ABSTRACT
The modulated-gradient method makes it possible to record the spatial distribution of a selected spectral line and/or to acquire the spectrum for a selected spatial element. In practice, the magnetic-field gradient is cosinusoidally modulated, and every spatial element r is distorted by a modulated field. The modulated-gradient technique is characterized by a cross-like distortion through the central peak. Gradient-modulation frequencies between 15 and 70 Hz were selected as an optimum with respect to Conditions 1 and 2, to a minimum induction of eddy currents, and for convenient averaging using the spectrometer time-constant. In the low-gradient case, distortions appear from the neighbor lines which are eliminated by the iterative one-dimensional deconvolution. The principle of the 90° phase sensitive detected modulated-gradient technique is based on an additional modulation that is generated by a small deviation of the phases of the gradient currents in the two symmetric coil systems.
