Circular planar coil as electromagnetic movement detector of inside organs in biosystems: the fundamental design relationships

A simple analytical approach is suggested to study the fundamental properties of a circular planar coil used as an electromagnetic movement transducer for internal organs in biosystems. First, for this purpose a general expression for the reactance of the coil is derived when put near a semi-finite lossy dielectric medium ?, secondly a new expression of the reactance variation is obtained in the case that a region ?, with different electromagnetic characteristics, moves inside ?. On this basis the existence is easily shown of a focal distance for this particular transducer as already foreseen by means of experimental measurements and numerical analysis. This characteristic is very interesting as it may be used to improve the very poor spatial resolution capability of this kind of transducers. © 1982 International Federation for Medical & Biological Engineering.