Abstract
This review discusses the theory and applications of the Continuous Wave Free Precession (CWFP) sequence in low-field, time-domain nuclear magnetic resonance (TD-NMR). CWFP is a special case of the Steady State Free Precession (SSFP) regime that is obtained when a train of radiofrequency pulses, separated by a time interval Tp shorter than the effective transverse relaxation time (T2*), is applied to a sample. Unlike regular pulsed experiments, in the CWFP regime, the amplitude is not dependent on T1. Therefore, Tp should be as short as possible (limited by hardware). For Tp < 0.5 ms, thousands of scans can be performed per second, and the signal to noise ratio can be enhanced by more than one order of magnitude. The amplitude of the CWFP signal is dependent on T1/T2; therefore, it can be used in quantitative analyses for samples with a similar relaxation ratio. The time constant to reach the CWFP regime (T*) is also dependent on relaxation times and flip angle (θ). Therefore, T* has been used as a single shot experiment to measure T1 using a low flip angle (5°) or T2, using θ = 180°. For measuring T1 and T2 simultaneously in a single experiment, it is necessary to use θ = 90°, the values of T* and M0, and the magnitude of CWFP signal |Mss|. Therefore, CWFP is an important sequence for TD-NMR, being an alternative to the Carr-Purcell-Meiboom-Gill sequence, which depends only on T2. The use of CWFP for the improvement of the signal to noise ratio in quantitative and qualitative analyses and in relaxation measurements are presented and discussed.
Highlights
Continuous Wave Free Precession (CWFP) is a special regime of the Steady-State Free Precession (SSFP) sequence introduced by Carr in 1958 [1]
In the SSFP regime, the nuclear magnetic resonance (NMR) signal is composed of a free induction decay (FID) after each pulse, and an echo signal preceding the following pulse
When Ernst and Anderson introduced pulse and Fourier transform NMR in 1966 [2], they studied the condition that optimizes the signal to noise ratio (SNR) in high-resolution NMR. They observed that the highest SNR was obtained when time between pulses (Tp) was shorter than the relaxation times
Summary
Continuous Wave Free Precession (CWFP) is a special regime of the Steady-State Free Precession (SSFP) sequence introduced by Carr in 1958 [1]. It is possible to acquire thousands of CWFP signals during one T1 period, enhancing the SNR by more than one order of magnitude during the same average time used for FID or echo signals in a conventional pulse sequences [12] Given these proprieties, CWFP methods have been used for quantitative analysis in conventional benchtop spectrometers and in line quality control systems, as is discussed . It is possible to acquire thousands of p , =i.e., CWFP signals during one period,regime enhancing the SNR by more is than one order during For these sequences, theT1CWFP of the magnetization obtained afterofamagnitude transition time average time used for FID or echo signals in a conventional pulse sequences [12]. Applications of CWFP in Steady State Regime first application of SSFP/CWFP sequences in low-field TD-NMR was the enhancement of
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