The recurrence of metastases after surgery is one of the main reasons for the poor prognosis of primary hepatocellular carci- noma (HCC). 61.5% of HCC recurrence occurs within 5 years after surgery, and even in small hepatocellular carcinomas, the recurrence rate is still as high as 43.5% 5 years after surgery [1]. The recurrence of hepatocellular carcinoma can be generally divided into two categories: first, intrahepatic recurrence and extrahepatic metastasis caused by preoperative and intraoperative dissemination of cancer cells from the original lesion. The second is the multicentric occurrence of hepatocellular carcinoma, i.e., the recurrence of new tumors after radical resection of liver cancer. Literature [2] reported that recurrence within 2 years after resection of hepatocellular carcinoma (recent recurrence after surgery) is mostly metastatic dissemination of the primary focus, while late recurrence (after 2 years after surgery) is multicentric occurrence, and the latter has a better prognosis than the former after treatment. Regular postoperative follow-up is the main way to detect and diagnose postoperative recurrence of hepatocellular carcinoma, which mainly relies on imaging diagnosis and serum tumor marker examination, among which imaging early diagnosis has an important position. If postoperative recurrence can be diagnosed early and treated timely, the survival rate and prognosis of patients with recurrence can be greatly improved. This article briefly summarizes the characteristics and application value of the main diagnostic imaging techniques (including ultrasound, CT, MRI, etc.) for postoperative follow-up. 1. Color ultrasound examination is the most important means for postoperative follow-up, and is also the most common means for early detection and diagnosis of recurrence. It is widely used in most hospitals in China for early detection and diagnosis of liver cancer as a non-invasive examination with simple operation, high sensitivity, accuracy and economy, and no radioactivity. Color Doppler imaging can observe the distribution of blood flow inside the lesion, thus improving the detection rate and qualitative ability of liver cancer. Ultrasound can diagnose HCC lesions of about 1 cm in diameter, and high-performance ultrasound instruments can even show and clarify small recurrent HCC lesions of 0.5 cm in diameter. In addition to being a common diagnostic and differential method for hepatocellular carcinoma, ultrasonography can also show the relative position and anatomical relationship between the lesion and large blood vessels, whether there is tumor dissemination and metastasis in the hilar lymph nodes of the remaining liver, and whether there is cancer thrombosis in the main trunk of portal vein and its branches. Ultrasonography, also known as enhanced ultrasound imaging, is a kind of gas microbubble agent injected intravenously on the basis of general ultrasound, which can observe the blood perfusion and distribution of microvascular network of tumor in real time and detect the dynamic changes of blood flow in tumor tissue in real time, which is a new and important diagnostic method in the field of ultrasound imaging in recent years. Ultrasound contrast agent is safe, the main component is gas microbubble, no iodine allergic reaction, the amount needed each time is small, and can still be tolerated by patients with heart and kidney failure. Because ultrasonography can observe the whole process of perfusion and withdrawal of blood from the lesion and liver parenchyma in real time, it can better distinguish the various temporal phases of liver perfusion, thus helping to more accurately determine the characteristics of the blood supply to the lesion. The qualitative diagnosis of occupying lesions in the liver can be made more accurately. It will play a greater role in the early diagnosis and differential diagnosis of HCC, especially in determining the thoroughness of local treatment such as radiofrequency ablation (RFA) and intratumoral anhydrous alcohol injection (PEI), and local tumor recurrence. Ultrasonography has the advantages of being economical, convenient, reproducible, non-invasive and non-radioactive, and can basically reflect the image characteristics of HCC lesions, which can be used as the preferred diagnostic imaging method for screening of high-risk patients and postoperative follow-up. However, ultrasound results are easily limited by the experience of the examiner, and smaller tumors located higher in the top of the hepatic diaphragm and more distant in the left outer lobe are easily missed, and the detection rate of lesions less than 1 cm in diameter is