Renal cystic lesions (also known as cystic nephropathy) are a group of diseases that manifest as single or multiple fluid-containing cystic foci in the kidneys. With the rapid development of medical imaging, the detection rate of renal cystic lesions has increased significantly after the application of high-resolution B-mode ultrasound and CT (especially multi-row spiral CT). Currently, renal cystic lesions have become the most common renal disorders in clinical practice, with the highest incidence of simple renal cysts, followed by polycystic kidneys, and then less common cystic lesions (including multicompartmental cystic nephroma/carcinoma) Bosniak Classification of Renal Cystic Lesions Twenty-five years ago, in order to more accurately evaluate and diagnose renal cystic lesions, Dr. Morton A. Bosniak proposed the Dr. Morton A. Bosniak proposed the well-known Bosniak classification of renal cysts 25 years ago in order to more accurately evaluate and diagnose cystic kidney lesions. After years of clinical practice and updating, the criteria have been gradually improved, and the criteria are classified into four types of renal cystic lesions based on imaging characteristics: Type I, simple renal cysts, with clear edges of the cyst wall, rounded, uniform density and CT value <20Hu, thin cyst wall and no enhancement, and no segregation or calcification within the cyst; Type II, mildly complex renal cysts, with mild irregularities in the edges of the cyst wall, accompanied by calcification, and small segments <1 mm in thickness; and Type II, mildly complex renal cysts, with irregular edges of the cyst wall, accompanied by calcification, and small segments <1 mm in thickness. Type II is a mildly complicated renal cyst with mildly irregular cyst wall margins, calcification, and small septations <1 mm thick, smooth septations without thickening or enhancement, or high-density cysts with uniform density, no enhancement, and a diameter of <3 cm, including type IIF, which is a high-density cyst with a slightly thickened septum or cyst wall that is not categorized as type II or type III, with a diameter of >3 cm, which requires 1-4 years of follow-up according to the degree of abnormality of the lesion. Type III is a complex cyst with uneven density, measurable enhancement, intracapsular septal thickness ≥2 mm, and wall nodules and irregular calcifications; type IV is a cystic renal carcinoma or other malignant lesion with a large cyst or necrotic component, with irregular thickening of the cyst wall and enhancement of the wall nodules. It is generally believed that type III and type IV lesions require surgical resection, but some retrospective studies have found that some surgically resected type III lesions are benign. There is some controversy over the Bosniak classification, which is highly subjective, and the differences between the evaluation criteria for each type are not well defined, ultimately leading to a wide variation among imaging evaluators. Experience and follow-up of the pathology of the specimens can help to minimize these issues, and the Borniak classification is more commonly used in the CT evaluation of cystic renal lesions, whereas gray-scale ultrasound or color Doppler ultrasound of cystic renal lesions do not allow for direct application of the Borniak classification criteria. Ultrasonography is important in the evaluation of some lesions that appear hyperdense or pseudo-enhanced on CT, and can differentiate type I and II (benign lesions) from type IIF, III, and IV (suspected malignant or malignant lesions). Some reports suggest that 59% of type III renal cystic lesions are malignant. In conclusion, Bosniak typing is important in the diagnosis of complex renal cysts and cystic masses and in guiding the principles of treatment, helping countless patients avoid unnecessary surgery. Diagnostic value of ultrasound Complex renal cysts are usually first detected by ultrasound, while ultrasound is also commonly used for further evaluation of cystic renal lesions detected by other imaging methods. Ensure that the optimal anatomic sound window is used to obtain the best ultrasound image of the kidney, and ultrasound is usually more effective in imaging the right kidney. Color Doppler technology is valuable in determining the benign or malignant nature of renal cystic lesions, as it can accurately distinguish the presence of blood flow within the lesion. In addition, tissue harmonic imaging and transvenous microbubble ultrasound contrast are useful in characterizing renal cystic space-occupying lesions, and the use of contrast-enhanced second-harmonic ultrasound has been shown to be useful for Bosniak staging of renal cystic lesions in a study by Ascenti et al. Ultrasound imaging of a typical simple renal cyst usually shows an echo-less area with smooth and neat margins, with posterior acoustic enhancement and a curved image of strong echo-reflections in the cyst wall. Simple