PHYSICS IS FUN: Enhancement vs. Shadowing

Good vs Evil.  Enhancement vs. Shadow.  How does that play into Ultrasound you may ask?  Last month we talked about how sound waves are affected by the media they travel through...think back to the tennis ball analogy.   Now let's now talk about how that affects the picture we see on the screen.

Dense objects block sound waves from penetrating further, just like a brick wall blocks the tennis ball from entering the next room.

Since the sound waves are blocked from penetrating further into the tissue, there is a void of sound (no tennis balls in the adjacent room).  This void prevents us from visualizing structures below the dense object.  This is the reason why we do not routinely use ultrasound to guide vascular access in the subclavian vein:  the vein is sub-clavicle and is thus shadowed from view.  This is called Acoustic Shadowing.

So what about the opposite: the tennis ball that goes through a doorway?

Fluid is our friend and allows sound waves to completely penetrate through liquids.  Therefore, few sound waves are returned to the machine before hitting the fluid interface.  Once in the fluid, all sound waves continue on, as if through a wormhole, to come out on the other side of the fluid interface.  Now you have a much stronger beam of sound distal to the fluid collection when compared to the surrounding tissue.  Since sound waves get progressively weaker as they travel father away from the sound source (probe), the sound waves that had to go through the soft tissue on either side of the fluid interface will be weaker. These weaker sound waves have less amplitude and therefore, do not look as bright.

The sound waves that went through our fluid wormhole will appear brighter or have a higher amplitude distal to the fluid when compared to the surrounding tissue.

This phenomenon is called Acoustic Enhancement.

This can be very useful to us.  It will allow us to visual distal structures better, such as the uterus.

It can also create problems though as well, such as during the FAST exam.  Imagine that there is a small fluid collection in the pouch of douglas.  The acoustic enhancement of the bladder can make visualization of this fluid appear overgained and not black, causing you to miss a positive FAST exam!
So when looking at structures distal to a fluid collection such as the bladder, be sure to use the distal TGC button to adjust the gain in that distal area, allowing fluid to become more visible.

Note that when the TGC is turned down, the distal structures start to differentiate.

Hopefully you can also tell the difference between acoustic shadowing versus acoustic enhancement now.