low. Therefore, other imaging methods should be combined to improve the sensitivity and accuracy of diagnosis. CTCT is one of the most important methods to detect and diagnose HCC recurrence. The majority of HCC is supplied by the hepatic artery, and CT multi-phase enhancement scans are typically characterized by the rapid passage of the arterial contrast into the lesion, resulting in rapid enhancement and high density. In the portal phase, the liver parenchyma supplied by the portal vein is rapidly intensified, while the contrast agent in the lesion is rapidly withdrawn and becomes low-density. Among them, the detection rate and qualitative diagnosis rate of small hepatocellular carcinoma are the highest in the hepatic artery phase, followed by the delayed phase and the lowest in the portal vein phase, and the combination of the three phases can improve the detection rate and qualitative diagnosis rate of lesions. The combination of the three phases can improve the detection rate and qualitative diagnosis of lesions. The portal vein phase shows the vascular structures inside and outside the liver most clearly, and it is easy to determine whether there is vascular invasion and cancer thrombosis. Dual arterial phase imaging (early arterial phase and late arterial phase) can improve the detection rate and correct diagnosis of HCC, and the probability of finding HCC lesions in the late arterial phase of dual phase is higher than that in the early arterial phase [5]. One study showed that the diagnostic sensitivity of multi-stage dynamic spiral CT scan for small hepatocellular carcinoma was 97.5%-97.6%, while the diagnostic sensitivity for microscopic hepatocellular carcinoma (≤1 cm in diameter) was 90%-95% [6]. In clinical practice, digital subtraction angiography (DSA) and iodine oil CT can be performed for lesions that cannot be clearly identified, or for those with high clinical suspicion of HCC but no positive findings on CT enhancement. DSA is an imaging method that reduces and eliminates structures other than the angiographic vessels and highlights the angiographic images of the organ being imaged. Since the majority of liver cancer lesions are blood-rich lesions, intrahepatic tumor staining will be seen on DSA. The angiographic manifestations of hepatocellular carcinoma are mainly tumor vessels and tumor staining, which are characteristic of small hepatocellular carcinoma. Large hepatocellular carcinoma lesions may be associated with arteriovenous fistulae, i.e. portal vein staining in the arterial phase. Selective hepatic arteriography is an effective method for diagnosing hepatocellular carcinoma, especially for small hepatocellular carcinoma with high sensitivity. Angiography not only plays a diagnostic role, but also allows for immediate chemoembolization at the time of angiography for some patients who are not suitable for surgery. Iodine oil CT is performed at the same time as DSA with direct injection of super-liquefied iodine oil through a contrast catheter and a CT scan 4 weeks after the procedure. Sometimes, some lesions that were not detected by CT-enhanced multi-phase scan and DSA can be detected by iodine oil CT examination (Figure 2, the patient had a recurrence of primary liver cancer 3 years after surgery. (Later, DSA and iodine CT were performed, and a 1 cm diameter tumor staining was seen in the right lobe of the liver during DSA, which was diagnosed as recurrence of primary liver cancer after comprehensive treatment). However, selective hepatic arteriography and iodine oil CT are invasive examinations, so they are not included in routine examinations and should be considered only when the above-mentioned non-invasive examinations cannot achieve satisfactory results. CT is the most common and routine diagnostic method for hepatocellular carcinoma diagnosis, and has an important position in the imaging diagnosis of recurrent hepatocellular carcinoma. CT enhanced multi-phase imaging can show the size, number, morphology, location, boundary, richness of blood supply and relationship with intrahepatic ducts of hepatocellular carcinoma lesions. However, the disadvantage is that it is radioactive, and the imaging resolution, especially soft tissue resolution, is lower than MRI, so the detection rate and sensitivity of small hepatocellular carcinoma are slightly worse than MRI.3. MRI has higher soft tissue resolution, and multi-sequence and multi-parameter imaging can analyze more lesions and obtain more information. Coupled with MRI enhanced multi-phase scanning imaging, MRI has more advantages than CT examination in the detection rate and diagnosis and differential diagnosis of small hepatocellular