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 tE, 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 tE = 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).

Keywords

• CPMG
• Ringing cancellation
• Phase cycling