Hopefully by now you have heard my US mantra:
Fluid is our friend and air is our enemy!
So how do we identify the enemy in our midst? There are times that visualization of gas can be useful: necrotising fasciitis, forniere's gangrene, intrabdominal free air, and emphysematous cholecystitis, to name a few. All of these diagnoses share the same basic findings on US.
Air on US has a characteristic appearance. Recall that as a media, it is poor at perpetuating sound in a straight line, but actually diffuses sound waves in all directions.
This is why you can hear a screaming aisle patient throughout the ED!
That scattering of sound waves creates a speckled line as the sound waves hit the tissue/air interface. The sound beam is then scattered diffusely. So what you see is the hyperechoic or bright line of the air interface and a gradual fade to black distally.
Notice that there is not a well demarcated shadow posterior to the area of gas as you see with something solid (gallstones).
It looks somewhat like a painter smeared the white line down the screen. Think Bob Ross, the famous PBS painter creating clouds, "place a little white here and drag your brush down!"
So let's look at the difference between a true acoustic shadow and a shadow created by an air interface.
- Acoustic Shadow: Sound waves hit the dense object and immediate get bounced back completely, creating a hyperechoic or white line. Notice that immediately under or distal to the solid object there is a void of sound, which creates the characteristic shadow.
- Gas: Sound waves hit the air interface and get scattered around. There is a hyperechoic white line and then a smear of the white distal to the gas. This smear "shadows" out the surrounding tissue not by creating a void of sound but by obscuring the surrounding tissue with the scattered sound waves.
Examples of gas in the soft tissue:
Emphysematous GB pics from USCases.info
Emphysematous Gallbladder video
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