Intracranial stenoses and occlusions, and circle of willis collaterals.

Stenoses and occlusions of the anterior, middle, pre- and postcommunicating posterior cerebral, and basilar and intracranial vertebral arteries are reliably detected by transcranial Doppler (TCD) and transcranial color duplex sonography (TCCS). Diagnosis of carotid siphon stenosis is consistently obtained by transorbital Doppler sonography. Internationally accepted criteria for diagnosing intracranial arterial obstructions are lacking. Both TCD and TCCS reliably identify collateral flow through the anterior and posterior communicating arteries. It is unclear whether TCCS is more accurate than TCD for detecting intracranial obstructions and collaterals, as no study has examined this issue.