Shimming: Theory and Practice - UCLA-DOE

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Field Mapping The field correction provided by the shims for a given change in shim current can be mapped similarly. The phase difference between images at two different shim settings is measured: γ[ΔB(r) shimset 1 – ΔB(r) shimset 2 ] = [Δφ shimset 1 (r) – Δφ shimset 2 (r)]/[TE 1 -TE 2 ] Therefore to map the shims, an image of the field is generated once with reasonable shims. Then a shim is moved significantly and another image is taken. This shim is returned to its original value. This is done with each shim in turn. The 1D field maps generated by missetting individual shims look something like this: Z Z 2 etc
3D images and field mapping G z G x,y 3D images can be done by performing many 1D scans, incrementing the strength (or length) of the X and Y gradients in a nested fashion. Each 3D experiment takes almost 2 minutes, so each 3D image takes almost 4 minutes to acquire (2 experiments are necessary, using two different echo times). You can see that 3D field mapping takes awhile. To map all 28 shims, 58 experiments must be run (2 + 2[28]). The field maps are empirical maps of the real effects of the shims.
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 and 18: Fourier Imaging Increment # 1 stren
Page 19 and 20: 1D image of field inhomogeneity a b
Page 21: Phase difference -90 -45 0 1 2 3 Im
Page 25 and 26: Heres a simple example: 1 shim (z),
Page 27: Spectrum Optimization Topshim perfo

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