Stimulated-echo filter

Fig. 7.1.3 [Blii2] NMR-timescale of molecular motion and filter transfer functions of pulse sequences which can be utilized for selecting magnetization according to the timescale of molecular motion. The concept of transfer functions provides an approximative description of the filters. A more detailed description needs to take into account magnetic-field dependences and spectral densities of motion. The transfer functions shown for the saturation recovery and the stimulated-echo filter apply in the fast motion regime.

Fig. 7.2.1 Pulse sequences for T and related magnetization filters, typical evolution curves of filtered magnetization components, and schematic filter transfer functions applicable in the slow motion regime. Note that the axes of correlation times start at Tc = Wo (a) Saturation recovery filter, (b) Inversion recovery filter, (c) Stimulated echo filter.

The inverse filter transfer function is obtained for the stimulated-echo filter (Fig. 7.2.1(c), cf. Fig. 2.2.10(c)). It consists of three 90° pulses. The second pulse generates longitudinal magnetization, which is modulated in amplitude by the precession phases accumulated during the evolution time t /2 between the first two pulses. The filter time ff is the time between the second and the third pulse. Here the modulated components relax towards thermodynamic equilibrium with the longitudinal relaxation times Ti(r), and the memory of the initial two pulses is lost as tf increases. Therefore, the amplitude of the stimulated echo is given by (cf. eqn (2.2.39))... 

The echo maxima are weighted by a function of both T and T2. Similarly, the stimulated echo (Fig, 7.2.1(c)) can be used as a combination filter to introduce T and T2 weights. The echo time (tf2 in Fig. 7.2.19(c)) determines the T2 weight and the mixing time between the second and the third pulses (tn in Fig. 7.2.19(c)) the T weight. Note that the filter transfer functions for T) contrast by saturation recovery and the stimulated echo are inverted (cf. Fig. 7.2.1 (a) and (c)), so that both combination filters introduce different contrasts (cf. eqn (7.2.3)). 

Figure 9.24. Solvent signal suppiession by a diffusion filter. The sample is 1 mM lysozyme in 50 50 H20 D20 and suppression in (a) was achieved using a H stimulated-echo-based sequence employing a diffusion delay A of 50 ms, bipolar gradient pairs of 4 ms total duration ( ) and gradient strengths of 0.43 Tm. The standard spectrum is shown in (b).

Concerning method implementations, a relevant paper deseribes two transverse-relaxation-optimized (TRO) N-filtered PFG stimulated-echo (STE) experiments for studies of translational dilfusion in solution, H-TRO-STE and N-TRO-STE, which include CRINEPT and TROSY elements. Preliminary results provide an extended platform for evaluating the NMR experiments available for dilfusion measurements in struetural biology projects involving molecular particles with diflerent size ranges. ... 

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