FIG 2. Sample Doppler spectra during the arrival of the contrast-agent bolus in two patients. Intravenous injection occurred at 0 seconds. Color change of the Doppler spectrum from blue to orange (arrows) indicates arrival of the bolus. The superimposed transient, high-frequency signals that fill the complete frequency range of the Doppler are artifacts caused by echo contrast-induced overmodulation (blooming effect). White lines in each spectrum represent the computer-generated graph of the calculated peak intensity values of the spectrum. Top: Spectra in patient 9, who was untreated. Note the typical, pulsatile, flow signal intensity in the ICA and the arterialized flow in the IJV. Moderate jugular vein compression at 16–19 seconds leads to a marked reduction in signal intensity. Bottom: Spectra in patient 5 after incomplete occlusion of the malformation. Note the still-prevailing, arterialized flow and the two-step pattern of intensity rise in the IJV; the moderate increase in venous intensity increase between 14.5 and 20.2 seconds; and the second, sharp intensity rise at 20.2 seconds.