Abstract

The standard pre-polarized and non-polarized measurement sequencies used in fast-field-cycling (FFC) NMR relaxometry [1] suffer from two major drawbacks:

(a) Due to the magnetization evolution during switching intervals, the longitudinal magnetization evolves with increasing τ towards non-zero values. Consequently, FFC-NMR decay curves must be fitted with an adjustable 'offset' parameter, in addition to the decay rates and weights of individual sample components (for example, three parameters in the case of a mono-exponential fit).

(b) Measurements are carried out using different sequences at high relaxation fields (NP) and at low relaxation fields (the crossover field being approximately half of the polarization field). For reasons which are not yet quite clear, this sometimes leads to a statistically significant discrepancy between the two 'sections' of the resulting NMRD profile.

We propose new measurement sequences consisting in a combination of RF pulse-phase cycling, receiver cycling and magnetic field cycling, such that all switching-time interval effects get cancelled, while the sample pre-polarization effects and the subsequent decay are enhanced.

In such sequences, the acquired signal S(τ) decays rigorously to zero when τ grows to infinity (which is why we call them null-biased). Essentially all classical FFC-NMR sequences can be cast in this way (the basic NP/PP, IR, IR_CPMG, ...), some of them in a way which does not at all reduce the efficiency of the measurement (i.e., the total signal range which can be achieved in a given time).

The null-biased sequences offer several important advantages:

(1) Since S(∞) is strictly null and does not have to be estimated experimentally, there is no need to acquire portions of the decay curve where the signal is already almost stable (this leads to a time saving).

(2) For the same reason, the data can be fitted without the adjustable offset parameter (for example, a two-parameter fit is sufficient in the case of a mono-exponential decay). Considering the effect of number of adjustable parameters on their confidance intervals, this fact alone improves the precision of the estimated T₁ by a large factor (~10).

(3) In most cases, the same sequence can be used to acquire the NMRD profile over the complete range of field values (for example, from 5 kHz to 40 MHz). As a result, the measured NMRD profiles are internally more coherent and no discrepancy between high-field data and low-field data can occur.

We discuss also an apparent drawback of the new sequences consisting in the fact that they enhance the visual impact of random field variations between consecutive scans. Analysis shows, however, that traditional measurements are burdened by the same instabilities, even though they are to a large extent masked by the switching-interval signal components.

References:

• G.Ferrante, S.Sykora, Technical Aspects Of Fast Field Cycling,
  in Adv.Inorg.Chem., Eds. R.van Eldik,I.Bertini, 2005, 57, 405-470.
• C.Radhakrishna Rao, Linear Statistical Inference and its Applications, John Wiley & Sons, 1973.