The AAPM/RSNA physics tutorial for residents. An introduction to MR angiography

This article provides an overview of the basic principles of magnetic resonance (MR) angiography. The parameters in MR imaging manipulated to generate high contrast between flowing nuclei and stationary tissue are discussed. Two primary strategies are used: time-of-flight (TOF) MR angiography, which creates differences in magnetization magnitude between flowing and stationary nuclei, and phase-contrast MR angiography, which induces changes in the spatial orientation, or phase, of flowing nuclei relative to stationary nuclei. The end result of an MR angiographic study is typically a three-dimensional data set composed of either sequential two-dimensional sections or true three-dimensional data. Two-dimensional TOF methods are sensitive to slow flow and are valuable for differentiating between slow flow and occlusion. Three-dimensional TOF methods have better resolution and are more useful in imaging tortuous vessels. Phase-contrast MR angiography can be effectively used to avoid problems of magnetization saturation that occur in three-dimensional TOF studies and to eliminate signal from high-intensity stationary material such as blood products, which may appear bright and mimic flow signal in TOF studies. Careful use of postprocessing tools aids in the assessment of vascular abnormalities once the MR angiographic data have been acquired.