A neonate born at 36 weeks period of gestation was found to have a cystic lesion in the posterior fossa (more than 10 mm) on routine cranial ultrasound and MRI was performed for further evaluation of the cystic lesion (cisterna magna/retrocerebellar arachnoid cyst) and rule out any congenital malformations.
Well-defined T2 high, T1 low and FLAIR suppressed (CSF attenuating) lesion seen in the left paramidline location in the posterior fossa. No flow voids were seen within the lesion. The lesion showed the mass effect in the form of scalloping of the adjacent inner calvarium and left cerebellar hemisphere. However, there is no evidence of the abnormal signal intensity within the adjacent cerebellar parenchyma. Fourth ventricle, vermis and cerebellar hemispheres appear normal. On CSF flow study, there is no evidence of flow within the cyst in both magnitude as well as phase images.
Flow of cerebrospinal fluid (CSF) in the brain has primarily two types of motion, namely bulk flow and pulsatile flow. Bulk flow is due to the continued hydrostatic pressure difference between the choroid plexus (from where the CSF is produced) and arachnoid granulations (where CSF is absorbed). There is continued slow flow of CSF from the ventricles to the subarachnoid spaces. The other flow pattern known as pulsatile flow depends on the pulsations in the choroid plexus and subarachnoid portions of cerebral arteries which are congruent with the cardiac cycle. It results in backward flow during cardiac systole and forward flow during cardiac diastole. Unlike the bulk flow, this pulsatile flow can be measured by the technique called Phase contrast MRI (PC-MRI). [1-3]
The basic principle of this technique is to create the bipolar gradient in moving and stationary tissues in the selected field of view. Spins moving in a same direction of the gradient will have the positive net shift whereas the spins moving in an opposite direction of the applied gradient will have the negative shift. Velocity encoding (VENC) is one of the crucial steps in the process and its selection depends on the fluid of the study. It should be slight less or equal to the velocity of the fluid which is to be measured. It is taken as approximately 10 cm/s for CSF traversing through the foramen magnum and 8 cm/s for the CSF through aqueduct. The magnitude image represents the CSF flow as a bright signal in the dark background which means that stationary tissue represents black signal and moving tissue represents bright signal. Phase image shows the forward flow and backward flow which is represented as white and dark signal respectively. The major disadvantage is its total time for obtaining the images (approximately 30 minutes) which is one of the issues in the paediatric population. Other common indications of CSF flow study is to evaluate cases of normal pressure hydrocephalus, syringomyelia, chiari malformation, communicating and non-communicating arachnoid cyst as well as after neurosurgical procedures. 
The main challenge in the diagnosis of posterior fossa cysts in the paediatric age group is differentiating between arachnoid cysts from mega cisterna magna. Mega cisterna magna is the enlarged cisterna magna (more than 10 mm in mid-sagittal plane) with intact cerebellar vermis and normal fourth ventricle. It freely communicates with the subarachnoid space in contrast to the arachnoid cysts. However, both arachnoid cysts and mega cisterna magna can produce scalloping of the inner calvarium and same signal intensity (T1 low, T2 high, FLAIR suppressed and no restriction in DWI) in MRI. Some differentiating features in MRI are the presence of the free falx cerebelli and posterior dural folds can suggest no mass effect in cisterna magna in contrast to arachnoid cysts. In CSF flow study, normal CSF flow can be seen within the cisterna magna with bright and black signal in phase images due to forward flow and backward flow depending on the diastole and systolic phases of cardiac cycle respectively. However, no flow is detected within the arachnoid cysts in the posterior fossa in the magnitude and phase images in PC-MRI. The treatment of the arachnoid cysts depends on location, size and presence of the neurological symptoms. [4-7]
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 Geres MM, Ozkurt H (2018) Is it an arachnoid cyst or a mega cisterna magna? What to and where to look for to make the correct diagnosis? European Congress of Radiology. Available at: https://posterng.netkey.at/esr/viewing/index.php?module=viewing_poster&task=&pi=143050
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