Coupling effects in volume selective 1H spectroscopy of major brain metabolites.

The effect of PRESS and STEAM sequences on the spectra of coupled substances are discussed using the examples of weakly coupled AX and A2X systems. Maximum differences compared to uncoupled spins occur if the RF pulses are applied to antiphase magnetization. In this case, the spin echo of the PRESS experiment shows a modified dependence on the refocusing flip angle, which may lead to an attenuation of the acquired signal. In STEAM spectroscopy the evolution within the middle interval tm is dominated by zero quantum coherences and longitudinal polarization, whose maximum efficiencies are 25 and 12.5%, respectively. Zero quantum coherences may lead to strong modulations when the tm value is varied. The effects on the spectra of important coupled metabolites of the human brain such as glutamate, GABA, inositol, and particularly lactate, are demonstrated. The observed modulations seem to make the quantification of the spectra rather difficult at echo times above 50 ms.