Quadrupole-Echo Techniques in Multiple-Quantum-Filtered NMR Spectroscopy of Heterogeneous Systems

Multiple-quantum-filtered quadrupole-echo pulse sequences for spin I = 1 and I = 3 2 are suggested. A general condition for obtaining simultaneously Zeeman and quadrupolar echo is formulated. A theoretical analysis of the various pulse sequences was performed on the basis of second-order perturbation approximation of the Liouville equation for the density matrix. The extent of refocusing as a function of the ratio of the residual quadrupolar interaction and the relaxation rates was calculated. Experimental results are presented for ²H and ²³Na in cartilage as an example of a heterogeneous system with residual quadrupolar interaction. The difference between relaxation times measured by the multiple-quantum-filtered echo techniques and those measured by conventional multiple-quantum-filtered NMR spectroscopy is a simple diagnostic of anisotropic motion that leads to a residual quadrupolar interaction. The results of the echo experiments are compared with the relaxation times computed on the basis of lineshape analysis of double-quantum-filtered spectra of a heterogeneous system.