Techniques have been developed to image the flowing blood. MRA and magnetic resonance venography (MRV) are used to study the flow in various vessels and have proved to be robust in routine clinical examination of vascular diseases. In this section the basic underlying principles of MRA and the major clinical uses are discussed.
There are two major MRA techniques that are used in MRI that exploit either the longitudinal spin magnetization or transverse spin magnetization of the protons in blood to visualize flowing blood. They are the time-of-flight (TOF) technique, which utilizes the inflow-outflow of the moving spins within the selected imaging volume to image the blood flow and the phase-contrast technique, which utilizes the flow-induced phase variations of the MRA signal caused by the motion of the blood. These MRA techniques depict the blood flow as bright in contrast to the dark surrounding stationary tissues. The MRA data are postprocessed with a maximum intensity projection algorithm and may be displayed in multiple angiographic projections. The MR method to examine the venous sinuses in the head is MRV. A two-dimensional TOF technique is used that has the advantage of sensitivity to slower-flowing venous structures and the ability to cover a large area.
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