title: Image-based gradient non-linearity characterization to determine higher-order spherical harmonic coefficients for improved spatial position accuracy in magnetic resonance imaging.
abstract: Spatial position accuracy in magnetic resonance imaging (MRI) is an important concern for a variety of applications, including radiation therapy planning, surgical planning, and longitudinal studies of morphologic changes to study neurodegenerative diseases. Spatial accuracy is strongly influenced by gradient linearity. This work presents a method for characterizing the gradient non-linearity fields on a per-system basis, and using this information to provide improved and higher-order (9th vs. 5th) spherical harmonic coefficients for better spatial accuracy in MRI.
  
  
  
  A large fiducial phantom containing 5229 water-filled spheres in a grid pattern is scanned with the MR system, and the positions all the fiducials are measured and compared to the corresponding ground truth fiducial positions as reported from a computed tomography (CT) scan of the object. Systematic errors from off-resonance (i.e., B0) effects are minimized with the use of increased receiver bandwidth (±125kHz) and two acquisitions with reversed readout gradient polarity. The spherical harmonic coefficients are estimated using an iterative process, and can be subsequently used to correct for gradient non-linearity. Test-retest stability was assessed with five repeated measurements on a single scanner, and cross-scanner variation on four different, identically-configured 3T wide-bore systems.
  
  
  
  A decrease in the root-mean-square error (RMSE) over a 50cm diameter spherical volume from 1.80mm to 0.77mm is reported here in the case of replacing the vendor's standard 5th order spherical harmonic coefficients with custom fitted 9th order coefficients, and from 1.5mm to 1mm by extending custom fitted 5th order correction to the 9th order. Minimum RMSE varied between scanners, but was stable with repeated measurements in the same scanner.
  
  
  
  The results suggest that the proposed methods may be used on a per-system basis to more accurately calibrate MR gradient non-linearity coefficients when compared to vendor standard corrections.
authors.affiliation: Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States.
authors.forename: Paul T
authors.initials: PT
authors.lastname: Weavers
authors.affiliation: Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States; Mayo Graduate School, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States.
authors.forename: Shengzhen
authors.initials: S
authors.lastname: Tao
authors.affiliation: Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States.
authors.forename: Joshua D
authors.initials: JD
authors.lastname: Trzasko
authors.affiliation: Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States.
authors.forename: Yunhong
authors.initials: Y
authors.lastname: Shu
authors.affiliation: Radiation Oncology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States.
authors.forename: Erik J
authors.initials: EJ
authors.lastname: Tryggestad
authors.affiliation: Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States.
authors.forename: Jeffrey L
authors.initials: JL
authors.lastname: Gunter
authors.affiliation: Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States.
authors.forename: Kiaran P
authors.initials: KP
authors.lastname: McGee
authors.affiliation: GE Heathcare, 3200 N Grandview Blvd, Waukesha, WI 53188, United States.
authors.forename: Daniel V
authors.initials: DV
authors.lastname: Litwiller
authors.affiliation: MD Anderson Cancer Center, 1515 Holcomb Blvd, Houston, TX 77030, United States.
authors.forename: Ken-Pin
authors.initials: KP
authors.lastname: Hwang
authors.affiliation: Radiology, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905, United States. Electronic address: mbernstein@mayo.edu.
authors.forename: Matt A
authors.initials: MA
authors.lastname: Bernstein
mesh_terms: D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results
keywords: Calibration; Gradient non-linearity; MRI; Treatment planning