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Dynamic CT of Hepatic Abscesses

Significance of Transient Segmental Enhancement

¹ All authors: Department of Radiology, Kanazawa University, School of Medicine, 13-1 Takara-machi, Kanazawa City, Ishikawa, 920-8641 Japan.

Received July 20, 2000; accepted after revision September 5, 2000.

 
Address correspondence to T. Gabata.

Abstract

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OBJECTIVE. The purpose of this study was to evaluate dynamic CT findings of hepatic abscesses, especially segmental hepatic enhancement, and to clarify the cause.

MATERIALS AND METHODS. Twenty-four abscesses in eight patients were examined by early (30 sec) and late phase (90 sec) dynamic CT. Patients underwent abscess drainage (n = 1), hepatic resection (n = 2), or antibiotic therapy (n = 5). CT during arterial portography and CT during hepatic arteriography were performed in one patient. We retrospectively observed the frequency and changes of segmental hepatic enhancement on dynamic CT and determined its cause using radiologic and pathologic correlation.

RESULTS. Sixteen abscesses (67%) showed transient segmental hepatic enhancement and three abscesses showed only segmental hepatic enhancement in the early phase. Four abscesses in one patient who underwent CT during arterial portography and CT during hepatic arteriography showed a segmental perfusion defect on CT during arterial portography and segmental enhancement on CT during hepatic arteriography. On follow-up dynamic CT performed 10-17 days after the initial CT, segmental hepatic enhancement surrounding hepatic abscesses decreased or disappeared in all abscesses. Pathologic examination of two patients showed marked inflammatory cell infiltration with stenosis of portal venules within the portal tracts surrounding hepatic abscesses without definite inflammation in the liver parenchyma.

CONCLUSION. Segmental hepatic enhancement on dynamic CT is frequently associated with hepatic abscesses and may be caused by decreased portal flow resulting from inflammation of the portal tracts.

Introduction

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Pyogenic liver abscess is caused by bacteria and a localized pus collection with destruction of the hepatic parenchyma. Causes of abscess include biliary inflammatory disease, infection via the portal vein from gastrointestinal inflammatory diseases, direct spread of infection (e.g., cholecystitis) from contiguous organs, trauma, infarction, and unknown causes. Delayed diagnosis of hepatic abscess may cause a high mortality rate; therefore, early diagnosis and prompt therapy are needed to reduce the morbidity and mortality associated with hepatic abscess. Diagnosis of hepatic abscesses has improved with CT.

Several CT features of hepatic abscesses have been reported [1,2,3,4,5]. Because hepatic abscesses sometimes mimic other hepatic lesions such as hepatic metastasis on unenhanced and contrast-enhanced CT, it is important to differentiate abscesses and other lesions to initiate appropriate therapy. Mathieu et al. [3] reported that transient segmental wedge-shaped hepatic enhancement associated with hepatic abscesses is sometimes visible in the early phase of dynamic CT and is of diagnostic value. We retrospectively analyzed the dynamic CT findings of hepatic abscesses, especially those of the transient segmental enhancement surrounding them. We examined the frequency of segmental enhancement and serial morphologic changes on follow-up dynamic CT after antibiotic therapy and clarified the cause of this finding with radiologic and pathologic correlation.

Materials and Methods

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Patients
Eight patients with 24 hepatic abscesses were examined using dynamic CT between January 1997 and April 2000. All patients were men who were 54-82 years old (mean age, 67 years). Three patients had hepatobiliary lithiasis, one patient had rectal cancer, and the other four patients had no underlying hepatobiliary or gastrointestinal diseases. Three patients had solitary abscesses and five patients had multiple abscesses (range, 2-7 abscesses). One patient underwent percutaneous abscess drainage with positive findings for Pseudomonas organisms. Two patients underwent partial hepatic resection. The other five patients were conservatively treated with antibiotics, with clinical improvement within 1 week of therapy.

CT Examinations
Dynamic CT was performed with a HiSpeed Advantage CT scanner (General Electric Medical Systems, Milwaukee, WI), a Remage CT scanner (General Electric Medical Systems), and an Xpeed CT scanner (Toshiba Medical, Tokyo, Japan). Unenhanced and contrast-enhanced dynamic CT were performed. Dynamic CT was initiated 30 sec (early phase) and 90 sec (late phase) after starting iodized contrast medium injection. One hundred milliliters of contrast medium was injected at a rate of 3 mL/sec with a power injector (Auto-Enhance A 50; Nemoto Kyorindo, Tokyo, Japan). CT images were obtained with 5-mm collimation, a pitch of 1.4, and 200 mA. CT during arterial portography and CT during hepatic arteriography were performed in one patient undergoing partial hepatic resection. Follow-up CT was performed on six patients at an interval of 10-17 days (mean, 13 days) after initial CT. The two patients who underwent surgery did not have follow-up CT.

