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Science at the Frontier: Volume I
phy has been developed to study blood vessels, but early indicators suggest MRI could largely supplant it, if techniques and cost considerations can be harnessed. "The routine angiogram requires sticking a needle about the size of a Cross pen into your groin, your femoral artery, where you feel the pulse, and then sending a catheter about a meter long up into the neck and injecting contrast into your carotid artery. It is not a pleasant procedure," commented Bradley, who has contributed to the development of "a painless procedure, MR angiography, that gives much the same information" in a matter of a few minutes, compared to the hour patients normally endure with the invasive and painful angiogram.
MRI studies can be directed not only toward the flow of blood, but also to the CSF in the central nervous system. Because of the abundance of protons in hydrogen and of water in the human body, MRI can detect and discern the movement of water anywhere in the body and is sensitive even at the smaller scales of molecules. MRI techniques are especially sensitive to the rate of flow. Blood traveling upwards from the heart through the aorta moves at nearly a meter a second. As it continues into the vascular network and moves into continually branching arteries, velocity is decreased proportional to the cumulative cross-sectional area. In major arteries in the brain, for example, blood is moving at about 10 centimeters per second. It continues to diffuse through the ever finer network until, reaching the capillaries, it is moving at about a millimeter per second. Generally, the more rapid the flow, the darker the MRI image: thus blood moving more slowly through the veins is lighter, and that pulsing through the arteries is darker on the image. Another convention used when imaging flow is to code for direction, which allows easy discrimination between inflowing and returning blood and CSF.
Before MRI practitioners could produce images of flow, techniques had to be developed to overcome a number of obstacles. Bradley pointed out that several "independent factors can result in decreased signal intensity of flowing blood: high velocity, turbulence, and odd-echo dephasing." Sometimes known as flow void, this decrease in signal can per se be revelatory, indicating under certain conditions the presence of aneurysms and arteriovenous malformations. To explain the mechanism behind the flow void, Bradley again described the spin-echo technique. Because the MRI equipment has been refined such that a thin slice on the order of 5 millimeters (or thinner) can be isolated for imaging, the speed of protons moving through the slice in a normal direction is problematic. "To give off a spin-echo signal," wrote Bradley, "a group of protons must be exposed to both a 90-and 180-degree RF pulse" (Stark and Bradley, 1988, p. 108). If