When CT of the liver shows cystic lesions, it may be a pathological reaction to many kinds of diseases. According to the etiology, they can be divided into: (1) congenital: such as liver cysts; (2) traumatic; (3) inflammatory: such as liver abscesses; (4) parasitic: such as liver encapsulated cysts; (5) neoplastic: such as liver cystic metastases; (6) others: such as liver infarcts. The CT manifestations of hepatic cystic lesions are divided into 7 groups and discussed as follows: 1. Cysts with homogeneous watery density lesions Congenital hepatic cysts belong to this type of lesions, and the CT manifestations are single or multiple round or oval homogeneous watery density shadows with smooth and sharp edges, and the CT values are about 0-15 Hu. Due to partial volume effect, CT values of small cysts are often high and should be scanned in thin layers to confirm the diagnosis. The cysts are not enhanced on enhancement scan. Its borders are more clearly defined against the surrounding normal liver parenchyma. The walls of the cysts are usually not observed, but if they are close to each other or close to the liver envelope, a very thin cyst wall is visible. Sometimes the cysts may be multifocal and may have septal-like manifestations within them. Liver cysts are usually associated with cysts of the kidney, spleen, pancreas, ovary, lung, brain, etc. This type of lesion is also seen in hepatic peritoneal cysts. There are 4 types of liver cysts: (1) simple type, which is indistinguishable from simple liver cyst; (2) internal cyst separation type, where a small number of cysts are seen; (3) multiple cysts type; (4) parenchymal calcification type, where calcification is seen in the cyst wall and material. 2.Increased density lesions in cysts Cysts with slightly higher density are commonly caused by intracapsular hemorrhage, infection, cyst fluid containing more protein and partial volume effect. In case of cyst infection or abscess, CT scan shows thick walls; if solid blood clots are seen in the cyst, it suggests intracystic hemorrhage; if air bubble shadow is present in the solid part, it is characteristic of infection, and history of trauma or fever is important for the diagnosis of hemorrhage or infection. However, sometimes the history of trauma is rather insidious and not easily perceived by the patients themselves, such as a very minor trauma in case of multiple cysts may cause bleeding. In addition, blood clots within cysts can resemble soft tissue portions of cystic tumors as well as cephalic nodes within liver peritoneal cysts. In this case, enhanced CT scan, Doppler ultrasound or MRI can provide valuable information. Sometimes, the CT presentation of a completely necrotic tumor resembles that of an abscess or liquefied hemangioma in the envelope formation phase, however, the former cyst wall will be more irregular and edema is often present around the abscess, whereas the hemangioma is more clearly demarcated. Preoperative aspiration or drainage is necessary to confirm the diagnosis and is necessary. If the tumor is all necrotic, it is difficult to make the diagnosis of tumor by aspiration of necrotic tissue, and it is not easy to make the diagnosis of other diseases. 3.When there is wall nodular lesion within the cyst, it is common to see necrosis in metastases, mucinous cystadenoma or cystic adenocarcinoma and other solid tumors. Cystic metastases can be divided into 4 types; (1) intracapsular segregated type; (2) intracapsular fluid type; (3) thick walled type; (4) intracapsular walled nodules type. The first 2 types of ultrasound are more sensitive than CT, and the last 2 types of enhanced CT or spiral CT are sensitive. Spiral CT, enhanced CT and MR can provide dynamic enhancement information of wall nodules. When color Doppler ultrasound is used to diagnose wall nodules in cystic lesions, it is easy to misdiagnose a limited thickening of the cyst wall due to blood vessels or hemorrhage as a wall nodule. In this case, the cyst wall is not enhanced on CT and MR-enhanced scans. Once wall nodule growth is detected, it should be considered as a tumor. Cystic metastases are commonly associated with epidermoid carcinoma, followed by ovarian, colon and smooth muscle sarcoma. Patients with primary malignancy should be considered for cystic metastases if multiple cystic lesions are seen in the liver parenchyma. Cystic dilatation of intrahepatic bile ducts without obvious stone or tumor compression should be considered as mucinous cystadenoma and cystic adenocarcinoma. Differentiation of these 2 diseases is more difficult and microscopy is unreliable. However, on CT, it is shown that the more solid portion is present, the more malignant it is. Biliary cystadenoma or cystic adenocarcinoma often presents as a single foci rather than multiple foci. If a cystic lesion with irregular margins is seen in the liver, along with cirrhosis and portal embolism, it should first be considered as a necrotizing lesion of hepatocellular carcinoma. Although pus, mucin or necrotic tissue resemble solid tumors on ultrasound images, ultrasound is more sensitive than CT in examining the internal structure of cystic lesions. 4. Neoplastic lesions with cystic component Both necrosis and hemorrhage of liver tumors can be cystic in appearance. Rich vascular tumors are common, which are due to insufficient blood supply or rupture of blood vessels. Spongiotic hemangioma is the most common benign rich hemangioma, its diagnostic criteria are clearer and has typical performance, CT dynamic enhancement scan lesion circumferential reinforcement, density increase, gradually filling to the center, isointense, lasting for a few minutes or even 1h; MRI performance is T2WI high signal. Cystic degeneration of spongiform hemangiomas is rare, and the central unenhanced portion is due to fibrosis or thrombosis. Another common vascular rich tumor is hepatocellular carcinoma. Once a hepatic tumor is necrotic with portal vein tumor thrombus, the most likely diagnosis is hepatocellular carcinoma. 