Chemical-Shift-Selective Acquisition of Multiple-Quantum-Filtered 23Na Signal

Equations describing the multiple-quantum (MQ) signal produced by an MQ pulse sequence are systematically derived in both absence and presence of refocusing RF pulses. When the RF pulses in an MQ pulse sequence satisfy certain conditions, these equations may be arranged in a factorized form. The off-resonance effects on the MQ signal due to chemical shift can then be analyzed separately during the preparation and evolution times. Using the reformulated equations, the dependence of the amplitude of an MQ signal on the phase shift induced by the resonance offset during the preparation and evolution times is demonstrated. By use of the new equations, it is shown that the off-resonance effects, occurring during both the preparation and evolution times, may be described in terms of the same physical process, i.e., interference between echo and antiecho. In applying the off-resonance effects for the elimination of the MQ signal in the presence of chemical shift, it is possible to suppress the MQ signal over a wider off-resonance bandwidth by use of the nonrefocused preparation and evolution times than by use of a single time. Furthermore, by taking an alternative approach in deriving the equations, the interference between echo and antiecho due to the resonance offset is shown to be insensitive to the flip angle of the creation RF pulse (usually the second pi/2 RF pulse). The theoretical findings were experimentally verified by use of a phantom containing sodium in agarose.