Shimming: Theory and Practice - UCLA-DOE
Shimming: Theory and Practice - UCLA-DOE Shimming: Theory and Practice - UCLA-DOE
a b 1D image of field inhomogeneity c 0 3 +10 -10 G z τ 3τ τ 1D gradient echo experiment with longer echo time 2 -5 0 0 1 -10 -10 +10 inhomogeneous sample 1 st gradient readout field gradient region of sample 1 a y b c acq x phase difference = -90 degrees freq. = -10 2 b c acq phase difference = -45 degrees freq. = -5 3 b c acq no phase difference freq. = 0
Phase difference -90 -45 0 1 2 3 Image of the field 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 is determined by the field inhomogeneity ΔΒ(r). Δφ(r) = γΔΒ(r)[TE 1 – TE 2 ] An image of the field is made by plotting the phase difference divided by the echo time difference against frequency. (remember that because of the acquisition gradient, frequency corresponds to position in the sample)
- 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: 1D image of field inhomogeneity a b
- 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
a<br />
b<br />
1D image of field inhomogeneity<br />
c<br />
0<br />
3<br />
+10<br />
-10<br />
G z<br />
τ 3τ τ <br />
1D gradient echo experiment<br />
with longer echo time<br />
2<br />
-5 0 0<br />
1<br />
-10 -10 +10<br />
inhomogeneous sample 1 st gradient readout<br />
field<br />
gradient<br />
region<br />
of sample<br />
1<br />
a<br />
y<br />
b<br />
c<br />
acq<br />
x<br />
phase difference<br />
= -90 degrees<br />
freq. = -10<br />
2<br />
b<br />
c<br />
acq<br />
phase difference<br />
= -45 degrees<br />
freq. = -5<br />
3<br />
b<br />
c<br />
acq<br />
no phase difference<br />
freq. = 0
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