80% of hepatocellular carcinomas are associated with cirrhosis; therefore, the thesis of hepatocellular carcinoma is most likely in cirrhosis with cystic lesions, which characteristically present with fissured necrosis in the center of the lesion. Intrahepatic angiogenic rich metastases are commonly seen in smooth muscle sarcoma, renal cell carcinoma, islet cell carcinoma, thyroid cancer, pheochromocytoma, melanoma, choriocarcinoma, and carcinoid tumors. If the primary lesion is known, especially from the organs mentioned above, and a cystic lesion is seen in the liver, the diagnosis of metastatic tumor is highly likely. However, biopsy is required for a definitive diagnosis. 5. Cystic lesions in the short term or disappearing When cystic lesions in the liver develop in the short term, the clinical history is particularly important in the CT diagnosis. If the patient is febrile or over immunosuppressed, microabscesses are likely. If it is post-traumatic it should be considered a liquefied hematoma or a bilious tumor. If the patient has been treated with hepatic artery embolization or local tumor injection with anhydrous alcohol a few days ago, a cholestasis or tumor necrosis is most likely. If the tumor progresses from solid to cystic, tumor necrosis should be considered. Although the presence of iodinated oil filling the hepatocellular carcinoma after arterial embolization may be considered for the survival status of the tumor, complete necrosis of the hepatocellular carcinoma manifesting as a purely cystic lesion is not a reliable sign of complete eradication of the hepatocellular carcinoma nodule. Experience shows that in this case, liver cancer will recur, so close observation is necessary. The disappearance of cystic lesions within a short period of time is most likely due to the absorption of small abscesses or small hematomas. 6. Stable small cystic lesions Stable small cystic lesions in the liver are most likely to be simple cysts, hemangiomas, purpuric hepatitis or bile duct malformation tumors. Differentiation between them is difficult and needs to be confirmed by liver parenchymal aspiration biopsy. Among them, early diagnosis of purpuric hepatitis is important to avoid its comorbidities. Such as hepatocellular dysfunction, portal hypertension and potential rupture of the liver. Purpuric hepatitis is caused by the application of steroid hormones and returns to normal as soon as they are discontinued. Because the disease is an intrahepatic space filled with blood, CT enhancement is isointense and resembles a hemangioma, or resembles a cystic lesion due to blood clots and emboli. Therefore, this group of lesions should be closely observed. 7.Tubular structures Dilated tubular structures in the liver may appear as cystic lesions in their cross-sections. On the contrary, the cross-section of portal thrombus may show solid nodules, which can be easily misdiagnosed as solid tumors. Usually bile duct dilatation is seen in the presence of bile duct obstruction, such as tumor compression, stones, and mucin embolism. If intrahepatic bile duct dilatation is disproportionate to the total bile duct dilatation and is columnar or cystic in shape, Caroli’s disease or sclerosing cholangitis should be considered, which is more extensive than sclerosing cholangitis. It is difficult to differentiate segmental Caroliization disease from simple cholangitis or bile duct dilatation due to stone obstruction. In summary, CT diagnosis of most cystic lesions in the liver is more accurate; however, the diagnosis of certain small cystic lesions is more difficult. The main reasons for misdiagnosis are: nodular shadowing due to partial volume effect, hepatic changes due to hepatic parenchymal steatosis or hemochromatosis, solid tumors with suboptimal enhancement, dual lesions and extrahepatic cystic lesions resembling intrahepatic ones. Therefore, CT value measurements, ultrasound and MR examinations, dynamic CT examinations, needle biopsies and short-term observation are commonly used for the diagnosis and differential diagnosis of intrahepatic cystic lesions. CT diagnosis and differential diagnosis of cystic lesions in the liver Convenient aids for diagnosis Cystic lesions in the liver are commonly seen clinically as simple non-parasitic cysts, parasitic cysts, congenital multicystic liver, intrahepatic bile duct dilatation, liver abscess, mucinous tumor, hepatic cystic adenoma, traumatic hematoma, hepatic cystic adenocarcinoma, central liquefaction necrosis of liver tumors and certain malignant lesions, liver metastatic lesions of gastrointestinal cancer, etc. 1.Hepatic cysts Smaller cysts have no change in liver morphology; huge cysts have increased liver volume and may have limited semicircular elevation on the liver surface. The cyst wall is thin and smooth and round. Small cysts of 3 mm to 5 mm can be detected by ultrasound at a minimum. In cysts with co-infection, in addition to systemic symptoms, ultrasound shows faint echogenicity within the cyst, and sometimes small floating signs can be seen. huge cysts (diameter >10cm or more) can occupy half of the liver or the entire upper abdomen, and large intrahepatic vessels are pushed and displaced or compress surrounding organs. Multiple hepatic cysts can be seen as multiple fluid dark areas within the liver, with varying morphology and size, and several cystic cavities can be fused and communicated or separated within a large cyst. In addition, congenital polycystic liver is often accompanied by polycystic kidney. 