Secondary Benign Liver Lesions 421

Pyogenic Abscess

Abscesses of the liver may be caused by bacterial, amebic or fungal infections, resulting in the localized collection of inflammatory cells and destruction of the surrounding parenchyma [82]. Hepatic abscesses can develop via five major routes [29]:

• the biliary route, due to ascending cholangitis, benign or malignant biliary obstruction and choledocholitisis,

• the portal vein route, due to pylephlebitis from appendicitis diverticulitis, proctitis, infected hemorrhoids, inflammatory bowel disease and others,

• the hepatic artery route, subsequent to septicemia,

Fig. 60a-e. Diffuse biliary abscess formation in ascending cholangitis. Diffusely distributed areas of high signal intensity can be noted on the unen-hanced T2-weighted image (a). On the corresponding unenhanced ^-weighted image (b) these areas appear hypointense. During the arterial phase of the dynamic series (c) peripheral hypervascularization of the affected areas (arrows) can be noted. In the portal-venous phase (d), the cystic-appearing regions remain hypointense. On fat suppressed images in the equilibrium phase (e) a hyperintense rim surrounding the affected areas (arrows) is indicative of an inflammatory process

• the direct extension route, from contiguous organ infections,

• the traumatic route, from blunt or penetrating injuries.

Before the era of antibiotics, pylephlebitis of the portal vein through seeding from appendicitis or diverticulitis was the most common cause of hepatic abscesses. Pyogenic abscesses today are most often associated with benign or malignant obstruction with cholangitis. About 50% of pyo-genic abscesses are caused by anaerobic organisms or mixed anaerobic and aerobic organisms. Escherichia Coli is most frequently isolated in adults, while Staphylococci organisms are most often isolated from hepatic abscesses in children. Abscesses of biliary tract origin are multiple and frequently involve both hepatic lobes (Fig. 60). Abscesses of portal vein origin are often solitary and mainly localized in the right lobe.

The clinical symptoms of patients with hepatic abscesses include fever, malaise, abdominal pain in the right upper quadrant, nausea and vomiting. Tender hepatomegaly is the most common clinical sign and leukocytosis, elevated serum alkaline phosphatase levels and hypoalbuminemia are the most common laboratory abnormalities. Generally the onset of symptoms is acute [29].

Ultrasound can detect hepatic abscesses as small as 1.5 cm with a sensitivity of up to 90%. Pyogenic hepatic abscesses are extremely variable in shape and echogenicity and may appear as ane-choic (50%), hyperechoic (25%) or hypoechoic (25%) (Fig. 61). Septa and fluid-fluid internal necrosis are frequently seen, while calcifications

Fig. 61. Pyogenic abscess on US. Ultrasound reveals a heterogeneous hypo- to isoechoic lesion with ill-defined margins (arrows)

and gas may also be detected. Early lesions tend to be echogenic and poorly demarcated [72].

CT is a valid method for detecting hepatic abscesses with high sensitivity. On CT, hepatic abscesses appear as hypodense lesions with an internal pattern of varying density. The lesions generally appear as rounded masses that show minimal contrast enhancement. Most abscesses have a peripheral rim that shows contrast enhancement predominantly in the equilibrium phase. The "cluster" sign is suggestive for abscesses and represents smaller lesions surrounding a large abscess. Another CT sign, the "double target", is seen with early abscesses, and represents a hypodense lesion surrounded by a hyperdense rim, and an outer low-density region (Fig. 62). The presence of central gas, either air bubbles or an air-fluid level, is a specific sign of pyogenic hepatic abscess, but is present in fewer than 20% of cases [6,87].

On MR imaging pyogenic abscess appears as an area of decreased signal intensity on T1-weighted images and increased signal intensity on T2-

Fig. 62a-d. Pyogenic abscess on CT. On an unenhanced CT scan (a) the abscess (asterisk) appears as a heterogeneously iso- to hypodense lesion. Peripheral rim enhancement is seen on the arterial phase image after the administration of contrast medium (b). This is better seen in the portal-venous (c) and equilibrium phase (d) images due to peripheral edema (arrowheadin c)

Fig. 62a-d. Pyogenic abscess on CT. On an unenhanced CT scan (a) the abscess (asterisk) appears as a heterogeneously iso- to hypodense lesion. Peripheral rim enhancement is seen on the arterial phase image after the administration of contrast medium (b). This is better seen in the portal-venous (c) and equilibrium phase (d) images due to peripheral edema (arrowheadin c)

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