Ringing cancellation in Carr-Purcell-Meiboom-Gill-type sequences

Abstract

We propose a four-step phase cycling scheme that can cancel ringings from both the excitation and the refocusing pulses in Carr-Purcell-Meiboom-Gill (CPMG) measurements. It allows accurate measurements of the first echo with short echo spacing t_E, yielding improved precision of porosity, permeability, and fluid type identification in porous media study, at a cost of ∼25% signal reduction. In our example, we achieved t_E = 100 μs on a 6.8 MHz unilateral NMR sensor without using a Q-switch. These methods can be integrated in all nuclear magnetic resonance (NMR) protocols using CPMG, such as relaxation (T₁, T₂) and diffusion measurements, and do not require additional hardware (e.g., Q-switches) to cancel ringing (although it can be used with a Q-switch to further reduce residual ringing).