Medical Imaging - Ultrasound Imaging; On the nature of sound waves

Ultrasound is a safe imaging method as it uses sound waves which are non-ionising form of radiation.  The sound waves used are of much higher frequency than the range that human ears are sensitive to.
Variations of ultrasound like Doppler-shifted Ultrasound provide measurement of flow rates (movement) and Harmonic Ultrasound can discriminate between different tissue type effectively.

In the following I reproduce the slides of my talk at James Watt Auditorium, East Kilbride, Scotland in the year 2008:

Change in frequency of sound waves (hence the shift in pitch) is proportional to the speed of the object divided by the speed of sound.

 Some references: This list should be updated ...