Spin-echo sequence

To understand how the chemical shift and multiplicity information is separated, we need to reconsider the spin-echo sequence described earlier. 

Figure 4. Dependence of the integrated intensity on the length between pulses, T, in the solid spin echo sequence. C6De/(Cs,Na)X at 2.4 molecules/supercage and 283 K. SW = 500 kHz.

The intense water signal and the broad protein resonances were suppressed by a combination of continuous secondary Irradiation at the water frequency and the Hahn spin-echo sequence (0[90 x-t-180 y-t-collect]). 

Fig. 4.4.7 (a) Reconstruction of the Stationary Helical Vortex (SHV) mode from MRI data acquired with the spin-tagging spin-echo sequence [27], The axial flow is upwards and the inner cylinder is rotating clockwise. The two helices represent the counter-rotating vortex streamtubes. (b) Construction of Poincare map for SHV [41]. The orbit of a typical particle is... 

All imaging experiments used the two-pulse spin-echo sequence a-T-2a-t-echo. In 3H experiments, a = 90 ° was used. For 2D 1H M RI of liquids, a 2-mm thick axial... 

Self-Diffusion of Desmopressin and Monoolein by NMR. The self-diffusion coefficient was measured with the NMR diffusion technique using a Bruker MSL 100 spectrometer. Two magnetic field gradient pulses were applied at either side of the 180-degree pulse in a [90x-T-180y-T-echo] spin echo sequence (7,8) (Figure 2). Due to diffusion, the amplitude of a component in the spin-echo spectrum is attenuated according to (7)... 

Both find their origin in the spin-echo sequence, devised hy Hahn in 1952 and used for the determination of relaxation times. 

Metais, A. and Mariette, F. 2003. Determination of water self-diffusion coefficient in complex food products by low field h-1 PFG-NMR Comparison between the standard spin-echo sequence and the T-l-weighted spin-echo sequence. J. Magn. Reson. 165, 265-275. 

Most clinical examinations apply robust spin-echo or fast spin-echo sequences. These types of sequences provide tissue contrast changes by variation of the chosen repetition time TR (time interval between succeeding RF excitations) and echo time TE (time delay between RF excitation and signal acquisition). 

Fig. 8. The Stejskal-Tanner pulsed gradient spin-echo sequence.

A(g) is the signal intensity observed with an applied gradient g, A(0) is the intensity in the absence of an applied gradient, y is the nuclear gyromagnetic ratio, and (5 and A are time intervals of the pulsed field gradient spin—echo sequence. 

It must be stressed that the spin-echo sequence is applied only during the detection period and its unique purpose is to estimate the signal amplitude (in a sense, it is a replacement for the simple 90° pulse). Consequently, in an arrayed multi-block experiment whose purpose is to measure Ti(Br), only the X value is varied, while the delays 5 and 8 are kept constant in order to make sure that no T2(Ba) effects leak into the experimental relaxation curves. Moreover, to avoid contamination of the echo by FID residues due to imprecise settings of RF pulses and to Bi inhomogeneity, proper phase cycling is highly recommended. 

Figure 12 Offset dependence of the intensity of the CT resonance in MAS NMR spectra of Na2MgEDTA-4H20, recorded with a single HS pulse placed before the spin-echo sequence. The experimental data (square points) were recorded using a 7mm probe (with 7kHz MAS rate) the solid red line is just a guide for the eyes. Also shown (blue line) is the simulated first-order quadrupole static powder pattern for the single Mg site in this compound.

Fig. 12. (a) 64 MHz H STEAM MRS spectrum of a brain tumor in vivo, and 500 MHz H HRMAS NMR spectrum of a biopsied sample of the same tumour using (b) presaturation, and (c) a spin echo sequence. Metabolites are identified directly on the spectra. For HRMAS, the samples were rotated at 4 kHz. Reproduced with permission from Ref. 128. Copyright 1999 Elsevier. 

Fig. 2. 37 illustrates the significance of a spin-echo sequence Components of transverse magnetization with different frequencies due to chemical shift or spin-spin coupling can... 

The antiphase relationship of the C13 —C13 doublet signals in the INADEQUATE spectrum can be eliminated by an additional spin-echo sequence (— 1/4Jcc — 180° — 1 /4 Jcc —) before the 90 monitor pulse [58]. The sensitivity of the experiment may be improved by the application of stronger magnetic fields or by using proton polarization transfer techniques [59]. 

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

Since JCH coupling is only in operation during one t,/2 period in the J-modulated spin-echo sequence with gated proton decoupling, as drawn in Fig. 2.51, the second Fourier transformation provides only one half of the actual Jal magnitudes. Therefore, resolution of smaller couplings will be poor. 

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