SpinEcho

Spin echo, sometimes written spin-echo or spinecho, refers to a family of radiofrequency pulse sequences used in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) to refocus dephasing spins and recover transverse magnetization that would otherwise decay due to inhomogeneities in the magnetic field and spin-spin interactions. The canonical two-pulse spin-echo sequence consists of a 90-degree pulse that tips net magnetization into the transverse plane, followed after a delay tau by a 180-degree pulse that flips the spins, causing them to rephase and form an echo at time TE = 2 tau.

In MRI, spin-echo sequences generate images with contrast dominated by T2 relaxation, since the echo amplitude after refocusing decays primarily as exp(-TE/T2). They are relatively robust to magnetic field inhomogeneities, yielding sharper images in regions with susceptibility variations compared with gradient-echo sequences. Fast-spin-echo or turbo spin-echo variants acquire multiple echoes per excitation to reduce scan time.

In NMR spectroscopy, spin-echo experiments (Hahn echo) are used to measure transverse relaxation times and to study molecular dynamics; the approach has many variants, such as Carr-Purcell (CP) and Carr-Purcell-Meiboon-Gill (CPMG) sequences, which apply trains of refocusing pulses to extend coherence and to separate relaxation from other effects.

The concept originated with Erwin Hahn who introduced the spin-echo principle in 1950; the term is sometimes written spinecho when used as a single word. The technique remains foundational in both imaging and spectroscopy, and has inspired related sequences designed to mitigate various sources of dephasing and to improve SNR.