In magnetic resonance, the term echo refers to the return of the MR signal a¦er it has decayed. As the name implies, the idea is similar to echoes produced by sounds re¥ecting o˜ walls or other large surfaces and that are probably more familiar.  e MR signal decays quickly a¦er a radiofrequency (RF) pulse, as described in Chapter 3, because all of the hydrogen nuclei in an object are not in exactly the same magnetic Ÿeld (due to Ÿeld imperfections, magnetic susceptibility di˜erences, interactions between hydrogen nuclei, etc., which contribute to T2* as described in Chapter 3). As a result, the nuclei precess at di˜erent speeds, and the directions of the transverse magnetization at di˜erent points in the object spread out, or the magnetization dephases, in the transverse plane. An echo can be produced by reversing this dephasing.