[ELi5] Can someone explain ultrasound jelly?

Question

Why do they have to use the jelly to get a reading ? What’s the purpose ? Also bonus if you tell me what it is.

In: 27

Answer 1

Ultrasound is transmitted as waves, and this wave nature is important.

Waves travel at a certain speed, which is related to the medium that the wave is travelling through and the type of wave. Whenever a wave changes in speed, some of the energy in the wave is reflected. The bigger the difference in speed the more reflects back.

The speed of sound is different in different types of material – it’s slow in air, and faster in water, and even faster in harder materials. This means that every time sound moves from air into water, or water into tough body tissue, some gets reflected. In fact, this is exactly how ultasound forms an image anyway. The machine sends out a burst of sound, and listens for the echo. The echos are produced whenever the sound goes from one type of tissue into another. Because the speed of sound in different body organs is quite close (but not identical), a weak echo comes back and most of the sound carries on going forward into the next organ.

Because the speed of sound in air is WAY slower than in body organs, the reflections that come when the beam hits air are extreme – almost all the sound echoes back, and almost nothing is left to keep going forward. Because not much sound gets into the body, you don’t get any echoes back from inside, so can’t see the organs.

The solution is to put a material between the probe and the skin, that has roughly the same speed of sound as the body. Water works fine, but it’s very liquid and just runs off (you could dam it up, but it’s messy and fiddly – although this is used sometimes for surgical ultrasound – for example, this is how ultrasound brain surgery for thigns like Parkinson’s disease is done).

So, water with some sort of thickener is usually used – typically a high molecular weight polyether such as polyethylene glycol (not propylene glycol as several other posts have stated – propylene glycol is something completely different)