In vivo bone vibration measurement by ultrasound

Unlike most structures usually investigated in experimental dynamics, dynamic testing on human patients presents challenges of different nature, such as reduced knowledge of the topology, inherent variability in time, and limited accessibility to name a few. In order to non-invasively measure the vibration of a bony structure underneath various layers of tissue, a method has been developed based on the calculation of the time of flight of ultrasound waves reflecting on the bones. A position-time plot, sampled at a frequency of 5 kHz and spanning over a number of vibration cycles, is obtained by means of a suitable post-processing, and the characteristics of the vibration are identified. The measurement apparatus avails of two channels for the simultaneous acquisition at two points, from which phase information can be drawn. The measurement system has been verified by means of a test object consisting of two masses interfaced with a viscoelastic material, and excited by means of a shaker. The out-of-phase vibration of the two masses has been measured with the ultrasound system for different frequencies, and the phase difference computed.