Ultrasound

Ultrasound or ultrasonic waves are sound waves with frequencies above 20,000 Hz. These waves are undetectable by humans as they lie above the audible range.

Ultrasonic waves have high frequencies and short wavelengths.

We know that the velocity of sound waves in air is 330 m/s. Therefore for an audible sound with frequency 1000 Hz we can calculate the wavelength using the wave equation.

velocity = wavelength x frequency

Rearranging equation,

wavelength = velocity/frequency

wavelength = 330/1000

wavelength = 0.33m

However, if the sound is in the ultrasonic range for example 100,000 Hz then the wavelength can be calculated as:

wavelength = 330/100000

wavelength = 0.0033 m = 0.33 cm

Hence the ultrasonic wave with is shorter wavelength can be transmitted as a beam and undergoes little diffraction; these properties of ultrasound are put to use in various applications.

Uses of Ultrasound

Echo Sounding

This technique is used to detect defects or flaws inside pieces of metal. It works on the principle of a pulse of ultrasound being transmitted into a metal component to be assessed. A receiver then picks up the echoes of the ultrasonic waves from different parts of the metal and shows the results on a cathode ray oscilloscope. The animation below shows how ultrasonic echo sounding works.

Pre Natal Scanning

Ultrasound waves are partially reflected at the boundaries between two different materials. Ultrasound reflects well at the boundaries between soft tissue and air or bone and air. The speed of ultrasound depends on the material in which it is travelling. Ultrasound waves travel faster in dense materials as the molecules a packed tightly together and slower in less dense materials. Thus in body tissue which is less dense ultrasound travels slowly with a corresponding decrease in wavelength and in dense material like bones it travels faster with a corresponding increase in wavelength. These properties of ultrasound are applied in pre natal scanners. The animation below explains how a pre natal scanner works.

Pre Natal Scanning

Ultrasound is used for pre-natal scanning to monitors the baby’s development in the uterus. Ultrasound is emitted from a transducer which also detects the echoes which are analysed by a computer to give a moving picture of the baby on a screen. Ultrasound is reflected at the boundaries between solid structures such as bones and muscles or tissues and body fluid present in the womb and displayed on the screen as grey and white areas. Hollow area like the eye sockets or heart chambers do not reflect the ultrasound as these appear as dark or black areas. This technique can also be used to analyse soft organs like the liver or gall bladder.

Advantages and Disadvanges of Ultrasound Scanning

Advantages of Ultrasound Scanning

Ultrasound is a non-invasive technique i.e. it does not need to go beneath the skin or enter any orifice making it painless.
Ultrasound does not produce any harmful radiations thus making it safe for monitoring unborn babies.
It is a low cost and easy procedure therefore reducing the demand of hospital resources.
It can display real time images of the movement and function of body organs and blood vessels.

Disadvantage of Ultrasound Scanning

Ultrasound does not reflect clearly from bone or air thus giving it limitations. For images of the lungs and bones other techniques are clearer and therefore preferred.