Cisterna magna (CM) measures > 10 mm on routine posterior fossa view

Cerebellar hemispheres and vermis otherwise normal

Most often normal variant : < 20% with other findings

Cerebellar hypoplasia: CM big because cerebellum is small

Blake pouch cyst: Dilation of posterior medullary vellum

• ○
  

  Cyst rotates vermis superiorly

  
  
• ○
  

  Axial view mimics mega CM

Anomaly of vermis

• ○
  

  Dandy-Walker malformation

  
  
  
  
  • –
    

    Absent vermis + ventriculomegaly

  
  
  
  
• ○
  

  Partial vermian dysgenesis

  
  
  
  
  • –
    

    Superior vermis present, inferior vermis missing

  
  

Arachnoid cyst of posterior fossa

• ○
  

  Look for mass effect on dural folds, vermis, cerebellum

Usually isolated incidental finding: Excellent prognosis

Can be part of multiple anomalies seen with trisomy 18

Steep scanning angle may cause false enlarged CM

• ○
  

  Posterior fossa view with 15° dorsal angulation, not more

Pay careful attention to cerebellum and vermis anatomy

• ○
  

  Look for normal bilobed cerebellar morphology

  
  
• ○
  

  Measure bicerebellar diameter

  
  
• ○
  

  Consider cine sweep to show vermis is completely intact

Consider 3D multiplanar imaging to show vermis in axial, coronal, and sagittal views

• ○
  

  Can measure vermis length on coronal and sagittal views

  
  
• ○
  

  Can see if vermis is elevated off midbrain on sagittal view

Turn on color Doppler to prove increased lucency behind cerebellum is not from vascular malformation