FFC data acquisition sequences

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR 

One thus arrives at the concept of pulser sequences (rather than RF pulse sequence) in which the programmable pulser generates a sequence of synchronized events and controls a number of distinct devices. Analogously, the term RF phase-cycling is no longer appropriate (in Stelar terminology, it is replaced by X-device cycling). 

In FFC relaxometry, the most conspicuous pulser-controlled device (apart from the RF excitation channel) is the magnet system. In other words, we generate Bq field pulses of considerable amplitude, often switching the magnet field between zero and a maximum value of over 1 T, and we rigorously synchronize such Bq pulses with the RF signal-excitation and/or preparation pulses. This, moreover, does not exclude the possibility to control other devices as well. 

Like any sequence of events, an FFC experiment can be intended as a sequence of elementary intervals during each of which all system control lines maintain constant values. One needs to keep in mind, however, that while a control line transition is always very fast (settling times of the order of Ins), the controlled device/parameter may require a much longer 

In order to control the impact of the field-transient periods on the measured data, it is necessary to devise FFC sequences in such a way that each elementary interval falls into one of two possible categories  

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