Ghosts are defined as secondary images appearing in locations that differ from the primary image. Ghosts occur along phase encoding direction regardless of direction moved. Distance between ghosts increases as the number of periodic motions between TRs increases – hence, increases with frequency of motion and with TR. The distance also increases with number of signal averages. Ghosts may be high or low signal depending on whether they are in phase or out of phase with the object they overlie. If the TR is so short that motion does not repeat between excitations, then periodic motion causes edge smearing (as with random motion) rather than ghost generation.