CT was consistent with their findings (16x16 GB fossa cystic mass) and it was thought to be a biloma, seroma or inclusion cyst. During her hospitalization IR drained this fluid, which was serous, but the final pathology is not yet back.
When we see a large anechoic structure on ultrasound, it is important to think about the differences between a cyst, an abscess, and free fluid.
Generally, a simple cyst should be smooth and uniformly anechoic throughout whereas an abscess will more likely have varying degrees of echogenicity (grayness) to it, but this is not always the case. And both can have septae (as demonstrated above).
The walls of a cyst are usually thin and linear while an abscess cavity may have more of a ragged appearance to it. With gentle compression of an abscess, you may even see the purulent material swirl, this is referred to as the "squish sign" (..really!).
Both cysts and abscess will be circumscribed within a hyperechoic wall whereas free fluid will flow freely around the organs and will not be contained by any walls. (this comes up a lot when we see renal cysts on the FAST exam…so take a look at this next time).
If there is anything unusual, always look in a different plane to add more sensitivity to your image.
Again, please feel free to email us with any thoughts or questions or interesting ultrasound cases! Happy Holidays everyone from your Ultrasound Team!