MRI [Neurosurgery Wiki]

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Pilocytic astrocytomas range in appearance:

Large cystic component with a brightly enhancing [[mural nodule]]: 67%
        
nonenhancing cyst wall: 21%
        
enhancing cyst wall: 46%
    
heterogeneous, mixed solid and multiple cysts and central necrosis: 16%
    
completely solid: 17%

Enhancement is almost invariably present (~95%). Up to 20% may demonstrate some calcification. 

Haemorrhage is a rare complication.

===MRI===

“Classic” MRI finding: posterior fossa cyst with an enhancing mural nodule. The cyst wall sometimes enhances, usually as a thin rim (biopsy negative for neoplasm, enhancement may be reactive
((Beni-Adani L, Gomori M, Spektor S, et al. Cyst wall enhancement in pilocytic astrocytoma: neoplastic or reactive phenomena. Pediatr Neurosurg. 2000; 32:234–239))).

Signal characteristics include:

T1: iso to hypointense solid component compared to adjacent brain
    
T2: hyperintense solid component compared to adjacent brain


{{::cerebellar_pilocytic_astrocytoma_mri.jpg?300|}}

[[Apparent diffusion coefficient]] (ADC) values have been shown to assist in differentiating cerebellar pilocytic astrocytomas and [[medulloblastoma]]s. Previous studies have applied only ADC measurements and calculated the mean/median values.

The 25th percentile for mean (MD) yields the best results for the presurgical differentiation between pediatric cerebellar pilocytic astrocytomas and medulloblastomas. The analysis of other [[DTI]] metrics does not provide additional diagnostic value
((Wagner MW, Narayan AK, Bosemani T, Huisman TA, Poretti A. Histogram Analysis
of Diffusion Tensor Imaging Parameters in Pediatric Cerebellar Tumors. J
Neuroimaging. 2015 Sep 2. doi: 10.1111/jon.12292. [Epub ahead of print] PubMed
PMID: 26331360.
)).