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Rapid in vivo proton shimming
1GE Medical System, Milwaukee, Wisconsin 53201.
Magnetic Resonance in Medicine
|April 1, 1991
Summary
A new automated method, AUTOSHIM, rapidly improves magnetic field (B0) homogeneity for MRI. While effective overall, it can create local distortions in specific anatomies, requiring targeted adjustments for optimal magnetic resonance spectroscopy.
Area of Science:
- Magnetic Resonance Imaging
- Spectroscopy
- Medical Physics
Background:
- Magnetic field (B0) homogeneity is critical for high-quality in vivo proton spectroscopy and imaging.
- Existing methods for B0 field adjustment can be time-consuming and manual.
Purpose of the Study:
- To describe a rapid, automated method for adjusting B0 homogeneity.
- To evaluate the efficacy of the automated shimming technique.
Main Methods:
- Generation of B0 inhomogeneity maps using a gradient-recalled echo pulse sequence.
- Derivation of low-order shim values via magnitude-weighted least-squares fits.
- Automatic application of shim values and use of chemical shift selective saturation for efficacy assessment.
Main Results:
- The AUTOSHIM technique demonstrates improvement in overall B0 homogeneity.
- Certain air/tissue and bone/tissue interfaces generate local high-order distortions uncorrectable by linear gradients.
- A "Zoom SHIM" approach can improve local homogeneity at the cost of other regions.
Conclusions:
- Automated shimming significantly enhances B0 homogeneity for MR applications.
- Morphological complexities necessitate advanced or targeted shimming strategies.
- Monitoring homogeneity during clinical MR spectroscopy is essential.