Simple cysts are the most common renal lesions in adults. The incidence, number, and size of renal cysts increase with age, with cysts reported in up to 27 % of patients greater than 50 years of age (Laucks and McLachlan 1981; Tada et al. 1983). Renal cysts can be solitary, multiple, or bilateral, and they commonly arise in the renal cortex. They are benign asymptomatic lesions, most of which are incidentally discovered on ultrasound or CT examinations.

On ultrasound, simple cysts classically present as round, anechoic lesion with acoustic enhancement and no appreciable wall thickness. On CT imaging, simple cysts are round with no perceptible walls and are well demarcated from the adjacent renal parenchyma (Fig. 1). The CT attenuation values of renal cysts are typically less than 20 HU (water attenuation). On MR imaging, simple renal cysts present with the same morphological features as demonstrated on CT. They appear very bright on T2-weighted images and maintain homogeneous signal void on T1-weighted images (Fig. 2) (Hartman et al. 2004).

The absence of contrast enhancement is critical to differentiate renal cysts from renal tumors with a cystic component. Although renal cysts should not enhance after administration of intravenous contrast material (Thomsen et al. 1997), some cysts may appear to enhance (pseudo-enhancement). This is because of multiple technical and image-related factors that are directly associated with the presence of iodinated contrast material within adjacent renal parenchyma (Fig. 3) (Bae et al. 2000; Coulam et al. 2000; Maki et al. 1999). The degree of pseudo–enhancement in CT is typically less than 10–20 HU but varies with multiple factors (size, location of cysts, CT scanners, reconstruction algorithm) (Bae et al. 2000; Birnbaum et al. 2007; Coulam et al. 2000; Wang et al. 2008; Tappouni et al. 2012). In addition to pseudo-enhancement, small cysts deep in the renal parenchyma may be subjected to a partial volume averaging with adjacent contrast-enhanced renal parenchyma and may appear to enhance (Silverman et al. 2008). To minimize the partial volume averaging effect on the enhancement assessment, it is critical to use appropriate image slice thickness; typically the thickness should be less than the diameter of the lesion.

Recent publications reported that dual-energy CT is useful in differentiating nonenhancing cysts from enhancing lesions on the basis of selective iodine display techniques (also known as color–coded iodine overlay or iodine–density images) (Graser et al. 2010; Mileto et al. 2014; Ascenti et al. 2012, 2013). In these techniques, the iodine signal of enhancing lesions is directly determined and visualized on a single-phase image dataset, whereas nonenhancing cysts lack a detectable iodine signal. Since renal cysts are avascular, they are devoid of iodine signal on iodine-specific images. In contrast, renal lesions enhanced by iodine uptake would show an iodine signal on color-coded iodine images (Fig. 4) (Graser et al. 2010; Mileto et al. 2014; Ascenti et al. 2012, 2013). In addition, iodine content (in milligrams of iodine per milliliter) can be quantified from the iodine-specific images to potentially improve the accuracy for the assessment of lesion vascularity (Mileto et al. 2014; Chandarana et al. 2011; Ascenti et al. 2013). It has been proposed that an iodine concentration threshold of 0.5 mg/mL more accurately differentiates enhancing solid renal lesions from nonenhancing cysts on dual-energy CT than does the conventional approach for the assessment of lesion enhancement on single-energy images (Mileto et al. 2014; Ascenti et al. 2013). However, this proposed threshold method is based on a single institutional experience with a single vendor CT scanner. Further studies would be required to test the generalizability of this specific threshold value in different vendor CT scanners.

Pseudo-enhancement may be also observed in contrast-enhanced MR imaging. Various features found in MR imaging, including motion artifacts, inherent noise, and partial volume averaging, can cause the appearance of enhancement (Ho et al. 2002). The optimal percentage of enhancement thresholds for distinguishing cysts from malignancies is reported to be 15 %, with the optimal timing for measurement at 2–4 min after the administration of contrast material (Ho et al. 2002). Some researchers advocate the review of the subtracted images (enhanced minus unenhanced MR images) for the detection of subtle contrast enhancement within renal lesions (Hecht et al. 2004) (Fig. 5).

Simple renal cysts may be complicated by internal hemorrhage, infection, or other processes that may appear on imaging with complex radiologic features suggestive of cystic renal neoplasm. On US imaging, complex cysts may present with internal echo, septum, wall thickening, calcification, and acoustic shadowing. Similarly, heterogeneous morphological features of complex cysts are delineated and characterized on CT and MR imaging. Some of the findings of complex cysts, such as irregular wall thickening, dense calcification, and multiple septa, can also be seen in cystic renal neoplasm (Hartman et al. 2004). A classification system of renal cystic lesions on CT, the Bosniak Classification of Renal Cystic Disease, grouped renal cysts into five categories based on imaging appearance in an attempt to predict the risk of malignancy and determine the risk and management of complex cysts and cystic neoplasm (Bosniak 1986). This will be discussed further in “Cystic Renal Masses.”

Complex cysts may present as hyperdense cysts (attenuation greater than water) in CT due to internal hemorrhage or proteinaceous content (Sagel et al. 1977). Since hyperdense cysts show no significant contrast enhancement, they can be differentiated from renal neoplasms based on the assessment of enhancement. However, hyperdense cysts discovered incidentally at CT with only a single phase of contrast enhancement may not be easily distinguishable from solid renal masses solely based on CT attenuation. A study reported that on portal venous phase contrast-enhanced CT scans, attenuation greater than 70 HU and moderate or marked internal heterogeneity favor a diagnosis of renal cell carcinoma (RCC) over a diagnosis of hyperdense renal cyst (Suh et al. 2003).

