Using Magnetic Resonance of Protons to Image Flowing Blood

Physicians often request angiograms or images of arteries. Conventional angiography requires catheterizing a patient, injecting an iodine compound into an artery, and X‐ray photography. All 3 steps have hazards and, compared to clinical nuclear magnetic resonance (NMR) examinations, the procedure is painful, time consuming, and expensive. Iodine distinguishes blood from other tissues in conventional angiography; blood motion serves the same purpose in several recent NMR experiments reviewed here. Blood spins can be inverted upstream of a region of interest. After they flow downstream, blood will be labeled in the region of interest. Comparing NMR images made with and without labeling identifies blood. Alternatively, equal and opposite magnetic field gradient pulses can mark moving blood by changing the phase of its magnetization. The second gradient cancels the phase effect of the first for spins which are still in the same place, thus only blood is affected. Two images with different blood phases are compared. The blood phases may differ because gradients used to produce the images differ or because blood velocities differ owing to different synchronization with the heart. Results of several NMR experiments are presented. All have much lower quality and higher convenience than the conventional X‐ray method.