Cross-sectional area measurement in collapsed tubes using the transformer principle

To avoid the necessity for intraluminal catheters as used with the axial impedance method of measuring the cross-sectional area of flexible tubes independently of their shape, while retaining the advantage of an immediate electrical output, an electromagnetic method was tested. The method uses a single-turn sensing coil attached to or embedded in the tube wall at the site of interest as the secondary winding of a transformer. One or more primary coils coaxial with the tube provide an alternating magnetic field parallel to the tube axis, and the resulting secondary voltage, after amplification and demodulation, is directly proportional to tube cross-sectional area. Tube pressure/area relationships measured thus were compared with those measured using both an ultrasonic imaging technique and liquid volume displacement. The method was shown to provide an accurate and relatively simple alternative to the impedance method. Various ways to fabricate tubes with or otherwise introduce the necessary sensing coil are discussed.