Dear all,
I have one question related to peak attenuation in Diffusion NMR.In the starting for minimum gradient(2%) we take area of the peak is 1. when we increase the gradient the intensity decreases.
my question is that is there any possibility that it slightly increase then decreases with gradient ? thanks in advance
I'd say that it's unlikely to be something "real". There are some scattering-like modulations in echo attenuation curves of very regularly organized open-pore materials (first described by Paul Callaghan over 20 years ago) but they require special samples and would not appear in this low gradient range. Nevertheless, it might help you to map the low gradient intensity range of echo attenuations a bit closer to track down possible sources of artifacts in your measurement. Sorting out these might need a lot of details on your diffusion pulse sequence and NMR hardware. Good luck!
You could get an effect as you describe, if you still have a B0 gradient across the sample after shimming. If the direction of that static gradient is the same or close to the direction of eddy-current gradient after your last diffusion weighting pulse and the polarities are opposite, then the two would cancel each other at a particular diffusion weighting gradient pulse amplitude. If what you are seeing is indeed such a shimming artifact, you should be able to make it disappear (well hide the effect actually) by flipping the polarity of the diffusion weighting gradients. Increasing the time between the last diffusion weighting gradient and the echo would also reduce the effect. Depending on the way your pulse sequence is written, you might be able to do this by increasing your echo time.
There is another explanation for the effect. If your sample is very concentrated there could be some radiation damping taking place. This broadens the strong lines and thus reduces their intensity. As the intensity decreases and the line narrows it could be come taller than the signal at low gradient strength.
The area of the signal is not affected and should show the normal decay.
Thanks all , for your answer . Can you refer me some literature regarding this?
Hello, you have not mentioned the deatails of the case (i.e. b-values, diffusion time, etc.). However it is possible to witness increased non-Gaussian effects on your signal. Usually the non-Gaussian effects result in positive kurtosis as in DOI: 10.1002/mrm.20508 which results in less decreases in signal than the exponential at lower b-values. But we have recently shown that it is possible to have negative kurtosis in biology and have introduced the prostate ducts as a candidate (DOI: 10.1002/mrm.26230 and DOI: 10.1002/mrm.26340)
If that is the case and you have further questions, please do not hesitate to contact me.
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