Fig. 1.

Reconstruction of an image encoded in a pure gradient field. (Inset) Two-dimensional image of a brain obtained in a commercial MRI system at 3 T. Each dimension has a total of 256 points, and the number of spins at each site was assumed strictly proportional to the intensity. Interpreted as a spin map, the grid was then stored in the computer memory and was taken as the starting point for a numerical simulation. Shown in the main figure is a numerical reconstruction of the brain image in the absence of a uniform static magnetic field. The used field-pulse scheme is explained in the text (see also Fig. 2 A). The dc-field pulses had an amplitude of 3 G(1G = 0.1 mT), and within each train the interpulse interval was 0.5 ms. The gradient amplitude was equal to 0.03 G/cm (130 Hz/cm). The number of points acquired was 1,024 in each of the 512 projections recorded, and data treatment was standard. The (elliptical) halo around the brain is an effect caused by projection/reconstruction (see Fig. 3).