2.Hepatic cystic adenoma The lesion with mainly liquid cystic cavity in the liver has the characteristics of hepatic cyst, but the inner wall of the cyst is uneven, there are papillary or irregular shaped substantial tissue convex in the cystic cavity, or there are substantial lumpy tissue echogenicity in the cyst, which makes the lesion appear as cystic solid mixed non-homogeneous heterogeneous echogenic image, which should be combined with other clinical examination to make a comprehensive judgment. 3, intrahepatic bile duct dilatation B ultrasound examination can be seen along the intrahepatic bile duct distribution of varying sizes of round or shuttle-shaped liquid cavity, in the size of a string of beads, the cystic wall is thicker echogenic enhancement, while the intrahepatic bile duct dilatation due to obstruction is more uniform and consistent duct expansion, regional or whole liver universal duct expansion, sometimes although there is a narrowing of the separation but more than a string of beads like round or shuttle-shaped changes. Intrahepatic bile duct dilatation is often accompanied by cystic dilatation changes of extrahepatic bile ducts, and there may be stone echogenicity and acoustic shadow in the cystic lumen. 4. Liver abscesses Bacterial and amoebic liver abscesses are common in clinical practice. Their sonograms are characterized by intrahepatic fluid-free echogenic occupying lesions. However, different sonograms appear with different stages of disease. In the early stage of abscess, liver tissue is mainly congested, edematous and inflammatory infiltrated, and the lesion is an enhanced echogenic cluster with thick and dense dots and uneven distribution, and the periphery of the lesion is indistinct. After necrosis and liquefaction of the abscess tissue, the lesion area is irregularly shaped echogenic liquefaction dark area, or irregular hypoechoic honeycomb structure of varying sizes; with the progression of the disease, the liquid dark area within the lesion expands, but the inner wall is not smooth, and there is a thick-walled envelope around the abscess, and the tissue around the abscess forms a wider slightly strong echogenic band due to edema, about 3-5 mm thick, in a wide band-like “halo “. In some cases, the abscess is thick with pus and contains exfoliated necrotic tissue, and the lesion area often shows irregularly distributed hypoechoic or disorganized slightly strong echogenic areas, which are often mistaken for substantial occupying hepatic lesions; because pus bacteria can produce gas, a gas-liquid surface can often appear in the lesion area, and the surface can move or disappear with position changes or appear as floating strong light spots within the surface; in addition, reactive pleural fluid can appear in the chest cavity on that side In addition, reactive pleural fluid may appear in that side of the chest cavity, and a liquid anechoic area is present. With reference to the medical history and clinical features, it is not difficult to diagnose liver abscess by ultrasound, and it can be treated with repeated puncture and pus aspiration, drug injection or tube drainage under the guidance of ultrasound. 5, liver worm cyst In ultrasound examination, typical single cyst liver worm has the same characteristics as simple liver cyst in morphology and internal echogenicity, which are round or similar round consistent liquid dark area with clear boundary with normal liver tissue. However, the encapsulated cyst has a rough cyst wall or appears as a double-layered cyst wall with a wall thickness of 3-5 mm and its posterior enhancement effect, which is often mistaken for a simple cyst. If the inner wall of the cyst is carefully observed, some of them can be seen to be uneven and slippery; cases containing subcapsules can be seen to have scattered light spots or small light clusters drifting in the dark area; some cases have thick cyst walls and varying degrees of calcification with acoustic shadowing, and individual cases can be seen to have part of the cyst wall detached in a “hundred lotus-like” figure. It is difficult to distinguish vesicular type cysts from hepatocellular carcinoma on ultrasound images, but the diagnosis can be made by combining medical history, clinical features and Casoni skin test. 6.Hepatic cystadenocarcinoma The ultrasound image features of this disease are the same as those of hepatic cystadenoma, with cystic solid echogenicity as the main feature, but the substantial mass inside the cyst often shows irregular or “cauliflower-like” changes and develops faster. It should be distinguished from cystadenoma by clinical history, systemic manifestations such as pain, anemia, fever, CEA, CA19-9, r-GT and other enzyme markers, presence of extrahepatic metastases, etc. The diagnosis can be confirmed by ultrasound-guided puncture and extraction of intracapsular fluid with blood. Certain tumors in liver, especially giant primary hepatocellular carcinoma, are often accompanied by necrosis in the central area, which appears as an irregularly shaped fluid-free echogenic area on the ultrasound image. The necrotic area appears as an irregular single-lumen liquefied area in the central part of the lesion. In contrast, liver abscesses can form irregular liquefaction in multiple locations within the lesion, with separated echogenic areas of varying sizes and fragmented necrotic tissue floating in the liquefied cavity. The distinction between the two should be made by a comprehensive analysis of the history, signs and various examinations. In addition, some malignant tumors of other organs such as adenocarcinoma and metastases to the liver may also present as cystic or cystic solid lesions, often multiple, and the ultrasound features of the lesions are different from those of simple cysts, which should be noted clinically.