Right vertebral artery entering the third transverse foramen diagnosed by magnetic resonance angiography: a report of two cases.

The vertebral artery (VA) usually enters the sixth transverse foramen (TF). The VA sometimes enters the fifth, and rarely enters the fourth or seventh TF. Reports describing a VA entering the third TF, identified by anatomical dissection or computed tomography angiography, are extremely rare. We herein report two cases in which magnetic resonance (MR) angiography showed the right VA entering the third TF. Coronal reformatted MR angiography source images were useful for identifying the level at which the VA entered the TF. Because the anomalous VA takes an anterior course, it poses a risk during anterior neck surgery. Thus, a correct diagnosis is important when interpreting cervical MR angiography.