A NEW MULTIPLE-QUANTUM FILTER DESIGN PROCEDURE FOR USE ON STRONGLY COUPLED SPIN SYSTEMS FOUND IN-VIVO - ITS APPLICATION TO GLUTAMATE

A numerical procedure is outlined that is appropriate for the design o f multiple quantum filter sequences targeted for the strongly coupled, multiple spin systems that occur in metabolites present in brain. The procedure uses numerical methods of solution of the density matrix eq uations, first, to establish the most appropriate resonance to target with the filter; second, to provide contour plots of a performance ind ex of the filter in terms of critical sequence parameters; and third, to produce the response signals of the target and the background metab olites to the optimized filter. The procedure is exemplified for the A MNPQ spin system of the amino acid glutamate at a field strength of 3 T. The 2.3 ppm peak of the PQ multiplet of glutamate was identified as the target resonance, and the performance of the filter so derived wa s evaluated experimentally on phantom solutions and in human brain. Th ese experiments clearly demonstrate that a linewidth of less than or e qual to 4 Hz is required for full resolution of glutamate from glutami ne at 3 T using this double quantum filter. Nevertheless, even at a li newidth of similar to 7 Hz in vivo, the 2.3 ppm peak of glutamate domi nates the filter response and thereby removes a significant cause of u ncertainty in measuring changes in glutamate by eliminating most of th e background observed in unedited spectra obtained using PRESS or STEA M.