Response of metabolites with coupled spins to the STEAM sequence

Abstract

This article demonstrates that a numerical solution of the full quantum mechanical equations for all metabolites with coupled spins is an efficient and accurate means, first, of predicting the optimum STEAM sequence design for quantifying any target metabolite in brain, and, second, for providing the basis lineshapes and yields of these metabolites to facilitate their accurate quantification. Using as illustrations the weakly coupled AX3 system of lactate, the ABX aspartyl group of N-acetylaspartate, which has only two strongly coupled spins, and the much larger strongly coupled AMNPQ glutamyl group of glutamate, the numerical solutions for the response to STEAM highlight the principal source of response variability, namely, the evolution of and transfer between zero quantum terms during the mixing time, TM. These highlights include the rapid oscillations of zero quantum terms due to the chemical shift difference of the coupled spins, the proliferation of oscillating zero order terms due to strong coupling, and the serendipitous smoothing of the response as the number of strongly coupled spins increases. The numerical solutions also demonstrate that the design of the selective 90 degrees pulses is a far less critical factor in determining the response than was the case for the selective 180 degrees pulses of the PRESS sequence (Thompson and Allen, Magn Reson Med 1999;41:1162-1169). The veracity of the method is demonstrated both in phantom solutions and in the parietal lobe of a normal human brain.