Chemical-shift selection

Flaase A and Frahm J 1985 Multiple chemical-shift-selective NMR imaging using stimulated echoes J. Magn. Reson. 64 94-102... 

Fig. 1.26 I maging of 129Xe using (a) regular imaging and (b) chemical shift selective imaging sequence along the cross section of three NMR tubes containing toluene, water...

S. Yao, A. G. Fane, J. M. Pope 1997, (An investigation of the fluidity of concentration polarisation layers in crossflow membrane filtration of an oil-water emulsion using chemical shift selective flow imaging), Mag. Reson. Imag. 15, 235. 

Hyperpolarized 129Xe not only allows for sufficient signal intensity for chemical shift selective gas phase MRI, it also provides the means for a unique type of contrast. The imaging contrast derived from the transport of hyperpolarized gases into the material can be utilized to obtain snapshots of the gas flow and diffusion into porous samples. In this context, it is important to appreciate that Figures... 

Fig. 5.3.2 (A) NMR spectrum of hyperpolar- abundance of approximately 25% of the, 29Xe ized 129Xe from a sample that contains bulk gas isotope. (B) 2D slice of 3D chemical shift phase (0.3 ppm) and xenon occluded within selective MRI of the bulk gas phase. (C-E) 2D aerogel fragments (25 ppm). The gas mixture slices of 3D chemical shift selective MRI of the used for the experiment contained 100 kPa of 25 ppm region for various recycle times T.

Fig. 5.3.3 (A) NMR spectrum of hyperpolarized 129Xe in NaX zeolites. (B) 2D slice in the flow direction of a 3D chemical shift selective MRI of gas in the zeolite pellets. (C) 2D slice perpendicular to the flow direction of the same 3D chemical shift selective MRI as in (A). Adapted from Ref. [14].

The 140-residue protein AS is able to form amyloid fibrils and as such is the main component of protein inclusions involved in Parkinson s disease. Full-length 13C/15N-labelled AS fibrils and AS reverse-labelled for two of the most abundant amino acids, K and V, were examined by homonuclear and heteronuclear 2D and 3D NMR.147 Two different types of fibrils display chemical shift differences of up to 13 ppm in the l5N dimension and up to 5 ppm for the backbone and side-chain 13C chemical shifts. Selection of regions with different mobility indicates the existence of monomers in the sample and allows the identification of mobile segments of the protein within the fibril in the presence of monomeric protein. At least 35 C-terminal residues are mobile and lack a defined secondary structure, whereas the N terminus is rigid starting from residue 22. In addition, temperature-dependent sensitivity enhancement is also noted for the AS fibrils due to both the CP efficiency and motional interference with proton decoupling.148... 

Technique for simultaneous slice selective and chemical shift selective excitation... 

Fig. 7. Double spin-echo imaging sequence for chemical shift selective imaging. The sequence applies spectral-spatial excitation pulses as presented in Fig. 6. Refocusing is performed with standard slice selective 180° pulses. For the examples in Fig. 8 echo times were chosen to TEi = 20 ms and TE2 = 60 ms.

Another approach to water and fat selective imaging which is also based on chemical shift elfects should be mentioned here The so-called Dixon method works with phase shifts in asymmetric spin-echo versus symmetric spin-echo acquisition. This method was shown to provide high-quality chemical shift selective images even in the presence of an inhomogeneous magnetic field. However, extreme high sensitivity to only one component seems to be obtained more difficult. 

Selective pulses and chemical-shift-selective filters... 

As pointed out in the Introduction, the essential part of a successful transformation of an nD experiment into its ID analog is the selective excitation of spins. There are two distinct tools available for this purpose selective pulses and chemical-shift-selective filters. 

The proposed ID TOCSY-NOESY experiment is illustrated by the assignment of NOEs from anomeric protons H-lc and H-ld of the polysaccharide 1. Because the resonances of H-lc and H-ld overlapped, this assignment was not possible from a ID NOESY spectrum as shown in fig. 3(b). Although these protons differed in their chemical shifts, it was not possible to separate them by chemical-shift-selective filtration because of the very fast spin-spin relaxation of backbone protons (20-50 ms) in this polysaccharide. Instead, a ID TOCSY-NOESY experiment was performed in which the initial TOCSY transfer from an isolated resonance of H-2c was followed by a selective NOESY transfer from H-lc. The ID TOCSY-NOESY spectrum (fig. 3(c)) clearly separated NOE signals of the H-lc proton from those originating from the H-ld proton and established the linkage Ic —> 6a. 

Chemical-shift-selective filters in multiple polarization transfer experiments... 

As an alternative to selective pulses, chemical-shift-selective filters (CSSF) were successfully used in ID COSY, ID RELAY and ID NOESY experiments when signals partially overlapping, but different in their chemical... 

An interesting possibility for assignment of NOEs from two or more overlapping anomeric protons is provided by a 1D RELAY-NOESY experiment [38] (fig. 13(d)). In this experiment the magnetization is transferred from H-1 protons to H-2 protons first and after a chemical-shift-selective... 

An alternative to using selective pulses in selective ID TOCSY has been proposed [52]. The frequency selection is instead accomplished by using a homonuclear ( H) chemical shift selective filter (CSSF) [53, 54]. The chemical shift filter for frequency selection consists of a non-selective 90° pulse which is set at the frequency of the selected signal, and a systematic increment of the chemical shift evolution between this pulse and the... 

G. Brix, M.E. Bellemann, U. Haberkorn, L. Gerlach, P. Bachert, W.J. Lorenz, Mapping the blodlstrlbutlon and catabolism of 5-fluorouracll In tumor-bearing rats by chemical-shift selective F-19 MR-lmagIng, Magn. Reson. Med. 34 (1995) 302-307. 

L. B. Moran, J.P. Yesinowski, Chemical-shift selective multiple quantum NMRas probe of the correlation length of chemical shielding tensors, Chem. Phys. Lett. 222 (1994) 363-370. 

Haase, A., Frahm, J., Haenicke, W., Matthei, D. (1985). H-NMR chemical-shift selective (CHESS) imaging. Phys. Med. Biol. 30,341-344. 

Pope et al. (1991) applied chemical-shift-selective imaging at microscopic resolution of various plant materials, including dried and undried fruits of fennel, to the study of selective imaging of aromatics and carbohydrates, water and oil. The non-invasive nature of the method gives it advantages over established methods of plant histochemistry, which involve sectioning and staining to reveal different chemical constituents. 

Pope, J.M., Rumpel, H., Kuhn, W., Walker, R., Leach, D. and Sarafis, V. (1 991) Applications of chemical-shift-selective NMR microscopy to the non-invasive histochemistry of plant materials. Magnetic Resonance Imaging 9(3), 357-363. 

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