Velocity encoding parameter

An artifact nniqne to PC MRA is that phase shifts exceeding 180° are interpreted as slow flow in the opposite direction. This aliasing artifact leads to incorrect determination of flow direction. In order to avoid this, a velocity encoding parameter (VENC) is selected which represents the maximum expected flow velocity in the imaging volnme. This value adjusts the strength of the bipolar gradients to prevent phase shifts from exceeding 180°. 

The higher-order gradient moments encode parameters of motion like velocity v and acceleration a and are related to the Fourier conjugate variables and e of these... 

In order to avoid flow artifacts it may be advisable to replace the spatial encoding pulses (right-hand box) by velocity compensated pulses such as shown in Figure 2.9.4(e) for phase encoding. The amplitude of the Hahn spin-echo is attenuated by hydrodynamic dispersion. Evaluation of the echo attenuation curve for fixed intervals but varying preparation gradients (left box) permits the allocation of a hydrodynamic dispersion coefficient to each voxel, so that maps of this parameter can be rendered. 

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