Multidetector computed tomographic (CT) angiography is used in many institutions for initial evaluation of the cerebral circulation for acute stroke and subarachnoid hemorrhage as well as in various preoperative settings. A comprehensive CT examination that includes a review of three-dimensional and maximum intensity projection images of the intra- and extracranial arteries and axial images of the skull base (obtained with bone window settings) allows identification of most abnormalities and normal variants. Knowledge of the presence and clinical relevance of normal variants such as fenestrations, duplications, and persistent fetal arteries plays a crucial role in the diagnosis and management of acute stroke and subarachnoid hemorrhage and may aid in surgical planning. For example, the preoperative detection of a medial or intrasellar persistent trigeminal artery may help surgeons avoid a potentially life-threatening hemorrhage in a patient undergoing transsphenoidal surgery for pituitary adenoma. However, the significance of normal variants diverges widely: A clinically important association has been observed between fenestration and intracranial aneurysm formation, whereas early branching of the middle cerebral artery is not associated with an increased risk of aneurysm formation and has little clinical significance.
Dimmick SJ, Faulder KC. Normal variants of the cerebral circulation at multidetector CT angiography. RadioGraphics 2009; 29:1027-1043