Results

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Dynamic CT revealed 24 intrahepatic abscesses in eight patients. In 10 (42%) of 24 abscesses, early phase dynamic CT showed a target appearance that consisted of a central hypodense area, intermediate ringlike enhancement, and a peripheral hypodense ring. Late phase dynamic CT showed a central hypodense area and a relatively thick ringlike enhancement of the intermediate and peripheral zones Figs. (1A,1B,1C,1D,1E,1F,2A,2B,2C,3A,3B). Nine (38%) of 24 abscesses showed ring-like enhancement on both early and late phase dynamic CT. In 16 abscesses (67%), transient wedge-shaped or segmental enhancement surrounding the hepatic abscesses was visible on early phase dynamic CT (Figs. 1A,1B,1C,1D,1E,1F,2A,2B,2C,3A,3B,4A,4B,4C,4D). This transient hepatic enhancement occurred only in the liver segment involved by the abscesses. This enhancement was invisible on late phase dynamic CT. In three abscesses, only wedgeshaped enhancement was visible on early phase dynamic CT. Two abscesses showed no enhancement on early or late phase dynamic CT. Four abscesses in one patient who underwent CT during arterial portography and CT during hepatic arteriography before hepatic resection showed a segmental perfusion defect on CT during arterial portography and segmental enhancement surrounding the hepatic abscesses on CT during hepatic arteriography (Fig. 4A,4B,4C,4D).

View larger version (159K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —Multiple pathologically proven hepatic abscesses in 64-year-old man. Unenhanced CT scan shows hypodense mass (arrow) with poorly defined margin at periphery of right posterior segment of liver.

View larger version (176K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —Multiple pathologically proven hepatic abscesses in 64-year-old man. Early phase (30 sec) dynamic contrast-enhanced CT scan shows double-target sign consisting of central cystic area with inner enhanced rim and outer hypodense zone. Note wedge-shaped enhancement (arrowheads) of hepatic parenchyma surrounding mass.

View larger version (166K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —Multiple pathologically proven hepatic abscesses in 64-year-old man. Late phase (90 sec) dynamic CT scan shows thick rim enhancement (arrow). Wedge-shaped enhancement is no longer seen.

View larger version (155K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1D. —Multiple pathologically proven hepatic abscesses in 64-year-old man. Photomicrograph shows organized abscess (AB) and prominent portal tracts caused by inflammation surrounding abscess. (H and E, x2.5)

View larger version (218K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1E. —Multiple pathologically proven hepatic abscesses in 64-year-old man. Photomicrograph shows organized abscess composed of inflammatory cell infiltration and fibrosis. (H and E, x50)

View larger version (173K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1F. —Multiple pathologically proven hepatic abscesses in 64-year-old man. Photomicrograph shows portal tracts surrounding abscess with marked periportal inflammatory cell infiltration and stenosis of portal venule (PV). HA = hepatic arteriole, BD = intrahepatic bile ductule. (H and E, x50)

View larger version (181K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —Hepatic abscesses in 64-year-old man undergoing antibiotic therapy. Early phase dynamic CT scan shows double-target sign (arrow) and segmental enhancement (arrowhead).

View larger version (175K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —Hepatic abscesses in 64-year-old man undergoing antibiotic therapy. Late phase dynamic CT scan shows central hypodense abscess cavity with peripheral enhancement (arrow).

View larger version (152K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C. —Hepatic abscesses in 64-year-old man undergoing antibiotic therapy. Follow-up early phase dynamic CT scan 10 days after antibiotic therapy shows decrease in segmental enhancement (arrowhead) and size of abscess cavity.

View larger version (173K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —Hepatic abscess in 65-year-old man treated with abscess drainage and antibiotic therapy. Early phase dynamic CT scan shows hypodense mass with double-target sign (arrow) and large segmental enhancement (arrowheads).

View larger version (157K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —Hepatic abscess in 65-year-old man treated with abscess drainage and antibiotic therapy. Follow-up early phase dynamic CT scan 16 days after abscess drainage and antibiotic therapy shows decrease in size of abscess (arrow) and segmental enhancement (arrowhead).

View larger version (166K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A. —Pathologically proven hepatic abscess in 59-year-old man. Early phase dynamic CT scan shows segmental enhancement (arrowheads) surrounding abscess (arrow) of left lateral segment.

View larger version (173K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B. —Pathologically proven hepatic abscess in 59-year-old man. Late phase dynamic CT scan shows rim enhancement (arrow).

View larger version (163K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4C. —Pathologically proven hepatic abscess in 59-year-old man. CT during arterial portography image shows segmental portal perfusion defect (arrowheads) surrounding abscess.

View larger version (169K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4D. —Pathologically proven hepatic abscess in 59-year-old man. CT during hepatic arteriography image shows segmental enhancement (Arrowheads).

