Flow measurement by magnetic resonance: a unique asset worth optimising.

Users and manufacturers of cardiovascular magnetic resonance (CMR) systems have, potentially, an unrivalled asset. Phase contrast mapping of velocities through planes transecting the great arteries should provide the most accurate measurements available of cardiac output, shunt flow, aortic or pulmonary regurgitation and, indirectly, of mitral regurgitation. But the reality is that phase contrast velocity mapping remains under-used, and may have become discredited in the eyes of some CMR users and referring clinicians. Even when appropriate methods of acquisition have been used, there can be inaccuracies of flow measurement on some CMR systems caused by background phase errors due to eddy currents or uncorrected concomitant gradients. Measurements of regurgitant or shunt flow can be seriously affected by these errors which should be minimised or corrected by appropriate hardware and software design. If they have not been, inaccuracies can be detected and corrected by repeating identical velocity acquisitions on a static phantom, and subtracting the corresponding apparent phantom velocities from those of the clinical acquisition. For accurate measurements of aortic regurgitation or mitral inflow, motion tracking and velocity correction with respect to the cyclic displacements of the valves are needed, but few if any commercial systems provide this facility. Measurements of jet velocity pose different challenges, mainly related to the size and placement of voxels relative to a narrow jet. Awareness of the potential problems and concerted efforts towards optimisation are needed from manufacturers and users to make appropriate use of phase contrast flow measurement - a unique strength of cardiovascular magnetic resonance.