Shaped gradient pulses

Many other pulsed NMR experiments are possible, and some are listed in the final sections. Most can be canied out using the standard equipment described above, but some require additions such as highly controllable, pulsed field gradients, shaped RF pulses for (for example) single-frequency irradiations, and the combined use of pulses at several different frequencies. 

Selected entries from Methods in Enzymology [vol, page(s)] Acquisition of frequency-discriminated spectrum, 239, 162-166, 170 sensitivity, 239, 169-173 constant-time, 239, 23-26 doublequantum filtered, 239, 236 gradient pulse experiments, 239, 185-189 protein structural information, 239, 377-379 pulse sequence and coherence transfer pathway, 239, 148-149 paramagnetic metalloprotein, 239, 494-497 data recording, SWAT method, 239, 166-169, 172 line shapes, effects of gradient pulses, 239, 162-166 identification of protein amino acid resonances, 232, 100 cyclosporin A, 239, 240-241. 

Figure 7.3 Sampling principles in 2D k space (a) Cylindrical coordinates. The angle of the field-gradient direction with respect to the x axis is given by 6= arctan Gy / Gx, (b) Cartesian coordinates. For rectangular gradient pulse shapes ky = -g Gy t1 and kx = -g Gx t2. Such sampling schemes are applicable to a slice which can be selected when the rf pulse is applied selectively in the presence of a gradient Gz. The areas of k space accessible by the pulse sequences shown are shaded in gray. TX transmitter signal ...

Fig. 4. Frequency-domain modulated gradient NMR rf and gradient pulse sequences, showing the (actual) gradient modulation wave form Git), the time integral of the effective gradient wave form Fit), and the spectrum of Fit). H )P directly samples the diffusion spectrum. The wave forms and spectra are for (a) double lobe/dc rectangular modulation, (b) single lobe/ac rectangular modulation, and (c) single lobe/ac sawtooth-shaped phase modulation. Note that pulse sequences (b) and (c) sample the diffusion spectrum at a single frequency.

Usually the rf pulses applied in an imaging sequence are not perfect. The pulses may differ in shape, phase, and amplitude from the required values. As a result, unwanted transverse magnetization may arise which leads to erroneous signals. This is so, in particular, for the 180° refocusing pulse of the Hahn echo. These transverse magnetization components can be dephased by application of homogeneity-spoil or crusher gradient pulses. 

The inhomogeneity in Bi, especially when using surface coils, can lead to a spatially dependent population of desired and undesired coherence pathways. Field gradient pulses in combination with shaped RF pulses lead to spatially selective excitation or refocusing and, in such cases, localization can be viewed as a type of coherence pathway selection. The inherent Bi gradients resulting from the inhomogeneous RF fields of surface coils have also been used for water suppression in in vivo experiments. ... 

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