Pyogenic abscesses are the most frequent form of focal hepatic infections resulting from an infectious process of bacterial origin.
Pathologically, pyogenic liver abscesses may occur as solitary or multiple lesions, ranging from millimeters to massive lesions.
Portal vein thrombosis is frequently associated with bacterial abscesses. The infected bland thrombus is characterized by low signal intensity on T2- and T1-weighted images and time-of-flight gradient-echo images. The thrombus does not enhance after contrast; however the vein wall shows a moderateto- intense enhancement after contrast administration, best seen on late-phase fat-suppressed images caused by the inflammatory reaction.
Characteristic MRI findings of pyogenic abscesses are high signal intensity on T2-weighted images, low signal intensity on T1-weighted images, moderate enhancement of stroma on immediate post-gadolinium images with persistent enhancement on interstitial phase images, and no enhancement of additional stroma or progressive fill in of the lesion over time (Balci and Sirvanci 2002).
Pyogenic abscesses also possess markedly thick walls and internal septations, which enhance moderately to intensely on early-phase images and demonstrate persistent enhancement on late-phase images that often appears more intense (Balci and Sirvanci 2002).
Abscesses typically have a moderate perilesional enhancement with indistinct outer margins on immediate post-gadolinium images because of a surrounding rim of granulation tissues and a hyperemic inflammatory response in adjacent liver. The perilesional enhancement rapidly diminishes, and is often nearly resolved by 1 min postinjection. Layering of debris and gas within the abscess cavity, mainly after biliary drainage, is also commonly appreciated.
Gas is identified as signal void on both T2- and T1-weighted images and debris are identified as low signal intensity on T2- and high signal intensity on T1- weighted images, since debris are usually composed of protein (Balci and Sirvanci 2002).