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Thursday, May 20, 2010

Gradient Spin Echoes for Selective Excitation

Shaped excitation pulses can replace the non-selective hard pulses typically used in a one-pulse measurement to achieve selective excitation. Another method of achieving selective excitation is the gradient spin echo using a selective 180° pulse. This technique is demonstrated in the figure below. A non-selective hard 90°x pulse is first given followed by a pair of identical pulsed field gradients sandwiching a soft selective 180° pulse about the y axis. The hard 90° pulse rotates all spin vectors onto the -y axis. During the first gradient pulse the spin vectors dephase and evolve according to their offset frequencies. The soft 180°y pulse flips a single resonance 180° about the y axis leaving all other resonances untouched. During the second gradient pulse, the "selected" resonance is rephased and its offset frequency evolution is refocused. The unselected resonances dephase more and continue to evolve according to their offset frequencies. The receiver is then turned on to collect the FID of the "selected" resonance, all others are dephased and therefore suppressed. This is demonstrated in the figure below which shows 1H NMR spectra for a mixture of methylence chloride and acetone. The bottom trace shows a standard one-pulse measurement. The middle and top traces show results from a selective gradient spin echo measurement with the selective 180° pulse set for methylene chloride and acetone, respectively.


levantine said...

Hello, Glenn.

I've been reading your blog for quite a long time with a great interest. You're always posting stuff that is really of high practical importance and it's quite difficult to find it anywhere else. Your pictures are also really very nice.

This time I have a question - what is the difference between two selective excitation schemes - selective 90-pulse and SPFGE? I mean, why use more complicated spin-echo sequence, if one can just apply selective 90-pulse? Also could you kindly explain, what are practical differences between SPFGE and DPFGE?

Thank you very much for your great job.

Glenn Facey said...

Dear Levantine,

Thank you for your comments and questions. If one is interested in simple selective excitation of a narrow resonance then collecting a simple one-pulse spectrum using a selective 90 degree pulse would certainly be simpler. The gradient spin echo shown here using a selective 180 degree pulse has the advantage that the gradients will suppress further anything which does not experience the 180 degree pulse. It therefore has better selection than a simple one-pulse experiment. It does however suffer from diffusion losses. As I understand it, the additional gradient pair in a double gradient spin echo measurement provides even better selectivity.


levantine said...

Thank you very much for your answer.

Anonymous said...

Hi Glenn,

The reason for the second gradient pair in a double gradient spin echo selective excitation is to refocus J coupling that evolves during the first echo period. A single spin echo would work fine for singlets but if you want to selectively excite a multiplet you will be in trouble. It is the same reason why CPMG T2 experiments are always done with an even number of 180's.