In the next minute, the reader will have approximately five quarts of blood completely circulated through an elaborately complex concourse of arteries, veins and capillaries. To track the channels and measure the velocities and quantities of blood flowing within the myriad of pathways has long been an important problem of medical instrumentation. Some of the importance of flow measurements applies to diagnoses of circulatory disease as well as to diagnoses of heart problems. A great deal of research has been directed towards developments of instruments to measure blood flow. There are, for example, injected radioactive and radio opaque tracers, electromagnetic, thermal, pressure, x-ray, and ultrasonic methods of blood flow assessment. All of these methods have their advantages and disadvantages. These methods are complex procedures. The results are sometimes difficult to interpret, and the procedures may be traumatic to the patient. The need for a simple, noninvasive, accurate and readily interpreted blood flow analysis system has been apparent for many years. We have recently demonstrated that NMR measurements of blood flow are noninvasive, easily visualized and interpreted, and provide accurate measurements of blood flow anywhere within the body. The implications of accurately measuring blood flow anywhere within the human body without damage are that whole new investigative areas are created. We have only to look at the practice of providing pharmaceuticals for cardiovascular problems such as hypertension or angina pectoris and we can see enormous advantages for monitoring their effects on blood flow during the testing of these pharmaceuticals on human subjects. Another application of visualizing and measuring blood flow is the treatment of arterial blockages or partial blockages. Catheterization procedures for plaque removal will be advanced enormously when one can view the artery, the catheter, and simultaneously measure the blood flow within the same artery. This visualization and blood flow measurement can now be accomplished using the systematic procedures we have developed. We can also utilize blood flow measurements to assess other therapeutic procedures. The choice of medicinal or surgical procedures such as bypass operations for cardiac patients can be determined by accurate quantitative measurements of blood flow in arteries and veins. Furthermore, the effects of diet, exercise, psychological stress, and trauma on both human and animal subjects now lend themselves to scientifically significant investigations by using blood flow monitoring.

The possibility of a new approach to blood flow measurement first occurred to us in 1957. As with many ideas, it needed a catalyst. The catalyst was a chance discussion with Professor Melvin Calvin. We were both involved in research projects in nuclear magnetic resonance (NMR) and could easily converse about the possibilities of an NMR blood flow measurement system. Calvin enthusiastically offered to let me use his laboratory and NMR equipment, including a supply of standard mice, and our first experiment was started. The initial experiment was to be extremely simple in order to test the validity of our belief that we could measure blood flow using the magnetic properties of the hydrogen nuclei which compose more than 90% of blood. Most of our blood is water, and the hydrogen nuclei of the water molecules are the tiny magnets of importance to our magnetic measurements.