Non-invasive dielectric measurements in bovine bones: a System Identification approach

RAMIRO M. IRASTORZA; M. MAYOSKY; J. RAÚL GRIGERA; FERNANDO VERICAT

Montevideo, Uruguay

Congreso; International conference of biological physics; 2007

Sociedad Argentina de Biofísica y Sociedad Uruguaya de Física

Changes in physiological properties (i.e: tissue water content, thickness) of human skin, subcutaneous fat, or deeper tissues can provide significant information from a clinical point of view. In some cases, these variations produce changes in the dielectric properties of the media under measurement. Open ended coaxial lines operating at microwave frequencies have been successfully used for measuring dielectric properties of layered materials. Their wide frequency response allows the study of different dielectric relaxation processes, providing relevant information not achievable with other kind of probes. As a non-invasive technique, it is well suited for application in biological measurements in vivo, without damaging the media under test. In search of suitable models, several authors have developed mathematical, empirical and numerical approaches. In particular, in this work a simple exponential approximation is applied and compared with a theoretical one. The resulting approach is useful to represent measurements in vivo of body areas that can be dielectrically modeled by layers. A fast procedure for estimating changes in the relaxation processes of layered biological tissues in a wide frequency spectrum is developed, using time domain measurements and Fourier analysis. Time Domain Reflectometry (TDR) and System Identification tools are used to obtain the frequency response. For both techniques the media under measurement is modeled as a causal linear time invariant (CLTI) system. The technique developed here is applied to measurements of bovine cortical and cancellous bones between 10 and 1300MHz. The results obtained are validated with frequency measurements and literature data. Application of the proposed methodology depends on a previous knowledge of the kind of tissues under study and hence, on the expected values their dielectric properties can take.