Tissue contrast in MRI owing to intermolecular multiple-quantum coherences (iMQC)

Dipole-dipole interactions in 1H NMR of liquids permit a new tissue contrast in MR imaging (MRI). For this purpose, the CRAZED (COSY revamped by asymmetric Z-gradient echo detection) sequence which simply consists of two radiofrequency pulses of delay τ and a magnetic field gradient pulse of strength G, is applied to create intermolecular multiple-quantum coherences of dipolar-coupled ensembles of water protons with spatial distance d = π(γGτ). As a consequence, the measured signal originates from ''sources'' that have a tunable dimension. CRAZED was first applied to MR imaging at 1.5 T by Zhong et al. To study the contrast properties and influence of sequence parameters of this technique, we implemented different sequences of this type on a clinical 1.5-T whole-body tomograph. The sequences were optimized in experiments with agar gel phantoms. We obtained images with dipolar contrast from the brain of healthy volunteers. The gradient system enabled correlation distances d in the range of 0.05 mm to 1 mm. (orig.)