Selective excitation profile

Figure 3.4. A single monochromatic radiofrequency pulse has an effective excitation bandwidth that depends inversely on the duration of the pulse. A short intense pulse is therefore able to excite over a wide frequency window (a), whereas a longer weaker pulse provides a more selective excitation profile (b).

Gaussian pulses are frequently applied as soft pulses in modern ID, 2D, and 3D NMR experiments. The power in such pulses is adjusted in milliwatts. Hard" pulses, on the other hand, are short-duration pulses (duration in microseconds), with their power adjusted in the 1-100 W range. Figures 1.15 and 1.16 illustrate schematically the excitation profiles of hard and soft pulses, respectively. Readers wishing to know more about the use of shaped pulses for frequency-selective excitation in modern NMR experiments are referred to an excellent review on the subject (Kessler et ai, 1991). 

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). 

Frequency-selective REDOR (fsREDOR) is a very powerful technique developed for the study of 13C and 15N uniformly labeled peptides or proteins [92]. The basic idea of this technique is to combine REDOR and soft n pulses to recouple a selected 13C-15N dipole-dipole interaction in a multiple-spin system. Usually one could use Gaussian shaped pulses to achieve the required selective n inversions. Other band selective shaped pulses have been developed for a more uniform excitation profile [93]. In its original implementation, fsREDOR was used to extract the intemuclear distances of several model crystalline compounds [92], In the past few years, this technique has proven to be very useful for the study of amyloid fibrils as well. For the Ure2p10 39 fibril samples containing 13C and 15N uniformly... 

It is well-known that the excitation profile by a periodic pulse also has a pattern of multiple bands in response to the multiple effective RF fields. The DANTE sequence,26 for instance, was one of the most frequently used periodic pulse in the past for selective excitation of a narrow centre band. It is constructed by a long train of hard pulses with a certain delay between two adjacent pulses. The advantage of using the DANTE sequence over the weak, soft RF pulses relies on that it is not necessary to change the RF power level in the pulse sequence. Consequently, phase distortions and certain delays accompanied by the abrupt changes of the RF power level are avoided. 

TOPHAT-shaped 90° pulses are used in other cases as the best compromise with respect to the excitation profile, the phase homogeneity and length. Depending on the type of the detected spin-spin interaction - being either scalar or dipolar coupling - each selected spin is initially perturbed only once (ID TOCSY, ID INADEQUATE, ID C/H COSY, 2D TOCSY-COSY and 2D HMBC), or for several times (ID NOE). With each of the selected spins initially perturbed only once the inherently smaller transient NOEs would be detected in the latter case, whereas with the multiple excitation of a selected spin within the NOE build-up period the stronger steady-state NOEs are more or less approximated. 

On the other hand when more than two polarization transfer steps are combined (ID analogs of 4D experiments) one can make a decision, without affecting the end result, as to whether the second, the third or both these steps should be selective. In order to avoid possible losses of magnetization during the selective pulses, due to either relaxation and/or nonperfect excitation profiles, it is usually possible to make one of these steps nonselective. 

Fig. 9.1. (A) Gaussian (a) and sine (b) excitation profiles. (B) Composite (G3) Gaussian pulse. (C) Train of soft pulses modified after the DANTE sequence to achieve selective off-resonance excitation. (D) Redfield 21412 sequence. (E) Binomial 11, 121, 1331, 14641 sequences. (F) JR (a) and compensated JR (or 1111) (b) sequences. (G) Watergate sequence. (H) Weft (Superweft) sequence. (I) Modeft sequence. (J) MLEV16 sequence. (K) NOESY sequence with trim pulse. (L) MLEV17 sequence with trim pulses. (M) Clean-TOCSY sequence.

The best approach, adapted from an earlier proposal by Tomlinson and Hill (19), is to specify the desired frequency-domain excitation profile in advance, and then syntheize its corresponding time-domain representation directly via inverse Fourier transformation. The result of the Tomlinson and Hill procedure is shown at the bottom of Figure 2, in which a perfectly flat, perfectly selective frequency-domain excitation is produced by the time-domain waveform obtained via inverse Fourier transformation of the desired spectrum. 

It is rather tedious to move the spectral window every time we want to select a peak with a shaped pulse, but it is necessary as the center of the Gaussian excitation profile is at... 

With the hard pulses that we have encountered, the time variation of excitation is simply a square pulse, in which the rf power is turned on and off as fast as possible, and the amplitude is constant during the pulse. However, the use of a simple square pulse leads to a far from ideal frequency excitation profile for a soft pulse, because the Fourier transform of the square pulse is a sine function with sidelobes, as we saw in Fig. 3.8. Over the millisecond range used for a selective (soft) pulse, it... 

FIGURE 9.6 Illustration of the E-BURP selective excitation pulse, (a) Depiction of the time excitation function. (b) Frequency domain excitation profiles for absorption (solid line) and dispersion... 

Figure 11 Excitation profiles for the DANTE-Z (top) and DANTE (bottom) pnlse seqnences. Note the almost complete suppression of the sine oscillation using DANTE-Z excitation. (Reprinted from J. Magn. Reson., 83, Boudot, D. Canet, D. Brondeau, J. Bouhel, J. C., DANTE-Z—A New Approach for Accurate Frequency-Selectivity using Hard Pulses , 428-439. 1989, with permission from Elsevier)...

Fig. 5. Schematic formula of the dendrimer and of the Re(I) guest which can complex via hydrogen bonds, at its periphery. Upon irradiation at 2=330nm, where the dendritic structure is almost selectively excited due to the different absorption profile with respect to the rhenium complex, an efficient energy transfer can occur from the dendrimer to the organometallic moiety.

Infrared and Raman spectra are also sensitive to the JT effect, which induces a breakdown of the usual selection rules of infrared absorption or modifies the Raman excitation profiles " . ... 

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 ... 

Figure 3 Excitation profiles of the Watergate sequence obtained using either a selective 180° pulse sandwiched between two hard 90° pulses (left panel) or a 3-9-19 binomial sequence in place of the S element of the gradient echo (right panel) (see also Figure 2).

The numerical diversity of Na channel genes and their variable expression patterns increase the probabihty that the anticipated channel subunit mutations will generate multiple CNS excitability profiles finked to distinct clinical seizure types. Each of these selective excitability increases in specific axonal pathways may act to synchronize large neuronal populations by synaptic or non-synaptic mechanisms. This review considers the anatomical aspects of Na channel involvement in epileptogenesis, namely, how the site of expression of the Na channel gene, both within the neuron and within specific brain pathways, acts as an important phenotypic determinant of the disorder. 

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