Lets start with how different tissue types create different images on the screen. The sound waves sent out from the probe strike the body and return to the probe. Depending on what they strike depends on how much or how little of the initial sound beam return.
Lets image that we have a bucket of tennis balls and we are ECC.
If we thrown ten tennis balls at the wall, how many come back to us?
All of them, correct? This is due to the fact that the wall is dense, just like a gallstone or bone is dense. In US, those sound waves strike this dense "wall" and cannot penetrate further, just like the tennis balls cannot go through the wall. Since the solid object blocks the sound waves from penetrating further into the body, there is a void of sound waves posterior to the solid. This void of sound is called Acoustic Shadowing.
This is a useful property as it can help us determine pathologic states or creates landmarks for us to follow.
So what if we throw our tennis balls at the curtain between resus beds 3 and 4, how many will come back to us?
It depends on the stiffness of the curtain, of course, but a certain amount will continue through the curtain and some will bounce back to us. This is what occur in soft tissues. Some sound waves bounce back to the machine and some penetrate through to hit structures underneath. As with the curtain, the strength to which the tennis balls bounce back varies with the stiffness or density of the curtain. Same for ultrasound: the amplitude of the returning sound wave varies depending on the density of the soft tissue it strikes. As the sound waves go through each progressive layer of soft tissue, the amplitude becomes weaker and weaker, until finally there is not any sound left.
This is how we can see different tissue planes on ultrasound and determine the difference between different tissue types.
Now let's image that we take our bucket of tennis balls and throw them down the aisle toward the fishbowl. How many will return to us?
None, right? They all go until they hit the bowl or the floor. This is what happens with liquids. Very little of the sound beam gets diverted as it goes through the liquid, so the vast majority of the beam hit the structures posterior to the fluid collection. This is way fluid is so great for ultrasound. It transmits sound waves to allow us visualization of distal structures, a property called Acoustic Enhancement.
So we have talked about solids, semi-solids, liquids, but what about gases? Let's go back to the tennis balls. Imagine if we set up the Bair Hugger hose, shooting air out the end. Now lets throw out tennis balls through that air current. What would happen to the tennis balls?
They would go in all directions, right? We couldn't predict where any of the balls would end up because the air current would disperse them in all directions! The same is true of ultrasound. When our sound beam strikes an air interface, the sound beam is scattered in all directions and we cannot see beyond it since the beam is now gone.
The dispersion of sound is what makes us able to hear that fishbowl patient yelling even when we're in Resus. Although this is an important property for ED management, it is awful for US.
Well, that's it for today. I hope you have enjoyed this first ED US Teaching Cast and will take home an important message: Air is our Enemy and Fluid is our Friend!
