Liver Abscesses In Dogs From South

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Fig. 63a-g. Pyogenic abscess after Gd-BOPTA. The abscess (arrows) has a high signal intensity on Turbo SE T2-weighted images (a) and T2-weighted images acquired with fat saturation (b). On unenhanced GE Tl-weighted images (c) the abscess has low signal intensity. Rim enhancement (arrowin f) is seen on dynamic phase images acquired after the administration of Gd-BOPTA (d-f). The central area of necrosis does not show enhancement. Central contrast agent pooling due to diffusion of the contrast agent is seen on the delayed hepatobiliary phase image (g)

weighted images. Perilesional edema, characterized by high signal intensity on T2-weighted images, is seen in one third of cases. The abscess cavity may appear with homogeneous or heterogeneous signal intensity. After administration of contrast material, abscesses typically show rim enhancement followed by a slower increase in signal intensity within the center of the lesion (Fig. 63). Small lesions may enhance homogeneously in a manner similar to that seen with small hemangiomas [5].

Peripheral edema may be seen on delayed phase images as a rim of high signal intensity after administration of contrast agents with hepatobil-iary properties. Similarly, decreased signal intensity after SPIO administration may be indicative of peripheral edema due to the high content of Kupf-fer cells and macrophages.

Amebic Abscess

Amebiasis caused by the parasite Entamoeba histolytica is an endemic disease of tropical areas, such as Mexico, Central and South America,Africa and Asia. Amebic liver abscess develops after infestation of colonic mucosa by the parasites, which lodge in the portal system. The liver can be invaded in one of three ways:

• through lymphatics,

• via direct extension through the colon wall into the peritoneum and then through the liver capsule.

Amebic liver abscess is the most common extraintestinal manifestation. Most patients with amebic liver abscesses present with a tender liver and abdominal pain in the right upper quadrant. Amebae are not usually found in the stool of patients with an amebic liver abscess. Because the clinical features and findings of stool examinations for amebae are usually not specific or are negative, serologic tests are helpful in detecting suspected amebic abscess; such tests are positive in about 90% of patients [61,84].

On US studies, amebic abscesses are usually large, round, sharply-defined, hypoechoic masses with fine, low-level internal echoes at high gain settings (Fig. 64) [64].

The CT appearance of amebic abscess is non

Fig. 64. Amebic abscess on US. The ultrasound scan reveals two large lesions (arrows) with different echogenicity specific and variable; the lesion is usually round or oval and demonstrates peripheral hypodensity. A slightly hyperdense peripheral rim can be seen on unenhanced scans, which generally shows marked enhancement after administration of contrast material (Fig. 65). Lesions may appear as unilocular or multilocular masses, with internal debris and nodularity of the margins [106].

Amebic abscesses are well-defined structures with rim-like areas of varying signal intensity on both T1- and T2-weighted MR images. Within the abscess cavity, the signal intensity is decreased on T1-weighted images compared with the normal hepatic parenchyma. On T2-weighted images the lesion is hyperintense with a homogeneous or heterogeneous appearance and is often surrounded by areas of even higher signal intensity that correspond to edema within the normal liver tissue. No enhancement is seen in the central necrotic area after contrast agent administration, whereas heterogeneous enhancement can be observed at the periphery of the lesion, corresponding to inflammatory tissue. Persistent enhancement on late he-patobiliary phase images can be observed in this inflammatory tissue when contrast agents with he-patobiliary properties are used (Fig. 66). MR also offers the advantage of multiplanar capabilities to clearly depict the extension of the lesion. It is also helpful in follow-up studies to evaluate response to therapy.

