Duplex color-coded ultrasonography for evaluation of the vertebrobasilar arteries

Abstract

Duplex color-coded ultrasonography performed for evaluation of the cerebrovascular arterial system consists of transcranial and carotid ultrasonogaphy. Vertebral arteries (VAs) pass through the foramina of the cervical transverse processes and unite to form the basilar artery, which affects the blood flow in each vertebral artery. Neck motions also affect the blood flow in the VAs. Carotid ultrasonography enables us to diagnose occlusion of the VAs, the subclavian steal phenomenon, VA dissection, and the Bow-Hunter syndrome. These disturbances of blood supply in the VAs often cause vertigo and dizziness, which result in vertebrobasilar insufficiency, sometimes resulting in cerebral infarction. The level of VA occlusion is diagnosed as follows; 1) Absence of apparent flow signals in the VAs indicates origin occlusion. 2) Preserved systolic velocity, but zero end-diastolic velocity indicates occlusion before the branching off into the posterior inferior cerebellar arteries (PICA). 3) Mean velocity (MV)<18cm/sec and MV-ratio (mean flow velocity in the contralateral VA divided by that in the target VA)≥1.4 indicate PICA-end or VA occlusion after branching off of the PICA. These could be further distinguished as follows: A diameter-ratio (diameter of the contralateral VA divided by the diameter of the target VA)≥1.4 indicates PICA-end occlusion. A diameter-ratio<1.4 indicates VA occlusion after branching off of the PICA. Retrograde VA flow indicates the subclavian steal phenomenon, which can cause vertebrobasilar ischemia during exercise of the upper extremeties. Neck motions sometimes damage the VAs and cause dissection, which produces flaps and stenoses in the VAs. Rotational VA occlusion, the so-called Bow-Hunter syndrome, can be diagnosed by transient change of the blood flow velocity to the occlusive pattern during neck rotations. Transcranial ultrasonography allows direct evaluation of the blood flow in the intracranial vertebrobasilar arteries. This method enables us to confirm the results of carotid ultrasonography, such as in basilar artery occlusion, in which retrograde flow is observed in the basilar artery. Duplex color-coded ultrasonography is less invasive and feasible for bedside examination, and allows real-time evaluation of the cerebrovascular arteries.