Transcranial duplex color-flow imaging is a new diagnostic method that allows visual display of blood flow in the basal cerebral arteries. This method allows determination of and correction for the Doppler angle of insonation. Conventional transcranial Doppler sonography has no imaging component and assumes a 0-degree Doppler angle for the calculation of flow velocities. The magnitude of the angle of insonation and the effect on flow velocity estimates have not been clearly defined. In order to evaluate the angle of insonation and the effect of angle correction on velocity readings, 50 patients referred for evaluation of cerebrovascular disease were studied. The mean age was 45 years (range, 24-62 yr). All were examined with conventional transcranial Doppler and transcranial duplex color-flow imaging. Specific vessels of the circle of Willis were identified by location, course, and direction of flow on color-flow images and by depth, direction of flow, and transducer orientation with conventional Doppler sonography. Visually controlled measurements of the Doppler angle of insonation were made by color-flow imaging. The data show that the mean angle of insonation was 33 degrees (+/- 15) in the middle cerebral artery, 35 degrees (+/- 17) in the anterior cerebral artery, 45 degrees (+/- 18) in the posterior cerebral artery, and 15 degrees (+/- 14) in the basilar artery. Angle-corrected peak systolic flow velocities were higher in all vessel segments (middle cerebral = 15%, anterior cerebral = 18%, posterior cerebral = 30%, and basilar = 3%), compared to uncorrected velocity readings by conventional Doppler sonography.(ABSTRACT TRUNCATED AT 250 WORDS)