Fig. 65. Amebic abscess on CT. The CT scan reveals hypodense lesions with a thin hyperdense peripheral rim. The hypodense appearance is due to the high liquid content

Fig. 64. Amebic abscess on US. The ultrasound scan reveals two large lesions (arrows) with different echogenicity

Fig. 65. Amebic abscess on CT. The CT scan reveals hypodense lesions with a thin hyperdense peripheral rim. The hypodense appearance is due to the high liquid content

Fig. 66a-g. Amebic abscess after Gd-BOPTA. HASTE T2-weighted images acquired in the axial plane (a) and True-FISP images acquired in the coronal plane (b) reveal a large heterogeneous hyperintense lesion (asteriskin a). The lesion is seen as an ill-defined iso- to hypointense mass on unenhanced GE T1-weighted images (c). Enhancement is seen mainly in the periphery of the lesion during the arterial phase (d) after the administration of Gd-BOPTA. This enhancement increases during the portal-venous (e) and equilibrium (f) phases when septations and internal necrosis are depicted more clearly. This is even better demonstrated on the T1-weighted fat suppressed image acquired during the delayed hepatobiliary phase (g)

Candidiasis Infection

Hepatic candidiasis is relatively frequent in im-munocompromised patients and it is found in more than 50% of patients with acute leukemia or lymphoma.

On US scans, three major patterns of candidia-sis are seen:

• "wheel within a wheel", in which a peripheral zone surrounds an inner echogenic area,

• "Bull's eye", a lesion with a hyperechoic center surrounded by a hypoechoic rim,

• uniformly hypoechoic, the most common appearance, attributable to progressive fibrosis. After therapy, the lesions may increase in echogenicity and decrease in size, although in some cases sonographic heterogeneity of the liver may persist for several years after treatment [33].

On CT the abscesses are generally multiple, small round hypodense areas on both pre- and post-contrast images. Calcifications can be seen within the lesions [97].

On MR imaging, candida lesions are generally hyperintense on fat suppressed T1-weighted images and have variable signal intensity on conventional T1-weighted spin-echo images. Contrast agent administration leads to the detection of more lesions, which are mainly round, ill-defined, focal hypointense areas. Frequently, percutaneous needle biopsy is needed to achieve a definitive diagnosis [93].

Echinococcal Cyst

Hydatid disease is caused by the parasite Echinococcus granulosus. The disease is mainly present in rural areas where dogs are used for herding live stock, especially sheep, and occurs frequently in Mediterranean countries, in Australia, and in South America.

Dogs are the normal host for the adult parasite, and hundreds of worms may exist in their intestinal tract. Sheep, cattle, herbivores and humans are intermediate hosts for the parasite and are infected after contact with dog feces. In heavily endemic areas, about 50% of dogs and up to 90% of sheep and cattle are infected with E. granulosus. Eggs are passed by the dogs and can be ingested by intermediate hosts.

Once inside the intermediate host, the parasitic eggs hatch and embryos penetrate the intestinal mucosa to enter lymphatic and venous channels. Most embryos are filtered by the liver and lungs with the remaining parasites reaching other organs, including the brain, spleen, kidneys, and the musculoskeletal system.Viable embryos transform into cysts which grow at a rate of approximately 1 cm per year. The wall of the hydatid cyst is composed of two layers: the endocyst, a germinal layer, and the ectocyst, a proteinaceous membrane. A dense fibrous capsule containing collagen, the per-icyst, is formed by the host.

Echinococcal cysts usually develop in the liver (75% of cases) but may occur in any part of the body. The lesions are often asymptomatic for many years and are discovered incidentally on US or CT scans. Hydatidosis can also be detected by serologic tests. Classic symptoms of hepatic hy-datid cyst include upper abdominal pain and hepatomegaly [2, 60].

Treatment consists of surgical removal of the cyst or antiparasitic drug therapy. If left untreated, a hepatic hydatid cyst may rupture into surrounding structures such as the liver parenchyma, biliary system, peritoneum, GI tract, or pleura. Hy-datic cyst rupture is the major complication of echinococcal disease [2,21,87].

On abdominal plain film, curvilinear or ringlike calcifications can be seen in the right upper abdominal quadrant in about 20-30% of cases. However, calcifications do not necessarily indicate death of the parasite.

