Carotid-anterior cerebral artery anastomosis with an anteromedial course variant diagnosed by magnetic resonance angiography.

Purpose: To describe a case of right carotid-anterior cerebral artery (ACA) anastomosis with a unique course in the proximal segment.

Methods: A 78-year-old woman with a history of right carotid endarterectomy 2 years prior underwent cranial magnetic resonance (MR) imaging (MRI) and magnetic resonance angiography (MRA) of the head and neck regions separately using a 3.0-Tesla scanner. MRA was performed using a standard 3-dimensional time-of-flight technique.

Results: MRA of the intracranial region revealed no pathological lesions. However, a large artery arose from the ophthalmic segment of the right ICA, took an anteromedial course and after making a hairpin turn, continued to the bilateral A2 segments of the ACA. Bilateral A1 segments of the ACA were absent. MRA source images and oblique sagittal reformatted images showed that the anomalous artery was running inferior to the right optic nerve, indicative of carotid-ACA anastomosis or an infraoptic course of the ACA. As some patients have an ipsilateral normal A1 segment, the latter name seems to be inadequate.

Conclusion: We encountered a case of right carotid-ACA anastomosis in which the proximal segment had a unique course, ran anteromedially, and made a hairpin turn. This type has not been reported in the relevant English-language literature. Usually, this variation takes a medial course and ascends between the bilateral optic nerves. Careful observation of MRA images, including their source images, is required for the correct diagnosis of this rare variation.