Hi Xue,
Yes, it is an interesting observation.
I will refer you to a document I have prepared on this topic.
You can find it in the tips and tricks area or click this link
https://birdultrasound.com.au/wp-content/uploads/2022/10/What-is-that-echogenic-area_.pdf
When I see echogenic material in the MSK system I have a few options to consider:
Gout: Monosodim Urate can be anywhere, likes to sit on the surface of hyaline cartilage, doesn’t show much on x-ray and is typically painful
CPPD (pseudo gout, chondrocalcinosis) : likes fibrocartilage and often sits inside hyaline cartilage. Shows up easily on x-ray
Hydroxyapatite: Needs an enthesis to migrate through as it is a bone salt, can be liquid or solid, can accumulate and regress, shows up on x-ray, is painful in the accumulation phase.
Dystrophic calcification: the result of tenocyte exhaustion with tendinosis or other injury to the tendon like trauma, Shows easily on x-ray, often asymptomatic.
Xanthoma: A lipid based deposition disease that looks exogenic on ultrasound but does not show on x-ray except for the soft tissue swelling.
Enthesophyte: Bone growth from an enthesis into a tendon, ligament, retinaculum etc. Shows easily on x-ray.
Osteophyte: Intra-articular bony irregularity at the margin of a joint.
Loose body: small piece of either bone or cartilage that has dislodged as part of a degenerative joint process e.g. OA or rheumatoid.
In your case the joint looks very typical of rheumatoid.
You can see significant synovitis and bony destruction.
The tiny echogenic foci could be a few things.
Gout crystals is certainly a possibility if you have a patient with rheumatoid and also a background of gout simultaneously.
The echogenic foci are tiny and this is often the case with gout.
But it is also possible these foci are simply tiny bits of bone / cartilage that have been displaced as part of the joint degeneration and erosion processes.
If you imagine dismantling a joint with a process like RA it seems impossible that the job can be done in such a tidy way that tiny pieces of debris are not created and these could potentially explain the echogenic foci.
Suck one up with a needle and send it off to the path lab !
Do Birefringent testing and also conventional microscopy and see what it shows.
Steve