The vast majority of these are usually unilateral and solitary, sometimes multiple or bilateral (Fig. 4.1). It is believed that the development is due to a progressive dilation of the Bowman’s capsule or the contorted proximal tubule, so they have a cortical origin.

They can spread everywhere throughout the renal tissue. The causes of the onset are not certain, although it is possible to assume damages to the basement membrane of the distal tubules. Other causes can be medullary interstitial fibrosis and ischemic renal parenchymal damages. They are more frequent in males than in females, particularly in middle-aged people; the frequency increases in the elderly population, particularly in males [5, 6]. On the contrary, they are much less frequent in the pediatric population.

The classic simple renal cyst demonstrates well-defined features. They have a completely anechoic content, round/oval shape, and sharp, thin posterior walls (Fig. 4.2a); an imperceptibly thin wall with no enhancement, delimited by a thin layer of fibrous tissue covered by epithelium single layer, is undetectable when the cyst is parenchymal.

It is important not to confuse the cysts with cortical medullary pyramids, taking hypoechoic appearance especially in the young (Fig. 4.2b).

The cysts generate the formation of artifacts due to the interaction of the ultrasound beam with the substrate analyzed; the ultrasonic beam crossing the cyst is neither attenuated nor absorbed and nor reflected, contrary to the surrounding tissues. The tissues located deeper than the fluid-filled structure emit signals that are reinforced, in comparison with the surrounding tissues. It follows the artifact of the increased through transmission (Fig. 4.3a). For this reason the back wall of the cyst appears thicker than how it really is. Another artifact is the lateral edge or acoustic shadows: thin acoustic shadow that appears behind edges of cystic structures (Fig. 4.3b). The speed of the ultrasounds in the fluid is faster than the propagation speed of the parenchymal tissues around the cystic formation. The matching between ultrasounds and the lateral profile of the cyst causes a refraction that makes the cystic lesion behave like an acoustic lens. This causes the formation of lateral acoustic shadows: the phenomenon of the refraction along the sidewalls. Anechoic renal cysts may show some artifact internal low-level echoes [7]. This may be improved by using harmonic imaging techniques.

Complications are rare with a reported range of 2–4 %. The most common complications are hemorrhage, infections, or rupture. Generally, the increase of the inner content echogenicity relates with hemorrhagic complication (Fig. 4.4).

A sudden intracystic hemorrhage can clinically occur with pain, and sometimes it is possible to document a fluid-fluid level of the content, with layers of echoes because of the concentration of the protein components deriving from intracystic bleeding.

Hemorrhage generally affects only one cyst, but frequently it is possible to notice multiple cysts with hemorrhagic content in the same kidney. Those with a diameter less than 3 cm, if homogeneous and without calcifications, can be regarded as lacking in particular clinical implications. Sometimes they develop residual calcification in a central pattern or within the cystic wall that becomes thickened and develops septa and the cyst becoming multilocular or multilobular, acquiring the features of a complex cyst (Fig. 4.5) [1, 8].

Complex or atypical cysts differ in shape and content from simple cysts. Sonographically, the criteria for a simple cyst are (1) an anechoic lesion, (2) sharply defined walls, and (3) acoustic enhancement behind the lesion. Complex or atypical cyst is defined as any deviations from at least one of these standards. A complex cyst includes irregular walls or non-anechoic content or calcifications or diffuse or focal low-level echoes [3, 9].

Oval, polygonal, or elongated shape is suspicious for a complex cyst (Fig. 4.6). As hemorrhagic cysts resolve, they develop residual calcification in a central pattern or within the cystic wall that becomes thickened and develops septa (Fig. 4.7) with the cyst becoming multilocular or multilobular; also the wall of infected cysts is often thickened markedly and calcified occasionally and has parietal irregularities such as endocystic vegetations [1–10] (Fig. 4.8). They have intrinsic echo content and sometimes clots that are indicative of bleeding.

The definition of complex cyst is made by the presence of one or more atypical aspects. Furthermore, the septum vascularization on color Doppler is suspicious for cancer. Also CEUS may be useful to show vascularity of septa or nodular protuberances in a renal cyst and can help differentiate a benign cyst from an indeterminate cyst or a malignant-appearing cyst [11]. During US examination, underlining this kind of aspects can raise suspicion of the presence of a malignant lesion. Therefore, a correct evaluation of the cystic lesion is essential in order to recommend an in-depth diagnosis by means of CT or MRI exams.