So we are scanning the left thorax in a patient with shortness of breath, in an effort to assess for pleural effusion. The following video was obtained:
https://gmep.org/media/11997
The operator correctly noted the presence of a pleural effusion, and a bit of lung tissue can be seen towards the left side of the screen floating in fluid. In addition, there are THREE shadows evident, each from a different source. Can you spot them?
https://gmep.org/media/11995
So let’s take these one at a time, with labels:
https://gmep.org/media/11996
Shadow A
Is the easiest one. It extends almost from the first pixel at the top of the screen down to the far field. We can’t even see the characteristic echotexture of skin or subcutaneous tissue in the near field. There’s no contact here between the transducer and skin, possibly due to:
- the probe not touching at all
- clothing or an EKG lead getting in the way
- not enough gel (the novice’s answer to everything but sometimes still true)
Shadow B
The most interesting one of the bunch. Probably two major factors at work here. First, this section of diaphragm is a particularly bright reflector so it can create a shadow behind it due to the sheer amount of reflection occurring. Second, the density difference between the diaphragm and pleural effusion is creating a refraction artifact, often referred to as an edge artifact. Beams of sound which were roughly parallel as they struck this interface get bent at different angles based on whether they hit the dense diaphragm or the less dense fluid. The space in between the formerly tightly spaced beams is displayed as blackness, or the absence of returning echoes.
Shadow C
That’s a rib shadow. Did you know that ribs grow back if you remove them?