k-Space detection and correction of physiological artifacts in fMRI.

Signal phase variations caused by physiology are a major source of instability in fMRI images produced by multiple RF pulses. k-Space phase variation maps show cyclic phase variations at the frequency of respiration combined with a cardiac variation of lower amplitude. The amplitude of the variation increases with gradient echo time and proximity to the chest, suggesting that the dominant cause of the phase variation is a B0 shift (approximately 0.01 ppm) produced by movement of organs in the chest. A simple k-space phase correction method is proposed and demonstrated for FLASH fMRI. The retrospective method requires no pulse sequence modification, and is more effective than navigator echo correction. Physiological noise is dramatically reduced, especially at inferior slice locations.