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2Brain

◆ Anatomic Variants and Incidental Findings

Common incidental findings on head CT include arachnoid cysts, prominent arach-noid granulations, choroid plexus and choroidal fissure cysts, remote lacunar in-farcts, focal encephalomalacia from prior infarct or trauma, and prominent perivas-cular spaces. The ventricles may be slightly asymmetric, and the septum pellucidum may contain a central CSF-filled cavity (cavum septum pellucidum). Unless symp-tomatic, these conditions usually do not require specific follow-up (Fig. 2.4).

Fig. 2.4a–f a,b Arachnoid granulation. Whenvisible, arachnoid granulations, which resorb CSF into the venous sys-tem, appear as lling defects in the opacied venous sinuses. They may also cause smooth erosion of the bone adjacent to the sinus. (a) Round lling defect in opacied left transverse sinus. Round osseous ero-sions near the internal occipital protruberance/torcular Hirophili.

c Choroidal ssure cyst. These small CSF attenuation cysts arise in the choroidal ssure and appear lat-eral to the midbrain or cerebral peduncle on axial images.

d Enlarged perivascular space. 8-mm CSF attenuation space located in the right posterior putamen. e Cavum septum pellucidum. A developmental variant due to failed embryonic fusion of the leaves of the septum pellucidum, it is present in ~15% of individuals. This CT image also shows cortical contusions, traumatic subarachnoid hemorrhage, and subacute bifrontal frontal subdural hygromas.

f Arachnoid cyst. CSF attenuation, extra-axial collection due to duplication in the arachnoid membrane, which compresses the adjacent brain and may smoothly expand the overlying calvarium. These are usu-ally asymptomatic; however, larger cysts may predispose to hemorrhage in minor trauma or can cause symptoms by compression of the brain. Epidermoids can have a similar appearance on CT, but they have characteristically high signal on FLAIR MRI.