CarrPurcellMeiboonGill

CarrPurcellMeiboonGill, commonly written as Carr-Purcell-Meiboom-Gill sequence (CPMG), is a radiofrequency pulse sequence used in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) to generate a train of spin echoes. It is designed to measure transverse relaxation and provide robust T2-weighted contrast in presence of field inhomogeneities.

The sequence combines contributions from two groups: Carr and Purcell showed that a series of refocusing pulses after an initial excitation can produce multiple echoes, while Meiboom and Gill (1958) modified the phase of these pulses to reduce sensitivity to pulse errors, yielding the widely used CPMG form.

Operation: after a 90-degree excitation, a train of 180-degree refocusing pulses is applied at intervals tau. Echoes appear at times TE = 2 tau, 4 tau, etc. The signal decays roughly as exp(-t/T2) rather than exp(-t/T2*), allowing T2 measurement that is less affected by B0 inhomogeneity. The Meiboom-Gill modification uses a 90-degree phase for the refocusing pulses, improving robustness against pulse imperfections and resonance offsets.

Applications: In NMR and MRI, CPMG is used to measure T2 relaxation times and to obtain T2-weighted images or maps. It is also used in solid-state NMR and various relaxation experiments that require accurate T2 estimates.

Limitations: The sequence can be sensitive to B0 and B1 inhomogeneities, pulse imperfections, and eddy currents. Proper calibration of pulse lengths and phases and selection of tau are essential to minimize biases.

See also: spin echo, T2 relaxation, Carr-Purcell sequence, Meiboom-Gill modification.