Spoiler gradients

The MQC intermediate state in coherence ( INEPT ) transfer can also be used to clean up the spectrum. In this case, we can apply a double-quantum filter (using either gradients or a phase cycle) to kill all coherences at the intermediate step that are not DQC. We will see the usefulness of this technique in the DQF (double-quantum filtered) COSY experiment (Chapter 10). As with the spoiler gradient applied to the 2IZSZ intermediate state, a doublequantum filter destroys any unwanted magnetization, leaving only DQC that can then be carried on to observable antiphase magnetization in the second step of INEPT transfer. 

PRESS, based on a double spin-echo, requires exact 90° and 180° pulses ideally. However PRESS too can be applied in the inhomogeneous surface coil Bi field, provided the out-of-voxel signal from nonideal pulses is removed using phase-cycling schemes and/or large spoiler gradient pulses. The same applies to the stimulated echo sequence STEAM. More complicated versions of PRESS may be implemented... 

Point resolved spectroscopy (PRESS) and stimulated-echo acquisition mode (STEAM) use three selective RE pulses in combination with linear field gradients to yield localization of the VOI in a single shot. Thus, the localization provided by these techniques is not degraded by subject motion or system instability. Generally, the use of three RF pulses leads to the formation of five coherent pathways for echo formation. Both PRESS and STEAM are designed to yield a particular echo that is localized to the VOI the other echoes are suppressed by the use of spoiler gradients and phase cycling of the RF pulses. The... 

Fig. 7 PANS Y-HSQC pulse sequence for parallel acquisition of H-X and F-X correlated spectra. Filled rectangles are 90° pulses while open rectangles are 180° pulses. Spoiler gradients are denoted G and G2...

Fig. 9 PANSY-HMBC pulse sequence for simultaneous H-X and F-X correlation experiments. Spoiler gradients are denoted Gs, coherence encoding gradients are denoted G. Gradients Gi and G2 are coherence decoding gradients...

A different approach to destroying the 12C-bound artifact is to use the gradient in a simpler way—as a spoiler that just kills all of the magnetization in the x -y plane while our desired signal is stored briefly on the z axis. To do this we go back to our discussion of intermediate states in INEPT coherence transfer (Section 7.10) and recall that instead of using simultaneous 90° pulses on lH and 13C to effect coherence transfer, we can start with the H 90° pulse and then, after a short delay, complete the INEPT transfer with the 13C 90° pulse ... 

A gradient spoiler pulse can be applied in intervals where the desired signal has coherence order p = 0. These applications include gradient-enhanced z and... 

Figure 6 Magnetization transfer technique. (A) Two-pool model used for the evaluation. This is certainly a simplification of reality, although imaging data quality does not justify further elaboration. (B) MTC imaging sequence. A gradient-recalled echo data acquisition is used once more. The spoiler shown is used to dephase residual transverse magnetization after the end of the off-resonance irradiation period (of length t). Practical sequences of all types use pulses like this to eliminate the unwanted signals from spins pursuing complex patterns of excitation and recovery.

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