Ghost phase cancellation with phase‐encoding gradient modulation

Motion artifacts are a dominant cause of magnetic resonance image quality degradation. Periodic or nearly periodic motion results in image replicates of the moving structures in spin‐warp Fourier imaging. The replicates, or ghosts, propagate in the image in the phase encoding, or y, direction. These ghosted images can be considered to consist of the time‐averaged spin density I 0 and a ghost mask g . A set of J ghosted images I j may be acquired in which the ghost mask is intentionally phase shifted by varying amounts relative to I 0 with interleaved acquisitions that have shifted phaseencoding orders or by acquiring multiple images during a single readout period in the presence of an oscillating phase‐encoding gradient. The resulting complex images I j have the same time‐averaged spin density I 0 but have ghost contributions g j that, on a pixel‐by‐pixel basis, trace part of a circle around I 0 . The source images I j can then be used to estimate I 0 . Simulations and experiments with the phase‐encoding gradient modulation method show good general ghost suppression for a variety of quasi‐periodic motion sources including both respiratory‐type artifacts and flow artifacts. The primary limitation of the method is the need for rapid gradient switching.