What are 'arachnoid granulations' incidentally found on head CT/MRI?
When performing head imaging, you may encounter findings that look like lesions. One of the most representative of these is the "arachnoid granulation".
Although they may look abnormal at first glance, most are normal structures, and it is important to understand them correctly.
What are arachnoid granulations?
Arachnoid granulations are normal structures where the subarachnoid space protrudes into the dural venous sinuses or the nearby skull.
Their main role is to absorb cerebrospinal fluid (CSF) into the venous system.
Therefore, a characteristic feature is that they are often discovered incidentally on head CT or MRI.
Typical appearance on imaging
1. Type protruding into the venous sinus
Protrudes into the transverse sinus, etc.
Appears as a low-attenuation area (defect-like) on CT
On contrast-enhanced MRI, it does not show contrast enhancement
Differentiation from thrombus is important
2. Type protruding into the skull
Seen in the occipital bone, etc.
On CT, it appears as a bone defect-like or osteolytic change
On MRI, it shows signal intensity similar to CSF (T2 high signal)
This is a point that is easily mistaken for bone tumors or metastases.
Points for image interpretation
To correctly identify arachnoid granulations, keep the following points in mind.
Most are normal variants
Within the venous sinus, it appears as a hypodense filling defect finding.
In the skull, it may present as bone defects or osteolytic-like changes.
On MRI, it often shows signal intensity similar to cerebrospinal fluid.
Differentiation from venous sinus thrombosis and osteolytic lesions is important.
Lesions that are easily confused in differential diagnosis
Arachnoid granulations can sometimes be confused with the following lesions:
Venous sinus thrombosis
Metastatic bone tumors
Multiple myeloma
Cystic lesions such as epidermoid cysts
It is particularly important to evaluate by combining 'shape,' 'signal,' and 'presence or absence of contrast enhancement.'
Summary
Arachnoid granulations are a classic example of normal structures that can look like lesions.
By firmly grasping the imaging characteristics, you can prevent unnecessary further examinations and misdiagnoses.
The perspective of pausing to ask, 'Is that really a lesion?' improves your image interpretation skills.
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