A critical assessment of noise‐induced errors in 31P MRS: Application to the measurement of free intracellular magnesium in vivo

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Abstract
Phosphorus magnetic resonance spectroscopy (31P MRS) is a noninvasive technique that has been used to estimate free intracellular magnesium concentration (free [Mg2+]). Free [Mg2+] is computed from the chemical shift separation between the α‐ and β‐phosphate resonances of ATP. The current study was undertaken to critically assess the influence of noise effects in estimating free [Mg2+] in rat brain subjected to moderate parasagittal fluid percussion‐induced injury. We show that contrary to published data, free [Mg2+] does not significantly change for up to 4 h after moderate trauma in different rat strains and using different surface coils. Before injury, free [Mg2+] = 0.56 ± 0.11 (mean ± SD, n = 36) and 4 h post‐trauma, free [Mg2+] = 0.56 ± 0.28. Our results suggest that explanations for this discrepancy comprise errors of chemical shift assignments accompanying low signal‐to‐noise ratios and the method of analysis employed. Indeed, the authors propose that spectra of β‐ATP signal‐to‐noise ratio less than 5:1 will produce significant noise‐induced errors. We conclude that without knowledge of the inherent errors in 31P MRS spectroscopy and appropriate statistical analysis, caution should be exercised in calculating free [Mg2+] and using these changes as a basis for proposing pharmacotherapeutic interventions.