The appearance of the hydatic cyst on US is variable and depends on the stage of evolution and maturity. The lesion may appear as a well-defined anechoic cyst, as an anechoic cyst except for hy-datid sand, as a multiseptate cyst with daughter cysts, as a cyst with a floating membrane, or finally, as a densely calcified mass (Fig. 67) [21,42].

US has also been used to monitor the efficacy of medical antihydatid therapy: positive responses include cyst size reduction, membrane detachment, increased echogenicity and mural calcification.

On CT, hydatid disease appears as unilocular or multilocular well-defined cysts. Daughter cysts are seen as areas of lower density and are usually oriented towards the periphery of the lesion (Fig. 68). Daughter cysts can also float in the lumen of the mother cyst. Curvilinear ring-like calcification or grossly diffuse calcification are also common features. The peripheral walls may show enhancement after contrast medium administration (Fig. 69) [70,87].

On MR imaging, the cystic component of echinococcal disease is similar to that of other cysts, with long T1 and T2 relaxation times. A low intensity rim around the cyst is present in most cases and is more conspicuous on T2- than T1-weighted sequences. This rim corresponds to the pericyst which is rich in collagen and has a short T2 relaxation time [2]. This rim and a multiloculated or multicystic appearance are distinctive features (Fig. 70). Floating membranes have low signal intensity on both T1- and T2-weighted images (Fig.

Fig. 67a-c. Echinococcal cyst on US. Ultrasound reveals either a well-defined anechoic cystic-like lesion (a), a cystic lesion with a floating membrane (arrow) (b), or a dense and heterogeneous nodule (arrowhead) (c)

Fig. 68. Echinococcal cyst on CT. CT after contrast medium administration reveals a large well-delineated cystic lesion. Peripheral round areas of lower density (asterisks) are indicative of daughter cysts

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Fig. 69a, b. Echinococcal cyst on CT. CT after contrast medium administration (a) reveals a multilocular well-defined cystic lesion (asterisk) with thick hyperdense walls and septa. Additional nodules with a heterogeneous appearance and gross calcifications (arrowheads) can be seen around the bigger lesion. The almost complete replacement of the lesion by central calcification indicates the death of the cyst (b)

Fig. 69a, b. Echinococcal cyst on CT. CT after contrast medium administration (a) reveals a multilocular well-defined cystic lesion (asterisk) with thick hyperdense walls and septa. Additional nodules with a heterogeneous appearance and gross calcifications (arrowheads) can be seen around the bigger lesion. The almost complete replacement of the lesion by central calcification indicates the death of the cyst (b)

Fig. 70a-e. Echinococcal cyst on MR. T2-weighted images acquired in the coronal and axial planes (a and b, respectively) reveal a large multiloculated hepatic mass. The cystic component is seen as hyperintense with a hypointense fibrous capsule. On the unenhanced T1-weighted image (c), the lesion is mainly hypointense with peripheral hypointense wall (arrows). Slight enhancement is seen in the wall but not in the cystic component on arterial phase images acquired after the injection of gadolinium (d). The cystic mass appears hypointense on the subsequent portal-venous phase image (e)

Fig. 71a, b. Complicated echinococcal cyst on MR. The T2- and T1-weighted images (a and b, respectively) reveal air within the upper portion of the echinococcal cyst and fluid level in the lower portion (duodenal fistula). A floating hypointense membrane can also be recognized (arrows)

Fig. 71a, b. Complicated echinococcal cyst on MR. The T2- and T1-weighted images (a and b, respectively) reveal air within the upper portion of the echinococcal cyst and fluid level in the lower portion (duodenal fistula). A floating hypointense membrane can also be recognized (arrows)

71). Small cystic extensions from the main lesion are seen as peripheral areas of increased signal intensity on T2-weighted images and probably represent the active portions of the disease [52,79].

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