Carr-Purcell echo train

In practice, Carr-Purcell echo trains usually result in measured Tg s that are too short because of cumulative errors of each pulses not being exactly 180° pulses and of inhomogeneity which spreads out the magnetization in a plane containing Hq and Hj. One way to compensate for these errors is to alternate the phase of each 180° pulse by 180° phase shifts as shown below. The first 90° pulse occurs with the rotating field along the rotating x axis. The first 180° pulse, how-... 

The trick introduced by Meiboom and Gill (14) is to dephase all n pulses in the Carr Purcell train by an angle of 90° with respect to the initial ti/2 pulse. It is easily shown that, without this phase change, imperfections of the 71 pulses are cumulative, whereas with the 90° phase change, a self-compensation occurs for all echoes of even number. The CPMG (Carr-Purcell-Meiboom-Gill) experiment can be handled in two ways ... 

Figure 7.22 The NMR-MOUSE (a) Schematic. The NMR sensor consists of an u-shaped permanent magnet with a solenoidal rf coil placed in the gap. (b) Photo of the NMR-MOUSE testing a tyre, (c) Example of a train of successive Hahn echoes generated according to Carr, Purcell, Meiboom and Gill (CPMG echo train) for carbon-black filled SBR measured by the NMR-MOUSE. The time constant of the echo-envelope defines T...

Phase considerations intrude even in the simplest experiments of observing an FID or an echo. Accurately adjusting the phases of rf pulses can be very important, particularly in experiments involving trains of pulses such as the Carr-Purcell Meiboom-Gill train or the multiple pulse line narrowing sequences. In other sections we have considered how phase shifts originate and how to cope with them. 

CPMG 3 205 Carr, Purcell, Melboom, and GIN spin echo train... 

Site populations in Al2Si205 kyanite and short range order parameters in CaAl2Si20g anorthite have been determined by using Al MQMAS NMR with quadrupolar Carr-Purcell-Meiboom-Gill echo train and FAM-II triple quantum to single quantum conversion pulses (MQ-QCPMG-MAS NMR). " ... 

Figures 2.13, 9.1, and 9.2 demonstrate the formation of an echo following a tt pulse. Application of additional tt pulses can be used to form a train of echoes. It is clear that the dephasing of magnetizations following an echo is of the same form as the initial dephasing during the FID and that application of a second tt pulse at 3T causes a second echo at 4t, etc. The envelope formed by the echo peaks decays according to the real T2, rather than T2, and Fourier transform of each echo provides a set of partially relaxed spectra, from which T2 of each line may be determined. (Carr and Purcell first recognized the value of such a long sequence of TT pulses,104 and their names are usually used to depict the method, but the technique that we described for the spin echo in Chapter 2 and that discussed here include a refinement by Meiboom and Gill,105 as discussed later.)...

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