Pre-emphasis
What is pre-emphasis and how does it reduce eddy currents?
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All manufactures use pre-emphasis or pre-compensation to correct for certain eddy current effects. This method does not eliminate or reduce eddy currents, but merely compensates for them. The technique involves purposely distorting the gradient driving currents, so that after eddy currents act, an output wave of the desired shape will be produced.
Effect of eddy currents on gradient waveforms. If the input current is in the form of a rectangular pulse, eddy currents will distort the output. To correct in advance for this distortion due to eddy currents, the input waveform is modified through the process known as "pre-emphasis." The final output waveform is now of the desired shape. (Note: the waveform distortions have been grossly exaggerated for didactic purposes).
Prior to the mid-1990's when active shielded gradients came into widespread use, pre-emphasis was the only method available to correct for eddy currents. In this not-so-distant era, eddy currents were a huge problem, with amplitudes up to 50% of the intended pulsed-gradient magnitude. Today, with active shielding, uncompensated eddy-current gradients have been reduced to the 1% level. Pre-emphasis techniques are still employed to ameliorate these much smaller (but still detrimental) effects.
References
Ahn CB, Cho ZH. Analysis of the eddy-current Induced artifacts and the temporal compensation in nuclear magnetic resonance imaging. IEEE Trans Med Imaging 1991; 10:47-52.
Doty FD. MRI gradient coil optimization. In: Blumler P, Blumich B, Botto R, Fukushima E (eds), Spatially Resolved Magnetic Resonance. Wiley; 1998, pp 647-674.
Kroot JMB, van Eijndhoven SJL, van de Ven AAJ. Eddy currents in a transverse gradient coil. J Eng Math 2008;62:315–331
Spees WM, Buhl N, Sun P et al. Quantification and compensation of eddy-current-induced magnetic-field gradients. J Magn Reson 2011; 212:116-23.
Trakic A, Liu F, Sanchez Lopez H et al. Longitudinal gradient coil optimization in the presence of transient eddy currents. Magn Reson Med 2007; 57:1119-1130.
Ahn CB, Cho ZH. Analysis of the eddy-current Induced artifacts and the temporal compensation in nuclear magnetic resonance imaging. IEEE Trans Med Imaging 1991; 10:47-52.
Doty FD. MRI gradient coil optimization. In: Blumler P, Blumich B, Botto R, Fukushima E (eds), Spatially Resolved Magnetic Resonance. Wiley; 1998, pp 647-674.
Kroot JMB, van Eijndhoven SJL, van de Ven AAJ. Eddy currents in a transverse gradient coil. J Eng Math 2008;62:315–331
Spees WM, Buhl N, Sun P et al. Quantification and compensation of eddy-current-induced magnetic-field gradients. J Magn Reson 2011; 212:116-23.
Trakic A, Liu F, Sanchez Lopez H et al. Longitudinal gradient coil optimization in the presence of transient eddy currents. Magn Reson Med 2007; 57:1119-1130.