A middle-aged male presented to the EDÂ with 2 weeks history of increasing exertional dyspnea.Â Air entry was reduced clinically. A focused bedside ultrasound demonstrated the aboveÂ findings. What artifact is missing?
1.The “normal” mirror image of the liver above the diaphragm is missing! It isÂ reassurring when present. The air in normal lung reflects about 99% of the ultrasound beam (i.e. you can’t see past the diaphragm, hiding all normal structures below it).Â So what’s above the diaphragm is an artifical reflectedÂ image of the liver or spleen, thanks to the diaphragm whichÂ behavesÂ as a specular reflector (see below). In this case, massive pleural effusionÂ replaces the normal aerated lung, allowing the ultrasound beam to shine through, revealingÂ the vertebral column “C” that was not normally seen. In addition,Â a tongue ofÂ atelectatic lung “E” is found sticking into the pleural fluid, and the superior edge of the diaphragm alsoÂ well defined
2. Closer look at the vertebral column shows that it isÂ brighter above the diphragm than below. Why? Fluid attenuates ultrasound beam energyÂ less than soft tissue. Therefore, more ultrasound energy passes through fluid than soft tissue for the same distance travelled.Â So there is actually a stronger ultrasound beamÂ reflected backÂ at “C” compared to “D” , thus making it look brighter. This artificial “brightness”Â is termed acoustic enhancement. In fact, all clear fluid structures is expected to demonstrate acoustic enhancement.
3. Why did the gerota’s fascia seems to disappear as we trace it from “A” to “B”?Â Anatomically, it didn’t. That’s an artifact from reflection.Â Fascia (like the diaphragm)Â form brightly reflective tissue interface (termed specular reflectors) as opposed to soft tissues (which are diffuse reflectors, having a grey appearance). At “A”, the approaching ultrasound beam hits the specular reflector head on, around 90 degrees. Acting like a mirror,Â the fascia reflectsÂ significantly moreÂ ultrasound beam back to the transducer, making it look bright. On the other hand, at “B”, the beam hits it at a much shallower angle. This fasica readily bouncesÂ the beamÂ away from the transducer to the left side of the image instead of mostly back towards the transducer,Â thusÂ making it look dark.
When something unreal is missing, something realÂ may beÂ happening.