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Fig. 6a-d. Multifocal fatty liver. On the US examination (a) multiple ill-defined, slightly hyperechoic nodules are detected (arrows). The corresponding pre-contrast CT scan (b) reveals numerous, ill-defined, slightly hypodense areas (arrows), which do not show significant enhancement during the arterial (c) and portal venous (d) phases after contrast medium injection. Note some vascular structures within the focal fatty areas (arrowheadsin d)

Fig. 7a-d. Diffuse fatty liver. On the pre-contrast HASTE T2-weighted image (a) and the GRE T1-weighted "in-phase" image (b) the signal intensity of the liver is homogeneously increased. Conversely, on the GRE Tl-weighted "out-of-phase" image (c) the signal intensity is markedly and characteristically decreased. GRE Tl-weighted fat suppressed sequences (d) are not sufficiently sensitive to small quantities of fat, and so the liver appears hyperintense as compared with the spleen

Fig. 7a-d. Diffuse fatty liver. On the pre-contrast HASTE T2-weighted image (a) and the GRE T1-weighted "in-phase" image (b) the signal intensity of the liver is homogeneously increased. Conversely, on the GRE Tl-weighted "out-of-phase" image (c) the signal intensity is markedly and characteristically decreased. GRE Tl-weighted fat suppressed sequences (d) are not sufficiently sensitive to small quantities of fat, and so the liver appears hyperintense as compared with the spleen

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Fig. 8a-f. Focal fatty liver. On the pre-contrast T2-weighted image (a) the liver appears homogeneously, slightly hyperintense, whereas on the pre-contrast GRE T1-weighted "in-phase" image (b) it appears heteogeneous, and ill-defined slightly hyperintense areas (arrows) can be seen. The corresponding pre-contrast GRE Tl-weighted "out-of-phase" image (c) shows diffuse hypointense areas (arrowheads) in both liver lobes indicating focal fatty infiltration. During the Tl-weighted dynamic study after contrast agent administration, weak and heterogeneous intralesional enhancement can be detected in the arterial phase (d). Note that some vascular structures are clearly visible in the affected areas. In the portal venous phase (e), areas of focal fatty infiltration (arrows) appear as slightly hypointense compared to surrounding normal liver tissue. In the hepatobiliary phase after Gd-BOPTA administration (f) the liver is relatively homogeneous in appearance, although some of the areas of focal fatty infiltration show slightly decreased signal intensity. The signal intensity of these areas is relatively unchanged compared with the unenhanced images; however, these areas appear slightly hypointense because of the increased signal intensity of the surrounding normal liver tissue

Fig. 9a-d. Focal fatty liver. An oval shaped, well-defined, slightly hyperintense area (arrowheads) in the posterior portion of segment IV can be detected on the pre-contrast GRE T1-weighted "in-phase" image (a). The lesion is heterogeneously hypointense on the pre-contrast GRE Tl-weighted "out-of-phase" image (b). In the hepatobiliary phase after Gd-BOPTA administration (c, d) the area of focal fatty infiltration appears isointense on the Tl-weighted "in-phase" image (c) and hypointense on the Tl-weighted "out-of-phase" image (d). The decreased uptake of Gd-BOPTA is due to the altered metabolic function in the area of focal fatty infiltration

Fig. 9a-d. Focal fatty liver. An oval shaped, well-defined, slightly hyperintense area (arrowheads) in the posterior portion of segment IV can be detected on the pre-contrast GRE T1-weighted "in-phase" image (a). The lesion is heterogeneously hypointense on the pre-contrast GRE Tl-weighted "out-of-phase" image (b). In the hepatobiliary phase after Gd-BOPTA administration (c, d) the area of focal fatty infiltration appears isointense on the Tl-weighted "in-phase" image (c) and hypointense on the Tl-weighted "out-of-phase" image (d). The decreased uptake of Gd-BOPTA is due to the altered metabolic function in the area of focal fatty infiltration a decreased uptake of SPIO in non-diffuse areas of fat deposition [18].

While focal fat depositions may mimic the appearance of focal liver lesions, it is equally the case that focal fatty lesions such as lipoma and an-giomyolipoma, but more frequently HCC or hepatic adenoma, may mimic focal fat. In order to distinguish focal fat depositions from lesions with fatty metamorphosis, the signal intensity on T2-

weighted images, the prominent vascularity, the presence of intratumoral areas of hemorrhage or necrosis, specific imaging signs such as pseudocapsule, and the enhancement behavior after administration of liver-specific contrast agents should all be taken into account (Fig. 10).

