Shimming: Theory and Practice - UCLA-DOE
Shimming: Theory and Practice - UCLA-DOE Shimming: Theory and Practice - UCLA-DOE
Mathematical Analysis Taken from: Concepts in Magnetic Resonance (1990), 2 131-149 Any steady-state magnetic field in a volume of space in which there are no charges or current obeys the Laplace equation: The field can be represented as a linear combination of any complete, orthogonal basis set. The spherical harmonics are a convenient basis set for this purpose. Thus, B 0 is described by the expansion in spherical harmonics: Where a is the average bore radius, r represents the sample position, and C nm and Ψ nm are constants. The calculations are easier when performed in spherical polar coordinates. Since the sample volume is small compared to the bore size, as the order (n) Gets higher, (r/a) n becomes negligible. Good thing too because the Number of terms in this equation is infinite, and we would need infinitely Many shims!
The functions P nm (cosθ) are polynomials in cosθ, and when m=0, P n0 are the Legendre Polynomials. The first few Legendre Polynomials Notice that all variation of B 0 with Φ disappears when m=0 because the final cosine term is unity. When m=0 the functions in the equation for B 0 are called zonal harmonics, and the zonal field is given by: You can think of these as the Z components of the field. That is, inhomogeneities that can be canceled by Z shims.
- Page 1: Shimming: Theory and Practice Dr. R
- 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 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
Mathematical Analysis<br />
Taken from:<br />
Concepts in Magnetic<br />
Resonance (1990), 2 131-149<br />
Any steady-state magnetic field in a volume of space in which there are<br />
no charges or current obeys the Laplace equation:<br />
The field can be represented as a linear combination of any complete,<br />
orthogonal basis set. The spherical harmonics are a convenient basis<br />
set for this purpose. Thus, B 0 is described by the expansion in spherical<br />
harmonics:<br />
Where a is the average bore radius, r represents the sample position, <strong>and</strong><br />
C nm <strong>and</strong> Ψ nm are constants.<br />
The calculations are easier<br />
when performed in spherical<br />
polar coordinates.<br />
Since the sample volume is small compared to the bore size, as the order<br />
(n) Gets higher, (r/a) n becomes negligible. Good thing too because the<br />
Number of terms in this equation is infinite, <strong>and</strong> we would need infinitely<br />
Many shims!
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