Flip angle selection

In a coupled spin system, the number of observed lines in a spectrum does not match the number of independent z magnetizations and, fiirthennore, the spectra depend on the flip angle of the pulse used to observe them. Because of the complicated spectroscopy of homonuclear coupled spins, it is only recently that selective inversions in simple coupled spin systems [23] have been studied. This means that slow chemical exchange can be studied using proton spectra without the requirement of single characteristic peaks, such as methyl groups. 

Figure 6.2 Pulse sequences for some common 3D time-domain NMR techniques. Nonselective pulses are indicated by filled bars. Nonselective pulses of variable flip angle are shown by the flip angle )8. Frequency-selective pulses are drawn with diagonal lines in the bars. (Reprinted from J. Mag. Reson. 84, C. Griesinger, et al, 14, copyright (1989), with permission from Academic Press, Inc.)...

The shape of any rf pulse can be chosen in such a way that the excitation profile is a rectangular slice. In the light of experimental restrictions, which often require pulses as short as possible, the slice shape will never be perfect. For instance, the commonly used 900 pulse is still acceptable, while a 1800 pulse produces a good profile only if it is used as a refocusing pulse. Sometimes pulses of even smaller flip angles are used which provide a better slice selection (for a discussion of imaging with small flip angles, see Section 1.7). 

Fig. 3. HNCA (a) and two implementations of HNCA-TROSY (b-c) experiments for recording intraresidual HN(/), 15N(/), 13C"(i) and sequential 1 HN(7), l5N(/), 13Ca(i — 1) correlations in 13C/15N/2H labelled proteins. Narrow and wide bars correspond to 90° and 180° flip angles, respectively, applied with phase x unless otherwise indicated. Half-ellipse denotes water selective 90° pulse to obtain water-flip-back.88,89 All 90°...

The peak rf amplitude required to achieve optimum excitation with a selective excitation pulse is given in comparison to the rf amplitude required to achieve an on-resonance 90° flip-angle with a selective rectangular pulse, the simplest conceivable shape. 

For selective irradiations with a flip angle of 180°, one can distinguish two groups of shaped pulses inversion pulses, which change the sign of Zeeman... 

In Section 2-4, we encountered a number of spectral acquisition parameters. The selection of these parameters is relatively direct and straightforward, with the exception of the flip angle a and the decoupler modulation frequency (for WALTZ decoupling). Neither a nor the decoupler modulation frequency is entered directly, and both parameters require that the spectrometer be calibrated. 

Fig. 15a, b Plot of the intensity corresponding to a 3Q excitation b 3Q to SQ conversion in a 3QMAS experiment, as a function of the flip angle and for several values of the Vq/v f ratio. The curves were obtained using programme PULSAR [35]. Intensities are normalised to the intensity of the signal obtained in a conventional MAS experiment using selective excitation of the central transition by a ti/2 pulse... 

The inhomogeneity of the excitation RF fields from transmit-receive surface coils can be used to advantage for selecting a sensitive volume slab based on approximate distance from the coil. Limited spatial selectivity can be achieved by adjusting the flip angle to best select the depth of interest. 

Typical flip angles are in the order of 15° for the slice-selective excitation pulses. The intensity of the FID after such a pulse corresponds to 25% (sin 15°) of the intensity after a 90° pulse. However, more than 96% (cos 15°) of the longitudinal magnetization are preserved, enabling fast repetition rates. The method can be employed in combination with the backprojection imaging scheme and with spin-warp imaging by phase encoding of spatial information. 

Fig. 6.3.3 [Hou2] Vector diagrams of magnetization illustrating the principle of phase modulation in rotating-frame imaging for a selected volume element. Following an initial y pulse of length ti with variable flip angle 6, the magnetization is placed into the xz plane (left).

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