carcinoma (including subclinical recurrent hepatocellular carcinoma). With the development and advancement of technology, lesion details are more clearly displayed, images are sharper, scans are faster, and with the application of hepatocyte-specific contrast agents, the detection rate of small hepatocellular carcinoma is greatly improved. In addition, MRI can clearly display intrahepatic vascular and bile duct structures, which is helpful for understanding the relative position and relationship between tumors and intrahepatic vessels and bile ducts, and for the diagnosis of hepatocellular carcinoma vascular and biliary invasion (Figure 3). In recent years, the research and development of liver-specific MRI contrast agents has progressed rapidly. The liver-specific contrast agents that have been developed for the examination of liver lesions can be generally divided into two categories: one is hepatobiliary-specific contrast agents that can be specifically taken up by hepatocytes and excreted through the bile duct system, which can reflect the functional status and perfusion characteristics of hepatocytes and facilitate the detection and characterization of lesions. The other type of contrast agent is iron oxide particles that can be specifically taken up by reticuloendothelial cells and are most taken up by the liver in vivo, also known as superparamagnetic iron oxide enhanced imaging. MRI with new contrast agents can provide more information about lesion morphology, blood supply, cell source and function in addition to the information available from conventional MRI enhancement scans, thus improving diagnostic sensitivity and accuracy. In recent years, due to the development of MRI technology, the sensitivity, specificity and accuracy of detecting liver occlusions, especially microscopic lesions, are significantly higher than those of spiral CT scan, which is beneficial to the early diagnosis of small liver cancer. With the continuous research on MRI contrast agents, new contrast agents with higher sensitivity for small hepatocellular carcinoma will be developed and the sensitivity of MRI for early diagnosis of small hepatocellular carcinoma will be further improved. However, the disadvantage of MRI is that it is contraindicated in patients with metallic bodies.4 Radionuclide imaging has been less used in the diagnosis of HCC in recent years due to the advancement of other non-nuclear imaging techniques. Posi- tron emission tomography (PET) CT, as a new imaging technique, is playing an increasingly important role in the diagnosis, prognosis and efficacy of HCC due to its functional imaging features at the molecular level. The combined application of multiple tracers can improve the sensitivity and specificity of PET-CT examination for the diagnosis of hepatocellular carcinoma. However, the literature [9] shows that nuclide imaging technique is less sensitive for the diagnosis of small and highly differentiated hepatocellular carcinoma, and more sensitive for larger hepatocellular carcinoma and metastatic carcinoma. It is currently not used as a routine imaging diagnostic method for hepatocellular carcinoma. In conclusion, comparing various imaging methods for liver cancer diagnosis, color ultrasound is the first choice for follow-up after treatment and early detection of recurrence because it is inexpensive, easily accessible and convenient to operate. MRI has a high soft tissue resolution and a high lesion detection rate. With the development of technology and the research and development of new contrast agents, the diagnostic sensitivity and specificity of MRI for hepatocellular carcinoma, especially small hepatocellular carcinoma, have exceeded those of CT examination, and currently, more clinicians choose enhanced MRI examination first for patients with hepatocellular carcinoma [10]. In order to reduce the risk of missed diagnosis due to ultrasound examination, if AFP is abnormally elevated but not detected by ultrasound examination, further CT or MRI examination is generally recommended. Even if AFP is not abnormally elevated, MRI or CT examination is recommended routinely once a year. Radionuclide imaging plays an important role in the diagnosis and treatment of hepatocellular carcinoma, but it is less sensitive in the diagnosis of small hepatocellular carcinoma and is not used as a routine examination. In the face of many evolving imaging methods, clinicians should consider the clinical diagnostic needs and select appropriate imaging methods to improve the early diagnosis and early treatment of liver cancer patients so as to further improve the survival rate and prognosis of liver cancer patients.