Magnetic resonance angiography of the carotid bifurcation

Abstract

MRA methods may be categorized into TOF or PC techniques. TOF techniques utilize flow related enhancement to provide high signal intensity blood. Two common TOF methods to visualize the bifurcations are sequential 2D or 3D imaging. In 2D imaging, the carotid bifurcation is visualized by obtaining a series of thin 2D axial gradient echo images. This stack of images may then be subjected to postprocessing to show just the vessel geometry, with the background stationary tissue suppressed. Advantages of 3D techniques include a reduction of T2* effects and a theoretical increase in signal to noise. While the scan time is increased with 3D imaging with the additional direction of phase encoding, overall imaging times are comparable for 3D and sequential 2D techniques, with both being shorter than the 3D PC technique. Advantages of PC angiography include direct and effective suppression of background tissues and definition of slow flow states. This method also has the potential for quantitative flow measurements. Because the signal of blood from this technique depends on flow induced phase change, signal loss from more complex flow is more problematic than with TOF methods. It is apparent from the plethora of methods available that no single MRA technique can answer all clinical questions and situations. Specific techniques and parameters will have to be tailored to individual patient needs. While this makes the routine application of MRA more complex, it also will ensure that the maximum diagnostic yield is achieved.