2.1 Case Presentation

2.1.2 Imaging Findings and Impression

MRI of the brain with contrast shows an oval lesion (white arrows; ▶ Fig. 2.1a,b) in the distal left transverse sinus, which is isointense to cortex on T2-weighted imaging (T2WI) and on fluid-attenuated inversion recovery (FLAIR; ▶ Fig. 2.1c) and hypointense on T1-weighted imaging (T1WI). The lesion is seen as a filling defect on postcontrast T1WI (▶ Fig. 2.1d) and does not show any susceptibility on susceptibility weighted imaging (SWI; ▶ Fig. 2.1e) to suggest a venous clot. An intralesional vein (black arrow) is seen draining the area. All these features are consistent with an arachnoid granulation.

2.2 Differential Diagnosis

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  Arachnoid granulation:

  
  
  
  
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    Represent enlarged arachnoid villi and are seen as nonenhancing filling defects within venous sinuses causing scalloping of the inner table when large.

    
    
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    Often seen is a superficial cortical vein draining into the arachnoid granulation.

    
    
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    Absence of susceptibility on gradient echo (GRE) images.

  
  
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  Venous thrombosis:

  
  
  
  
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    Usually involves an entire segment of one sinus or multiple sinuses.

    
    
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    May be associated with mastoid inflammation.

    
    
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    Can extend into cortical veins and may involve the distal sigmoid sinus and jugular vein, which does NOT occur with arachnoid granulations.

    
    
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    Unlike arachnoid granulations, acute thrombus is hyperdense to the brain on CT, hyperintense on T1WI, and hypointense on FLAIR sequences and shows susceptibility artifact on GRE/SWI.

  
  
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  Intrasinusal tumor:

  
  
  
  
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    Enhances with gadolinium administration.