Hepatic abscess presents as a relatively complicated fluid collection, which is composed of central areas with low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Particularly, on T2-weighted images, the central cavity may show septa and debris. Central fluid-containing cavity is often surrounded by a few millimeters (most cases: 1–5 mm; some cases: >5 mm) thick inflamed liver parenchyma (wall of the abscess), which most likely contains microabscesses. Perilesional (wedge-shaped) edema may be present. Most abscesses show early persistent enhancement of the wall (Figs. 22.1, 22.2, and 22.3a, b). Although in most patients the diagnosis and follow-up are carried on CT, MR imaging is increasingly performed in the acutely ill patients; therefore, it is important for radiologists to understand the appearance of hepatic abscess at MR imaging.

In ambiguous cases, the differential diagnosis may include (1) metastases (the ring enhancement progresses in a centripetal fashion with decrease in intensity on delayed images), (2) infected metastases (difficult to differentiate, thicker and more irregular wall, clinical history important), (3) hepatosplenic candidiasis (multiple lesions <10 mm in diameter), (4) hydatid cysts (internal septa), and (5) echinococcus abscesses (thicker septa and daughter cysts) (Fig. 22.3c, d).

Management options include: (1) percutaneous drainage, (2) open surgical drainage, and (3) antibiotic therapy. Single dominant hepatic abscess with a large fluid cavity can be treated with percutaneous drainage. Treatment should be tailored to each patient.

On the T2-weighted images, hepatic abscess is visible as a focal lesion with an overall moderately high signal intensity; close look at the lesion, however, reveals a fairly characteristic appearance with high signal of the central cavity, surrounded by an intermediate signal infected but non-necrotic liver tissue, and finally a thick rim of compressed (almost normal or minimally inflamed) liver parenchyma, which will be nearly isointense with the background liver (Fig. 23.1a). On the T1-weighted images, hepatic abscess, like many liver lesions, has a nonspecific low-signal-intensity appearance. In many ways, the T1 appearance of the liver lesions is comparable to the CT appearance with lack of internal tumor anatomy (Fig. 23.1b). On the arterial phase images, hepatic abscess shows a faint enhancement of the thick rim immediately around the central necrotic cavity as well as a slightly more intense enhancement of the thicker compressed liver tissue around the abscess (Fig. 23.1c). On the delayed (4–5 min postinjection) phase images, hepatic abscess has a more distinct non-enhancing central cavity compared to the more homogenously enhancing surrounding infected but viable and normal liver tissue (Fig. 23.1d). On the diffusion-weighted (DWI) and apparent diffusion coefficient (ADC) images, the central cavity containing complex fluid and debris will show restricted diffusion (bright on DWI and dark on ADC). This is similar to many (benign and malignant) solid liver lesions (Fig. 23.2e–g). In the clinical setting, we rely more on the T2 and post-contrast findings rather than DWI and ADC (Fig. 23.2a–c). The contrast-enhanced CT images show findings similar to contrast-enhanced part of MRI (Fig. 23.2e).

Over the years, we have observed three imaging patterns of liver abscesses: (1) a single lesion with a necrotic cavity, (2) multiple small microabscesses, and (3) a branching biliary abscess. Minimum-intensity projections (MinIPs) based on the post-contrast delayed phase images particularly facilitate the recognition of the branching biliary abscess (Fig. 23.2d, f–h).

In order to distinguish liver abscesses from liver masses, the clinical correlation and the imaging findings are important. Rapidly changing or sudden new lesions should trigger the diagnostic possibility of an abscess. It remains however a challenge for radiologists and clinicians to correctly diagnose a liver abscess and avoid dreadful situations such as placing a drainage catheter in a necrotic malignant liver mass.

Management options include: (1) percutaneous drainage, (2) open surgical drainage, and (3) antibiotic therapy. Single dominant hepatic abscess with a large fluid cavity can be treated with percutaneous drainage. Treatment should be tailored to each patient.

On MR images, tumors are small (usually <1 cm), often multiple, and well defined. The high fluid content renders these lesions low signal on T1, high signal on T2, and negligible enhancement on early and late post-gadolinium images. Although this appearance resembles simple cysts, biliary hamartomas demonstrate a thin rim of enhancement on early and late post-contrast images (Figs. 24.1 and 24.2).

The major potential diagnostic error is to misclassify these lesions as metastases due to the presence of ring enhancement. The thin enhancing rim of biliary hamartomas, visualized on imaging, may be correlated histopathologically with the presence of compressed hepatic parenchyma bordering the lesion. In contrast, the pattern of ring enhancement displayed by metastases relates histopathologically with the outer most vascularized portion of the tumor. Peritumoral enhancement is also observed in some metastases. MR imaging further corroborates the different histologic profiles of the two processes through the observation that enhancement in biliary hamartoma does not progress centrally, while enhancement in metastases most often progresses centrally.

Histopathologically, biliary hamartomas consist of a collection of small, sometimes dilated, irregular, and branching bile ducts embedded in a fibrous stroma. A few of the ducts may contain inspissated bile. In general, biliary hamartomas contain no or few vascular channels (Fig. 24.3).

No treatment or follow-up with imaging is required for typical biliary hamartomas.

At MR imaging, cysts are low in signal intensity on T1-weighted images, high in signal intensity on T2-weighted images, and retain signal intensity on longer echo time (e.g., >120 ms) T2-weighted images. After injection of contrast, cysts do not show any enhancement. On delayed post-gadolinium images (up to 5 min), cysts remain unenhanced. The latter findings may be useful to ensure that lesions are cysts and not poorly vascularized metastases that show gradual enhancement. T2-weighted sequences with long TE in combination with delayed phase images are especially effective at showing small (≤5 mm) cysts. MRI is particularly valuable when lesions are small and the patient has a known primary malignancy (Figs. 25.1 and 25.2). Typical and relatively larger cysts can be characterized at CT as well as US (Figs. 25.3a, b).

