Hi Samuel,
I think this is subjective,
I would say a cyst is anechoic (no internal echoes at all),
Low level echoes I would classify as some faint echoes within the cyst seen when you turn the gain up higher.
Medium level echoes I would classify as about half the echogenicity of the thyroid gland tissue.
High level echoes I would classify as similar or greater echogenicity to the normal thyroid tissue
BUT,
I don’t think this matters much in terms of diagnostic outcomes,
Echoes in a cystic structure are less important than if there is vascularity within the echoes in a cyst.
If there are some echoes within a cyst and they are evenly distributed this is likely due to some haemorrhage within the cyst. Blood in a cyst may also form angular lines as the clot retracts similar to anhaemorhagic ovarian cyst. In fact it is worth looking at the IOTA classification for ovarian masses as the same logic can be applied.
Once you see Doppler signal inside the echoes within a cyst this changes everything. You are now not dealing with a cyst that has had some bleed within it but in fact you have a solid tissue element within a cyst and this makes it a more suspicious lesion that may require a biopsy. The determination to biopsy or not can be made using the TiRADS system.
TiRADS does not use vascularity as a criteria, however if there is vascularity within the cyst this indicates it is not a simple cyst and in fact is a cystic / solid mixed lesion and if the solid component has only faint echoes that will be classified as very hypo echoic solid element and this will score high on the TiRADS system making the lesion likely requiring a FNA.
Steve