Focal Hepatic Infections

Cholangiography

Ascending cholangitis is the most common cause of pyogenic hepatic abscess, and cholangiography has become an important aid to diagnosis in many cases. Percutaneous transhepatic cholangiography and endoscopic retrograde cholangiography can accurately define the level and cause of biliary obstruction; they are the first step in biliary drainage procedures and can accurately define biliary anatomy for the surgeon. These procedures increase intrabiliary pressure and can precipitate deterioration of a patient who is already septic. Accordingly, biliary drainage procedures should be anticipated. Magnetic resonance cholangiopancreatography is an important tool in diagnosis of obstructive biliary tract lesions.

Ultrasound

Real-time ultrasound can detect hepatic abscesses as small as 1.5 cm with a sensitivity of 75% to 90%. Pyogenic hepatic abscesses are extremely variable in shape and echogenicity. They are usually spherical ( Fig. 55.1 ) or ovoid but may be lobulated or lentiform. Mural thickness is variable, and the wall typically is irregular and hypoechoic. On sonography, abscesses appear anechoic (50%), hyperechoic (25%), or hypoechoic (25%). Septa, fluid-fluid levels, internal debris, and posterior acoustic enhancement may also be seen. Early lesions tend to be echogenic and poorly demarcated; they may evolve into well-demarcated, nearly anechoic lesions. If gas is present in an abscess, brightly echogenic reflectors with posterior reverberation artifact may be noted.

The sonographic differential diagnosis includes amebic or hydatid infection, necrotic or cystic neoplasm, hematoma, complicated biloma, and simple cyst with infection.

Magnetic Resonance Imaging

Hepatic abscesses, like most other focal hepatic processes, prolong T1 and T2 relaxation times. ^, At MRI, air within the abscess appears as a signal void and is therefore more difficult to differentiate from calcifications. However, the shape and location (air-fluid level) should enable the correct diagnosis. After administration of gadopentetate dimeglumine (Gd-DTPA), abscesses typically show rim enhancement ( Fig. 55.2 ), which is secondary to increased capillary permeability in the surrounding liver parenchyma (the double target sign). Small lesions (<1 cm) may enhance homogeneously, mimicking hemangiomas. ^, Abscess wall enhancement on dynamic postgadolinium images may be considered a distinctive feature of pyogenic liver abscesses. Abscess wall shows a fast and intense enhancement that persists on portal venous and late phase images. Some of the lesions may contain internal septations, which also reveal persistent enhancement on late phase images. ^, Perilesional edema, shown as high signal on T2-weighted images, is associated with 50% of abscesses. However, it may also be seen in 20% to 30% of patients with primary or secondary hepatic malignant neoplasms. Therefore, the presence of perilesional edema can be used only to differentiate a hepatic abscess from a benign cystic hepatic lesion. Resolution of perilesional edema may indicate response to therapy. ^,

Computed Tomography

By virtue of its good spatial and contrast resolution, CT is the single best method for detecting hepatic abscess, with a sensitivity as high as 97% ( Fig. 55.3 ). On CT scans, abscesses appear as generally rounded masses that are hypodense on both contrast and noncontrast scans. The attenuation ranges between 0 and 45 HU and thus overlaps with the appearance of cysts, bilomas, and hypodense neoplasms. Most have a peripheral rim or capsule that shows contrast enhancement in a pattern similar to that seen on MRI (see Fig. 55.1 ). Most abscesses are sharply defined, but a minority have a grossly lobular contour and circumferential “transition zones” of intermediate attenuation.

Another helpful finding is the transient segmental or wedge-shaped enhancement of the hepatic parenchyma surrounding an abscess on the arterial dominant phase of a contrast-enhanced dynamic CT scan.

Some abscesses show the cluster sign (see Fig. 55.3C–E ), in which small, pyogenic abscesses appear to cluster or aggregate in a pattern suggesting coalescence into a single large cavity.

All of these findings are nonspecific and require aspiration for diagnosis. Central gas (see Fig. 55.3A ), as either air bubbles or an air-fluid level, is a specific sign, but it is present in less than 20% of cases. A large air-fluid or fluid-debris level is often associated with communication with the gut.