Ultrasound is a medical imaging technique that uses high frequency sound waves and their echoes. The sound waves are above human hearing, and the echoes from the sound waves bouncing off tissue inside the body are turned into electrical pulses that can be put together into a digital image.
A probe is placed on the patient's body with some water-based gel used to transmit the sound waves. As the sound waves hit boundaries between tissues, some waves are reflected back to the probe and others keep travelling. The probe picks up the reflected waves and a computer works out the relative distances of tissues from these waves and creates a picture.
With two probes, it is possible to create three-dimensional images, and to measure fluid flow, for example, blood.
The equipment is relatively small and the images can be seen at the same time as the patient is undergoing the ultrasound.
Ultrasound is very good at showing muscle and soft tissue and clearly showing solid and fluid-filled spaces. Ultrasound doesn't work well through bone, so is not used to look at the brain. One of the common uses of ultrasound is to check on the development of a foetus during pregnancy. Ultrasound is generally considered safe for this purpose.
From bats to 3D
Bats use very high pitched sound to navigate – the sounds they make are reflected back and they use these reflections to locate objects in their path – and dolphins and whales use the same technique underwater. This principle was used to develop radar and sonar during World Wars 1 and 2.
Between the wars, ultrasound was being developed as a tool to detect flaws in ships and other metal structures. About the same time, ultrasound started to be used as a therapeutic tool in medicine. One of the most important early researchers was Scottish obstetrician and gynaecologist Ian Donald, an RAF officer during the war, who became fascinated by the use of radar and sonar. After the war he became Professor of Midwifery at the University of Glasgow. The husband of one of his patients was a director of a boiler-making firm in Glasgow and invited Donald to view a demonstration of an ultrasound flaw detecting machine. Donald was fascinated by the demonstration and later visited the factory with a collection of assorted gynaecological lumps, fibroids and cysts to test with the scanner. The results convinced Donald that ultrasound would be a very useful diagnostic tool and he obtained his own flaw scanner. His work with the scanner attracted the attention of Tom Brown, a young engineer, who joined Donald’s team.
Their work was initially greeted by scepticism by doctors, but this was overcome when the scanner was used to diagnose an ovarian cyst in a woman who had previously been thought to have inoperable stomach cancer.
The output from these early scanners did not look at all like the ultrasound pictures we know today – it was in the form of a curved line on an oscilloscope that had to be interpreted by the person doing the scan. Donald and Brown realised the limitations of such a display and, in 1957, began work on a scanner that would produce a two dimensional picture in black and white.
An Australian research team lead by George Kossof developed greyscale pictures in the late 1960s, and real-time scanning was developed in the 1970s. In the 1980s and 1990s, the same computer technology used to produce 3D animated films was adapted to produce 3D ultrasound scans.