## Materials and Methods
### Image Processing Tasks Related to Finding and Characterizing Liver Nodular Lesions
#### 2.5.3 Guidelines for CT Image Processing for Decision Support Purposes  
During our research, we faced several problems regarding the cooperation of people with medical and engineering backgrounds. This is the reason why a short summary of our rules for cooperation are given in the following points.  
1. Biology means diversity. Liver is an especially diverse an organ, it can have many shapes, sizes, and density ranges. The lesions in the liver are diverse as well.
2. The goal should be to build a method that is good enough to help medical experts, not replacing them.
3. If a disease is found, it does not mean that there cannot be another, different or maybe the same type.
4. There are lots of types of lesions that can be found in a liver CT. Some of them are extremely rare, yet very dangerous. It is very difficult to build a sufficiently large training set for all the possible outputs of a purely deep learning liver diagnosis program.
5. There is a possibility to suggest other diagnostic methods, if the data from the CT is not sufficient to give reliable results. It is possible to have an exit code \"order another type of diagnostic procedure\" (even biopsy) for a medical image classification program.
6. It is necessary to clarify the goal of the classification and the mathematical measure in which the hit rate should be high. It is necessary to describe the usual measures of hit rates to medical experts and to discuss if they are a sufficient or necessary measure for them. It might take more rounds to clarify the goals.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Focal Liver Lesion** & \multicolumn{2}{l}{**Characteristic CT Features**} \\ \hline \multirow{4}{*}{Cyst} & \(\bullet\) & near water-density fluid content \\  & \(\bullet\) & no enhancement \\  & \(\bullet\) & round \\  & \(\bullet\) & solitary or multiple \\ \hline \multirow{4}{*}{Hemangioma} & \(\bullet\) & contrast enhancement follows the blood pool \\  & \(\bullet\) & nodular peripheral enhancement in the arterial phase \\  & \(\bullet\) & graduate filling-in from the periphery in the later phases \\  & \(\bullet\) & prolonged enhancement in delayed phase can be observed \\ \hline \multirow{4}{*}{Focal Nodular Hyperplasia (FNH)} & \(\bullet\) & often beneath the liver surface \\  & \(\bullet\) & well-demarcated borders \\  & \(\bullet\) & can be lobulated \\  & \(\bullet\) & usually soliaire \\  & \(\bullet\) & characteristic low-density central scar (delayed enhancement) \\  & \(\bullet\) & spoke-wheel enhancement pattern (most convenient is early arterial phase) \\  & \(\bullet\) & isodense in portal and delayed phase \\ \hline \multirow{4}{*}{Hepatic Adenoma} & \(\bullet\) & sharp contour \\  & \(\bullet\) & can be inhomogeneous (especially if large)—bleeding, necrosis, calcification \\  & \(\bullet\) & arterial enhancement, can be undistinguishable later phases \\ \hline \multirow{4}{*}{Hepatocellular Carcinoma (HCC)} & \(\bullet\) & different tissue types can show different appearances \\  & \(\bullet\) & avid enhancement in the arterial phase \\ \cline{1-1}  & \(\bullet\) & washout in portal and delayed phase \\ \cline{1-1}  & \(\bullet\) & rim-enhancement in the delayed phase \\ \hline \multirow{4}{*}{Metastases} & \(\bullet\) & variable appearance \\ \cline{1-1}  & \(\bullet\) & marked washout in portal and delayed phase \\ \cline{1-1}  & \(\bullet\) & can be heterogeneous (especially if larger) \\ \hline \hline \end{tabular}
\end{table}
Table 1: The properties of the focal nodular lesions.  
A medical method should be tested for a long time (several years) before validation. It is necessary to help the medical experts understand how the image processing or classification method works and what are the critical points for testing.