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1、精選優(yōu)質(zhì)文檔-傾情為你奉上(b)(a)R1M1R i1L1iNRNL2MNiL FIGURE 10.13 Lumped circuit model for eddy currents.The gradient coil (a) and each eddy-current conductor (b) are modeled as LR circuits.Eddy currents result from mutual inductance Mk between the gradient coil and each eddy-current conductor.The mutual inducta

2、nce allows a current i in the gradient coil to induce current ik in the eddy-current conductors.Each conductor produces a magnetic field proportion to ik+ (Adapted from Van Vaals and Bergman 1990;Jehenson et al.1990.)Rdadc -rt +rR -R1 +bPSgradient coil>>PE(1)computerPSZ0 - coil>>PE(0)FIG

3、.3. (a) Preemphasis circuit (for one exponential term).The high-pass RC filter generates an exponentially decaying signal with adjustable time constant (p1=RC) and relative amplitude(V1=(r/rt)(R1/Rd).Summation of the signal to the input step function produces the desired pulse shape.Other RC filters

4、 with different time constants and gains can be added in parallel to generate multiexponentials.The direct path d should only exist in the preemphasis for the gradient coil,not for other coils like the Z0 coil.(b) General circuit structure.The computer provides the input function to the preemphasis (PE) units;the power supplies (PS) convert voltages to currents which drive the gradient,Z0,or any other coil. CB0(t) 40Hz20Hz0 -20Hz -40Hz -60Hz 1ms 10ms 100m