الفهرس 
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Abstract
Summary
Brain tumours represent the most common solid neoplasm in children and second most common pediatric malignancy following leukaemia.
Approximately 65% of all brain pediatric tumours arise in the posterior fossa.
The commonest tumours are ;pilocytic astrocytoma (commonest),medulloblastoma, ependymoma ,brain stem glioma,atypical tyratoid rhabdoid tumour (ATRT),haemangio-blastoma(uncommon except with VHL), teratoma(in infant).
Earlier diagnosis of brain tumours in children and young adults improve long term outcome.
Conventional MRI is an essential tool for diagnosis and evaluation of location, quality and extent of posterior fossa tumours, also the follow up, but offers limited information regarding grade and type.
Advanced brain MRI techniques provide incremental diagnostic value over conventional MRI. However, no single advanced technique is perfect but different techniques typically complement one another.
DWI and apparent diffusion coeffetient map increase both sensitivity and specificity of MRI and provide information about cellularity of the tumour and have an important role in preoperative differentiation of different tumour type.
ADC value is simple and available technique for evaluation of pediatric cerebellar neoplasm and accurately differentiate between the most common tumour types.
Proton MR spectroscopy is non invasive invivo technique that provides additional metabolic diagnostic indices and has been used widely in the imaging of brain tumours as it can help differentiation of common pediatric brain tumours.
In our study we prospectively evaluate recent role of MRI regarding the benefit of adding DWI, ADC value and MRS in imaging protocol for characterizing and differentiating posterior fossa tumors in pediatric patients.
The study include twenty five patients with posterior fossa tumors ten cases medulloblastoma (MB) 40% ,five cases juvenile pilocytic astrocytoma (JPA) 20%, one case of ependymoma 4%,four cases brain stem glioma 16% ,two cases ganglioglioma 8% and three cases low grade glioma 12%. Age ranged between 1 year and 15 years.
In our study DWI with ADC was highly useful in the diagnosis of tumors, as high tumour grade e.g medulloblastoma show restricted diffusion and low ADC value. Pilocytic astrocytoma show free diffusion and higher ADC value than medulloblastoma.
ADC value can differentiate medulloblastoma from pilocytic astrocytoma or low grade glioma.
Also MRS data has important rule in tumour grading as high grade tumours show elevated coline, reduced NAA ratios.
Based on these results that, DWI, ADC value calculation and MRS data provide important data helping the diagnosis, we recommend that they should be integrated into the pre-operative MRI protocol of evaluation of posterior fossa tumors in pediatric patients.