Fig. 10a-f. HCC with fatty metamorphosis. The pre-contrast Turbo SE T2-weighted image (a) and the corresponding GRE T1-weighted "in-phase" image (b) reveal a well-defined round nodule (arrow) which is slightly hyperintense and heterogeneously isointense compared to the normal liver parenchyma, respectively. The signal intensity is reduced on the GRE Tl-weighted "out-of-phase" image (c) due to the fatty content of the lesion. On dynamic Tl-weighted imaging after Gd-BOPTA administration (d, e) the lesion shows strong enhancement in the arterial phase (d) and a thin, hypointense pseudocapsule (arrowhead) in the portal venous phase (e). On the delayed hepatobiliary phase image (f) the nodule is slightly hypointense, suggesting that the lesion is malignant in nature. In this case, however, the neoplastic cells in the well-differentiated HCC still have some capability to take up Gd-BOPTA and to produce bile

Fig. 10a-f. HCC with fatty metamorphosis. The pre-contrast Turbo SE T2-weighted image (a) and the corresponding GRE T1-weighted "in-phase" image (b) reveal a well-defined round nodule (arrow) which is slightly hyperintense and heterogeneously isointense compared to the normal liver parenchyma, respectively. The signal intensity is reduced on the GRE Tl-weighted "out-of-phase" image (c) due to the fatty content of the lesion. On dynamic Tl-weighted imaging after Gd-BOPTA administration (d, e) the lesion shows strong enhancement in the arterial phase (d) and a thin, hypointense pseudocapsule (arrowhead) in the portal venous phase (e). On the delayed hepatobiliary phase image (f) the nodule is slightly hypointense, suggesting that the lesion is malignant in nature. In this case, however, the neoplastic cells in the well-differentiated HCC still have some capability to take up Gd-BOPTA and to produce bile

Focal Spared Areas in Fatty Liver

Focal sparing of fatty infiltration most frequently occurs around the gallbladder and in the dorso-medial portion of the medial segment where supply to the hepatic parenchyma may derive from systemic veins such as the cystic vein of the gallbladder or an aberrant right gastric vein, rather than from the portal vein. Focal sparing can also occur adjacent to a tumor due to the presence of an arterioportal shunt or as a rim around an expansively growing tumor.

Unlike focal fat deposits, focal spared areas have a hypoechoic appearance on US. However, neither focal fat deposits, nor focal spared areas determine a mass effect with respect to vessels (Fig. 11).

On multiphasic CT, focal sparing has a hyper-dense appearance that is variable with the amount of fatty liver infiltration. As in the case of focal fat infiltration, round areas of focal sparing may mimic hepatic tumors (Fig. 12).

On hepatobiliary phase Tl-weighted MR images after the administration of Gd-BOPTA, focal sparing in fatty liver has an isointense or slightly hyperintense appearance (Fig. 13). A similar appearance is seen after the administration of other hepatobiliary agents such as Gd-EOB-DTPA or Mn-DPDP. Conversely, on T1- and T2-weighted images after SPIO administration, focal spared areas in fatty liver are seen as areas of relatively low signal intensity reflecting the relatively high uptake of SPIO in these areas compared with reduced uptake in fatty areas of the liver [18]. Whereas focal fatty infiltration and focal spared areas with a round, regular appearance may mimic hepatic tumors (Figs. 12, 13), irregular or diffuse areas of infiltration or sparing may obscure focal liver lesions (Fig. 14).

Inflammatory Pseudotumors

Hepatic inflammatory pseudotumor is an unusual and rare tumor-like condition that is increasingly recognized as an important differential diagnosis in patients presenting with liver masses. Synonyms used to define this lesion include xanthogranulo-ma, fibrous xantoma, plasmacellular granuloma, histiocytoma, pseudolymphoma, and plasmocy-toma, all of which reflect the histologic components of the lesion. Most commonly, the condition occurs in children and in young men. Although the etiology is unknown, some authors have suggested oblit-erans phlebitis starting from the portal vein as a possible cause, with secondary biliary stasis and degeneration and necrosis of the biliary ducts,leading to periductal abscess or xanthogranuloma [27].

