Computed tomography of the liver

Segmental structure of the liver

When planning a biopsy or radiation therapy for the liver, you need to know exactly which segment the pathological entity is in. In the course of the main branch of the portal vein in the horizontal direction, the liver is divided into the cranial and caudal parts. In the cranial part, the segments of the segments are the major hepatic veins. The border between the right and left lobes of the liver does not pass through the crescent ligament, but along the plane between the middle hepatic vein and the pit of the gallbladder.

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

Selecting a window

With traditional (nonspiral) computed tomography, a liver without contrast enhancement is evaluated in a special hepatic window. Its width is 120 - 140 HU. This special narrowed window helps to more clearly differentiate pathological formations from normal hepatic parenchyma, because it provides better contrast of the image. If there is no fatty hepatosis, intrahepatic vessels are defined as hypodense structures. With fatty hepatosis, when the absorbing capacity of the tissue is reduced, the veins can be isodensic or even hypergrade with respect to the unresponsed parenchyma of the liver. After intravenous administration of KB, a window with a width of about 350 HU is used, which smoothes out the contrast of the image.

Passage of bolus of contrast medium

Spiral scanning is performed in three phases of passage of the bolus of the contrast preparation. The early arterial phase, the portal vein phase and the late venous phase are distinguished. If no pre-scan was performed, then the scan in the last phase can be used as not amplified for comparison with other phases. Hypervascularized pathological formations are much better differentiated in the early arterial phase than in the late venous phase. The late venous phase is characterized by almost identical densities of arteries, portal and hepatic veins (equilibrium state).

CT-portography

The true extent of the pathological formations of the liver (eg, metastasis) is significantly better determined by scanning in the portal vein phase after selective administration of a contrast medium to 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 the hyperdense unchanged parenchyma of the liver intensified by the contrast drug, pathological formations become hypodense. When compared with a cut in the early arterial phase, the same patient can see that without contrast portography, the prevalence of metastases is greatly underestimated.

Liver cysts

Liver cysts contain a serous fluid, clearly delineated by a thin wall from surrounding tissues, have a uniform structure and a density close to water. If the cyst is of small size, then due to the effect of a private volume, it does not have clear boundaries with the surrounding tissue of the liver. In doubtful cases, it is necessary to measure the density within the cyst. It is important to establish the area of interest exactly in the center of the cyst, away from its walls. In small cysts, the average density can be quite high. This is due to the ingress of the surrounding hepatic tissue into the measurable region. Pay attention to the lack of intensification of cysts after intravenous administration of contrast medium.

Echinococcus cysts (Echinococcus granulosus) have a characteristic multi-chambered appearance, often with radially divergent septa. But with the death of a parasite, it is sometimes difficult to differentiate a spasured parasitic cyst with other intrahepatic formations. The right side of the liver is more often affected, although sometimes the left lobe or spleen is involved in the process. In sections without contrast, the density of the cystic fluid is usually 10 - 40 HU. After intravenous administration of the contrast medium, the strength of the outer capsule is determined. Often there is partial or complete calcification of the walls of the cysts. The differential diagnosis includes infectious E. alveolaris infection (not shown) and hepatocellular carcinoma, which is difficult to distinguish from other abnormal liver pathologies.

Metastasis in the liver

If multiple focal lesions are visualized in the liver, one should think about the presence of metastases. The most common sources are neoplasms of the colon, stomach, lungs, breast, kidneys and uterus. Depending on the morphology and vascularization, several types of metastases in the liver are distinguished. Spiral computed tomography with contrast is performed to evaluate the dynamics of the process both in the early arterial and in both venous phases. At the same time, even the smallest metastases are clearly visible, and you will not confuse them with the hepatic veins.

In the venous phase, hypo- and hypervascularized metastases are hypodense (dark), because of them the contrast substance is quickly washed out. If it is not possible to perform a spiral scan, you will be helped by comparing the slices without gain and with gain. To assess the native images, it is always necessary to increase the contrast of the liver parenchyma by installing a special tapered window. This allows you to visualize even small metastases. Small hepatic metastases, unlike cysts, have a fuzzy contour and high density (gain) after intravenous administration of contrast medium. The average density level is 55 and 71 HU.

In doubtful cases and to assess the dynamics during treatment it is useful to compare CT images with ultrasound data. As well as on KT, ultrasound signs of metastases are different and can not be reduced only to a typical hypoechoic rim. Ultrasound diagnosis can be difficult, especially when calcification with acoustic shade occurs in metastases. But this is rare enough, except for slowly growing metastases of mucosal cancer (for example, gelatin gut), which can almost completely calcify.

Solid liver formations

Hemangioma is the most common benign liver formation. In native images, small hemangiomas are defined as clearly delineated homogeneous zones of reduced density. After intravenous administration of the contrast medium, amplification is first characteristic at the periphery of the formation, and then gradually spreading to the center, which resembles the closure of the optical diaphragm. With dynamic CT examination after the introduction of the contrast drug bolus, the enhancement progresses centripetally. In this case, a bolus of contrast agent is administered and scanning is performed to obtain a series of CT images every few seconds at the same level. The accumulation of a contrast agent within the hemangioma results in a 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 found in women aged 20-60 years, who took oral contraceptives for a long time. They grow from hepatocytes and can be single and multiple. Adenomas are usually hypodense, sometimes hypervascularized and may be accompanied by zones of infarction or central necrosis of reduced density and / or areas of increased density, reflecting spontaneous hemorrhage. Surgical removal is recommended because of 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, focal nodular hyperplasia is defined as hypodense, sometimes isodense, but clearly delineated formations. After intravenous administration of a contrast agent in the area of nodular hyperplasia, a central area of blood supply of irregularly shaped lower density often appears. However, this sign is determined only in 50% of cases.

Hepatocellular carcinoma is often found in patients with prolonged cirrhosis of the liver, especially in men older than 40 years. In one third of all cases, a single tumor is defined, in others - a multi-focal lesion. Thrombosis of the portal vein branches due to the germination of the tumor into the lumen of the vessel are also found in one third of patients. Manifestations of hepatocellular cancer on CT images are very diverse. In native images, the tumor is usually hypodense or isodendous. After the introduction of the contrast medium, the enhancement is diffuse or annular with a zone of central necrosis. If hepatocellular cancer develops against the background of liver cirrhosis, it can be very difficult to determine the boundaries of the tumor.

When conducting differential diagnosis, you should always keep in mind the secondary lymphoma because of its ability to infiltrate the liver parenchyma and cause diffuse hepatomegaly. Of course, one should not think that any hepatomegaly has developed due to lymphoma. Non-Hodgkin's lymphomas resemble hepatocellular carcinomas, because they have similarities in vascularization and nodal growth.

Diffusive lesions of the liver

With fat hepatosis, the density of the unrespended hepatic parenchyma (normal about 65 HU) can decrease so much that it becomes isodensic or even hypodense in comparison with blood vessels. In the case of hemochromatosis, the accumulation of iron leads to an increase in density above 90 HU and even up to 140 HU. At the same time, the natural contrast between the liver parenchyma and the vessels is significantly increased. Cirrhosis as a result of chronic liver damage leads to the appearance of diffuse nodal structure of the organ and uneven, tuberous edges.

[1], [2], [3], [4], [5], [6], [7], [8], [9]