The Carr-Purcell-Meiboom-Gill sequence (CPMG) experiment allows to measure transverse or spin-spin T₂ relaxation times of any nucleus. The experimental half-height linewidth (d) of a given resonance is directly related to T₂^* by the equation d=1/(pi*T₂^*) In the measurement of the  T₂ relaxation times, the magnetic field inhomogeneities (T₂^inh ) must be considered: 1/ T₂^* =(1/T₂ )+ (1/T₂^inh )

Easy implementation on AVANCE spectrometers.

The basic pulse sequence of the CPMG experiment is based on the spin-echo pulse sequence and consists of the following steps:

• A 90º pulse creates transverse magnetization
• An spin-echo period (delay-180º-delay block) determines the decay of the M_xy magnetization. This period is repeated n times (see T₂ filter).
• Acquisition is performed as usual

This procedure can be used to measure T₂ relaxation times of any nucleus. For instance, in the case of ¹³C, all pulses and acquisition are applied on ¹³C channel while broadband proton decoupling is applied during all sequence.

The CPMG experiment can be acquired, processed and plotted in a fully automated way. Minor changes from a predefined parameter set are required. Important parameters to consider are:

• Relaxation delay must be set to 5*T₁
• The experiment is repeated n times, by incrementing the number of spin-echo blocks.
• Quantitative analysis can be performed using the relaxation T₁ /T₂  software.

For further details on practical implementation of the inversion-recovery experiment on AVANCE spectrometers see Tutorial: T2 1H measurements

The CPMG experiment affords conventional 1D spectra. The signal intensities will shown a dependence with the overall spin-echo length:

• Using a short spin-echo period, all signals will shown positive intensities
• The intensity of the signals will decrease when the spin-echo period is increased
• Using longer spin-echo periods, the signal intensities become null.

Related selective inversion-recovery experiments are efficient methods of measuring proton spin-spin cross-relaxation rates in crowded regions ( 93JB335 )