Typical simple cyst at MR imaging does not cause any diagnostic problems and can be distinguished from other cystic lesions with confidence. Atypical cysts or cysts with septa may show overlapping features with other entities such as (1) mucinous or cystic metastases (look for multiple septa and perilesional enhancement) and (2) ciliated foregut cysts (congenital lesions, frequently located at the anterior margin of the liver with mucinous content and slightly bulging the liver contour). The presence of a cystic lesion with an enhancing wall and extension beyond the contour of the liver may also be observed in some forms of metastatic disease such as hepatic metastasis from ovarian malignancies. For this reason, a diagnosis of foregut cyst on imaging studies should only be made in the absence of peritoneal disease and a clinical history of malignancy.

Pathologically, the lining of the cyst shows a single layer of cuboidal to columnar epithelial cells. Lining epithelium rests on an underlying fibrous stroma (Figs. 25.3c, d).

No treatment or follow-up with imaging is required for typical small hepatic cysts.

Very large liver cysts have similar MR imaging appearance as smaller cysts. Cysts are low in signal intensity on T1-weighted images, high in signal intensity on T2-weighted images, and retain signal intensity on longer echo time (e.g., >120 ms) T2-weighted images. Cysts, as nonsolid liver lesions, share this T2 characteristic with hemangiomas in the liver. Because cysts do not enhance with gadolinium on MR images, delayed post-gadolinium images (up to 5 min) may be useful to ensure that lesions are simple cysts without any solid nodules or thickened septa. In addition, MR imaging can reliably exclude hemorrhage on T1-weighted GRE with fat suppression as well as any biliary dilatation (Figs. 26.1 and 26.2). CT is also well able to visualize these lesions (Fig. 26.3). CT may be less accurate for small amount of hemorrhage or solid components.

Differential diagnosis of a large cystic liver lesion includes (1) infectious cystic lesions, such as abscess (clinical history and enhancing wall at imaging) and intrahepatic hydatid cyst (clinical history and daughter cysts in 75 % at MRI and CT and calcifications in 50 % at CT), and (2) neoplastic lesions, such as undifferentiated embryonal sarcoma, biliary cystadenoma or cystadenocarcinoma, and cystic metastases. Hallmarks of neoplastic lesions are solid components with intralesional or perilesional enhancement. Such findings should not be present in large simple developmental liver cysts.

Several types of treatments have been advocated for large (symptomatic) hepatic cysts, including (1) percutaneous drainage and sclerosis using alcohol, tetracycline, doxycycline, or a combination (this method is associated with recurrence of cysts and symptoms); (2) drainage in combination with various types of surgery; and (3) drainage in combination with minocycline injection (more recent type of treatment). Percutaneous treatments may be contraindicated in patients with bleeding tendency, echinococcal cysts, and cysts communicating with biliary tree.

The appearance and the ability to characterize small hepatic cysts differ considerably on MR imaging, CT, and US (Figs. 27.1, 27.2, and 27.3). At MR imaging, hepatic cysts can be characterized based on a unique combination of findings including very bright signal on T2-weighted sequences with sharp margins and the absence of enhancement on all phases of dynamic gadolinium-enhanced as well as on delayed phase images. Due to large intrinsic tissue differences between the background liver (dark) and cysts (very bright), even very small cysts of only a few millimeters in diameter can be detected and characterized on images with much larger slice thickness. CT and ultrasound lack these properties, and hence the characterization of small hepatic lesions (mainly distinction between solid and nonsolid lesions) may be difficult. Detection and characterization on CT are mainly based on two parameters: (1) differences in X-ray attenuation and (2) differences in enhancement. On contrast-enhanced CT images, cysts appear grayish compared to the enhanced liver. On contrast-enhanced MR images, cysts appear almost as signal voids (very dark) compared to the enhanced liver. This suggests that MRI is much sensitive to gadolinium than CT is for iodine contrast media. At US, cysts appear very dark with sharp margins and increased sound transmission through the lesion. Although with the application of real-time image optimization possibilities such as beam steering, increased sound transmission may not be obvious. In addition, in many centers, it is a common practice to have a confirmation of US findings at CT or MRI, especially in patients with an underlying malignancy.

At MR imaging, typical cysts will have no differential diagnosis. At US and CT, smaller lesions may have differential diagnosis with solid liver lesions, such as metastases.

MR imaging often plays a decisive role in the management of patients with small hepatic cysts, particularly with an underlying malignancy.

At MR imaging, multiple hepatic cysts can be characterized based on a unique combination of findings including low signal intensity on T1-weighted sequences, very bright signal on T2-weighted sequences with sharp margins, and the absence of enhancement on all phases of dynamic gadolinium-enhanced as well as on delayed phase images. Cysts complicated with hemorrhage contain fluid-fluid levels or diffuse high signal intensity on (fat-suppressed) T1-weighted sequences. Hemorrhage is particularly present in renal cysts (Figs. 28.1 and 28.2). CT can also visualize liver and renal cysts, but hemorrhagic cysts may be difficult to detect on CT exams (Fig. 28.3). MR imaging is also preferred to CT in this patient group with renal dysfunction. Infected cysts will have imaging feature of a pyogenic liver abscess with intraluminal debris and enhancing wall.