Measurements and analysis of the microwave dielectric properties of tissues
Knowledge of the microwave dielectric properties of human tissues is essential for the understanding and development of medical microwave techniques. In particular, microwave thermography relies on processes fundamentally determined by the high frequency electromagnetic properties of human tissues. The specific aim of this work was to provide detailed information on the dielectric properties of female human breast tissue at 3-3.5GHz, the frequency of operation of the Glasgow microwave thermography equipment. At microwave frequences the frequency variation of the dielectric properties of biological tissues is thought to be determined mainly by the dipolar relaxation of tissue water. Water exists in different states of binding within the tissue; the relaxation of each component of this water may be parameterised by the Debye or Cole-Cole equations. At a single frequency an average relaxation frequency may be calculated for a given tissue type. Mixture equations may be used to describe the dielectric properties of two-phase mixtures in terms of the dielectric properties and volume fractions of the component phases. Biological tissues are very much more complex than these two phase models. However, comparisons of the observed dielectric properties as a function of water content, with models calculated from mixture theory allow some qualitative conclusions to be drawn regarding tissue structure. Human and animal dielectric data at frequencies between 0.1 and 10GHz have been collected from the literature and are displayed in tabular form. These comprehensive tables were used to examine the widely-held assumption an animal tissue is representative of the corresponding human tissue. This assumption was concluded to be uncertain in most cases because of lack of available data, and perhaps wrong for certain tissue types. The tables were also used to compare in vivo and in vitro dielectric data. These may be expected to be different because the tissue is in a physiologically abnormal state in vitro. However at microwave frequencies in vitro data was found to be representative of the tissue in vivo provided gross deterioration of the tissue is avoided. A new resonant cavity perturbation technique was designed for dielectric measurements of small volumes of lossy materials at a fixed frequency of 3.2GHz. This technique may be used to measure materials of a wide range of permittivities and conductivities with accuracies of 3-4%. The major sources of error were found to be tissue heterogeneity and sample preparation procedures. Using this technique in vitro dielectric measurements were made on human female breast tissues. A large number of data were gathered on fat and normal breast tissues, and on benign and malignant breast tumours. Each data set was parameterised using the Debye equation. Results from this suggest that all breast tissues measured in this work contain a component of bound water. A smaller proportion of water is bound in fat than is bound in other tissues. Comparisons were made of the dielectric properties of breast tissues with values calculated from mixture theories. Permittivity data largely fall within bounds set by mixture theory: conductivity data often fall outside these limits. This may imply that physiological saline is not a good approximation to tissue waters; or it may imply that another relaxation process is occurring in addition to the dipolar relaxation of saline. Comparisons of tissue type indicate that a dielectric imaging system could be designed which would detect breast diseases, but that severe problems could arise in distinguishing disease types from dielectric imaging alone.