These vessels are the vertebral arteries. They ascend from the upper chest and enter the skull at the base of the brain- the foramen magnum. These two vessels form the “Circle of Willis” and bring blood supply to the base of the brain and the cerebellum.
The reason these arteries can have different appearances is the movement of the blood while the scan is being taken. MRI scans are like old time pictures in the wild west. If you have ever seen these pictures, some individuals look like ghosts as their appearance is smudged if they move while the picture is being taken.
The same thing happens with movement while the scan is taken. You can’t ask blood to stand still and this motion will “color” the blood depending upon the phase of the scan taken.