Many of us will have had the very pleasing experience of finding ourselves in a place with really good echo acoustics. The echoes work because the sound reflects off anything where there is a change in density. So where there is a change from air to rock, the sound bounces off and you hear it back a short time later. It doesn’t have to be rock. Many of you will know that bats can use echoes to locate small insects. It works in water as well – dolphins use it for find and stun fish.
The same trick can be used in the human body. If you get the note just right (way above human hearing), then the sound will travel through the body. It will bounce back whenever it meets a change in tissue. Blood, fat, muscle, bone etc. By timing the echoes, we can work out where the boundaries are and can form images. The images are a little grainy, but they can be very useful – below you can see applications to gall stone detection, pregnancy screening and breast cancer.
Just like MRI, there is a solution which is widely used – very small bubbles can be injected into the blood stream. The bubbles are like the ones you blow – a thin film of a soap-like material surrounding air. The air makes excellent echoes and the contrast can be remarkable.
Learn about the other imaging techniques.