Magnetization-Grid Rotating-Frame Imaging Technique for Diffusion and Flow Measurements

A method for NMR imaging of magnetization patterns generated by a preparation radiofrequency pulse is reported. The technique is suitable for the simultaneous spatially and spectroscopically resolved acquisition of diffusion, flow, and spin-lattice relaxation data. The procedure is based on gradients of the RF amplitude B₁. A first preparation RF pulse produces a z-magnetization grid. After a certain evolution interval, the grid is imaged by a rotating-frame imaging technique using the same RF coil. Neither rotary nor Hahn echoes are intrinsic to the method. Transverse relaxation in the free-evolution intervals is irrelevant. High-power transmitters in combination with suitable probeheads normally produce RF pulses which are short relative to transverse relaxation in the presence of RF, so that spin-lattice relaxation is the only time-limiting factor. Gradients of the main magnetic field induced by variations of the magnetic susceptibility are uncritical. The proposed "real-space detection" method is compared with stimulated or rotary-echo "wave number encoding" procedures for diffusion experiments. It is shown that the imaging procedure presented not only makes visible the spatial (apart from the spectral) distribution of transport properties which otherwise are concealed in the wave-number encoded signal, but also renders the measuring procedure insensitive to inhomogeneities of the B₁ gradient, which needs neither to be constant nor to be uniformly oriented. Extremely inhomogeneous B₁ gradient distributions should even make single-scan diffusion experiments feasible. The magnetization-grid rotating-frame imaging procedure can be employed for the two-dimensional measurement and representation of the probability P(z₁, 0|z₂, t) that a particle is at a position z₁ at a time 0 and at a position z₂ at a time t.