Inversion composite pulse

In this section, three experiments are going to be discussed. Two of them, a broadband inversion and a Hahn spin echo, are well-known in the rotating frame. They need to meet the requirement of the phase coherence in PIPs in order to work properly in the Eigenframe. The third is a composite pulse with offset modulation. 

As shown in Fig. 21a, the simulated broadband inversion profile by the three PIPs resembles the profile by the composite pulse 90°180°90° except for a different excitation region. The inversion profile is severely distorted (Fig. 21b) if the three initial phases, phase relationship in the rotating frame is the wrong one in the Eigenframe. The phase coherence in PIPs needs to be considered even for PIPs with the same frequency shift, A/ = 50 kHz in this case. 

FIGURE 9.4 (a) Rotation of magnetization vectors in response to a 180, pulse.The vectors shown are off resonance, with (to — to /yB, ranging from 0.4 to 0.6. (b) Rotation of the same off-resonance magnetization vectors by a composite pulse, 90i240),90 produces much more effective inversion of the magnetization than the simple 180° pulse. From Freeman.19... 

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). 

Originally, the concept of prograde and retrograde composite pulses was developed for inversion pulses [Shal]. For instance, a prograde inversion pulse, is obtained for (Fig. 5.3.11(b)) [Shal]... 

One complicating factor associated with the implementation of composite pulses arises from the fact that many composite sequences have been designed with a particular initial magnetisation state in mind and may not perform well, or give the expected result, when the pulses act on other states. The two principal applications are the use of composite 180° pulses for population inversion, for example in the inversion-recovery experiment, or in the ubiquitous spin-echo for refocusing, in which they act on longitudinal and transverse... 

Numerous such composite pulse decoupling sequences have been developed along these lines over the years (Table 9.2), the most widely used being WALTZ-16 whose basic inversion element is 90 180 c270 c (or more succinctly 123, hence the name). This provides very effective decoupling over a band-... 

Example 3 Composite pulses are designed to overcome the imperfections of "ideal hard pulses". The Bloch simulator is a suitable tool to illustrate the superiority of a composite 180° pulse inversion compared to a standard 180° pulse. The application of composite pulses in decoupling techniques is discussed in sections 5.3.2 and 5.5.3. In Check it 4.3.4.3 the composite 180° pulse 90x/180y/90x (the indices x and y denote the pulse phase) is demonstrated. These simulations reveal how effective a composite 180° inversion pulse is for different rf offsets. 

M. H. Levitt and R. Freeman, "NMR population inversion using a composite pulse," J. Magn. Resonance 33, 473-476 (1979). 

The moral of all this is that considerable care must be taken when composite pulses are introduced into pulse sequences. It is wise to test the compensated pulse sequences vs the original uncompensated sequences on a known sample to see if improvements are achieved in reality. This offers a check on whether the composite elements have been correctly introduced and on whether the spectrometer is capable of correctly executing the desired sequences, many of which demand accurate control of rf amplitudes and phases for extended periods. The experimental performance of the composite pulse itself is best tested with a simple experiment, such as an inversion sequence (180 t, y -90 r-FIDA ) or a spin-echo sequence (90 180 c,y, c, y—FID with... 

Composite pulse A sequence of RF pulses designed to generate a desired spectral response. The individual pulses within a composite pulse may vary in time, frequency, and orientation to achieve the desired response. Composite pulses are commonly used to invert spins over a broad frequency range where a single pulse gives imperfect inversion far from resonance. Composite pulses are also used for excitation of selected frequencies. 

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