Patent US 4,577,152

Two-dimensional projection measurements are formed of the NMR activity within a volume. Measurements are made using different intensities of the main magnetic field, using the different dependencies of relaxation times on magnetic field. The projection measurements are combined to obtain projection images of specific materials within the volume. Projection images are obtained of specific regions within the volume by insuring that the unwanted regions remain insensitized. For improved SNR, projection measurements are repeated to obtain spin-echo signals. The measurements of relaxation times are interleaved so that the data acquisition can occur rapidly.