Nevertheless, when a cystic lesion that does not meet the criteria for a simple cyst is detected on a routine enhanced CT, patients are usually referred to either (1) an ultrasound imaging (which is often indefinite, especially if the attenuation of the lesion is greater than +40 HU due to the containment of blood or proteinaceous debris), (2) an additional delayed CT scan to assess lesion “de-enhancement”, or (3) a repeat CT or MRI scan that includes both unenhanced and contrast-enhanced imaging. We may be able to eliminate these additional tests with the use of dual-energy CT. Particularly, when a questionable high-attenuating renal lesion (>30 HU) is detected on contrast-enhanced images without available unenhanced or delayed CT images, dual-energy CT offers an additional opportunity to improve the differential diagnosis between benign high-attenuation cysts (Bosniak category II) and solid enhancing lesions (e.g., renal cell carcinoma).

Recent data (Mileto et al. 2014; Silva et al. 2011) suggest that the creation of spectral attenuation curves – which are based on the attenuation analysis across a spectrum of synthesized monochromatic energies – can help guide toward the correct diagnosis. Notably, enhancing renal lesions on single-phase nephrographic images can be differentiated from nonenhancing high-attenuation cysts by means of spectral attenuation curves in that iodine attenuates greater at lower energies, while cyst attenuation remains flat at a broad range of CT energies (Figs. 6 and 7) (Mileto et al. 2014; Silva et al. 2011).

MR imaging is useful to evaluate complex cystic renal masses. Hemorrhage or protein within complex cysts may cause T1 shortening and present with higher signal intensity than water on T1-weighted image (Brown and Semelka 1995; Marotti et al. 1987) (Fig. 2). Hemorrhagic cysts may show variable signal intensities on T2-weighted images, depending on the stage of blood products (Brown and Semelka 1995; Hilpert et al. 1986; Marotti et al. 1987). Although the presence of calcification within renal lesions is difficult to identify with MR imaging, the same morphological features can be used to characterize complex cystic lesions, as with CT. Lack of gadolinium contrast enhancement is an important MR feature to differentiate complex cysts from renal cystic neoplasms. Since complex cysts are typically hyperintense on T1-weighted images, a careful comparison between the pre- and postcontrast MR images and the use of subtraction technique are often required to accurately assess the enhancement within complex cysts (Fig. 5) (Brown and Semelka 1995; Hilpert et al. 1986; Marotti et al. 1987).

Extraparenchymal cysts commonly occur in the renal sinus. These cysts are usually called parapelvic or peripelvic cysts because of their locations adjacent to the renal pelvis (Amis and Cronan 1988; Rha et al. 2004). Although parapelvic cysts (originating in the renal parenchyma and extending into the renal sinus) and peripelvic cysts (originating in the sinus presumably from lymphatic obstruction) differ in their origins, they are radiographically indistinguishable and are described interchangeably. These cysts are often discovered incidentally and are frequently multiple in nature. They replace the renal sinus fat and may displace or compress adjacent structures, including the renal pelvis and calyces, parenchyma, and vessels (Schwarz et al. 1993).

Renal sinus (parapelvic or peripelvic) cysts are benign and do not require treatment or imaging follow-up. Although these cysts do not communicate with collecting system, they may simulate hydronephrosis because of their proximity to the collecting system. One useful sign for the differentiation of renal sinus cysts from hydronephrosis is that whereas renal sinus cysts are centered predominantly in the renal hilum with relatively sparing of the upper and lower apexes of the renal sinus (Fig. 8), hydronephrosis is usually evident throughout the entire renal sinus (Fig. 9). When the diagnosis is still uncertain, postcontrast delayed CT and MR images can help differentiate unenhanced renal sinus cysts (which compress or displace enhanced collecting system) from a contrast-enhanced dilated pyelocalyceal system (Fig. 10) (Morag et al. 1983; Nahm and Ritz 2000). It should be noted that the signal intensity of the contrast-enhanced collecting system in MR imaging varies depending on the delayed phases and on the concentration of gadolinium contrast excreted into the collecting system (EL-Merhi and Bae 2004).

Localized cystic disease is a benign condition of unknown pathogenesis characterized by a cluster of simple cysts localized in only one kidney (Slywotzky and Bosniak 2001) (Fig. 11). It is also known as segmental cystic disease of the kidney (affecting only part of the kidney) or unilateral cystic disease of the kidney (affecting the entire kidney). Cysts are limited to one kidney and the contralateral kidney is normal. No association with renal tumor or renal insufficiency has been reported (Brenner and Rector 2008). Differential diagnoses for localized cystic disease include multiloculated cystic neoplasm, such as multilocular cystic nephroma and multiloculated RCC (Hartman et al. 1987, 2004). The multilocular cystic neoplasm usually has a thick capsule that may demonstrate contrast enhancement. The absence of a capsule surrounding the cluster of cysts, the presence of normal parenchyma between the individual cysts, and the presence of cysts outside the cluster confirm the diagnosis of localized cystic disease (Slywotzky and Bosniak 2001). Finally, when presenting in children, autosomal dominant polycystic kidney disease (ADPKD) may be a differential diagnostic consideration and can be further evaluated by phenotype screening of family members or by long-term follow-up imaging (Slywotzky and Bosniak 2001).