Shimming: Theory and Practice - UCLA-DOE

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Gradient <strong>Shimming</strong> from: J. Magn. Reson. A (1994), 111, 203-207 A phase image of the sample reflects the evolution of the magnetization. When the carrier is on resonance, the phase difference Δφ(r) obtained at a certain position in the sample from images taken at two echo times (TE) is determined by the field inhomogeneity γΔΒ(r): Δφ(r) = γΔΒ(r)[TE 1 – TE 2 ] The effect of the shims can be similarly mapped by measuring the phase difference between images at two different shim settings: γ[ΔΒ(r) shimset 1 - γ[ΔΒ(r) shimset 2 ] = [Δφ shimset 1 (r) - Δφ shimset 1 (r)] / [TE 1 – TE 2 ] Optimum shim settings can then be calculated using the measured field inhomogeneity, <strong>and</strong> the known effects of each shim.
1D image of field inhomogeneity a b c G z τ τ τ 0 3 +10 -10 1D gradient echo experiment 2 -5 0 0 1 -10 -10 +10 region of sample inhomogeneous sample 1 st gradient readout field gradient 1 a y b c acq x freq. = -10 2 b c acq freq. = -5 3 b c acq freq. = 0
Page 1 and 2: Shimming: Theory and Practice Dr. R
Page 3 and 4: The functions P nm (cosθ) are poly
Page 5 and 6: Modern shims are coils that produce
Page 7 and 8: Since the sample is not centered at
Page 9 and 10: As we misset shims of higher order,
Page 11 and 12: Shimming the higher order z shims:
Page 13 and 14: Effect of 2,2 (xy or x 2 -y 2 ) inh
Page 15 and 16: Now look at the spectrum and evalua
Page 17: Fourier Imaging Increment # 1 stren
Page 21 and 22: Phase difference -90 -45 0 1 2 3 Im
Page 23 and 24: 3D images and field mapping G z G x
Page 25 and 26: Heres a simple example: 1 shim (z),
Page 27: Spectrum Optimization Topshim perfo

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