Arachnoid granulations on MRI

47 yr old being investigated for extrapyramidal symptoms shows an appx 14 mm  sized CSF signal intensity on loa sequences lesion in close proximity to left sigmoid sinus with no restricted diffusion/ blooming on SW  suggesting Arachnoid granulation (pacchionian bodies)

Teaching points by Dr MGK Murthy, MRI Technologist: Mr Hari

1.Normal variants  and can potentially produce   CSF otorrhoea  in  a few cases and whether  produce or  occur as result of benign  intracranial  hypertension is debated .  Possible internal  stenosis of transverse sinus due to  large  arachnoid granulation  playing  a role in Idiopathic intracranial hypertension  is  a matter of scientific deliberations at present . 

These  increase in size , number with age and may calcify as well .

2. Are absent at birth and occur after fontanelle closure in areas of weaknesses of dura mater and hence could  be regarded as developing form CSF pulsation at higher pressure through dural defects  in to sinuses at lower pressure 

3.Needs to be understood to avoid  misinterpretation as extra axial/ parenchyma/ sinus diverticula/ calvarial(destructive) /  dermoid cyst/ other aetiologies   

4. Sizes are usually small, and can be unilateral, large ones referred to as Giant granulations 

5. More common clustering the parasinusoidal blood lakes along side the Superior sagittal sinus 

6. Are bulbous aggregates of fibro elastic tissue  that are continuous with  subarachnoid space. CSF circulates through these and enters venous system. Do increase in size and number with age.