Measurement of short NMR relaxation times: Effect of radio-frequency pulse length

In certain situations, such as in viscous liquids or nano-porous media, the transversal relaxation time T2 may become so short that the pulse duration or the detector dead time is no longer negligible. These cases raise fundamental questions about relaxation during the radio-frequency pulses used in classical CPMG or Inversion-Recovery sequences. For an ideal system having a single relaxation time, we examine the effect of the finite pulse duration on the magnetization decay after the pulses occurring in FID, CPMG and Inversion-Recovery sequences. We solved analytically the Bloch equations during and after the pulses and compared the theoretical predictions with experimental data. Finally, we propose approximate simple expressions to correct the sequences for the magnetization attenuation during the pulses. IR curves are affected by transversal relaxation during the pulses, yielding asymmetric curves even if T1 is very large. The magnetization decay obtained during a CPMG sequence is not affected by relaxation during the pulses. This is valid provided the time origin is chosen in the middle of the first pulse.