stimulatedecho

Stimulated echo is an MRI and NMR signal arising from a multi-pulse coherence pathway in which magnetization is stored along the longitudinal axis and later reconverted to transverse magnetization to form an echo. A common implementation uses a sequence of three 90-degree radiofrequency pulses separated by two delays. The first pulse creates transverse magnetization that dephases during the first delay. The second pulse stores part of the magnetization as longitudinal z-magnetization, during which transverse dephasing is effectively halted. The third pulse converts the stored magnetization back to transverse form, which then rephases to produce the stimulated echo at a later time.

The stimulated echo sequence makes use of magnetization storage to extend the effective evolution time compared with conventional spin-echo experiments. This enables measurement strategies sensitive to relaxation and diffusion properties over longer time scales, and can be adapted to encode diffusion weighting or specific relaxation contrasts that are difficult to obtain with standard two-pulse sequences. The exact signal and its dependence on tissue properties are governed by the timing of the delays and the pulse angles used.

Compared with spin echoes, stimulated echoes rely on a different coherence pathway that includes storage along the z-axis, which provides distinct sensitivity to inhomogeneities and relaxation processes. They can offer advantages in terms of allowing longer observation windows and specialized contrast mechanisms, but may come with lower signal-to-noise ratio and increased radiofrequency energy deposition due to multiple pulses. Stimulated echoes remain a standard tool in multi-pulse NMR and MRI for research applications involving relaxation and diffusion measurements.