Abstract
A direct real-time recording of the cross-sectional area of the heart valve is useful for the fluid dynamic study of the cardiovascular system. Electronic circuitry is described that is capable of driving the transmitter coil assembly placed outside the animal and detecting an area-related signal induced in the one-turn coil in vivo. When a piece of fine pliable metal thread encircles the area of interest (e.g. the mitral/aortic valve orifice) so as to form a single loop, the electrical potential between the ends of the loop is linearly related to the size of the area irrespective of its shape. The principle of measurement, construction of transmitter coil assembly, and simple but accurate direct calibration are also described.