Spatial resolution in echo planar imaging: shifting the acquisition window in k-space

Single-shot echo planar imaging (EPI) is one of the most suitable techniques for very fast image acquisition, especially in functional MRI. In standard EPI schemes the k-space center is sampled in the middle of the acquisition train. This leads to longer echo times for higher spatial resolutions, as well as reduced signal intensity and signal-to-noise ratio. Therefore, echo shifting to lower echo times is often used. After a brief overview on the theoretical background of various point-spread-functions (PSF) computational simulations are presented, which quantify the modulation amplitude of a binary test object sampled with either standard, zero-filled or shifted k-space acquisition. The results suggest that echo-shifting with zero-filling is not advantageous, not even with small matrix sizes, and that echo-shifting with additional acquisition of outer k-space lines decreases the modulation amplitude only slightly. Simulations were also performed on noise-corrupted test objects, indicating that the use of the echo-shifted scheme causes resolution loss of up to 30% compared to the standard scheme for a 128 by 128 pixel matrix at a noise level of 20%. Finally, in vivo experiments using different echo shifts are presented and the characteristics of signal and noise with varying TE are quantified.