Pulse-field gradient-stimulated echo sequence

Fig. 2. 500 MHz pulsed field gradient stimulated echo spectra of a mixture of choline chloride, acetone and TSP in D2O, obtained using the LED sequence as described in the text. A signal from an unidentified impurity is visible just to high field of the acetone resonance.

Pulsed field gradient spin echo NMR is generally the method of choice for diffusion measurements on liquid samples (see Section 5.1 in this chapter). With modem high field instruments, however, severe problems can arise when it is applied to samples with very high proton concentrations because of the presence of radiation damping. Connel et al reduced the problems by a suitable choice of experimental parameters, in particular the use of modified stimulated echo pulse sequences with a reduced flip angle for the first pulse. 

Figure 15.1. Pulse sequences for the standard pulsed field gradient spin-echo (a) and pulsed field gradient stimulated spin-echo (b) techniques. Note that in the latter case, the length (5) and strength (C) of the gradient pulse is omitted for clarity but have the same meaning as in part (a)...

Diffusion-relaxation correlation has been utilized to study biological tissues, e.g., compartmentalization in tissues [32-35]. In many reports, a sequence that combines a stimulated echo-type sequence with a pulsed field gradient and a CPMG as a detection has been described [35]. Other pulses sequences have also been used to study the diffusion-relaxation correlation, e.g., Ref. [36]. 

Fig. 2.7.2 Diffusion-relaxation correlation se- The detection (2nd) segment for both is a quences using pulsed field gradients, (a) The CPMG pulse train that is similar to that in first segment is a spin-echo with the echo Figure 2.7.1. The amplitude or the duration of appearing at a time 2tcpi after the first pulse, the gradient pairs in both sequences is (b) The first segment is a stimulated echo incremented to vary the diffusion effects, appearing at a time tcpi after the third pulse.

Fig. 4.5.5 Pulsed field gradient sequences to obtain velocity and diffusion data (a) spin-echo (PGSE) and (b) stimulated-echo (PGSTE). The application of imaging gradients C Gy and Gz allows the measurement of velcocity maps and spatially-resolved diffusion coefficients and size distribution in emulsions.

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... 

Fig. 1. The pulse sequences for the pulsed field gradient echo NMR experiment (a) Hahn echo, (b) stimulated or three-pulse sequence.

Figure 10 shows the incorporation of the velocity-encoding field gradient pulses in a stimulated-echo imaging pulse sequence which was used in this work. A remarkable advantage of using the stimulated echo is that the relaxation... 

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.

PRESS, based on a double spin-echo, requires exact 90° and 180° pulses ideally. However PRESS too can be applied in the inhomogeneous surface coil Bi field, provided the out-of-voxel signal from nonideal pulses is removed using phase-cycling schemes and/or large spoiler gradient pulses. The same applies to the stimulated echo sequence STEAM. More complicated versions of PRESS may be implemented... 

Figure 8 Pulse sequences commonly used for PGSE measurements sequences with (a) spin-echo (b) stimulated echo (c) stimulated echo and longitudinal eddy-current delay (LED) (d) stimulated echo with bipolar pulsed field gradients and LED. Narrow and wide black rectan es represent 7t/2 and tt radiofrequency pulses, respectively. Narrow and wide open rectangles are field-gradient pulses of duration /2 and...

Fig. 5.3.4 (A) Stimulated echo dynamic NMR microscopy pulse sequence. The first field gradient pulse (g,) of duration 8 serves to encode spatial positions of spins and the second field gradient pulse has a refocusing effect.

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