A gradient echo (GRE) is simply a clever manipulation of the FID signal that begins by applying an external dephasing gradient field across the specimen or tissue. This gradient (produced by special coils hidden within the magnet housing) causes a calibrated change in local magnetic fields and hence alters the resonance frequencies slightly across the specimen. This results in accelerated dephasing and 'squelching/ scrambling' of the FID.
In step 2, the process is reversed. A rephasing gradient is applied with the same strength but opposite polarity to the dephasing gradient, reversing/ undoing the phase scramble. A small GRE has been generated! Note that the rephasing gradient has only refocused spins scrambled by the dephasing gradient itself. T2 and T2* processes are unaffected.
The rephasing of spins by gradient reversal is often illustrated by analogy to runners on a track, such as the tortoise and hare shown right. The fast hare represents spins precessing rapidly (and accumulating phase) by virtue of their location in a stronger portion of the gradient; the tortoise represents more slowly precessing spins in a weaker part of the gradient.
The fast hare travels much farther initially (corresponding to a larger phase accumulation). The reversal of direction half way through the race corresponds to the gradients being applied with opposite polarities (rephase lobes). The hare again runs faster but in the opposite direction, having more distance to make up. Finally both return to the starting line at the same time (equivalent to net phase shift = 0).
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The reason a "double-wide" rephase lobe is typically used for frequency-encoding (readout) is that it places the peak of the GRE at the center of k-space. It also allows sampling of both the rising and trailing portions of the echo, improving signal-to-noise. However, it is possible to arrange the gradient areas so they are asymmetrical and the peak occurs early or late in readout. This is the principle underlying "fractional echo imaging" discussed in a later Q&A.
Bernstein MA, King KF, Zhou XJ. Handbook of MRI Pulse Sequences. Oxford: Elsevier, 2004, pp 267-297. (Although missing newer sequences developed in the last 10 years, this text is a classic/must read for anyone wishing to understand the details of RF and pulse sequence design)
Elster AD. Gradient echo imaging: techniques and acronyms. Radiology 1993; 186:1-8.
Winker ML, Ortendahl DA, Mills TC et al. Characteristics of partial flip angle and gradient reversal MR imaging. Radiology 1988;166:17-26.
Where does the MR signal come from?
How do you produce multiple GRE's from a single pulse?