Flow compensation (flow compensation, FC), also known as gradient moment elimination (GMN), is the use of specially designed gradient field combinations to reduce or eliminate flow artifacts technology. 1, the generation of flow artifacts: SE, for example, 90-degree pulse excitation, to the TE moment, 180-degree focusing pulse before and after the readout gradient field is also symmetric, the area of action just offset each other, for the stationary tissue there is no accumulation of phase shift. However, the situation is different for tissues moving along the direction of the readout gradient field (e.g., flowing blood, cerebrospinal fluid). Due to the 180 degree focus pulse before and after the flow of the proton position changes, the accumulated phase shift in the TE moment can not be fully corrected, so the phase error, so in the Fourier transform will be such a phase shift error as the phase encoding direction of the position of the information, the signal of the fluid will appear in the phase encoding direction of the wrong position, become the flow of artifacts. Principle: The phase error brought by the flow can be corrected by the special design of the gradient field. there are many kinds of gradient combination modes in FC technology. Through several transformations of positive and negative gradient fields of different areas, the phase shift of fluids of various speeds can eventually be close to zero, thus eliminating flow artifacts. 3, Clinical application: FC technology can reduce or eliminate mainly the artifacts caused by the flowing fluid along the direction of the applied FC gradient field. In SE and GRE, after selecting FC, the FC gradient field is applied in the three directions of level selection, frequency encoding, and phase encoding; whereas in the FSE sequence, FC can generally be applied in only one of the two directions of level selection and frequency encoding. Clinically, the FC direction should be set to the direction of fluid flow. In addition, FC is more effective in eliminating flow artifacts caused by fluids within the layers, but less effective in eliminating flow artifacts caused by fluids perpendicular to the layers. GE: Select FC in the imaging options. (1) Reduce the flow artifacts of blood vessels, especially during enhancement scanning. ② Reduce the signal loss caused by flow out-of-phase and improve the quality of MRA. Reduce cerebrospinal fluid flow artifacts. ④Improve the signal of cerebrospinal fluid on T2WI. It should be noted that after the FC technique is applied, the shortest TE that can be used for e.g. SE sequences and GRE will be extended to varying degrees, thus affecting the acquisition speed. Therefore, FC techniques are generally not used in ultra-fast gradient echo sequences such as Balance-SSFP and CE-MRA.