Answer:

Penumbra sign in subacute osteomyelitis of calcaneum.

Discussion:

The images shown here belongs to that of a 25 year old female, a known case of
osteomyelitis. The Intraossseous abscess in calcaneum which are multiple here exhibits
Penumbra sign.

One imaging finding reported to be helpful in differentiating a subacute or chronic abscess
from tumor has been called the penumbra sign. This sign was originally described in cases
of subacute osteomyelitis, but it has been found to be useful in identifying soft tissue
abscesses as well. It consists of a thin, intermediate signal intensity rim along the periphery
of a bone or soft tissue abscess on unenhanced T1W images . Although this appearance is
not specific for an abscess, it is highly suggestive, and is thought to be related to a layer of
granulation tissue along its wall.

Reference- Helms: Musculoskeletal MRI second edition, musculoskeletal infections.
Contributed By: Dr. Karunakaran M. Kalathi MD,  Dr. Arun Chelladurai MD 
Consultant Radiologists, Aarthi Scans, Thirunelveli, Tamilnadu, India