Small flip-angle pulses

For this reason ki(t) is also called the impulse-response function. For excitation of the linear response in NMR, that is, for excitation with small flip-angle pulses, k t) is identical to the FID (Fig. 4.1.1(b)). If the input is a weak continuous wave with adjustable frequency co, then x(t) = exp in>r, and the response is given by the input wave attenuated by the spectrum K (to) of the impulse-response function ki (r),... 

The basic principle is depicted in Fig. 6.2.16(a) [Cho2]. The rf excitation is a train of small flip-angle pulses forming a DANTE sequence, i.e., the sum of flip angles is 90° (cf. Section 5.3.4). They are applied while the read gradient Gx is tuned on. The response to the DANTE excitation is refocused in a Hahn echo, which itself consists of a train of echoes. This echo train is detected in the presence of the read gradient Gx... 

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). 

With the popularity of the pulse Fourier transform nmr spectrometers, recent CIDNP experiments were largely performed in a FT spectrometer. The technique has been reviewed by Kaptein (80), who has emphasized the important facts initially pointed out by Ernst et al. (55) in connection with using a FT spectrometer for CIDNP experiments. For homonuclear multiplet effects a small flip angle of less than 20° should be used to exhibit the multiplet features. As a 90° pulse turns the magnetization vector from the z axis to the xy plane, it would eliminate the homonuclear multiplet effects but not the net effects. The... 

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