The cluster sign is a finding on MRI and CT that is associated with pyogenic hepatic abscesses and can help differentiate pyogenic abscesses from other types of liver lesions.
The cluster sign is best seen on MRI T2-weighted and postcontrast T1-weighted sequences. Small necrotic/purulent areas in the pyogenic abscess "cluster" together and then coalesce into a larger necrotic/purulent areas, eventually becoming a larger septated abscess cavity.
At the periphery of these clusters
- there is rim enhancement on the early arterial phase of the postcontrast dynamics sequence
- there is reactive change in the surrounding liver manifesting as an increase in T2 signal and altered perfusion around the cluster