Abstract： Objective To evaluate the curative effect of cystocistern fenestration for the treatment of middle cranial fossa cyst in children and the value of post-operative evaluation using phase-contrast MRI.(PC-MRI).Methods 28 patients with intracranial arachnoid cysts (IAC) underwent PC-MRI assessment.Cystocistern fenestration was decided in those who were found to have NCIAC.A retrospective study was carried out analysing the clinical manifestations before and after surgery,postoperative decrease of the cyst size using phase-contrast cine MRI scan in these cases.The fistula flow of cerebrospinal fluid,the cerebrospinal fluid dynamics index (peak velocity,flow and flow wave) were obtained.Results Phase-contrast cine MR imaging showed no communication between IAC and the adjacent subarachnoid space in 20 of 28 patients pre-operation and they underwent cystocistern fenes tration.In these patients,arachnoid cysts reduced in size post-operatively.Furthermore,the clinical symptoms were improved or cured.CSF flow direction in the fistula of the basal cistern were bi-directional,which was similar to normal aqueduct flow waveform,suggesting patent fistula.In the 8 patients who were diagnosed to have communicating arachnoid cyst (CIAC),there was no clinical manifestation and no progressive increase in cyst size.Conclusions 1.Cystocistern fenestration could be an effective treatment of children with middle cranial fossa cysts; 2.PC cine MRI can be used to diagnose and assess post-operative progress of non-communicating IAC.