title: Gradient nonlinearity calibration and correction for a compact, asymmetric magnetic resonance imaging gradient system.
abstract: Due to engineering limitations, the spatial encoding gradient fields in conventional magnetic resonance imaging cannot be perfectly linear and always contain higher-order, nonlinear components. If ignored during image reconstruction, gradient nonlinearity (GNL) manifests as image geometric distortion. Given an estimate of the GNL field, this distortion can be corrected to a degree proportional to the accuracy of the field estimate. The GNL of a gradient system is typically characterized using a spherical harmonic polynomial model with model coefficients obtained from electromagnetic simulation. Conventional whole-body gradient systems are symmetric in design; typically, only odd-order terms up to the 5th-order are required for GNL modeling. Recently, a high-performance, asymmetric gradient system was developed, which exhibits more complex GNL that requires higher-order terms including both odd- and even-orders for accurate modeling. This work characterizes the GNL of this system using an iterative calibration method and a fiducial phantom used in ADNI (Alzheimer's Disease Neuroimaging Initiative). The phantom was scanned at different locations inside the 26 cm diameter-spherical-volume of this gradient, and the positions of fiducials in the phantom were estimated. An iterative calibration procedure was utilized to identify the model coefficients that minimize the mean-squared-error between the true fiducial positions and the positions estimated from images corrected using these coefficients. To examine the effect of higher-order and even-order terms, this calibration was performed using spherical harmonic polynomial of different orders up to the 10th-order including even- and odd-order terms, or odd-order only. The results showed that the model coefficients of this gradient can be successfully estimated. The residual root-mean-squared-error after correction using up to the 10th-order coefficients was reduced to 0.36 mm, yielding spatial accuracy comparable to conventional whole-body gradients. The even-order terms were necessary for accurate GNL modeling. In addition, the calibrated coefficients improved image geometric accuracy compared with the simulation-based coefficients.
authors.affiliation: Department of Radiology, Mayo Clinic, Rochester, MN, USA. Mayo Graduate School, Mayo Clinic, Rochester, MN, USA.
authors.forename: S
authors.initials: S
authors.lastname: Tao
authors.forename: J D
authors.initials: JD
authors.lastname: Trzasko
authors.forename: J L
authors.initials: JL
authors.lastname: Gunter
authors.forename: P T
authors.initials: PT
authors.lastname: Weavers
authors.forename: Y
authors.initials: Y
authors.lastname: Shu
authors.forename: J
authors.initials: J
authors.lastname: Huston
authors.forename: S K
authors.initials: SK
authors.lastname: Lee
authors.forename: E T
authors.initials: ET
authors.lastname: Tan
authors.forename: M A
authors.initials: MA
authors.lastname: Bernstein
mesh_terms: D000465:Algorithms; D000544:Alzheimer Disease / Q000473:pathology*; D002138:Calibration; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D017711:Nonlinear Dynamics; D019047:Phantoms, Imaging