Putative Physiopathological Explanation for the "Sock Sign" in Bone Scans

Four patients were referred to our department to undergo a bone scan for suspected reflex sympathetic dystrophy in the upper limbs. After TC-HMDP injection in the foot, they developed a "sock sign," defined by highly increased uptake in the injected lower limb. The sock sign has been previously described in only 1 case in the literature and attributed to intra-arterial injection. Our images suggest that the sock sign might instead be the consequence of tracer extravasation at the injection site.