Dear all,
I read some 1H single voxel MRS papers and the majority of them show metabolites quantification for the range of 0.2 to 4.5 ppm.
Is there any reason to don’t quantify metabolites whose ppm are at the left of the H2O peak? Don’t you think that this way leads to an underestimation of some important metabolites since we don’t quantify all of their peaks? Like :Cr (missed pic at 6.64 ppm), pCr (missed pics at 6.58 and 7.29ppm), Ins (missed pic at 6.39 ppm), Glc (missed pic at 5.21 ppm)…
Also, I made a comparison between a “classical” ppm range (0.2-4.2 ppm) and an “extended” ppm range (0.2 to 8.2 ppm) and I found a significant difference between the two quantification ways of the metabolites ratios (/tCr) . How can someone choose the correct way/ratios in this situation?
The peaks on the ‘other side’ of the water peak are frequently referred to as the ‘downfield’ part of the spectrum. You are right that they contain a lot of interesting information - I really like this summary here.
Very briefly speaking, the reason why these resonances are usually not included in linear-combination modeling of the upfield spectrum is that they don’t scale well with the upfield part of the spectrum - there are many effects that act on the downfield, but not the upfield spectrum, for example chemical exchange of amide protons (which is exploited by CEST imaging). They are also typically strongly broadened, and difficult to disentangle.
You would have to appropriately model (or correct for) these effects in order to simultaneously model both the upfield and the downfield spectrum. I’m sure we will see a lot of research coming out in the next couple of years, as the downfield spectrum is increasingly well characterized. A good example is this paper, where the homocarnosine estimate from the downfield spectrum is used together with J-difference editing of the upfield spectrum.
I hope this helps for now. Let me know if I can help with anything else.
Dear @admin, Dear Georg,
I’m sorry for my late reply ( I was absent).
Thank you so much for this excellent answer and very interesting papers that k read with a great pleasure. I hope that more work will be done in this direction and then we can better quantify the dowfield peaks.
Best regards,
Salem