Composite pulses spin locks

For many applications, the basis sequence can be iteratively constructed from simplw tarting sequences (Tyko, 1990). MLEV-4-type super cycles RRRR or RRRR (Levitt et al., 1983) are examples of simple iterative schemes for the construction of basis sequences with vanishing effective fields from a starting sequence R, which is a (approximate) composite inversion pulse R. Here, the composite pulse R is identical to R, except that the phases of all square pulses are shifted by 180°. The MLEV-16 super cycle RRRR RRRR RRRR RRRR (Levitt et al., 1983) suppresses effective fields even better. MLEV-4- and MLEV-16-type supercycles are often used in the construction of broadband Hartmann-Hahn mixing sequences. In these sequences, an effective spin-lock field can be introduced by adding an uncompensated additional pulse after each complete supercycle (see Section X). 

X and -X axes is altered by adding an additional uncompensated spin-lock period (Davis and Bax, 1985). The bandwidth of the DB-1 sequence can be further improved if composite pulses (60° 300°) and (60°300° ) are applied when the phase of the spin lock is changed from +x to —x and from -X to +x, respectively, in order to align the magnetization with the respective effective spin-lock axes (sequence DB-2 Bax and Davis, 1986). 

Although in principle the simple scheme presented in Fig. 5.59 should provide TOCSY spectra, its suitability for practical use is limited by the effective bandwidth of the continuous-wave spin-lock. Spins which are off-resonance from the applied low-power pulse experience a reduced rf field causing the Hartmann-Hahn match to breakdown and transfer to cease. This is analogous to the poor performance of an off-resonance 180° pulse (Section 3.2.1). The solution to these problems is to replace the continuous-wave spin-lock with an extended sequence of composite 180 pulses which extend the effective bandwidth without excessive power requirements. Composite pulses themselves are described in Chapter 9 alongside the common mixing schemes employed in TOCSY, so shall not be discussed here. Suffice it to say at this point that these composite pulses act as more efficient broadband 180 pulses within the general scheme of Fig. 5.60. 

There are essentially two approaches based on composite-pulse methods in widespread use for the practical implementation of the TOCSY experiment (Fig. 5.68). The first of these [51] (Fig. 5.68a) is based on the so-called MLEV-17 spin-lock, in which an even number of cycles through the MLEV-17 sequence are used to produce the desired total mixing period. To ensure the collection of absorption-mode data, only magnetisation along a single axis should be retained, so it is necessary to eliminate magnetisation not parallel to this before or after the transfer sequence. In this implementation, this is achieved by the use of trim-pulses applied for 2-3 ms along the chosen axis. 

The first of these arises when the long spin-lock pulse acts in an analogous fashion to the last 90" pulse of the COSY experiment so causing coherence transfer between J-coupled spins. The resulting peaks display the usual antiphase COSY peak stmcture and tend to be weak so are of least concern. A far greater problem arises from TOCSY transfers which arise because the spin-lock period in ROESY is similar to that used in the TOCSY experiment (Section 5.7). This may, therefore, also induce coherent transfers between J-coupled spins when these experience similar rf fields, that is, when the Hartmann-Hahn matching condition is satisfied. Since the ROESY spin-lock is not modulated (i.e. not a composite pulse sequence), this match is restricted to mutually coupled spins with similar chemical shift offsets or to those with equal but opposite... 

Table 9.2. Selected composite-pulse sequences for broadband decoupling and spin-locking...

Proton relaxation under multiple pulse conditions has also been used to characterise phase composition in, for example, PET [95,96], and polyethylene [120]. The technique is particularly useful in the case of PET because the phases present generally do not show large differences in the decay times of their FID components, so FID analysis would be particularly problematic. Although the problem of assessing the extent to which spin diffusion is suppressed also applies to relaxation under multiple pulse conditions, the available experimental evidence suggests that they may be more effective in practice than the corresponding off-resonance spin-locking experiment [96]. This is almost certainly due to practical considerations rather than theoretical ones. 

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