Artifactual absense of flow in the internal carotid

MIP of TOF angiography in the of the skull base shows absence of flow in the right internal carotid.

Color Doppler showed normal and symmetric velocities in the internal carotids.
Contrast enhanced image shows normal enhancement of both internal carotids. A comment or explanation will be appreciated. More images in http://www.mypacs.net/cgi-bin/repos/mpv4_repo/wrm/repo-view.pl?cx_subject=64901426&cx_from_folder=1259318&cx_repo=