Figure 2

The phase across a small vessel with no flow is ideally flat with infinitely steep sides (a-red); off-resonance causes a distortion of the flat phase such that it becomes curved (simulated off-resonance) (a-blue/green) and can be seen in the LOW flow phase images in Figure 2 where both Ref ph and Vel ph are similar. Velocity mapping induces phase relative to velocity within a pixel, the ideal Ref. phase over the vessel with laminar flow remains flat (a-red) and for Vel. phase has parabolic profile (b-red). When the Vel. image is acquired with off-resonance, the phase from these 2 effects can add constructively or destructively resulting in distortion of the velocity profile (Figure 1-PEAK VM). Signal loss in edge pixels with high velocity shear can be increased (b-blue) or decreased (b-green) depending on the direction of the off-resonance phase resulting in the implosion (Figure 1-I) and explosion (Figure 1-E) artefacts in Vel. mg images.