Response of metabolites with coupled spins to the STEAM sequence

This article demonstrates that a numerical solution of the full quantum mec hanical equations for all metabolites with coupled spins is an efficient an d accurate means, first, of predicting the optimum STEAM sequence design fo r quantifying any target metabolite in brain, and, second, for providing th e basis lineshapes and yields of these metabolites to facilitate their accu rate quantification, Using as illustrations the weakly coupled AX(3) system of lactate, the ABX aspartyl group of N-acetylaspartate, which has only tw o strongly coupled spins, and the much larger strongly coupled AMNPQ glutam yl group of glutamate, the numerical solutions for the response to STEAM hi ghlight the principal source of response variability, namely, the evolution of and transfer between zero quantum terms during the mixing time, TM. The se highlights include the rapid oscillations of zero quantum terms due to t he chemical shift difference of the coupled spins, the proliferation of osc illating zero order terms due to strong coupling, and the serendipitous smo othing of the response as the number of strongly coupled spins increases, T he numerical solutions also demonstrate that the design of the selective 90 degrees pulses is a far less critical factor in determining the response t han was the case for the selective 180 degrees pulses of the PRESS sequence (Thompson and Alien, Magn Reson Med 1999;41:1162-1169), The veracity of th e method is demonstrated both in phantom solutions and in the parietal lobe of a normal human brain, (C) 2001 Wiley-Liss, Inc.