Stimulated echo sequence

Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. 

The reference scan is to measure the decay due to spin-lattice relaxation. Compared with the corresponding stimulated echo sequence, the reference scan includes a jt pulse between the first two jt/2 pulses to refocus the dephasing due to the internal field and the second jt/2 pulse stores the magnetization at the point of echo formation. Following the diffusion period tD, the signal is read out with a final detection pulse. The phase cycling table for this sequence, including 2-step variation for the first three pulses, is shown in Table 3.7.2. The output from this pair of experiments are two sets of transients. A peak amplitude is extracted from each, and these two sets of amplitudes are analyzed as described below. 

Fig. 4.1.5 A stimulated-echo sequence for detecting spin displacements. This sequence detects spin displacements along the Zt direction. TM is mixing time.

For the basic PFGE experiment a spin-echo experiment (either the two-pulse Hahn echo sequence, Fig. la, or the three-pulse stimulated echo sequence. Fig. lb) is combined with two magnetic field gradient pulses with duration 8 and separated by the time duration A. The gradient pulses generate a magnetic... 

Rapid stepping of the magnetic field, instead of using a second microwave frequency, has been used to measure interspin distances of the order of 20 A at X-band.28 A microwave pulse, called the pump pulse, is synchronized with the field step and occurs between the second and third pulses of a stimulated echo sequence. The effects of nuclear ESEEM were removed by dividing data obtained with a pump pulse (at the stepped magnetic field) by data obtained without a pump pulse. 

Fig. 6. ESEEM spectra of Ni in the F4j0-reducing and methyl viologen (MV)-reducing hydrogenases from M. thermoautotrophicum (A/f strain). Spectra (a) were obtained using a three-pulse stimulated-echo sequence, the time T between the second and third pulses being varied, (b) Fourier transform of FH2ase data (c) simulated spectra. Spectra are the average of recordings from g = 2.0 to 2.34. Reproduced, with permission, from Ref. 56.

Fig. 2.2.10 Echoes in NMR. (a) Two-pulse Hahn echo, (b) CPMG sequence with multiple refocusing pulses, (c) Stimulated echo sequence showing both, the Hahn echo (HE) or primary echo and the stimulated echo (SE). (d) Gradient echo.

For optimum signal strength in liquid spin-i systems, all pulses are 90° pulses. In this case, the stimulated-echo sequence can be written as... 

Fig. 9. (A) Selective excitation and destruction of magnetization using a magnetic field gradient pulse. PGSE sequences used for diffusional attenuation of the solvent signal, based on the Hahn spin-echo sequence (B) and the stimulated-echo sequence (C). In the Hahn spin-echo sequence the magnetization is always subject to spin-spin relaxation. However, in the stimulated-echo sequence the delays can be set such that A is mainly contained in t2 where the relaxation is longitudinal and thus this sequence is preferable for large solute molecules since the condition T2 < usually holds.

Diffusion-based methods such as DRYCLEAN are also applicable to cases where the solute of interest, although not having an inherenffy small diffusion coefficient, has an effectively small diffusion coefficient by virtue of its environment (i.e. restricted diffusion such that would occur if the solute were in the intracellular matrix). For example, the intracellular solutes of perfusing cells were able to be selectively observed whilst the extracellular water and solutes in the surrounding medium were suppressed using the stimulated-echo sequence. In a later work Potter et al used the same idea to suppress extracellular water in bacterial suspensions in porous media. 

For a Mims ENDOR sequence, the ENDOR response R" for a nucleus n depends on A" and varies with time t between the first and second pulses of the stimulated echo sequence according to the relation... 

Investigations of the spin-lattice relaxation time of single crystals of Feg at T = 1.27 K, utihzing the stimulated echo sequence, revealed two relaxation processes with time constants of 1.0 ps and 0.95 ms, respectively [153]. The authors attribute the former time constant to spectral diffusion, and speculate that the latter may reflect true spin-lattice relaxation, although the time constant is two orders of magnitude longer than values found by magnetization-detected EPR (see above) [129, 131]. 

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