A method for obtaining spin-density distributions of selected NMR, periodicity in the effective magnetic field leads to layers in annular shear flow is introduced. A transverse rotating spinning sidebands (8). In this paper, we extend our method magnetic field gradient is generated by oscillating orthogonal grafor obtaining spectra of selected fluid layers in sheared dients in quadrature. The equivalence of the frames of reference Couette flow based on their different rotational frequencies of a rotating sample with a fixed magnetic field gradient and a (9). We show that, for a sheared fluid, a given layer can be rotating magnetic field gradient with a fixed sample is demonsampled by stroboscopic application of pulses timed to the strated with modulation sideband patterns. When the magnetic rotation period for that slice. The selection was not ideal as field gradient and sample rotation frequency differ, sidebands apthe signal from the rest of the sample is not fully selfpear at integer multiples of the difference between the two rotation destructive (9). Further, the intensity is necessarily T 2 frequencies. When the gradient is synchronized with the sample rotation, the spectrum is a spin-density projection of the sample weighted and low-rotation-frequency layers may have perionto the gradient. When the sample and gradient are counterrotatods much longer than T 2 .
ing, high audio-frequency sidebands appear with consequent spec-Rotational Couette flow is an elementary flow in fluid tral simplification. When the orthogonal components used to gendynamics (10). Stable, two-dimensional flow is generated in erate the rotating gradient are imperfectly matched, sidebands fluid contained between two cylinders when the outer cylinappear at integer multiples of the sum and difference frequencies der is rotated and the inner cylinder is fixed. A continuum of V s and V g . A novel, low-cost electronic system is described to of concentric circular streamlines is obtained with rotational create the rotating gradient on a standard high-resolution specfrequencies which range from the cylinder frequency at the trometer. Spectra of layers of fluid moving at a chosen angular outer wall to zero at the inner cylinder (Fig. 1). Alternatively, frequency in a sample undergoing Couette flow shearing are shown. The radial fluid layer is selected by stroboscopically sam-if the outer cylinder is fixed and the inner cylinder is rotated, pling with a Hahn-echo refocusing pulse applied at integer multicomplex three-dimensional flow fields-Taylor vortices ples of the rotation period. A two-dimensional analysis is given (11)-result. In this paper, we present images of two-dimenthat separates the signal arising from the layer that is synchronized sional Couette flow. with the rotating gradient from the signals arising from the rest We previously reported (9) a time-domain treatment of of the sample. Complex phase distortions occur when the echo the sideband patterns generated when a cylindrical sample delays do not match the rotation period. A radial frequency map is rotated in a linear, transverse magnetic field gradient. Magof the Couette streamline flow is generated using the 2D technique netization created in the transverse plane by a 90Π pulse which compares favorably with the calculated flow field. α§ 1995 experiences a sinusoidally oscillating magnetic field which Academic Press, Inc.