The response of the strongly coupled AB system of citrate to typical 1H MRS localization sequences

The response of the strongly coupled AB system of citrate to the STEAM and PRESS spatial localization sequences is developed analytically and then verified experimentally at 2.35 T. At this field strength, the response predicted by the complete strong-coupling calculation is shown to differ greatly from that of the simpler weak-coupling approximation. It is shown that the strong-coupling effects induce a significant field-strength dependence in the time-dependent modulations of the response to either sequence. These effects are illustrated for the STEAM and PRESS responses at field strengths of 1.5, 2.35, and 4.7 T. Independent of the chosen sequence, the increase in the signal-to-noise (S/N) of the frequency response with increasing field strength is shown to be significantly less for the citrate system than for uncoupled spins. The factors which weaken the S/N dependence of the citrate AB signal with increasing field strength are quantified and discussed.