Program Information
Gradient Nonlinearity Calibration and Correction for Head-Only Asymmetric Gradient System
S Tao1, J Trzasko1, Y Shu1, P Weavers1, J Huston III1, S Lee2, J Mathieu2, T Foo2, M Bernstein1. 1. Mayo Clinic, Rochester, MN; 2. GE Global Research, Niskayuna, NY
Presentations
TH-CD-207-11 (Thursday, July 16, 2015) 10:00 AM - 12:00 PM Room: 207
Purpose: Due to engineering limitations, the gradient fields in clinical MRI inevitably contain high-order, nonlinear components. The presence of gradient nonlinearity (GNL) causes image geometrical distortion. Standard correction methods are based on parameterization of the simulated gradient fields with spherical harmonic polynomials. Conventional whole-body gradient systems typically employ symmetric designs, and the GNL for such systems usually contains only odd-order terms (up to 5th-order). Recently, a high-performance, head-only gradient system was developed. Due to asymmetric design, this new system exhibits more complex GNL profiles. Here, we demonstrate measurement-based high-order(N>5) GNL-correction on this new system using a fiducial phantom and iterative model-fitting procedure.
Methods: The Alzheimer’s Disease Neuroimaging Initiative (ADNI) phantom was scanned with a 3D IR-FSPGR sequence (sagittal acquisition plane, Nx=Ny=256, Nz=196, Δx=Δy=1.05mm, Δz=1.3mm, BW=125kHz) on the head-only gradient system operating at 80mT/m, 500T/m/s. This phantom contains 160 fiducial spheres (diameter=1.0 or 1.5cm) that are distributed within a diameter=20cm spherical shell. The spatial positions of the fiducials were then measured from the distorted images and fit using a spherical harmonic polynomial model and iterative fitting procedure. The model order was increased from 1 to 10 step-by-step to test the effects of including high-order terms (N>5). Then, the coefficients were used to correct distortion. The residual root-mean-square-error (RMSE) was calculated from fiducial positions tracked from GNL-corrected images.
Results: The RMSE analysis shows that GNL-distortion is reduced from 3.93 to 0.39mm by using correction terms up to 7th-order including both even- and odd-order terms. The addition of higher orders (N>7) provides negligible benefit.
Conclusion: The GNL of a high-performance, head-only gradient system was successfully measured. The use of up to 7th-order correction terms was found to be sufficient for correcting GNL-distortion in a typical brain scan. These techniques could also improve geometric accuracy for whole-body MRI systems used for radiation therapy planning.
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