Sources of variability in the response of coupled spins to the PRESS sequence and their potential impact on metabolite quantification.

Using a numerical method of solving the equation of motion of the density matrix, an evaluation is presented of the sources of the marked variability in the response to the point resolved spectroscopy (PRESS) pulse sequence of coupled proton spin systems. The consequences of an inappropriate 180 degrees pulse design and of the limitations on radiofrequency power are demonstrated for a weakly coupled example, lactate. The dominating role of strong coupling, which is present in most brain metabolites, is demonstrated for glutamate, in which 160 terms in the density operator were tracked to monitor the gross changes in lineshape and signal intensity as a function of the two echo times. The predictions of the numerical solutions were confirmed by experiments on phantoms of aqueous metabolite solutions.