Macroscopically, the inflammatory pseudolesion is usually yellow-grey in color and solitary, although multifocal inflammatory pseudotumor of the liver has been described. The most frequent microscopic components are plasmacellular cells, although variable amounts of histiocytes, macrophages, fibromy-oblasts, and fibrous tissue are also observed. Three histologic subtypes have been identified on the basis of the prevalence of single components, thus xanthogranulomatous type lesions have a histiocyt-ic prevalence, plasmacellular type lesions contain mainly plasma cells, and sclerotic type lesions have a predominantly fibrotic component [46].

Typically, symptoms and laboratory findings indicate an acute inflammatory process, and recurrent pyogenic cholangitis is the most frequent clinical manifestation. Large lesions may cause a sensation of right upper abdominal quadrant fullness or discomfort, with malaise, fever and weight loss. Liver function tests sometimes demonstrate the elevation of alkaline phosphatase and y-glutamyl-transferase.

Features are non-specific on US, with lesions presenting as heterogeneously hypoechoic or mosaic patterns, similar to those observed in other focal liver neoplasms such as HCC [19]. Similarly, the appearance of an inflammatory pseudolesion is non-specific on unenhanced CT, with lesions invariably appearing hypodense. After contrast medium administration, an early intense and peripheral enhancement is usually followed by homogeneous, complete and persistent enhancement (Fig. 15). After a few minutes, peripheral enhancement and a hypodense core can be observed, the former comprising fibroblastic cells, and the latter chronic inflammatory cells [19].

On unenhanced T1-weighted MR images, inflammatory pseudotumor is typically hypointense, particularly in the central portion. Conversely, on T2-weighted images the lesion frequently demonstrates isointensity or slight hyperintensity (Fig. 16). However, the appearance is variable in relation to the histologic components: for example, slight hypointensity may be observed on T2-weighted images in lesions with a strong fibrotic predominance while a stronger hyperintense appearance is indicative of a greater predominance of inflammatory cells.

Early peripheral enhancement is typically seen on T1-weighted dynamic imaging after bolus injection of contrast agent, reflecting the cellular components and inflammatory changes within the lesion. Hepatobiliary phase imaging after administration of Gd-BOPTA or another hepatospecific contrast agent, frequently reveals a hypointense area representing the absence of hepatocytes within the lesion (Fig. 17) [19,37].

A drop in signal on T2-weighted images after SPIO administration may reveal residual Kupffer cell function in liver parenchyma in and sur-

Fig. 11a, b. Focal sparing on US. B-mode US (a) reveals a hypoechoic area (arrowhead with a triangular shape near the surface of the liver. On color Doppler US (b) an intralesional vessel is clearly visible. Note the absence of any mass effect. This is typical of focal sparing in fatty liver

Fig. 12a-d. Focal sparing. Patient with Burkitt lymphoma after chemotherapy. On the US examination (a) the liver is extremely bright due to hepatic steatosis, and a round, hypoechoic nodule (arrowhead) is visible in segment IV of the liver. On the CT study (b-d) the lesion (arrowhead) does not show significant enhancement. This is indicative of focal sparing in fatty liver

Fig. 12a-d. Focal sparing. Patient with Burkitt lymphoma after chemotherapy. On the US examination (a) the liver is extremely bright due to hepatic steatosis, and a round, hypoechoic nodule (arrowhead) is visible in segment IV of the liver. On the CT study (b-d) the lesion (arrowhead) does not show significant enhancement. This is indicative of focal sparing in fatty liver

Fig. 13a-f. Focal sparing. Patient with history of breast cancer and chemotherapy. US evaluation (a) reveals an oval shaped, hypoechoic area (arrowhead) within a diffuse fatty liver. This is considered suspicious for metastasis. On the MR examination, this focal area (arrow) appears slightly hypointense on the pre-contrast TSE T2-weighted image (b), isointense on the GRE Tl-weighted "in-phase" image (c), and hyperintense on the GRE Tl-weighted "out-of-phase" image (d). On the dynamic images after Gd-BOPTA administration (e, f) the lesion does not reveal increased perfusion or wash-out. This is more indicative of an area of focal sparing in a fatty liver than of a metastasis

