Dielectric Imaging Research Articles

Application of conjugate harmonics to electrical process tomography

An analytical method based on conjugate harmonics is described to allow prediction of the field distribution for electrical tomography. It is demonstrated that such an approach is useful for design of sensor geometry for dielectric imaging and could also be extended to impedance imaging in conducting media. The possible use of this methodology to assist in the design of sensors for industrial measurement problems in which capacitance sensors are to be used to image and control the flow of powders in a dense phase conveyor is discussed.

Dielectric imaging of biological cells

A dielectric technique that can image local permittivity and conductivity has been applied to living biological cells in an aqueous environment. The local permittivity and conductivity were measured between 10 kHz and 10 MHz with a fine probe electrode, which was laterally scanned over cells on a plate electrode. The dielectric images of the cells depended on frequency, indicating dielectric relaxation that is due to interfacial polarization. The low-frequency image (at ∼10 KHz) in which the cells have high permittivity and low conductivity compared with the medium results from the presence of the plasma membrane with high resistivity. The dielectric image of the cell interior is obtainable at high frequencies (∼10 MHz), where the plasma membrane is short-circuited.

Resolution in collection-mode scanning optical microscopy

The use of small apertures or sharpened tips as sensing elements in scanned-probe optical sensing devices has led to the development of a number of instruments that provide lateral spatial resolution much finer than that available in conventional optical imaging instruments. Such a device might generally be classified as a scanning optical microscope, or SOM. One particular mode of SOM operation involves the use of a sharpened optical fiber to collect light emanating from a surface. The lateral spatial resolution of such a collection-mode SOM is discussed in terms of the electromagnetic mode solutions of the probe tip. Numerical results indicate that, though bound modes solutions exist for increasingly fine unclad tips, classical diffraction effects limit resolution to a finite fraction (approximately 1/3) of the source wavelength λ. A second mechanism for signal transduction is shown to involve molecular scattering at the probe tip. An analysis of signal collection efficiency demonstrates that at tip radii below λ/5 for metallic-clad probes, and λ/10 for probes in a dielectric ambient, scattering dominates and imaging resolution scales with tip size, thus defeating limits imposed by diffraction.

Dielectric properties of female human breast tissue measured in vitro at 3.2 GHz

Complex permittivities of in vitro diseased and undiseased human female breast tissues have been measured at 3.2 GHz using a resonant cavity technique. Experimental data are compared with models predicted from mixture equations. Measured permittivity data lie within limits set by two-phase mixture theory, but some conductivity data are in excess of those expected. At any particular microwave frequency in all tissue of a given type, the relationship between permittivity and conductivity may be parameterized using the Debye relaxation equations. It is concluded that the dielectric relaxation of tissue water is not the only dispersive process occurring at this frequency: dielectric relaxation of bound water and the tail end of a beta -dispersion may also contribute to the dielectric properties. The similarity of the dielectric properties of benign and malignant breast tumours measured suggest that in vivo dielectric imaging methods will not be capable of distinguishing them.

Phantoms for hyperthermia and impedance tomography

The tissue-heating effect of radiofrequency and microwave radiation is used in diathermy treatments of soft-tissue injuries and inflammation, it can be employed for the rapid thawing of cryopreserved organs, and it forms the basis of hyperthermia as a cancer therapy. The fact that different tissues and organs exhibit different dielectric properties has also led to the development of electrical impedance tomography, which amongst other applications might lead to radiofrequency and microwave radiation being used for the dielectric imaging of tumours, and also as a non-invasive monitor of the heating effect of hyperthermia. Progress in the effective use of electromagnetic hyperthermia and dielectric imaging is greatly aided by the availability of phantoms that, as far as possible, simulate the electrical and thermal properties of human tissue.