Lactate InversionWhy does the lactate peak flip upside down? None of the other peaks do.
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 Chemical structure of lactate showingJ-coupling between the methyl (blue) and methine (red) protons. Lactate, the conjugate base of lactic acid, is produced during the anaerobic metabolism of glucose. It is present in small quantities in normal tissues, but accumulates significantly during states of hypoxic stress.
The principal ¹H spectral peak of lactate is derived from its methyl protons (−CH3) and is centered at δ = 1.32 ppm. Because of J-coupling interaction with the solitary methine proton (−CH), lactate's primary peak is split into a doublet. The methine proton (at δ = 4.10 ppm), is in turn split into a quartet by the three methyl protons. This quartet is seldom identified on routine MRS studies, however, due to its small amplitude and proximity to the broad, dominant water peak at δ = 4.65 ppm. |
Lactate doublet (arrow) at 1.32 ppm is inverted at TE = 144 ms but upright at TE = 288 ms.
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The physical basis of J-coupling has been described more completely in a separate Q&A. For lactate, the measured value of the coupling constant (J) is ~7 Hz. This means that the resonant frequency of the methyl protons go in and out of phase in the spectrum every (1/J) = (1/7Hz) ≈ 144 ms. The protons are initially in phase with one another, so that images acquired at very short TE values (e.g., TE = 30) will show an upright lactate doublet. Images acquired at TE ≈ 144 ms will show an inverted doublet, while those at TE ≈ 288 ms will again show an upright doublet. The process is analogous to the more familiar in-phase/out-of-phase phenomenon between water and fat protons commonly seen in gradient echo imaging.
Although lactate is perhaps the most famous and easily visualized spectral doublet, it is not the only metabolite with J-coupling whose peak(s) may be flipped upside down. For example, the small secondary peaks of NAA and taurine are doublets that can also be inverted with appropriate choice of TE. The reason the major peaks (such as NAA, Cho, and Cr) always remain upright in ¹H NMR spectra is because they are singlets, arising from protons without nearest neighbor hydrogens that do not experience J-coupling.
References
Lange T, Dydak U, Roberts TPL, et al. Pitfalls in lactate measurements at 3T. AJNR Am J Neuroradiol 2006; 27:895-901.
Liu H-S, Chung H-W, Juan C-J. Anomalous J-modulation effects on amino acids in clinical 3T MR spectroscopy. AJNR Am J Neuoradiol 2008; 29:1644-1648.
Lange T, Dydak U, Roberts TPL, et al. Pitfalls in lactate measurements at 3T. AJNR Am J Neuroradiol 2006; 27:895-901.
Liu H-S, Chung H-W, Juan C-J. Anomalous J-modulation effects on amino acids in clinical 3T MR spectroscopy. AJNR Am J Neuoradiol 2008; 29:1644-1648.
Related Questions
Why do some spectra split into smaller peaks while others do not?
What is meant by in-phase vs out-of-phase imaging?
Why do some spectra split into smaller peaks while others do not?
What is meant by in-phase vs out-of-phase imaging?