Ultrasound Physics Fundamentals
Producing a sound wave: Ultrasound waves are produced by a transducer. A transducer is a device that takes power from one source and converts the energy into another form eg electricity into sound waves. The sound waves begin with the mechanical movement (oscillations) of a crystal that has been excited by electrical pulses, this is called the piezoelectric effect.The sound waves are emitted from the crystal similar to sound waves being emitted from a loud speaker. The frequencies emitted are in the range of (2- 15MHz) and are unable to be heard by the human ear. Several crystals are arranged together to form a transducer. It is from the transducer that sound waves propagate through tissue to be reflected and returned as echoes back to the transducer.
Sound is produced using Piezoelectricity which is the ability of some materials (notably crystals and certain ceramics) to generate an electric charge in response to applied mechanical stress, the reverse applies when the word is derived from the piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effectconverse piezoelectric effect (the production of stress and/or crystals will exhibit a maximum shape change of about 0.1% of the original dimension.
Precise electrical pulses from the ultrasound machine make the transducer create sound waves at the desired frequency. The sound is focused either by the shape of the transducer (Curved, Linear, Sector) or a set of control pulses from the ultrasound machine. This focusing produces the desired shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth. On the face of the transducer a rubber material enables the sound to be transmitted efficiently into the body. This rubber coating is required for impedance matching and allows good energy transfer from transducer to patient a vice versa. To help with the transmission of sound waves a water based gel is placed between the patient’s skin and the probe.The gel establishes good acoustic contact with the body, since air is a very good acoustic reflector.