Nonlinear Gradient Field Mapping Using a Spherical Grid Phantom for 3 and 7 Tesla MR Imaging Systems Equipped with...

Area of Science:

• Medical Imaging
• Magnetic Resonance Imaging (MRI)
• Gradient Coil Technology

Background:

• High-performance gradient coils are crucial for advanced MRI applications.
• Spatial linearity of gradient fields is often compromised in high-performance systems.
• Accurate magnetic field linearity is essential for precise image reconstruction.

Purpose of the Study:

• To measure and quantify the spatial nonlinearity of gradient fields in MRI systems.
• To evaluate the efficacy of vendor-provided geometric distortion correction methods.
• To develop and validate a method for correcting gradient field nonlinearity.

Main Methods:

• Utilized 3D gradient echo sequences and a spherical phantom with a lattice structure for measurements.
• Approximated spatial gradient field variations using 3rd order polynomials.
• Calculated polynomial coefficients via the steepest descent method.
• Corrected geometric distortion in 3D MR images using derived polynomials and compared with vendor methods.

Main Results:

• Quantified significant spatial nonlinearity in the gradient fields of two MRI systems.
• Demonstrated that vendor-provided harmonic function-based corrections were insufficient.
• The developed polynomial-based correction method showed improved accuracy.

Conclusions:

• Vendor-supplied nonlinearity correction formulae for MRI gradient fields require verification.
• Geometric phantoms are essential tools for assessing and correcting gradient field nonlinearity.
• Accurate correction of gradient field nonlinearity is critical for reliable 3D MR imaging.