Frequency-selective pulse

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.)...

This is a variation of the proton-detected shift-correlation experiment via long-range couplings proposed by Bax and Summers (Bax and Summers, 1986), with the difference that the first C pulse is substituted by a frequency selective pulse (Fig. 7.14) (Bermel et al., 1989 Kessler et al., 1989b,1990). This significantly increases resolution in the F dimension. For example, this can be used to remove the overlap between the cross-peaks of the carbonyl resonances of peptide bonds in proteins that all occur within a... 

Fig. 9 (a) Constant-time frequency-selective pulse sequence used for indirect measurement of 13C -13C distances in MB(i + 4)EK with uniformly 15N,13C-labeled L-alanine at Ala9 and AlalO. The 2D spectra in (b) and (c) and the extracted data points in (d) show that the dephasing is more rapid, with hence a shorter intemuclear distance, for 13C nuclei in the helical conformation (represented by squares) than for those in the non-helical conformation (represented by circles) (reproduced from [149] with permission)... 

For single crystals with STs well separated from the CT line, saturating or inverting STs without disturbing the CT is in principle straightforward, with use of frequency-selective pulses this is illustrated in Figure 5. For powders, on the other hand, the quadrupolar first-order broadened ST... 

The normal H spectrum of tissue consists of the dominant resonance of water at about 4.7 ppm and a weaker resonance from the methylene of lipid (fat) at about 0.9 ppm. This spectrum never changes significantly and is therefore usually uninteresting. However, by suppressing these resonances, many others, much weaker in intensity but far more diagnostically useful, are revealed. There are a number of suppression techniques. A frequency-selective pulse may first be applied to saturate the unwanted resonances. Water, being by far the... 

MRS applies frequency selective pulses in order to generate signal from a single volume of interest defined using MRI. The signal is acquired in the absence of a magnetic field gradient in order to detect the minor frequency shifts due to... 

The constant amplitude pulses are usually easier to implement and they do not require special hardware. A typical example and also the most frequently used kind of constant amplitude, frequency selective pulse is the DANTE pulse train [4]. In the simplest case it consists of a sequence of square pulses interleaved with periods of free precession. Unfortimately, the excitation profile of the DANTE sequence has extensive sidelobes. It also produces sidebands at frequencies ... 

Fig. 5.3.13 Hadamard encoding and decoding for simultaneous four-slice imaging. The encoding is based on four experiments, A-D. In each experiment, all four slices are excited by a multi-frequency selective pulse. Its phase composition is determined by the rows of the Hadamard matrix H2. The image response is the sum of responses for each individual, frequency selective part of the pulse. Thus, addition and subtraction of the responses to the four experiments separates the information for each slice. This operation is equivalent to Hadamard transformation of the set of image responses. Adapted from [Miil21 with permission from Wiley-Liss. Inc., a division of John-Wiley Sons, Inc.

In solid-state imaging, frequency-selective pulses are difficult to apply because of the rapidly decaying FID. Nevertheless, for rare spins, selective excitation may be possible [Davl], but also saturation sequences which suppress longitudinal magnetization at selected frequencies for durations of the order of the spin-lattice relaxation time Ti can be applied (cf. Section 5.3.2) [Dodl, Nill]. For abundant spins, demanding line-narrowing sequences composed of hard pulses must be employed. 

Hadamard spectroscopic imaging (HSI) is a technique to obtain localized spectroscopic information from n regions of interest in n scans [Boll, Hafl, Goel, Goe2, Goe4, MU14]. It is a straightforward extension of the multi-frequency selective-pulse technique... 

The measured responses to the combinations of multi-frequency selective pulse excitation can be unscrambled for each volume element by transformation with a super-Hadamard matrix. The dimension of this matrix equals the product of the dimensions of the Hadamard matrices used for encoding each space axis. 

DANTE-type frequency-selective pulses have also been used to selectively excite specific nuclei in the spin system and this has been employed to simplify MQ spectra. This approach is limited to relatively simple spin systems where the resonances of some nuclei are well separated from the normally overlapped signals in the spectrum.28,29... 

In many instances, it is important that some form of chemical selectivity be applied in magnetic resonance imaging so as to distinguish nuclei in one or more specific molecular environment(s). There are many ways of doing this and we discuss here just three. The first option is to ensure that one of the excitation RF pulses is a narrow bandwidth, frequency selective pulse applied in the absence of any gradient [22]. Such a pulse can be made specific to one particular value of the chemical shift and thereby affects only nuclei with that chemical shift. In practice this can be a reasonable method for the specific selection of fat or oil or water in a mixed hydocarbon/water system. 

The upper indexes, 00), 01), 110), 111), are to emphasize that the corresponding operators only execute a true Hadamard gate when they act on the indicated states. The indexes 01, 12, and 23 indicate the pulse transition as indicated in Figure 4.2. However, the operators c), d) and e), g) can be implemented by a single pulse sequence if we use two-frequency pulses to excite simultaneously two transitions. For example, Uhj. = where 01-23 indicates a two-frequency selective pulse that act simultaneously on the transitions 01 and 23 see Figure 4.2, will implement a operation independently of the initial state. All the Hadamard transformations indicated in (4.2.11) are self-reversible. 

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