The LED and BPLED Sequences

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. 

The BPLED sequence shown in Fig. 6.4b is a modification of the LED sequence shown in Fig. 6.4a. In this sequence, each gradient pulse (G), which has a duration of J is replaced in the LED sequence by two pulses of different polarity (G and — G) separated by a 180° rf pulse with a duration of dfl. The G-180-(—G) sequence causes the eddy currents to be cancelled out, while the diffusion gradients accumu- 

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