Zipper artifacts appear as dashed lines. There are various causes for this MRIartifact.
Most of zipper artifacts result from inhomogeneities of the magnetic field caused by interferences with radio frequency from various sources ('your radio is working in the scanner room means your shielding is not working'). Software and equipment problems can also cause zipper lines in both directions.
Image Guidance
Scanning room door during the acquisition of images is closed, shielding is working, and all disturbing devices are removed?