Fig. 13a-f. Focal sparing. Patient with history of breast cancer and chemotherapy. US evaluation (a) reveals an oval shaped, hypoechoic area (arrowhead) within a diffuse fatty liver. This is considered suspicious for metastasis. On the MR examination, this focal area (arrow) appears slightly hypointense on the pre-contrast TSE T2-weighted image (b), isointense on the GRE Tl-weighted "in-phase" image (c), and hyperintense on the GRE Tl-weighted "out-of-phase" image (d). On the dynamic images after Gd-BOPTA administration (e, f) the lesion does not reveal increased perfusion or wash-out. This is more indicative of an area of focal sparing in a fatty liver than of a metastasis

Fig. 14a-f. Focal fatty liver. Patient with history of renal cell carcinoma and chemotherapy. On the CT examination (a-c), and on the pre-contrast GRE Tl-weighted "in-phase" and GRE Tl-weighted "out-of-phase" images (d, e), the heterogeneous, diffuse fatty infiltration does not permit the confident definition of any lesion and in particular a small and ill-defined area (arrowhead) in liver segment II. On the corresponding HASTE T2-weighted image (f) two markedly hyperintense lesions (arrows) can be seen, and the signal intensity is suggestive of hemangioma

Fig. 14a-f. Focal fatty liver. Patient with history of renal cell carcinoma and chemotherapy. On the CT examination (a-c), and on the pre-contrast GRE Tl-weighted "in-phase" and GRE Tl-weighted "out-of-phase" images (d, e), the heterogeneous, diffuse fatty infiltration does not permit the confident definition of any lesion and in particular a small and ill-defined area (arrowhead) in liver segment II. On the corresponding HASTE T2-weighted image (f) two markedly hyperintense lesions (arrows) can be seen, and the signal intensity is suggestive of hemangioma

Fig. 15a-e. Inflammatory pseudotumor. Patient with primary immunodeficiency. On the US examination (a) a well-defined hypoechoic nodule (arrow) is detected. On the corresponding pre-contrast CT examination (b) a well-defined, oval, homogeneously hypodense lesion (arrow) is demonstrated in segment IV of the liver. After the bolus administration of contrast medium, early peripheral enhancement (arrowhead is evident in the arterial phase (c), while an isodense homogeneous appearance is seen in the portal venous (d) and equilibrium (e) phases

Fig. 16a-g. Inflammatory pseudotumor. On the pre-contrast HASTE T2-weighted image (a) a slightly hyperintense lesion (arrows) in the area of the liver hilum is visible. On the corresponding pre-contrast T1-weighted image (b) the lesion is hypointense. The lesion shows heterogeneous enhancement in the arterial (c) and portal venous (d) phases of dynamic imaging after injection of Gd-BOPTA. In the equilibrium phase (e) central wash-out of contrast agent is evident, and the lesion now demonstrates a hyperintense rim (arrowheads). This most likely corresponds to an inflammatory reaction and edema of the surrounding liver tissue. In the hepatobiliary phase (f) the lesion is hypointense in the center, surrounded by a slightly brighter rim. A HASTE T2-weight-ed image (g) acquired during a follow-up examination performed six months after antibiotic therapy reveals complete restitution and no residual tumor

Fig. 17a-f. Inflammatory pseudotumor. Same case as demonstrated in Fig.15. On the pre-contrast T2-weighted sequence (a) and on the GRE Tl-weighted image (b) the lesion (arrow) appears hyperintense and heterogeneously hypointense respectively, compared to the surrounding normal liver parenchyma. Enhancement is seen in the periphery of the lesion during the arterial phase (c) after Gd-BOPTA administration. During the portal venous phase (d), the lesion appears slightly hyperintense, particularly in the central portion. During the hepatobiliary phase (e) the nodule appears hypointense, due to the absence of hepatocytes. Follow-up MR imaging after six months (f) does not reveal any focal lesions

Fig. 17a-f. Inflammatory pseudotumor. Same case as demonstrated in Fig.15. On the pre-contrast T2-weighted sequence (a) and on the GRE Tl-weighted image (b) the lesion (arrow) appears hyperintense and heterogeneously hypointense respectively, compared to the surrounding normal liver parenchyma. Enhancement is seen in the periphery of the lesion during the arterial phase (c) after Gd-BOPTA administration. During the portal venous phase (d), the lesion appears slightly hyperintense, particularly in the central portion. During the hepatobiliary phase (e) the nodule appears hypointense, due to the absence of hepatocytes. Follow-up MR imaging after six months (f) does not reveal any focal lesions rounding the inflammatory pseudotumor. However, some authors have described lesions that show no SPIO particle uptake [37].

