## Appendix A About the Liver
### The Most Important Diseases of the Liver from an Image Processing Perspective
#### a.1.4 Hepatic Adenoma  
Hepatic adenoma is a true benign tumor of the liver, which has the potential to transform into a malignant carcinoma. It affects mostly young women and it has a presumed relation with oral contraceptive usage. It can be solitary or multiple.  
Its contour is clear; it usually has no capsule but can have one. It is quite common to see bleeding or necrosis at the center of the lesion. In non-enhanced CT images, the tumor appears as a hypodense lesion. Commonly it is bigger at the time of recognition and is inhomogeneous, but smaller ones can be homogeneous. In arterial phase images, the lesion shows moderate enhancement followed by uniformly isodense state at the later phases, except for the necrotic, calcified, or otherwise degenerated areas.  
Appendix A.1.5. Hepatocellular Carcinoma (HCC)  
HCC arises from the liver cells and is one of the most frequent tumors [48]. There are many types, and their appearances differ.  
They can be focal (large mass), nodular (multifocal), or diffuse (infiltrative). A hepatocellular carcinoma mostly receives its blood supply from the hepatic artery, hence its characteristic fast arterial enhancement, followed by early washout. Washout means that because of the arterial blood supply, the lesion has no (or little) blood from portal vein, so in the portal phase, it enhances less than the other parts of the liver; contrast material washes out from the lesion, and in the portal phase the non-enhanced arterial blood makes a darker area than the normal tissue. Rim-enhancement in delayed images is another characteristic sign of HCC. A complete ring around a lesion always indicates a malignant disease. Similarly, the foci of arterial enhancement inside a hypodense lesion usually refer to small HCC within a regenerative nodule. There are other, more or less specific signs that can indicate a HCC: wedge-shaped perfusion abnormalities, vascular invasion, or any lesion in a cirrhotic liver.  
The size of the HCC at the moment of recognition can vary. Small nodules are usually ill-defined, hypo-or isodense lesions with arterial enhancement (can be confused with benign lesions!), or intranodular focal enhancement. Larger, focal, or nodular nodules are well-demarcated and often, but not always, encapsulated lesions. Internal necrosis, hemorrhage, or scars can also occur, creating an inhomogeneous appearance. Calcification can be seen, but is rare. Sometimes, in smaller, well-differentiated tumors, arterial hypoenhancement can occur. These lesions, because of their special vascularity, can show a so-called corona enhancement surrounding the tumor in the arterial or portal phase.  
A fibrous capsule is demonstrated as a hypodense rim in non-enhanced, arterial and portal phases and shows enhancement in the delayed phase. The larger the tumor, the more frequent the occurrence of the capsule.  
Fatty changes, internal fibrous stroma, necrosis, or hemorrhage are atypical in HCC, which does not mean they do not occur.  
Appendix A.1.6. Metastases  
Liver metastases are many times more common than primary liver tumors. The most common liver metastases are derived from the gastrointestinal tract.  
Most of the metastases are hypovascular metastases, meaning that they contain less vasculature than the normal liver and therefore they seem darker in every phase, although they enhance the contrast material. Hypervascular metastases are mostly derived from endocrine cancers, renal cell cancer, or melanomas. Enhancement of a metastasis is typically peripheral, with or without a complete filling-in in the portal phase. The delayed phase shows washout, which can help differentiate them from a benign lesion.  
Metastatic liver cancer forms nodules, whose size and distribution depends on the type of the primary malignancy.