renal cysts with intracellular hemorrhage or proteinaceous components may appear as hypoechoic areas or layered echoes. Intracystic segregation can produce strong posterior echoes and may result in intracystic phonocardiographic antiphonal artifacts. Calcified components of the cyst may produce a posterior acoustic shadow, resulting in failure to visualize the remainder of the cyst. The typical ultrasound presentation of a solid component within a cystic renal lesion is moderately echogenic with no posterior enhancement effect of the lesion. Color Doppler ultrasound technology is helpful in determining the solid component within cystic lesions. Ultrasound imaging is particularly well suited to guide interventional treatments (e.g., radiofrequency ablation and cryotherapy) for cystic renal space-occupying lesions, and can also be used for intraoperative evaluation of cystic renal lesions. Ultrasound can accurately localize and directly treat lesions, minimizing damage to residual renal tissue. Diagnostic value of CT CT is the main method for imaging and evaluation of renal cystic lesions, especially MSCT (Multi-Slice CT) which can provide sub-millimeter spatial resolution, and combined with contrast-enhanced scanning technology, MSCT is widely used in the evaluation of renal cystic lesions.The introduction of MSCT has also made the CT imaging of renal cystic lesions more complicated than before, and even has to be set up according to the manufacturer. The settings must be customized according to the manufacturer. Of course, some scanning principles must be adhered to, such as maintaining the same exposure parameters, field of view (FOV), and layer thickness in the plain scan, arterial phase (30 s delay), and nephrogram phase (70-80 s delay), to ensure the accurate assessment of the degree of enhancement of the lesion; in addition, the smaller image layer thickness (<1 mm) and the overlap of reconstructed images (overlap of about 20%-50%) can reduce the volume effect, which of course, will result in a larger image data volume and a larger image size, and will lead to the increase of image data volume and the increase in image size. In addition, smaller image layer thickness (<20%-50%) and overlap of reconstructed images can reduce some of the volume effect, but of course, they also result in a larger amount of image data and radiation intensity. Although MSCT can be used to evaluate renal cystic lesions using image reconstruction techniques (including multiplanar reconstructive MPR, volumetric reproduction VR, etc.), it is uncertain whether these reconstruction techniques can actually improve the accuracy of diagnosing renal cystic lesions. The enhancement characteristics and morphologic features of a lesion on CT images determine the benign or malignant status of the lesion.The enhancement characteristics of a lesion on CT images are determined by its histologic characteristics, especially whether it is rich in blood vessels or not.For example, the implementation component of multicompartmental cystic renal carcinoma or renal cell carcinoma cystic lesions usually exhibits transient marked enhancement, and papillary renal carcinoma usually exhibits mild enhancement. The peak of enhancement may be earlier (e.g., arterial phase) or delayed (e.g., nephrogram phase) with the type of solid structural cells within the cystic lesion in the kidney. Determining whether a lesion is enhancing or not requires reference to the plain image; cystic lesions with no enhancement (or enhancement <10 HU) are basically considered to be avascular or avascular and can be diagnosed as simple renal cysts. Enhancement >15 HU within a cystic renal lesion implies the presence of a solid lesion, and although the lesion may be malignant (multilocular cystic renal carcinoma or renal carcinoma cystic lesion), there is also the possibility of a cystic angiomyolipoma, a large eosinophilic nephroma, or an infectious lesion. An increase in the CT value of a cystic lesion in the range of 10-15 HU after intravenous contrast injection is suggestive of a neoplastic lesion, and this change in CT value may also be due to volume averaging, ROI (region of interest) placement, respiratory motion, or artifactual enhancement (discussed in more detail below). The most common renal cystic lesion is the simple renal cyst, which has a prevalence of 10% in adults and increases in size and number with age, with a prevalence of up to 27% in people over 50 years of age. CT images of simple renal cysts have significant features, usually presenting as a well-defined sharp-margined lesion in the renal parenchyma with no wall or obvious septum, the lesion is watery density with a CT value of less than 20 HU, intra-lesional hemorrhage or infection may lead to increased density or thickening of the wall of the cyst; enhancement of arterial and nephrographic stages of simple renal cysts in enhanced CT images is less than 10 HU. despite the fact