MR flow quantification with cardiovascular applications: a short overview.

We present a short overview of the potential of magnetic resonance imaging (MRI) for quantification of flow in the cardiovascular system. The most widespread method for creation of MRI flow information utilized flow-sensitizing magnetic field gradients. Objects that move in the varying magnetic field introduced by such gradients change their precession frequency and therefore obtain a velocity-dependent offset phase angle. The exact phase behaviour for different types of gradients and motion patterns can be calculated and a very simple linear relationship is predicted by theory as well as confirmed in experiments between constant velocity and phase angle. Phase-sensitive flow MRI (velocity mapping) is frequently performed as a two-dimensional gradient-echo technique with flow sensitivity (flow encoding) in the through-plane direction, but other encoding directions are possible. Parameters that can be determined from a velocity map are linear velocity in each voxel, vessel area and flow rate. In the case of a stenotic vessel, the trans-stenotic pressure gradient can also be estimated. The velocity mapping method has been extensively used for cardiac flow studies in adult patient groups, e.g. for indirect measurements of coronary artery flow and for the study of aortic valve diseases. In children, the method has recently been used to determine shunt volumes in congenital heart disease. We conclude that flow investigation with MRI may in the future present a good alternative for the clinical evaluation of cardiovascular disorders.