Eddy-current delay

Fig. 6.27. The signal decay as a function of the gradient strength (C) of the water peak (a-d) and one representative peak of 15c (e-h) in the CDCI3 solution of [(15c)6(H2O)g] collected with a BPLED sequence when the eddy-current delay time (te) was (a, e) 150 (b. f) 50 (c, g) 14 and (d, h) 4 ms, respectively [62]. (Reproduced with permission from ref. [62]. Copyright 2005 American Chemical Society.)...

The final sequence also included an additional longitudinal eddy-current delay and bipolar gradients to compensate for eddy-current effects. Vastly improved results were achieved in terms of the absolute value of D measured and the temperature dependence of D, compared to the standard non-compensated sequence. 

Fig. 9. Modifications to the basic PFG spin-echo pulse sequence (a) stimulated echo (b) addition of alternating pulse gradients (c) addition of longitudinal eddy-current delay period.

Figure 9.4. The (a) BPP-STE and (b) BPP-LED diffusion sequences. The encoding gradients of the stimulated-echo are apphed as symmetrical bipolar pulse pairs of total duration 8 and the LED sequence is extended with an eddy current delay period T. Optional purging pulses Gpi and Gp2 may also been employed.

In the original paper describing this technique [9], the acronym LED appears to have been derived from the term longitudinal encode-decode, a reference to the pulse-gradient combinations employed, whereas this term is now taken to mean longitudinal eddy-current delay [10]. 

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...

Q-space diffusion MR was proposed as means to obtain structural information on a micron scale." The effect of diffusion pulse sequence on the signal decay was examined in q-space diffusion MR performed on micron-scale phantoms of different geometries and homogeneities. The effect of the pulse gradient stimulated-echo, the longitudinal eddy current delay (LED) and the bipolar LED (BPLED) pulse sequences was evaluated. 

One of the best alternatives to reduce the effects of eddy currents in NMR diffusion measurements is to use the LED and the BPLED sequences shown in Figs. 6.4a and 6.4b, respectively. From Fig. 6.4a, it is clear that the difference between the LED sequence and the STE diffusion sequence is the addition of two 90° pulses separated by a delay (te) at the end of the STE diffusion sequence. As a result of the fourth 90° rf pulse, the magnetization is again stored in the longitudinal direction while the eddy currents can decay during an eddy current settling period, Then, the fifth 90° rf pulse rotates the magnetization into the x-y plane where it can be detected. 

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