As it is difficult to diagnose inflammatory pseudotumor of the liver on MR imaging alone, supplemental lesion biopsy should also be performed [26].

Peliosis Hepatis

Peliosis hepatis is a rare entity characterized by blood-filled cystic cavities in the liver. Peliosis he-patis frequently develops in association with malignancies and chronic wasting diseases, such as tuberculosis. However, it has also been described in association with renal transplantation, hemato-logical disorders and infection with human immunodeficiency virus (HIV), as well as in patients on long-term treatment with anabolic steroids, oral contraceptives, hormones, estrogen or Azathi-aprine. Regression is generally observed after such agents have been stopped or after appropriate antibiotic therapy [44,45].

Macroscopically, peliosis is characterized by cystic dilated sinusoids filled with red blood cells and bound by cords of liver cells. Two varieties have been described: the phlebectatic type, in which the blood-filled spaces are lined with en-dothelium and are based on aneurysmal dilatation of the central veins, and the parenchymal type, in which the blood spaces are not lined with endothelium and are usually associated with he-morrhagic parenchymal necrosis. Peliosis can be differentiated from hemangioma by the presence of portal tracts within the fibrous stroma of the blood-filled spaces. Numerous theories have been proposed for the cause of peliosis hepatis, including outflow obstruction and hepatocellular necrosis leading to cystic blood-filled formations. Peliosis hepatis is usually found incidentally at autopsy but, rarely, it can cause hepatic failure or liver rupture with hemoperitoneum or shock. Patients sometimes have non-specific signs such as hepatomegaly and portal hypertension [44].

US findings are not specific for the diagnosis of peliosis hepatis; the hepatic echopattern is usually non-homogeneous with both hyperechoic and hy-poechoic areas [30].

On CT images after the bolus administration of iodinated contrast material, these lesions usually appear initially hypodense, and become isodense over time [42].

On unenhanced T2-weighted MR images, peliosis hepatis frequently demonstrates high signal intensity similar to that seen in hemangioma. Conversely, low signal intensity is usually seen on un-enhanced T1-weighted images. After gadolinium administration, the lesions may show homogeneous or heterogeneous hypervascularization depending on flow, and may appear iso- or hyperin-tense on portal venous and equilibrium phase images after Gd-BOPTA administration.

In the hepatobiliary phase after the administration of Gd-BOPTA, Mn-DPDP or Gd-EOB-DTPA, the lesion again appears hypointense because of the absence of hepatocytes within the cystic dilated sinusoids (Fig. 18).

Fig. 18a-j. Peliosis hepatis. Patient with non-Hodgkin lymphoma during chemotherapy. The pre-contrast HASTE T2-weighted MR image

(a) reveals multiple ill-defined areas (arrowheads) with high signal intensity. On the corresponding unenhanced GRE Tl-weighted image

(b) these areas have low signal intensity. During the dynamic evaluation after administration of Gd-BOPTA, the lesions show homogeneous hypervascularization in the arterial phase (c) and remain hyperintense during the portal venous (d) and equilibrium (e) phases. Because of the absence of hepatocytes within the dilated sinusoids, the lesions appear hypointense on Tl-weighted (f) and Tl-weighted fat-suppressed (g) images acquired during the hepatobiliary phase after Gd-BOPTA administration. A follow-up examination performed one year after chemotherapy (h-j) shows complete resolution of the parenchymal changes ^

Fig. 18a-j. Peliosis hepatis. Patient with non-Hodgkin lymphoma during chemotherapy. The pre-contrast HASTE T2-weighted MR image

(a) reveals multiple ill-defined areas (arrowheads) with high signal intensity. On the corresponding unenhanced GRE Tl-weighted image

(b) these areas have low signal intensity. During the dynamic evaluation after administration of Gd-BOPTA, the lesions show homogeneous hypervascularization in the arterial phase (c) and remain hyperintense during the portal venous (d) and equilibrium (e) phases. Because of the absence of hepatocytes within the dilated sinusoids, the lesions appear hypointense on Tl-weighted (f) and Tl-weighted fat-suppressed (g) images acquired during the hepatobiliary phase after Gd-BOPTA administration. A follow-up examination performed one year after chemotherapy (h-j) shows complete resolution of the parenchymal changes ^

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