Page 533 - Atlas of Small Animal CT and MRI

Hepatobiliary Disorders 523

differentiation to be made on multiphase CT and MR administration is associated with higher‐grade lesions.
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scans in people. 7,8 Liver‐specific contrast medium has been investigated
and is effective in differentiating benign from malignant
Benign hepatic masses lesions, with similar imaging characteristics compared to
The term nodular hyperplasia is often used to describe gadolinium‐DTPA contrast medium. 13
apparently spontaneous development of a somewhat Cholangiocellular carcinomas (biliary adenocarcino­
unorganized mass consisting of relatively normal hepatic mas) have been reported in cats and dogs. 8,9,11 They are
cellular elements. A regenerative nodule suggests a poorly encapsulated with heterogeneous contrast enhance­
similar response to a known or suspected hepatic insult. ment and regions of absent contrast enhancement
Despite the nuanced differences, the terms are often used (Figure 5.3.11).
interchangeably. Nodular hyperplasia is nonencapsu­ Metastatic lesions to the liver may be hypervascular or
lated and may be either hypoattenuating during the hypovascular, depending on their degree of arterial blood
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arterial phase (Figure 5.3.3) or diffusely hyperattenuat­ supply. Hypovascular lesions are more common in people,
ing (Figure 5.3.4). Nodular hyperplasia is often with hypoattenuation on the arterial phase (Figures 5.3.12,
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isointense to liver in the unenhanced, portal, and delayed 5.3.13). A target lesion appearance in the delayed phase
phases, making detection difficult in protocols that do images has been identified as specific for hypervascular
not include arterial phase imaging. Nodular hyperplasia metastasis, such as those of neuroendocrine tumors
lesions tend to be smaller than hepatocellular adenoma (Figure 5.3.12). Metastatic lesions tend to be hypointense
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or carcinoma. On MR images, regenerative nodules are on T1 and hyperintense on T2 images with contrast rim
similar to hepatic parenchyma with T1, T2, and contrast enhancement and multifocal distribution. 11
imaging characteristics similar to normal liver. 11 Lymphoma often occurs as a diffuse hepatic abnormal­
Hepatocellular adenomas are diffusely contrast ity, and is hypointense to skeletal muscle on T2‐weighted
enhancing in the arterial phase, similar to nodular MR images, compared to normal dogs that have hyperin­
hyperplasia. They may also be isoattenuating in the tense liver intensity. Mass lesions may also occur, for
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unenhanced and delayed phases, appearing similar to example in the bile duct, causing partial obstruction
normal liver parenchyma (Figures 5.3.5, 5.3.6). (Figure 5.3.14). Hepatic lymphoma is hypoattenuating on
Myelolipomas are benign tumors composed of fat and CT and may have a central region of low intensity indic­
myeloid elements, which occur in cats and wild felines. ating necrosis. Enhancement may be absent, patchy, or
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On CT images, they are irregular, lobular masses with peripheral. 16
fat‐attenuating characteristics (Figure 5.3.7). Hepatic sarcomas have a variable appearance in people
Cholangiocellular adenoma (biliary cystadenoma) is depending on the specific cell type. Common features
a benign neoplasm occurring more frequently in cats. include hypoattenuation compared to normal liver on
These masses are composed of multiple variably sized unenhanced images, an unorganized multicameral
cystic regions with little or no peripheral contrast appearance when cysts are present, and variable but often
enhancement and central fluid attenuation on CT images inhomogeneous or peripheral contrast enhancement.
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(Figure 5.3.8). Primary sarcomas, such as hemangiosarcoma, spindle
cell sarcoma, and histiocytic sarcoma, occur in the liver
Malignant hepatic masses of dogs as large masses (Figure 5.3.15). On MR images,
On arterial phase CT images in people, hepatocellular hemangiosarcoma is T1 hypointense and T2 hyperin­
carcinoma has increased arterial enhancement compared tense. On contrast‐enhanced images, rim enhancement
to benign nodules and early washout in the portal phase. is seen in masses with internal hemorrhage and masses
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This is beginning to be explored in veterinary medicine enhance progressively on delayed phase images. 11
with CT and MRI although our experience suggests that Neuroendocrine tumors, or carcinoids, arise from
variability in enhancement may limit diagnostic utility. neuroendocrine cells present within the biliary tree,
Hepatocellular carcinoma is more cystic and has central gallbladder, or hepatic progenitor cells. Hypervascularity
hypoattenuating regions with peripheral arterial is common on histopathology; however, imaging fea­
enhancement on CT images (Figure 5.3.9). This may be tures have not been described. 12
8,9
partially due to the large size of typical hepatocellular
carcinomas at time of diagnosis, with regions of poor Degenerative and other disorders
perfusion and necrosis. On MR images, hepatocellular
carcinoma is heterogeneous on T1 and T2 images Hepatic cysts develop for a variety reasons but can be
(Figure 5.3.10) with increased signal intensity in the early associated with hepatic neoplasia, polycystic kidney
contrast phase. Greater heterogeneity following contrast disease, parasitic disease, and congenital hepatopathy.
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