Analysis and correction of gradient nonlinearity and B0 inhomogeneity related scaling errors in two-dimensional phase contrast flow measurements.

Phase contrast flow measurements will be increasingly biased at eccentric positions, where nonlinearity of gradients and inhomogeneity of the main field become important. In theory, they scale the result of phase contrast flow values in two ways: incorrect velocity encoding of moving spins and geometric distortion of the vessel cross-sectional area. A flow phantom, consisting of a 3D grid of interconnected tubes, was used to determine the spatial dependence of the associated scaling factors, which demonstrate that scaling errors in flow can be as large as 20% within the examined volume of 336 x 336 x 336 mm(3). The same phantom was also used to determine and minimize concomitant gradient effects. Correction of the off-center flow values with the local scaling factors and the concomitant gradient phase improves the measurement accuracy substantially, both in the flow phantom and in a volunteer study. Copyright 2004 Wiley-Liss, Inc.