Multiple-quantum filters of spin-3/2 with pulses of arbitrary flip angle

The influence of inhomogeneous RF fields on the double- and triple-quantum filtering of spin-3/2 nuclei in the presence of biexponential relaxation is analyzed. In this analysis, spherical tensor operators have been used for density-matrix calculations. In the presence of inhomogeneous RF fields, it is shown that the three-pulse triple-quantum filter (TQF) without a refocusing 2 theta pulse (with delta omega = 0) is on average about 100% more sensitive than the corresponding double-quantum filter (DQF), and in the case of four-pulse DQF and TQF with a refocusing 2 theta pulse (with delta omega not equal to 0), two relaxation coefficients f(1)11(tau) and f(1)33(tau) also contribute to the observed DQ and TQ coherences. It is also shown that the three-pulse filters are more sensitive than the corresponding four-pulse filters. In both three- and four-pulse cases, when used with surface coils, these filters act as depth pulses and thus yield spatial localization. The experimental results obtained with homogeneous RF coils are in excellent agreement with the theoretical results.