In a nuclear magnetic resonance (NMR) measurement, referring to the cycle of radio frequency pulses designed by Carr, Purcell, Meiboom and Gill to produce pulse echoes and counteract dephasing due to magnetic field inhomogeneities. In the CPMG sequence, an initial radio frequency pulse is applied long enough to tip the protons into a plane perpendicular to the static magnetic field (the 90^o pulse). Initially the protons precess in unison, producing a large signal in the antenna, but then quickly dephase due to the inhomogeneities. Another pulse is applied, long enough to reverse their direction of precession (the 180^o pulse), and causing them to come back in phase again after a short time. Being in phase, they produce another strong signal called an echo. They quickly dephase again but can be rephased by another 180^o pulse. Rephasing is repeated many times, while measuring the magnitude of each echo. This magnitude decreases with time due to molecular relaxation mechanisms surface, bulk and diffusion. One measurement typically may comprise many hundreds of echoes, while the time between each echo (the echo spacing) is of the order of 1 ms or less.

Carr HY and Purcell EM: ?Effects of Diffusion on Free Precession in Nuclear Magnetic Resonance Experiments,? Physical Review 94, no. 3 (1954): 630-638.

Meiboom S and Gill D: ?Modified Spin-Echo Method for Measuring Nuclear Relaxation Times,? The Review of Scientific Instruments 29, no. 8 (1958): 688-691.