Computed tomography of the liver

Segmental structure of the liver

When planning a liver biopsy or radiation therapy, it is necessary to know exactly in which segment the pathological formation is located. Along the main branch of the portal vein in the horizontal direction, the liver is divided into cranial and caudal parts. In the cranial part, the borders of the segments are the main hepatic veins. The border between the right and left lobes of the liver does not pass along the falciform ligament, but along the plane between the middle hepatic vein and the gallbladder fossa.

Left lobe	I caudate lobe II lateral segment, cranial part III lateral segment, caudal part IV quadrate lobe (a: cranial, b: caudal)
Right share	V anterior segment, caudal part VI posterior segment, caudal part VII posterior segment, cranial part VIII anterior segment, cranial part

Window selection

In traditional (non-spiral) CT, the liver is assessed without contrast enhancement in a special liver window. Its width is 120 - 140 HU. This special narrowed window helps to differentiate pathological formations from normal liver parenchyma more clearly, because it provides better image contrast. If there is no fatty hepatosis, the intrahepatic vessels are defined as hypodense structures. In fatty hepatosis, when the tissue absorption capacity is reduced, the veins can be isodense or even hyperdense relative to the non-enhanced liver parenchyma. After intravenous administration of KB, a window with a width of about 350 HU is used, which smoothes the image contrast.

Contrast bolus passage

Spiral scanning is performed in three phases of the contrast agent bolus passage. An early arterial phase, a portal venous phase, and a late venous phase are distinguished. If no preliminary scanning was performed, the scan in the last phase can be used as non-enhanced for comparison with other phases. Hypervascularized pathological formations are differentiated much better in the early arterial phase than in the late venous phase. The late venous phase is characterized by practically identical densities of the arteries, portal veins, and hepatic veins (equilibrium state).

CT portography

The true extent of the spread of pathological liver formations (e.g., metastases) is much better determined by scanning in the portal vein phase after selective administration of a contrast agent into the superior mesenteric or splenic artery. This is due to the fact that the blood supply of most metastases and tumors is carried out from the hepatic artery. Against the background of hyperdense unchanged liver parenchyma enhanced by the contrast agent, pathological formations become hypodense. When compared with a section in the early arterial phase in the same patient, it is clear that without contrast portography the spread of metastases is significantly underestimated.

Liver cysts

Liver cysts contain serous fluid, are clearly delimited by a thin wall from the surrounding tissues, have a uniform structure and a density close to water. If the cyst is small, then due to the effect of private volume it does not have clear boundaries with the surrounding liver tissue. In doubtful cases, it is necessary to measure the density inside the cyst. It is important to set the area of interest exactly in the center of the cyst, away from its walls. In small cysts, the average density value may be quite high. This is due to the surrounding liver tissue entering the measured area. Note the lack of cyst enhancement after intravenous administration of a contrast agent.

Echinococcal cysts (Echinococcus granulosus) have a characteristic multi-chambered appearance, often with radially diverging septa. However, when the parasite dies, it is sometimes difficult to differentiate the collapsed parasitic cyst from other intrahepatic lesions. The right lobe of the liver is most often affected, although the left lobe or spleen is sometimes involved. In sections without contrast, the density of the cystic fluid is usually 10-40 HU. After intravenous administration of a contrast agent, an increase in the external capsule is determined. Partial or complete calcification of the cyst walls is common. The differential diagnosis includes infectious E.alveolaris (not shown) and hepatocellular carcinoma, which is difficult to distinguish from other abnormal liver lesions.

Liver metastases

If multiple focal lesions are visualized in the liver, metastases should be considered. Most often, the sources are neoplasms of the colon, stomach, lungs, mammary gland, kidneys and uterus. Depending on the morphology and vascularization, several types of liver metastases are distinguished. Spiral computed tomography with contrast is performed to assess the dynamics of the process both in the early arterial and in both venous phases. In this case, even the smallest metastases become clearly visible, and you will not confuse them with hepatic veins.

