The “rim sign”: FDG PET/CT pattern of pulmonary infarctus

Objectives Incidental pulmonary embolism (PE) is not an uncommon diagnosis in patients with cancer. PE may be associated with pulmonary infarctus (PI), appearing as a non-specific peripheral consolidation. PI is associated with variable FDG uptake and can mimic a lung tumor or lead to a tumor burden overestimation. We present the PET/CT results in 6 patients with confirmed PI exhibiting the “rim sign”, which may help to evoke this difficult diagnosis.

Methods Six patients (median age: 56 years, 53-89) with PI exhibiting the “rim sign” were retrospectively included. Patients were referred for lung cancer initial staging (n=3), solitary pulmonary mass (n=2) and follow-up of chemotherapy (n=1). All PI were confirmed on MDCT showing acute PE (n=4) or tumoral arterial obstruction (n=2) in the same lobe or segment as PI. The “rim sign” was defined as a continuous slight FDG uptake along the border of a subpleural consolidation without uptake within the consolidation. Morphologic CT characteristics of PI were recorded.

Results Eight PI were diagnosed in the 6 patients. All PI exhibited the "rim sign" with moderate uptake (median SUVmax: 3.6, 2.2-6.8). Median size of PI was 48.5 mm (30-74). On MDCT, central lucency, triangular shape and vessel sign were observed in 5/8, 4/8 and 1/8 cases, respectively. Two out of the eight PI appeared as solitary pulmonary mass and exhibited only the “rim sign” and none of the suggestive MDCT sign.

Conclusions The “rim sign”, defined as a moderate and continuous FDG uptake at the border of a peripheral lung consolidation is easily recognizable at FDG PET/CT and is strongly suggestive of PI. This pattern appears to be very specific and can be observed even in the absence of suggestive findings of PI on MDCT. The “rim sign” is consistent with pathologic data about PI showing central blood alveolar filling with peripheral inflammatory reaction