Abstract: An improved method for measuring very slow flow rates using nuclear magnetic resonance techniques is disclosed. The basic technique is that of steady state free precession, in which a sequence of radio frequency pulses are applied to nuclei in a magnetic field having a substantial gradient, so that a driven equilibrium state is obtained and which is characterized by a spatial periodicity in the magnetization response of the nuclei. Two images are generated. The two images may be generated using different time intervals between the application of the radio frequency pulses. Alternatively, the two images may be generated using different effective gradients. The spatial periodicity, and the NMR response of flowing nuclei to the spatial periodicity, is thus different during the two image formations. One image is subtracted from the other, which cancels signals from static nuclei in the signal, while relatively fast flowing nuclei, namely in the larger blood vessels or the like, never reach the equilibrium state.

Abstract: The present invention provides an NMR image in which the individual image element intensities are proportional to the amount of nuclei flowing within a window of velocities. The invention is predicated on the fact that static nuclei in a static reference frame provide the most intense portions of the NMR signal. A moving reference frame for image creation is created moving at the velocity of the nuclei to be imaged. Nuclei moving at the same velocity as the reference frame thus have zero velocity with respect to the reference frame, and hence provide maximum signal intensity. In the described embodiment, the moving reference frame is created by satisfaction of two conditions, which relate to tracking the Larmor frequency and phase of the nuclei of interest moving in a gradient. These conditions may be satisfied by variation of the main NMR magnetic field H.sub.o as a function of time. The field may be varied simply by addition of an additional coil to conventional NMR equipment.