Aliasing
Aliasing occurs when the FOV in the phase-encoding direction is smaller than the anatomy being imaged. The data are under-sampled and the image appears to be wrapped from one side to the other along this direction. This can occur in two directions when 3D data are acquired, but it does not occur along the frequency-encoding direction.

Solutions
? Increase the FOV along the phase-encoding axes. This will solve this problem but at the expense of decreased spatial resolution.
? Increase the phase FOV percentage or enable the no-phase-wrap feature.
? Swap the readout and phase-encoding directions, unless this increases flow artifacts to an unacceptable degree.
? Apply parallel imaging techniques to “unwrap” the artifact. This will solve the problem but can decrease signal-to-noise ratio in the image.
? Reposition the center of the FOV so that the aliased data are not included in the anatomy of interest. Signal-to-noise ratio and resolution are not decreased, but several attempts may be required to position the aliased tissue outside of the region of interest.

RF Zipper
An RF zipper results when RF energy from a source other than the patient is detected by the RF coil used for imaging. The signal is not spatially encoded and appears as a line whose intensity varies from bright to dark along the phase-encoding direction. The typical source of this noise is (but is not restricted to) a pump or other electromechanical device in the room. Also, any conducting cable that enters the room can act as an antenna, propagating noise from outside into the scan room. Finally, a leak in the RF shield of the MR scan room can allow external RF noise to enter. These RF leaks occur
most commonly around doors and windows.

Note that zipper-type artifacts that occur along the frequency-encoding axis arise from sources internal to the MR scanner such as stimulated echoes and pulse sequence timing errors.

Solutions
? Identify and remove all noise sources within the room.
? Have a qualified service engineer check for leaks in the RF shield of the scan room.