We have seen that each of the three orthogonal gradients does a completely different job: the z gradient defines the slice; the x gradient imposes a frequency change; and the y gradient produces a phase change. It is important to realize that each gradient is the same physical phenomenon: a change in the magnetic field from one point in space to another. Also, each gradient produces the same basic effect on protons: the vector precession frequencies are changed. Therefore, the particular task each gradient performs is determined by essentially one thing: when in the pulse cycle the gradient is applied.
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