New Perspectives in Rheoencephalography

One of the most important advances in biomedical engineering has been the ability to inspect inside the body without opening it. In this sense, rheoencephalography (REG) is an electromedical technique used to assess the cerebral blood flow (CBF) by noninvasive electrical impedance methods, using electrodes attached to the scalp surface. This technique was first proposed by Polzer and Schuhfried (1950), and emerged as an extrapolation of impedance plethysmography applied to the head. An electric current flowing through a biological tissue causes a potential difference between any pair of electrodes that can be measured. This potential difference depends on the amplitude of the injected current, the shape of the conductor, the arrangement of the electrodes, and the electrical characteristics of the tissue. For instance, the electrical conductivity of the lung tissue is much lower than that of the cerebrospinal fluid (CSF), since alveolar sacs are nonconductive. Furthermore, the electrical conductivity depends on the frequency of the electric current, the orientation of the tissue fibers relative to the current flow, and the amount of extracellular fluid that surrounds the cells. For example, electrical conductivity is higher in the blood than in most tissues, since plasma acts as a truehighway for ions (Malmivuo & Plonsey, 1995). he ejected blood fills the arterial vascular bed, and a small and abrupt change in the electrical impedance of the tissues occurs, due to two reasons: the crosssectional area of the tissue increases, and tissue effective conductivity rises. This is the physical principle of impedance plethysmography, and explains why the electrical impedance of any part of the body throbs synchronised to the heartbeat some tens or hundreds of milliohms around a basal impedance value.