What does it mean when brain mri results state "arteries are patent?"

Good news. : The blood vessels supplying your brain are not narrowed or blocked. These are the cerebral arteries.

No blockage: The MRI detects a signal caused by changes of the properties of water under high-strength magnets. The water in blood, however, flows through & does not really get a chance to get magnetized; this is detected as a dark spot in cross section where the arteries r open & blood flowing (arteries patent). If there were a blockage or absence, then the vessel or tissue would "light up" unexpectedly.