that after injection of contrast medium Despite the fact that simple renal cysts do not show enhancement after contrast injection, the effect of many factors may result in an increase in the measured CT value of the cyst after enhancement (usually less than 10 HU), which is known as pseudo-enhancement of renal cysts. Partial volume effects in the dense renal parenchyma surrounding the renal cyst or adjacent dense tissue structures during enhancement scanning can lead to pseudointensification. In addition, smaller renal cysts (<1 cm in diameter), cysts located within the renal parenchyma, CT scans using a higher number of detector rows, and higher tubulo-spherical voltages are more likely to lead to pseudointensification of renal cysts. Imaging Evaluation of Renal Cystic Lesions Calcifications Compared to ultrasound, which is susceptible to acoustic shadows and reflections from calcifications in the evaluation of calcific renal cystic lesions, CT plain scanning is more advantageous for the visualization of the calcific component of the lesion. Benign calcific foci usually appear as a small number of smooth-margined calcified foci distributed in the cystic wall or septum, and the presence of a papilla within a cystic lesion is also considered to be a benign indication. Surgical excision of cystic lesions should be considered when there is enhancement, nodularity, or thickening of the cyst wall around the calcifications. High-density cysts Cystic lesions of the kidney with a CT value greater than 20 HU on plain scan are called high-density cysts, and most of them are caused by cystic hemorrhage, hemolytic products, high protein content, or colloid-like substances. If the margins of a high-density cyst are smooth and sharp, the density within the lesion is homogeneous, and no enhancement of the lesion is seen on enhancement scans usually suggests benignity, and ultrasound imaging is usually only suggestive of a cystic lesion. High-density cysts with non-smooth margins, non-uniformity of internal density on plain or enhanced scans, significant enhancement on enhanced scans, or a realistic component on ultrasound suggest malignancy and require surgical treatment. Regular follow-up is required for homogeneous high-density cysts >3 cm in diameter. Compartmentalization Intra-lesional compartmentalization of cystic lesions can be produced by healing or mechanization after hemorrhage or infection; compartmentalization can be produced by two adjacent cystic lesions sharing the cystic wall; CT is the best method to evaluate the curved calcification foci on the compartmentalization of the lesion, and ultrasonography is better than CT for thin compartmentalization. If the septum is thickened (>2 mm), irregular, nodular or markedly intensified, it is malignant and requires surgery; smooth but slightly thickened septum in the capsule (1-2 mm) suggests that the lesion is benign, but needs to be followed up for observation. A cystic lesion in the kidney with more segments (>3) is called a multilocular cystic lesion. The most common multilocular cystic lesions in the adult kidney are multilocular cystic nephroma and renal cell carcinoma cystic lesion (multilocular cystic renal carcinoma), which are usually common in females, while multiple cystic renal carcinomas are more common in males. Cyst wall thickness The cyst wall of solitary renal cysts is characterized by a thin cyst wall without nodules, which can only be revealed on enhancement scans, and the thickness of the cyst wall cannot be measured accurately if the cyst is located entirely within the renal parenchyma. Cystic renal carcinoma is usually characterized by localized thickening or nodular changes of the cystic wall, with a thickness of >2 mm; however, it is difficult to detect cystic wall thickening in small cystic lesions. Cyst wall thickening can also be seen in non-neoplastic lesions, including cyst infections, abscesses, cyst hemorrhage, pancreatic pseudocysts, and pseudoencapsulation of hematomas. Intermittent multiple nodules in the cyst wall are another sign of cystic renal carcinoma, and the fluid in the cyst provides an acoustic window that allows ultrasound to visualize small cyst wall nodules. Simple renal cysts are easily diagnosed by ultrasound and CT scan based on their characteristic imaging. Any other fluid-containing renal space-occupying lesions that do not meet the diagnostic criteria for simple renal cysts require a differentiation between benign and malignant lesions. On ultrasound and CT images, the majority of benign lesions should be definitively diagnosed by careful analysis of calcification, density, and segregation of renal cystic lesions, and some of these lesions require close follow-up. Renal cystic lesions with multicompartmental renal occupations, significant enhancement, and nodularity or segregation in the cystic wall or septum require surgical resection, meticulous histologic analysis, and final diagnosis.