Figure 4.

The effect of an offset of the resonance frequency or “off-resonance” on slice selection (a) and imaging readout (b). During slice selection, the yellow and green spins resonate below the expected rates. When the RF pulse is centered at the rate of the gray spins in the orange region, the green spin is not excited, while the yellow one is excited, resulting in signal loss (green) and pile-up (yellow). Next, when the spins are imaged, the yellow spin will again resonate below the expected rate (b), and be detected at the incorrect postion in the A/P direction (c), resulting in in-plane pile-up artifact. Note that although just a few spins are shown here to illustrate the concept, there is actually a continuous distribution of magnetization in both directions and the excited slice can show displacement, broadening, thinning or even splitting depending on the off-resonance frequency distribtuion.