A critical assessment of noise-induced errors in 31P MRS: application to the measurement of free intracellular magnesium in vivo.

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 alpha- and beta-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 beta-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.