In the venous phase, hypo- and hypervascularized metastases are hypodense (dark) because the contrast agent is quickly washed out of them. If spiral scanning is not possible, comparison of unenhanced and enhanced sections will help. To evaluate native images, it is always necessary to increase the contrast of the liver parenchyma by installing a special narrowed window. This allows visualization of even small metastases. Small liver metastases, unlike cysts, have an unclear outline and high density (enhancement) after intravenous administration of contrast agent. The average density level is 55 and 71 HU.

In doubtful cases and to assess the dynamics of treatment, it is useful to compare CT images with ultrasound data. Just as on CT, ultrasound signs of metastases are different and are not limited to the typical hypoechoic rim. Ultrasound diagnostics can be difficult, especially when calcification with acoustic shadowing appears in metastases. But this is quite rare, with the exception of slowly growing metastases of mucous cancer (e.g., intestinal gallbladder), which can become almost completely calcified.

Solid liver lesions

Hemangioma is the most common benign tumor of the liver. On native images, small hemangiomas are defined as clearly demarcated homogeneous zones of low density. After intravenous administration of a contrast agent, enhancement is characteristically first at the periphery of the formation, and then gradually spreads to the center, which resembles the closing of the optical diaphragm. In dynamic CT examination, after the administration of a bolus of contrast agent, enhancement progresses centripetally. In this case, a bolus of contrast agent is administered and scanning is performed with a series of CT images every few seconds at the same level. The accumulation of contrast agent inside the hemangioma leads to homogeneous enhancement in the late venous phase. In the case of large hemangiomas, this may take several minutes, or the enhancement will be inhomogeneous.

Liver adenoma is most often detected in women aged 20 to 60 years who have taken oral contraceptives for a long time. They grow from hepatocytes and can be single or multiple. Adenomas are usually hypodense, sometimes hypervascularized, and may be accompanied by areas of infarction or central necrosis of low density and/or areas of increased density reflecting spontaneous hemorrhage. Surgical removal is recommended due to the risk of significant bleeding and malignant degeneration. In contrast, focal nodular hyperplasia is not prone to malignancy and contains bile ducts. On native images, areas of focal nodular hyperplasia are determined as hypodense, sometimes isodense, but clearly demarcated formations. After intravenous administration of a contrast agent, an irregularly shaped central blood supply zone of low density often appears in the area of nodular hyperplasia. However, this sign is determined in only 50% of cases.

Hepatocellular carcinoma is common in patients with long-standing liver cirrhosis, especially in men over 40 years of age. A single tumor is detected in one third of all cases, while multifocal lesions are found in the rest. Thrombosis of the portal vein branches due to tumor growth into the vessel lumen also occurs in one third of patients. Manifestations of hepatocellular carcinoma on CT images are very diverse. On native images, the tumor is usually hypodense or isodense. After the introduction of a contrast agent, enhancement can be diffuse or ring-shaped with a zone of central necrosis. If hepatocellular carcinoma develops against the background of liver cirrhosis, it can be very difficult to determine the tumor borders.

In differential diagnosis, secondary lymphoma should always be considered because of its ability to infiltrate the liver parenchyma and cause diffuse hepatomegaly. Of course, one should not assume that all hepatomegaly is due to lymphoma. Non-Hodgkin lymphomas resemble hepatocellular carcinoma because they have similarities in vascularization and nodular growth.

Diffuse liver lesions

In fatty hepatosis, the density of the non-enhanced liver parenchyma (normally about 65 HU) can decrease so much that it becomes isodense or even hypodense compared to the blood vessels. In the case of hemochromatosis, iron accumulation leads to an increase in density above 90 HU and even up to 140 HU. In this case, the natural contrast between the liver parenchyma and the vessels increases significantly. Cirrhosis as a result of chronic liver damage leads to the appearance of a diffuse nodular structure of the organ and uneven, bumpy edges.

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