The dielectric properties of rabbit tissue, pure water and various liquids suitable for tissue phantoms at 35 GHz.

Newly developed microwave apparatus was used to measure the complex permittivity of some rabbit tissues and water over the temperature range 20-37 degrees C. Various liquid mixtures which might be suitable as phantom tissue equivalents at 35 GHz were also considered. Some of the measured tissue data were compared with previous lower frequency data and it was concluded that the observed behaviour was not anomalous.

The modelling of biological systems in three dimensions using the time domain finite-difference method: I. The implementation of the model.

A computer method has been developed which uses the time domain finite-difference (TDFD) algorithm to calculate the deposition of the electromagnetic (EM) field in three-dimensional biological models. This, the first of two papers, describes the algorithm and the computer programs developed. The method is demonstrated by calculating the penetration of the EM field from a rectangular waveguide radiating into a homogeneous model, the calculation being carried out in two dimensions for simplicity in this paper.

Dielectric properties of mammalian brain tissue between 1 and 18 GHz.

A newly developed frequency domain technique was used to measure the dielectric properties of white matter, grey matter and macerated rabbit brain at 20 and 37 degrees C. An analysis of the data showed that between 1 and 4 GHz there were substantial contributions from processes other than the gamma dispersion. However, above 7.5 GHz it appeared that mainly free water was relaxing although evidence of a small spread of relaxation times was found for the bulk water in the white matter. Mouse and rat brain were also measured but no significant differences were found between the species. The quantity of bound water was estimated but there was no evidence of a difference in the amount between either the tissues or the temperatures.

Dielectric properties of human blood at microwave frequencies.

Waveguides systems have been used to obtain measurements for the complex permittivity of human blood in the frequent range 29-90 GHz. The data are reported and analysed in conjunction with lower frequency work, in particular that of Wei. There are two distinct regimes of behaviour in the spectra obtained: a high-frequency relaxation process in parallel with a lower frequency fractional power-law response.

Dielectric properties of ocular tissues at 37 degrees C.

Values of the relative permittivity, dielectric loss and conductivity are given for various rabbit ocular tissues at frequencies in the range 10 MHz-10 GHz. The tissues measured were cornea, retina, choroid, iris, and the cortical and nuclear zones of the lens. The dielectric parameters were determined using the technique of multiple response time domain spectroscopy. For all tissues the water relaxation could be characterised by a Debye dispersion with a relaxation time longer than that of pure water, indicating that its dielectric behaviour was affected by the presence of the biological macromolecules.

Dielectric properties of insulin in solution.

Values of the relative permittivity of a 5% solution of despentapeptide insulin (DPI) are given at six temperatures over a frequency range 0.2-50 MHz. The solvent was a mixture of water and ethylene glycol at pH = 3. From the dispersion curves a dipole moment of 72 +/- 1 was obtained for the insulin molecule at 25 degrees C. By combining the dielectric measurements with values of solvent viscosity the volume of the insulin molecule in solution was found to be only slightly larger than that in the crystal. This indicates that insulin possesses a markedly lower quantity of water of hydration than has been observed for globular proteins containing a higher proportion of hydrophilic groups on the surface of the molecule.

Microwave dielectric measurements (0.8-70 GHz) on Artemia cysts at variable water content.

Dielectric permittivity measurements are reported for cysts of Artemia, a crustacean known as the brine shrimp. Using coaxial and waveguide techniques we examined the frequency range from 0.8 to 70 GHz. Taking advantage of the ability of this system to reversibly lose essentially all intracellular water, we determined the permittivity over the entire range of cyst water contents. Although experimental errors prevent a rigorous treatment of the data, we advance the general conclusion that little of the water in this system behaves dielectrically like pure water, regardless of water content. This conclusion is supported by, and is consistent with, the results of previously published studies that probe the motional properties of water in this system using nuclear magnetic resonance spectroscopy and quasi-elastic neutron scattering.

The modelling of biological systems in three dimensions using the time domain finite-difference method: II. The application and experimental evaluation of the method in hyperthermia applicator design.

The three-dimensional time domain finite-difference (TDFD) method was used to design a 433 MHz circular waveguide applicator for electromagnetic hyperthermia. This applicator has a water-filled lens at one end, providing surface cooling as well as some shaping of the emergent electromagnetic field. An experimental system for scanning the EM field in three dimensions is described. The experimental data were compared with those calculated from the TDFD program and good agreement was found. A realistic three-dimensional model based on an ultrasound scan of a surface tumour was built. Both the EM field and the specific absorption rate (SAR) within the model when irradiated by the applicator were calculated.

Electrical properties of lens material at microwave frequencies.

Values of the complex permittivity of human and rabbit lens material in the frequency range 100--6000 MHz are reported. The data were obtained by using an on-line computer-based time-domain spectrometer. The lenses were divided into an outer (cortical) zone and an inner (nuclear) zone and the dielectric properties of each zone were measured separately for both species of lens. The results are analysed in terms of the aqueous and protein constituents, assuming a molecular model whereby the hydrated protein molecule is represented by a spherical particle embedded in an aqueous continuum. It is shown that for lens material taken from the nuclear zone the particle can be separated into protein and bound water (water of hydration). For the cortical zone the amount of water of hydration is smaller and, within the limitations of the model and uncertainties due to experimental error, cannot be distinguished from zero.

The dielectric properties of the cerebellum, cerebrum and brain stem of mouse brain at radiowave and microwave frequencies.

The relative permittivity and conductivity of the cerebellum, cerebrum and brain stem of mouse brain were measured at a temperature of 37 degrees C over a frequency range of 72 MHz to over 5 GHz using time-domain spectroscopy. An analysis of the data suggests that the water exists in various forms of binding with an average relaxation frequency less than free water.