A volume-selective NMR method is presented for very exact long mixing times TM where zero-quantum relaxation redetermination of the difference of the Larmor frequencies between duces the signal intensity of coupled spin systems such as the coupled resonances of homonuclear AX 3 spin systems, as, for lactate much faster than single-quantum relaxation in spinexample, lactate. The frequency difference can be determined with echo sequences. Furthermore, too small a variation of TM an accuracy of 0.003 ppm even if only the doublet of the spin leads to ambiguous results for the zero-quantum frequency. system can be detected. The method is based on the effects of In the following we describe a method based on HOPE homonuclear polarization transfer which occurs in localized douwhich allows one to determine a precise and unequivocal ble-spin-echo spectroscopy. It is used for determination of the value for the chemical-shift difference of weakly coupled chemical-shift difference of the lactate multiplets, which depends homonuclear spin systems. The method is used for investigaon the degree of dissociation and consequently on pH. Measuretion of the influence of pH on the chemical-shift difference ments were performed with a 1.5 T Siemens Magnetom SP 63
whole-body imager on solutions of lactate and acetic acid in physi-of the lactate multiplets. Additionally, these investigations ological sodium chloride solution with pH from 1 to 11. As a lead to the pH-dependent evaluation of the recently proposed consequence of these measurements, conclusions are possible for optimum echo time for measurements of lactate with the the optimum echo time for PRESS measurements which avoid symmetric PRESS sequence (6) at TE Å 2/J. signal losses from polarization transfer. Furthermore, the possibility of localized pH determination by this effect is discussed.
THEORY AND METHOD