A simple classroom exercise for teaching principles of ultrasound diagnostic using two springs

Abstract

Abstract We present a novel method for demonstrating the physical principles of ultrasound imaging at a level suitable for educational programmes up to the university level, using a simple mechanical model that is very inexpensive and accessible to a broad variety of educational institutions. The method revolves around the use of one or two steel springs that can be extended on a flat surface. The spring represents a tissue and the longitudinal wave traveling along the spring represents the propagation of an ultrasound wave in tissue. This method allows students to gain direct experience with wave propagation, reflection and transmission in tissue as well as insight into the physical processes underlying ultrasound imaging. These include equally the ultrasound diagnostic device measurement of the depth of various tissue boundaries, modelled as a simple measurement of the time elapsed between the emission of a wave and the detection of a wave reflected from an object intersecting the spring. An ultrasound device probes the boundary between a formation (an organ or a lesion) and surrounding tissue by registering the amplitude of reflection on the formation, which is proportional to a difference in acoustic impedance between the formation and the tissue. The amplitude of the reflected wave in our apparatus is also proportional to acoustic impedance between the spring and an object like a plastic ruler intersecting the spring, whose stiffness and consequently acoustic impedance can be adjusted by its offset. It is also easy to clearly see the reflected wave on the boundary of two coupled springs (two tissues) with different acoustic impedances. We represent a series of representative measurements using our mechanical model demonstrating its good precision.