Gradient echo sequence

The basic principle is similar to spin echo imaging. An initial excitation pulse of radiofrequency energy is supplied at the Larmor frequency to the brain in the presence of a powerful static magnetic field. Protons are excited to a state characterized by increased transverse magnetization and a coherent phase of precession around the axis of the external field. Immediately the radiofrequency pulse has ceased, protons begin to relax back to their equilibrium state of maximum longitudinal magnetization and random phase of precession, emitting a radiofrequency signal by free induction decay.

In gradient echo imaging, the process of spin-spin relaxation (dephasing) is first accelerated by briefly applying a gradient to the magnetic field shortly after the excitation pulse. Then, at some time (TE/2) after the excitation pulse, a second gradient is applied to reverse the process of dephasing, causing rephasing and a signal maximum or echo some time (TE) after excitation. The sequence is repetitively applied with a constant time interval between consecutive excitations (TR).

The objective is to manipulate spin-spin relaxation by brief perturbations of the external magnetic field rather than by supplying additional pulses of radiofrequency energy as in spin echo imaging. Frequency- and phase-encoding gradients are applied to locate the sources of signal in three-dimensional space (see Ch§pteri2.3.7).

One advantage of gradient echo imaging is that TE and TR can both be shorter than in spin echo imaging, allowing an overall reduction in scanning time. However, if TR is short, spoiler gradients or radiofrequency pulses may be needed to ensure that the protons have returned to equilibrium before the next excitation pulse is supplied. The flip angle a induced by radiofrequency excitation can be adjusted to generate images weighted by different sources of tissue contrast. Trweighted images are generated by radiofrequency pulses causing flip angles of the order of 10° to 20°. For functionally sensitive T2-weighted images, more radiofrequency energy must be supplied in the excitation pulse to give a flip angle approaching 90°.

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