WISE pulse sequence

Fig. 13. (a) The J-WISE pulse sequence.18 (b) The constant time J-WISE pulse sequence.18 The 0m pulse angle is the angle between the applied magnetic field of the NMR experiment and the direction of the effective field in the homonuclear decoupling sequence applied to the 1H. 6m> — 90°—6m. 

Fig. 1.19 Spin-echo based pulse sequence to each gradient pulse, A the separation between encode velocity change. The gradients are each pair of bipolar gradient pulses and tm the stepped pair-wise independently (2D VEXSY) mixing time between the bipolar gradient pairs, or simultaneously (1 D VEXSY). For a VEXSY The opposite polarity of the bipolar gradient experiment, 7q to k4 are usually applied along pair is realized by an inversion 180° pulse, the same spatial direction. 8 is the duration of...

Fig. 10. The pulse sequence for the WISE experiment.21 This two-dimensional experiment separates H wideline spectra according to the isotropic l3C chemical shift of the 13C each H is bonded to. An initial H 90° pulse creates transverse H magnetization that is allowed to evolve in t. A short cross-polarization step then transfers the remaining H magnetization to the nearest 13C spin, i.e. the bonded one. The resulting 13C transverse magnetization is then allowed to evolve in ti under magic-angle spinning, where an FID is recorded.

Pulse Sequence. The pulse sequence for WISE experiments can be considered as a derivative of the CP pulse sequence (Figure 49). After the initial (tt/2) pulse, the H magnetization is allowed to evolve during t. Subsequently, the remaining H magnetization is transferred via CP to C. The resulting C magnetization is then recorded in 12 The very short CP step (50 150 p,s) is essential to prevent H spin diffusion. 

The pulse sequence for the 2D WISE experiment (Fig. 20a) consists of three... 

Fig. 20. The pulse sequence for 2D (a) WISE and (b) WIM/WISE NMR. H evolution time, H spin diffusion (mixing) time, t2 detection time. (Figure 1 in the original literature X. Qiu and P. A. Mirau, J. Magn. Reson., 2000, 142, 183.)...

Barzykin, A.V. Theory of spin echo in restricted geometries under a step-wise gradient pulse sequence, /. Magn. Reson., 139, 342, 1999. 

Fig. 12. The WISE experiment16,17 pulse sequence as applied to organic solids, i.e. 1H and 13C. See text for details.

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... 

Another method to optimize image contrast includes the addition of an inversion recovery pulse before the typical SE sequence to suppress signal from cerebrospinal fluid (CSF). This yields so-called fluid-attenuated inversion recovery (FLAIR)-or CSF-suppressed DWI images or ADC maps. With this approach, the partial-volume effect of cerebrospinal fluid can be substantially reduced, which is particularly relevant in cortical regions due to the frequent CSF contribution to individual voxels (Latour and Warach 2002). The technique may enhance lesion conspicuity in the vicinity of CSF and has been shown to be of value in research studies utilizing pixel-wise analyses to predict tissue fate. The technique is not yet implemented in many centers, although this may change in the near future. 

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