of cats in vivo and in situ immediately following the animal's death were measured at frequencies from 10 kHz to 100 MHz. A novel multi-ring capacitive sensor and a computer-controlled automatic network analyser (ANA) were employed. The results were compared with the data available from literature for the same species in the frequency range between 10 and 100 MHz. It was found that at frequencies from 10 to 100 kHz the in vitro dielectric constant for all tissues except spleen was smaller than the in vivo one. In contrast, in the range from 1 to 100 MHz the in vitro dielectric constant was larger than the in vivo one. At intermediate frequencies from 0.1 to 1 MHz both the dielectric constant in vivo and in vitro were the same within the experimental uncertainty. The dielectric constant of the spleen in vivo was quite similar to that in vitro. The in vivo conductivity of all tissues appeared to be higher than in vitro from 10 kHz to 10 MHz, while at frequencies above 10 MHz the two conductivities were within the experimental uncertainty.

Dielectric properties of human B and T lymphocytes at frequencies from 20 kHz to 100 MHz.

Dielectric properties of the human B and T lymphocytes were measured. The experiments were performed at frequencies from 20 kHz to 100 MHz for different cell concentrations at 24 and 37 degrees C. An end of the line capacitive sensor and a computer-controlled automatic network analyser were utilised. Cole-Cole dielectric parameters were determined by curve fitting using a computer program. Specific membrane capacitances (Cm) were calculated from the experimental data (assuming a membrane charging mechanism) to be 0.77 and 2.89 microF cm-2 for the T and B lymphocytes, respectively. The fitted relaxation time is located in the centre of the calculated relaxation time distribution spectrum for the B lymphocytes, while it is shifted towards higher values of the relaxation time equivalent to the larger cells for the T lymphocytes. The distribution of the relaxation times was estimated on the basis of the Maxwell-Wagner dispersion reflecting membrane-charging processes.

Use of the loss-tangent function in dielectric spectroscopy.

A new method for finding the dielectric parameters of biological substances is presented. The method makes use of the loss-tangent as a function of frequency to identify the dominating relaxation process. The method was tested for a few cell suspensions (blood and lymphocytes) and two tissues (liver and spleen). The obtained parameters agree well with those calculated from Maxwell-Wagner theory (beta dispersion).

Postmortem changes of the dielectric properties of bovine brain tissues at low radiofrequencies.

The dielectric properties of the bovine brain grey and white matter in the frequency range from 20 kHz to 100 MHz were measured at different times following animal death. Changes in the dielectric parameters versus time are interpreted in terms of the reduction of the cell volume fraction that results from either cell disintegration or cell size reduction. Good agreement between the computer fitted parameters and the values calculated from the Maxwell-Wagner model of the static dielectric constants was found. At frequencies above 1 MHz the changes of the dielectric properties are less pronounced, confirming earlier observations made by other investigators for different species.

Radiofrequency dielectric properties of animal tissues as a function of time following death.

The dielectric properties of three bovine tissues, liver, kidney and spleen, as a function of time following death, were measured in the frequency range from 20 kHz to 100 MHz using an automatic network analyser and an end-of-the-line sensor. The dielectric constant of kidney and spleen decreases as a function of time following death, particularly at frequencies below 1 MHz. However, all tissues measured show a characteristic increase in the frequency-independent ionic conductivity. This is believed to reflect changes in the conductivity of the extracellular region of tissues after death. The dielectric parameters, i.e. the static dielectric constant, the relaxation time and the coefficient of the relaxation time distribution, obtained by a curve-fitting process, do not change within the first 10 h following death in the case of liver, whereas early changes occur for both kidney and spleen. High initial values of the static dielectric constant for these tissues decrease significantly within a few hours following death. Similarly, the relaxation time which is relatively long for kidney and spleen, as compared with liver, decreases with time. Our data compare favourably with those reported by several investigators for similar tissues in other species (dog, cat, swine and cattle).

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.