Simple Renal Cysts
The occurrence of simple renal cysts is dependent on the patient’s age. Simple renal cysts mostly originate from the renal cortex, are round, demonstrate a smooth thin wall and do not exhibit any septa. There are often multiple cysts in affected kidney parenchyma and cysts can be bilateral.
On T1-weighted images, kidneys show homogeneous low SI. On T2-weighted images, there is homogenous high SI. The cyst wall should be smooth and almost not appreciable. After intravenous injection of contrast agent, the cyst must not enhance. Peripelvine renal cysts are mainly differentiated by their more variable shape. They have, however, the same signal characteristics. One can distinguish a peripelvine renal cyst from an ampullary renal pelvis simply by looking at post-contrast T1-weighted images, in which peripelvine renal cysts do not enhance.
Simple renal cysts can turn into complex cysts by hemorrhage, infection, rupture, calcification, or the occurrence of an intracystic malignant tumor. Hemorrhage in renal cysts is mostly secondary to hemorrhagic diathesis and trauma but in most cases of unknown origin. Depending on the age of the hemorrhage, the SI of the cysts is very different. Even within a single cyst, there can be a signal gradient due to sedimentation of the degradation products of the hemoglobin. The dependent part of the cysts shows a higher SI in the T1-weighted sequences and a lower SI on the T2-weighted sequences, respectively.
|Bosniak category||Name||Imaging features||Malignant lesions (%)|
|I||Simple cyst||Hairline-thin wall, no septa, No contrast enhancement, No calcifications||1,7|
|II||Probably benign cyst||Hairline-thin wall, few thin septa, Fine calcifications, No contrast enhancement||18.5 (II and IIF)|
|IIF||Indeterminate cyst||More septa, Minimal contrast enhancement, Minimal wall thickening, Nodular to thick calcifications||18.5 (II and IIF)|
|III||Probably malignant cyst||Irregular walls, Enhancing septa||33,0|
|IV||Malignant cyst||Enhancing soft tissue masses||92,5|
During the further temporal course, a hemorrhagic cyst may behave like a protein rich fluid with high SI on the T1-weighted images. MR imaging does not allow safe differentiation between a hemorrhagic cyst and a hemorrhagic renal cell cancer based on the signal intensities on T1-weighted and T2-weighted images. Contrast-enhancement of the cyst wall, which can be appreciated much better with MRI than with CT, suggests potentially malignant tissue. To visualize even faint contrast uptake, subtraction techniques are suitable and make MRI superior to CT in the detection of malignant lesions.
Cysts with partially enhancing wall or nodular components should be closely followed or even surgically removed. Infection of renal cysts can occur spontaneously, during a bacteremia or after the puncture of the cyst. Morphologically, a thickened or nodular appearance of the cyst wall can be appreciated. Due to the higher protein content and the resultant increased relaxivity, infected cysts may also exhibit higher SI than normal cysts on T1- weighted images.
Calcification of the cyst wall may be due to hemorrhage or infection. In contrast to the irregularly distributed calcifications of renal cell carcinoma, calcifications of cyst walls are typically located in the periphery of the cyst. The calcifications cannot be directly visualized using MRI. Rarely, they can be indirectly seen as signal voids.
Calcifications are a minor characteristic in the Bosniak cyst classification and are often used in CT. In studies comparing CT and MRI with regard to the assessment of cystic lesions according to the Bosniak classification (Table 1), a slightly higher sensitivity for MR has been reported. Seven out of 69 lesions were upgraded due to the higher soft tissue contrast of MRI, which helps in depicting a greater number of thin, hairline septa and allows better appreciation of enhancing wall structures.