In gradient-moment nulling, additional gradient lobes are added prior to signal readout to compensate in advance for motion-induced dephasing at the time of the echo. As a simple example, let us analyze the velocity-compensated GRE sequence shown right. Note that the readout gradient has a more complex appearance than in the usual GRE sequence, with the addition of the two orange lobes. These additional gradient lobes give this sequence its flow-compensation properties.
These two extra GMN lobes play a clever little game with the phase. The first orange lobe creates a phase gain, but the second orange lobe during the next time interval, when added to the phase changes from the first blue lobe, create a strongly negative phase loss. By the middle of readout, however, the net phase of the moving spins using GMN has returned to zero (orange circle).
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The above example describes first-order (velocity) gradient-moment nulling. The method can be easily extended by to compensate for phase dispersions due to second-order (acceleration), third-order ("jerk"), and even higher degrees of motion. These higher order corrections require more gradient lobes (specifically, n+1 additional lobes, where n = motion order to be compensated). These extra lobes take time to play out, so using higher order GMN even further lengthens minimum TE and reduces number of slices for a given TR. Also, the effectiveness of the method decreases as the order increases. In practice, therefore, velocity-compensated GMN is nearly exclusively used, with acceleration-compensated GMN performed only occasionally.
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Richardson DN, Elster AD, Williams DW III. Gd-DTPA-enhanced MR images: accentuation of vascular pulsation artifacts and correction using gradient-moment nulling (MAST). AJNR Am J Neuroradiol 1990; 11:209-210.
Can you explain even-echo rephasing?