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Analysis of IDEAL from a signal-averaging standpoint shows that echo spacings of 2π/3 (solid lines) and 4π/3 (dashed lines) both yield an NSA of three. Doubling the echo spacing narrows the pass-bands and halves the spacing between aliased copies of the spectrum. The narrowed pass-bands and nearby aliased sensitivity bands for off-resonant spins increase both the importance of and the difficult in obtaining an accurate field map estimate.
