Medical Physica Ultrasound

Medical Physics.

Ultrasound

Sounds, including ultrasounds, are mechanical longitudinal waves, meaning they are created by vibrations in a medium.

Hearing range is ~20 Hz to 20 000 Hz. Sounds with a frequency greater than 20 000 Hz are ultrasounds. Diagnostic ultrasound, however, uses frequencies ranging from 1 to 10 MHz.

Ultrasounds generally have a very short wave length and as a result are much more easily reflected than sound in the normal hearing range.

Ultrasounds are produced by applying a high AC frequency voltage to a piezoelectric crystal, and using the piezoelectric effect, to create the very rapid vibrations in the medium.

Piezoelectric crystals are capable of converting an oscillating potential difference, applied to the crystal, into mechanical vibrations and mechanical vibrations into an oscillating potential difference. This is known as the piezoelectric effect.

The acoustic impedance of a medium is a measure of its opposition to the propagation of sound waves. The acoustic impedance of a medium is the product of the density of the medium and the speed of sound in the medium.

Z = pu

The velocity of sound in most materials is different and the densities of most materials are different. Since the acoustic impedance of a material is defined as the product of its density and the velocity of sound through it, then different materials will generally have different acoustic impedances.

Both density and velocity of sound through a medium are variable so acoustic impedance is not directly proportional to velocity of sound or density.

Even if two materials have the same density, the speed of sound in the different materials will most likely be different; hence their acoustic impedances will differ.