Cystic malformations of the posterior cranial fossa, current state of classification, CT and MR appearance

The cerebellum has the longest development in comparison with other large structures of the brain, and this may be the cause of many developmental abnormalities. [1,2]. They are divided into two groups: cystic malformations and disturbances of cerebellar histiogenesis process. The paper focuses on the former of the two large groups mentioned above [3,4,5]. Cystic malformations leading to hypoplasia of the vermis and cerebellar hemispheres often cause enlargement of the posterior cranial fossa. The aim of this study is to summarize and systematize the contemporary knowledge of cystic lesions of the posterior fossa, especially their imaging. Imaging of posterior fossa malformations is practically based on computed tomography (CT) and magnetic resonance imaging (MRI). On CT studies, the posterior fossa is imaged with 5mm or 2-3 mm slices to gain more precise visualization. However, CT is often followed by MRI, mainly in T1-weighted, sagittal and axial images, which allow for the best evaluation of the posterior fossa structures. 2D-FISP imaging technique may be also useful. Thorough analysis of the literature allowed to put forward the following conclusions: classification of these cystic malformations is frequently changed; it is necessary to evaluate disorders both in CT and MRI; one should use the descriptive term - retrocerebellar cyst, if an image is not clear-cut; the radiologist's goals include finding a lesion, evaluation of its topography and effect on the brain, but not necessarily establishing its particular type.