The water resonance as an alternative pH reference: Relevance to in Vivo31P NMR localized spectroscopy studies

Abstract

PH measurements require a suitable pH reference within the ³¹P NMR spectrum with respect to which the chemical shift of P₁ and hence pH, may be calculated. In muscle spectra PCr is prominent and provides a reference frequency. However, recent localized tumor studies have reported the absence of PCr, for example, in breast tumors. The use of the α, β, and γ-ATP peaks as suitable references has been suggested, but the position of the β- and γ-ATP peaks is dependent upon the intracellular Mg²⁺ concentration. The α-ATP is not affected by ionic concentrations; however, it contains UDPG and NAD⁺, the presence of which can lead to peak-shape distortion. This paper considers the use of the H₂O resonance from the proton spectrum used for shimming as a suitable pH reference, provided this is also localized to the same region of interest, using a sequence giving rise to eddy current effects comparable to those of the ³¹P NMR sequence. Localized in vivo measurements in the muscle and brain of volunteers indicate good agreement between the proton and phosphorus chemical shifts, allowing the PCr position to be predicted to within 0.01 ppm in all cases. © 1991 Academic Press, Inc.