CT of small pyogenic hepatic abscesses: the cluster sign

Of 36 consecutive patients with pyogenic liver abscesses evaluated by CT, five (14%) had multiple small abscesses less than 2 cm in size. The CT appearance of the small pyogenic abscesses was compared with that of 10 patients who had either fungal or mycobacterial abscesses and with that of 50 patients who had hepatic metastases. In all five patients who had small pyogenic abscesses, the abscesses appeared to cluster, or aggregate, in a pattern that suggested the beginning of coalescence into a single, larger abscess cavity (cluster sign). This cluster appearance was not seen in in any of the patients who had fungal or mycobacterial microabscesses. It was present in only one of the patients who had confirmed hepatic metastasis. Despite the small size of the abscesses, guided needle aspiration was successful in recovering pyogenic organisms in four of the five patients. In our experience, the presence of the cluster sign suggests that the lesions are pyogenic abscesses.