We measured end-diastolic flow velocities (EDFVs) of the internal carotid artery (ICA) bilaterally by duplex ultrasonography and calculated their side-to-side ratios in 14 healthy volunteers (normal group) and 29 patients with unilateral occlusion of the middle cerebral artery (MCA, n=5), distal internal carotid artery (distal ICA, n=7) or the origin of the ICA (ICA origin, n=17) . The site of arterial occlusion was confirmed by digital subtraction angiography, magnetic resonance angiography or transcranial color Doppler ultrasonography. The side-to-side ratio was 1.11±0.06 in the normal group. In the MCA occlusion group, it was 1.73±0.39 and exceeded 1.23 (the mean+2SD of the side-to-side ratio in the normal group) in four patients. In four patients with distal ICA occlusion, EDFVs were zero. However, in the other three patients with a flow to the posterior communicating artery or to moyamoya vessels, there was residual flow in the end-diastolic phase, and EDFV ratios were greater than 1.23. In the ICA origin occlusion group, no flow could be detected throughout the systolic and diastolic phases. In conclusion, measurement of EDFV and calculation of EDFV ratios of the ICA seem useful as screening measures to determine the site of occlusion of the ICA or MCA.
