In adults, the frequency of pineal cysts found on routine autopsy varies between 20 and 40% [1, 2], but these figures include all cystic transformations of 2 and 5 mm in size. This explains the high prevalence found in autopsy studies compared with imaging studies, which—except in a few reports [8, 9]—use a minimum size of 5 mm. Histologically, the wall of a benign cyst of the pineal gland is ≤2 mm thick and has three distinct layers from the periphery to the centre: an outer fibrocollagenous layer (capsule), a middle pineal cell layer that sometimes contains calcifications, and an inner glial tissue layer [2, 6, 10, 11]. Hypotheses proposed to explain the genesis of a cyst include physiological involution of the pineal gland, the persistence of a pineal diverticulum arising from the pineal recess of the third ventricle, and ischaemic degeneration of an intrapineal glial plaque followed by necrosis and cavitation [2, 10, 12].
It is generally agreed that on MRI the normal pineal gland appears as a solid nodule of tissue in only about 52% of cases [8, 12], and that its appearance can be more crescent-like (26%) or ring-like (22%), a change believed to be a cystic transformation with no pathological significance, provided its largest diameter is less than or equal to 5 mm. Our study took these criteria into account, as only lesions with at least one diameter ≥5 mm were considered to be cysts. Under these conditions, we found a prevalence of 10–11% in both groups.
In MRI studies, the reported frequency of pineal cysts, all age groups combined, is somewhere between 1.2 and 10.8%, depending on both the technical parameters—and thus on the date of the study [6–8, 11, 13, 14]—and the minimum cyst size used, which ranges from 2 to 5 mm . Most of the MRI studies carried out in children involve patients investigated for headaches and/or various neurological disorders (seizures, vertigo, visual problems, mental retardation, etc.) [5, 7, 8], contrary to our study, which only included patients with no neurological signs. Yet we found pineal cysts in very young children—six of the patients were under 3 years old and one of these was under 1 year—in accordance with some previous reports .
The 3D gradient-echo sequences (so called CISS or FIESTA)—which have also been used in other series —allows acquisition of very high contrast resolution images (liquid/solid). It is a sequence well-adapted to the analysis of fluid structures, thanks to its heavy T2-weighting. The well-circumscribed appearance, regular contours, wall thickness less than 2 mm, and especially the lack of nodules that characterise these cysts fit the usual criteria for benign, probably glial, cysts . While the multilocular appearance and internal septa are described in some histopathological studies , the multiple septations found in 74% of the cases in our series have rarely been reported in MRI studies, most of which describe cysts as unilocular [6, 10, 16]. One exception is the recent series by Pastel et al.  who, using a similar sequence, demonstrated the presence of intracystic septa in six patients.
The signal intensity of the cysts in our series is consistent with previous data [6, 10, 13, 18], i.e., a fluid signal subtly hyperintense relative to CSF on T1-weighted images and slightly hypointense relative to CSF on T2-weighted images. We found no haemorrhagic changes, which are still reported only in adults [12, 19]. Some authors have described cysts that are very hyperintense—and hyperintense to CSF—on T2-weighted images [18, 20]. This could be due to the cysts having a higher protein content than the LCR or to the stagnant nature of the cyst fluid [18, 20]. The FLAIR (fluid-attenuated inversion recovery) sequence could have demonstrated the lack of fluid suppression for these cysts, which unlike CSF remain moderately hyperintense .
Several MRI studies in large young adult cohorts have shown a higher frequency of pineal cysts in women in their 3rd decade [7, 13, 14]—as high as 5.8%. More recently, Al-Holou  studied MRIs from a population of children and young adults, and found a significantly higher prevalence among female subjects. The current report did not show any significant sex difference in the prevalence of cysts, but may lack the necessary statistical power (i.e., a larger cohort) to discern it.
We did not use contrast material routinely to explore ISS or precocious puberty in children if unenhanced studies did not disclose pituitary abnormalities, as the additional risk could not be justified by its diagnostic value. However, pineal cysts frequently show thin peripheral contrast enhancement [6, 11, 13, 21–23], sometimes associated with enhancement of the cystic cavity by diffusion of the contrast agent on late images (30 min post-injection). Stability over time—which also argues in favour of a benign cyst [2, 3, 6, 13]—can be appreciated on the heavily T2-weighted 3D GE sequences. Simple cysts can however show atypical features on MRI as enhancing septa or haemorrhagic signal [21, 24].
Precocious puberty may be associated with some malignant tumours of the pineal region (choriocarcinoma and germinoma), and very rarely, if ever, with pineal parenchymal tumours such as pineocytoma or pineoblastomas, which represent two distinct patterns of tumours, well- and non-differentiated [12, 24–26]. Most pineocytomas are described in middle-aged adults [16, 26], but can also be found in children. They are often small (less than 3 cm), and they may show some cystic areas, a purely cystic form being rare .
In conclusion, pineal cysts are a commonplace and incidental finding (11%) in children with no neurological signs. The frequent presence of septations with high-resolution MRI (74%) should be recognised to avoid superfluous controls or IV contrast media.