Seventeen lesions in six patients who underwent follow-up CT decreased in size after either antibiotic therapy (n = 5) or abscess drainage with antibiotic therapy (n = 1). In these six patients, 12 abscesses that showed segmental enhancement surrounding hepatic abscesses on early phase dynamic CT before antibiotic therapy were observed. Five abscesses decreased in size, and the remaining seven abscesses disappeared (Figs. 2A,2B,2C and 3A,3B). In two patients who underwent hepatic resection, the lesions showed organized abscesses composed of fibrosis and inflammatory cell infiltration. The portal tracts (Glisson's capsule) of the hepatic parenchyma surrounding the hepatic abscesses revealed marked inflammatory cell infiltration and stenosis of the portal venules caused by inflammation (Fig. 1A,1B,1C,1D,1E,1F). The hepatocytes surrounding the hepatic abscesses did not show definite pathologic changes in two patients.

Discussion

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Several CT features of hepatic abscesses have been reported in the literature [1,2,3,4,5]. Hepatic abscess is depicted as a single or multiloculated hypodense mass that frequently shows a peripheral rim or capsule that is clearly enhanced on contrast-enhanced CT [5]. In our patients, nine (38%) of 24 abscesses showed a single enhanced rim in the early and late phases of dynamic CT. Mathieu et al. [3] reported a double-target sign consisting of a hypodense central abscess cavity surrounded by an inner hyperdense ring, and an outer hypodense zone on dynamic contrast-enhanced CT. This double-target sign was found in 30% of the cases reported by Mathieu et al. and 42% (10/24) of our patients. Overall, 19 (79%) of 24 abscesses in our study showed enhancement of the abscess wall on dynamic CT. This target appearance (double or single) of the rim sign is considered a characteristic sign of hepatic abscess. Mathieu et al. surmised that the inner enhanced ring corresponded to a capsule of the abscess and the outer hypodense zone to a localized zone of edema of the hepatic parenchyma surrounding the abscess [3]. Rim enhancement has also been reported on contrast-enhanced MR imaging [6, 7]. Small abscesses have a tendency to cluster or aggregate and coalesce into a single larger abscess cavity (cluster sign) [4].

Mathieu et al. [3] also reported that the hepatic parenchyma surrounding an abscess showed transient segmental or wedge-shaped enhancement on the arterial dominant phase of dynamic CT in 12 (30%) of 40 patients. They hypothesized that this transient enhancement was a result of localized hepatic venous obstruction caused by acute inflammation of the hepatic parenchyma surrounding the abscess, but they could not obtain pathologic confirmation. Other researchers also have described the same segmental or wedge-shaped contrast enhancement associated with hepatic abscesses on dynamic CT and dynamic MR imaging [7, 8]. In our patients, 16 (67%) of 24 abscesses showed segmental or wedge-shaped transient hepatic enhancement surrounding hepatic abscesses; three abscesses showed only wedge-shaped enhancement. Four abscesses in one patient who underwent preoperative angiography showed a wedge-shaped portal flow decrease on CT during arterial portography and wedged-shaped enhancement on CT during hepatic arteriography, which were consistent with the enhancement visible on dynamic CT. From findings on CT during arterial portography and CT during hepatic arteriography, it is possible to establish a hypothesis that segmental or wedge-shaped enhancement reflects portal flow decrease and compensatory arterial flow increase. We performed pathologic examinations on the specimens of two patients who underwent partial hepatic resection to confirm our hypothesis. Pathologically, the portal tracts (Glisson's capsule) of the hepatic parenchyma surrounding hepatic abscesses showed marked inflammatory cell infiltration and stenosis of portal venules, which may result in reduction of portal flow and a compensatory increase in arterial inflow. On follow-up dynamic CT performed 10-17 days (mean, 13 days) after initial CT, the segmental enhancement decreased or disappeared after antibiotic therapy. We believe that inflammation of the portal tracts is alleviated by antibiotic therapy, with the portal flow recovering and arterial flow decreasing.

Several causes of segmental hepatic enhancement have been considered, including portal vein compression or thrombosis, arterioportal shunt, local hyperemic change, aberrant blood supply, steal effect and hepatic venous outflow obstruction or thrombosis, and cholangitis [8]. It is important to differentiate hepatic abscesses from tumorous or nontumorous arterioportal shunt [9, 10] or tumors associated with segmental enhancement caused by portal or hepatic vein stenosis or occlusion [11, 12]. Unenhanced and contrast-enhanced dynamic CT are indispensable for differentiating these diseases. The distinctive features of the segmental enhancement associated with hepatic abscesses are the wedge shape and a decrease in size on follow-up dynamic CT. Differentiation between multiple abscesses with rim enhancement and multiple necrotic metastases with ring enhancement is sometimes difficult. Segmental enhancement surrounding the abscesses would be a clue to differentiation of these abscesses.

In conclusion, when hepatic abscesses are suspected clinically, it is necessary to perform dynamic contrast-enhanced CT. Segmental or wedge-shaped hepatic enhancement and the rim sign or double-target sign are useful to diagnose hepatic abscess. To differentiate hepatic abscess from hepatic tumors with segmental enhancement, it is important to observe the serial change in size of segmental enhancement on follow-up dynamic CT.

References

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