Shimming: Theory and Practice - UCLA-DOE
Shimming: Theory and Practice - UCLA-DOE Shimming: Theory and Practice - UCLA-DOE
To shim well, one must look at the FID and/or the spectrum, not just the lock level. Getting an idea what these indicators look like when certain shims are off is very helpful. Here are some examples with the z shims: Good shimming Z shim off When the z shim is off, the initial decay of the FID is much faster and can have beats. The spectrum broadens and manifests structure. Here what look like individual peaks within the single peak are due to B 1 (the field generated by the pulse) inhomogeneities. Local maxima in B 1 close to probe coil conductors cause these peaks.
As we misset shims of higher order, the initial fall off of the FID gets faster: z 2 shim misset in one direction (a), and in the other (b) Even orders give a hump on one side of the peak. Why? Z 2 is an even function: There is much sample volume in the vicinity of field strength A. This gives rise to the main peak. At higher field (like at B) there is some sample (this gives rise to the hump), and there is no field lower than A so theres a sharp cutoff on one side. The hump ends suddenly because the sample tube ends. For higher orders the hump is lower on the peak but has a longer tail.
- 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: Since the sample is not centered at
- 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
To shim well, one must look at the FID <strong>and</strong>/or the spectrum, not just the<br />
lock level. Getting an idea what these indicators look like when certain<br />
shims are off is very helpful. Here are some examples with the z shims:<br />
Good shimming Z shim off<br />
When the z shim is off, the initial decay of the FID is much faster <strong>and</strong> can<br />
have beats. The spectrum broadens <strong>and</strong> manifests structure.<br />
Here what look like individual peaks within the single peak are due to B 1<br />
(the field generated by the pulse) inhomogeneities. Local maxima in B 1<br />
close to probe coil conductors cause these peaks.
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