Quantitative MRI mOOCB0 Mapping · Exercise 5-3
Shimming
In this exercise, we will cover:
- How to find the shim currents that make the field more uniform
- How your shim compares with the optimal one
- Volume-wise and slice-wise shimming
Exercise 5-3 · (a)
Shimming the B0 map
Find the shim currents that optimize the shim, that is, that minimize the mean and the SD of B0.
Input
Output
Exercise 5-3 · (b)
Your shim and the optimal shim
Submit your shim on the previous slide.
Your shim Optimal shim
Exercise 5-3 · (c)
Volume-wise vs slice-wise shimming
Find the shim currents that minimize the mean and the SD of B0: first over the whole brain, then over axial slice 35 only.
Input
Output
Data: Chan K-S, Marques JP, Spincemaille P, Wang Y. QSM consensus example data (Siemens). Zenodo, doi:10.5281/zenodo.7795492, CC BY 4.0; echoes 1 and 2, 2 mm, phase difference unwrapped in 3D.
Exercise 5-3 · (d)
Volume-wise and slice-wise shims
Submit your shim on the previous slide.
Exercise 5-3 · Summary
Summary
- B0 maps let us compute the shim currents that reduce the field inhomogeneity.
- Volume-wise shimming minimizes the mean and the SD of Δf over the whole masked volume; slice-wise shimming does it over a single slice only.
- Slice-wise shimming can give better shims for multislice acquisitions, with the shims updated for each slice.
To compute shims for your own data, you can use Shimming Toolbox, an open-source tool developed at NeuroPoly.
