Medical Physics - Ultrasound

Despite us not being able to hear it with the human ear, we can create detectors which allow us to map out an image of ultrasound waves which can show us the internal structure of objects. It is possible to use ultrasound in this way because the sound waves have a small amplitude and so they are low energy and therefore less harmful to living tissue.

An ultrasound scan works by sending high frequency sound waves into the body. A special gel is applied to the skin to make sure that the sound waves are transferred efficiently. Without the gel, the big density difference between the skin and the air would mean most of the ultrasound would be reflected and would never get into the body. Inside the body, some of the sound reflects off different tissue surfaces and bounces back to the detector. The rest of the sound continues on. When it reaches another layer, a little more is reflected and so on.

The reflected sound is then either presented as a graph or converted into an image by a computer. The image is displayed on a screen and can also be printed. To an untrained eye, an ultrasound scan image looks very blurred and out of focus. Medical staff who are trained to read ultrasound scans can see a lot more detail. When compared with an image produced from X-rays, the latter is much sharper and clearer because the wavelength of X-rays is a lot shorter than ultrasound. An advantage of ultrasound scanners over X-rays is that sound waves are non-ionising and so no damage is done to cells, making it safer for the patient than X-rays.

Its most commonly known use is to examine the fetus inside the womb during a pregnancy but it is also used to examine organs such as the liver, kidneys, gall bladder and heart. It is also possible to use it to measure the speed at which the blood is flowing round a patient's body (using a Doppler ultrasound scanner) and to break up kidney stones - they are vibrated by the ultrasound waves until they shake themselves into smaller fragments that can pass through the urethra and out of the body. Away from the body, ultrasound can be used to clean industrial components and medical instruments.

The computer in an ultrasound scanner calculates the depth of each layer of tissue from which the ultrasound has been reflected. The time taken for the reflections (echoes) to reach the detector is measured and the distance calculated from that using an equation that you should already know - distance = velocity x time (s = v x t).