Basic principles of magnetic resonance angiography.

The flow of blood through magnetic field gradients and radiofrequency fields produces signal changes that can be used to distinguish blood vessels from surrounding stationary tissue. The field of magnetic resonance (MR) angiography attempts to overcome this limitation by creating images that depict blood vessels in a projective format similar to a conventional invasive angiogram, but without the need for ionizing radiation or a contrast agent. This article reviews basic concepts involved in MR angiography, including signal changes from time-of-flight and phase effects, maximum intensity projection algorithm for postprocessing of two-dimensional (2D) and three-dimensional (3D) image sets, and methods for flow quantification. Potential problems with MR angiography are considered, as well as means to overcome them.