Flow compensation (FC), also known as gradient torque cancellation (GMN), is a technique that uses a specially designed gradient field combination to reduce or eliminate flow artifacts.
The role of flow compensation: (1) Proton phase re-convergence that causes uniform phase motion to occur, thereby eliminating motion artifacts caused by CSF and slow-flowing blood flow. Not suitable for motion artifacts caused by acceleration and rapid flow, such as the effects of large blood vessels in heart and abdominal imaging.
The generation of flow artifacts: taking SE as an example, after the 90-degree pulse excitation, to the TE time, the readout gradient field before and after the 180-degree focus pulse is also symmetrical, and the action areas just cancel each other out, and there is no accumulation for the stationary tissue. Phase offset. However, the situation is different for tissues moving in the direction of the readout gradient field (such as flowing blood, cerebrospinal fluid). Since the position of the proton flowing before and after the 180-degree focusing pulse changes, the accumulated phase shift cannot be completely corrected at the time of TE, so a phase error occurs, so that the phase is shifted during the Fourier transform. When the position information in the phase encoding direction is shifted, the fluid signal appears in the wrong position in the phase encoding direction and becomes a flow artifact.
Principle of flow compensation: Phase errors caused by flow can be corrected by the special design of the gradient field. There are many types of gradient combination modes for FC technology. Commonly, there are "+1~-1", "+1~-2~+1", "+1~-3~+3~-1". Through the transformation of multiple positive and negative gradient fields of different areas, The phase offset of the various velocity fluids can eventually be close to zero, eliminating flow artifacts.
Clinical application of flow compensation: FC technology can reduce or eliminate artifacts caused primarily by flowing liquid in the direction of the applied FC gradient field. In SE and GRE, after FC is selected, the FC gradient field is applied to the three directions of layer selection, frequency coding and phase encoding. In the FSE sequence, FC can only be selected in the two directions of layer selection and frequency coding. Apply in one direction. Clinically, the FC direction should be set to the direction of fluid flow, and flow compensation will work. In addition, FC has a better effect on eliminating flow artifacts caused by fluids in the layer, while eliminating the flow artifacts caused by fluids perpendicular to the layer is less than ideal. (1) In liver imaging, lesions (darker) and thrombus (bright) can be identified on the T1 image. (2) Signal loss caused by loss of phase loss is reduced, and the quality of MRA is improved. (3) Reduce cerebrospinal fluid flow artifacts, improve the signal of cerebrospinal fluid on T2 during brain and spinal imaging. (4) Reduce vascular flow artifacts, especially when enhancing scanning, such as reducing vascular pulsation artifacts in liver imaging. It should be pointed out that after the application of the FC technology, the shortest TE that can be used in the SE sequence and the GRE will be extended to different extents, and the number of scanning layers will also be reduced, thereby affecting the acquisition speed. Therefore, FC technology is generally not used in ultra-fast gradient echo sequences such as Balance-SSFP and CE-MRA. When in the spinal imaging, both the frequency encoding direction and the flow compensation direction are set in the front-rear direction, blurring occurs at both ends of the image in the up and down direction, missing artifacts (generating Maxwell artifacts); when multi-echo scanning is used, Only double echo imaging can be performed after FC;
Setting method :
GE: FC technology is selected in the parameter adjustment interface “imaging optionsâ€; Siemens: FC technology is selected in the “Sequence†card, and the direction can be selected. Philips: Select “FC†in the “Motion†card and select “Yesâ€.
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