CarrPurcell

CarrPurcell is a term used in nuclear magnetic resonance to refer to the Carr-Purcell sequence, a pulse sequence designed to generate multiple spin echoes and measure T2 relaxation. The sequence was developed by Eric R. Carr and Murray Purcell in 1954. It begins with a 90-degree excitation pulse that flips the net magnetization into the transverse plane, followed by a train of 180-degree refocusing pulses applied at a fixed interval tau. Each refocusing pulse creates an echo at times 2tau, 4tau, 6tau, and so on. The set of echoes provides repeated samples of the transverse magnetization, allowing a more robust estimate of T2 and an extended observation window compared with a single spin-echo measurement. In practice, imperfections in pulse calibration and inhomogeneities can distort the echoes and bias T2 estimates.

A widely used variant is Carr-Purcell-Meiboom-Gill (CPMG), which employs phase cycling of the refocusing pulses to suppress errors from flip-angle miscalibration and off-resonance effects, yielding more accurate T2 measurements. The CP sequence remains foundational in NMR spectroscopy, solid-state physics, and magnetic resonance imaging, and has inspired numerous turbo and fast-relaxation experiments.

Limitations include sensitivity to pulse imperfections, diffusion in liquids during long echo trains, and hardware constraints on repetition rate. The term CarrPurcell is sometimes encountered as an informal spelling variant in older literature, but Carr-Purcell is the standard designation.