Gradient echo signal selection

Fig. 9.1.5 [Akol] Timing of rf and gradient signals for the PRESS method. Transverse magnetization of a volume element is selected by formation of a double Hahn echo with selective pulses in the presence of orthogonal field gradients.

Figure 5.44. The ID double-quantum filter. The sequence is derived from the 2D experiment by replacing the variable ti period with a fixed spin-echo optimised to produce antiphase vectors (A = 1 /2J) as required for the generation of double-quantum coherence. Signal selection is then as for the 2D experiment, and gradient selection may be implemented as in Fig. 5.41.

When implementing this sequence it may be necessary to add attenuation to the transmitter to increase the duration of each pulse so that the shorter elements do not demand very short (< 1 xs) pulses (note the similarity with the requirements for the DANTE hard-pulse selective excitation described above). The binomial sequences can be adjusted to provide an arbitrary overall tip angle by suitable adjustment of the tip angles for each element. For example, inversion of all off-resonance signals can be achieved by doubling all elements relative to the net 90 condition. Exactly this approach has been exploited in the gradient-echo methods described below. 

Branca et alP reported a detailed and understandable analysis of the evolution of various coherence orders in a Correlated 2D spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED) like pulse sequence, used to select a signal from intermolecular multi quantum coherences (iMQCs). Because the signal to-noise-ratio of iMQC is much lower than the signal from conventional single quantum coherence (SQC), an optimization of experimental parameters is a necessity when measurements are made with iMQC. For this purpose a phase cycle is shown that not only allows a simpler selection of a particular quantum coherence order, but also removes receiver artifacts. 

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