6.1 Case Presentation

6.1.2 Imaging Findings and Impression

Axial (▶ Fig. 6.1a) and coronal (▶ Fig. 6.1b) CT scans of the head without contrast demonstrate a well-circumscribed cerebrospinal fluid (CSF) density lesion (white arrows) measuring approximately 10 × 10 mm in the choroidal fissure. The lesion has an imperceptible wall with no evidence of a solid component within the cyst.

However, MR was performed to exclude the less likely possibility of a small cystic lesion. The cyst follows CSF signal intensity on axial T1-weighted (black arrow on ▶ Fig. 6.1c), axial fluid-attenuated inversion recovery (FLAIR; ▶ Fig. 6.1d), and coronal high-resolution fast spin-echo (FSE) T2-weighted (▶ Fig. 6.1e) images (white arrows). Coronal image (▶ Fig. 6.1e) clearly shows that the 10-mm cyst lies within the choroidal fissure. The distinctive spindle shape of the lesion is best seen in the T1-weighted sagittal plane (asterisk on ▶ Fig. 6.1f). The lesion does not show restriction on diffusion-weighted and ADC (apparent diffusion coefficient) map images (arrows on ▶ Fig. 6.1g,h).

6.2 Differential Diagnosis

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  Choroid fissure cysts:

  
  
  
  
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    Choroid fissure cysts are a normal variant occurring in the choroidal fissure and follow the CSF signal intensity on all sequences, with the absence of nodular enhancement.

  
  
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  Mesial temporal sclerosis:

  
  
  
  
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    This is rarely an incidental finding but rather a common cause of epilepsy.

    
    
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    The hippocampus (uni- or bilateral) is atrophic with abnormal signal intensity on FLAIR images and with loss of normal internal architecture on T2-weighted images.

    
    
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    The ipsilateral fornix is also often noted to be small.

    
    
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    This atrophy may result in ipsilateral widening of the temporal horn and choroidal fissure.

    
    
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    The primary abnormality of the hippocampus and secondary widening of adjacent CSF spaces should distinguish this from a choroid fissure cyst in which the adjacent brain is displaced due to mass effect but is otherwise normal. ¹

  
  
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  Epidermoid cysts:

  
  
  
  
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    They are more commonly found in the CPA (cerebellopontine angle) cistern.

    
    
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    Epidermoid cysts do not suppress on FLAIR images completely and are bright on diffusion-weighted images, unlike choroid fissure cysts.

  
  
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  Parasitic cysts (neurocysticercosis):

  
  
  
  
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    They are more common in tropical countries.

    
    
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    Typically, they are parenchymal but can be present in the subarachnoid space.

    
    
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    The vesicular stage of neurocysticercosis (tapeworm infection) can look similar, but the cysts are most often multiple.

    
    
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    Nonenhancing T2 hypointense/T1 hyperintense scolex can be identified within the cysts.

    
    
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    They evolve into colloidal vesicular, granular nodular, and nodular calcified stages over time.

  
  
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  Hippocampal sulcus remnants:

  
  
  
  
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    They are multiple tiny cysts within the hippocampus itself and are therefore seen just medial to the temporal horn of the lateral ventricle but between the dentate gyrus and subiculum.

    
    
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    They are incidental findings and follow the CSF signal intensity on all sequences.

  
  
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  DNETs (dysembryoplastic neuroepithelial tumors):

  
  
  
  
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    Intra-axial mass/masslike abnormalities as opposed to a choroid fissure cyst that may compress adjacent brain but is extra-axial in location.

    
    
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    They often occur in the temporal lobes and have a pseudocystic/bubbly appearance.

    
    
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    They do not suppress on FLAIR sequences and often have a bright rim, which should help distinguish them.

    
    
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    Calcification and cortical remodeling may be seen.

    
    
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    Contrast enhancement is unusual. ²