Phantom-based field maps for gradient nonlinearity correction in diffusion imaging

Abstract

Gradient coils in magnetic resonance imaging do not produce perfectly linear gradient fields. For diffusion imaging, the field nonlinearities cause the amplitude and direction of the applied diffusion gradients to vary over the field of view. This leads to site- and scan-specific systematic errors in estimated diffusion parameters such as diffusivity and anisotropy, reducing reliability especially in studies that take place over multiple sites. These errors can be substantially reduced if the actual scanner-specific gradient coil magnetic fields are known. The nonlinearity of the coil fields is measured by scanner manufacturers and used internally for geometric corrections, but obtaining and using the information for a specific scanner may be impractical for many sites that operate without special-purpose local engineering and research support. We have implemented an empirical field-mapping procedure using a large phantom combined with a solid harmonic approximation to the coil fields that is simple to perform and apply. Here we describe the accuracy and precision of the approach in reproducing manufacturer gold standard field maps and in reducing spatially varying errors in quantitative diffusion imaging for a specific scanner. Before correction, median B value error ranged from 33 – 41 relative to manufacturer specification at 100 mm from isocenter; correction reduced this to 0 – 4. On-axis spatial variation in the estimated mean diffusivity of an isotropic phantom was 2.2% – 4.1% within 60 mm of isocenter before correction, 0.5% – 1.6% after. Expected fractional anisotropy in the phantom was 0; highest estimated fractional anisotropy within 60 mm of isocenter was reduced from 0.024 to 0.012 in the phase encoding direction (48% reduction) and from 0.020 to 0.006 in the frequency encoding direction (72% reduction).

Mapping gradient nonlinearity and miscalibration using diffusion‐weighted MR images of a uniform isotropic phantom

“…Different methods have been proposed to permit users to compute accurate b‐matrices, although none is routinely used in either research or clinical diffusion MRI studies. Rogers et al 26 suggests measuring the GCFMs using phase contrast mapping. 28 Their method involves multiple scans of a large oil phantom with different values of the linear shim settings.…”

The effect of gradient nonlinearities on fiber orientation estimates from spherical deconvolution of diffusion magnetic resonance imaging data

“…Water phantom studies (Nagy et al, 2009;Rogers et al, 2018Rogers et al, , 2017 reveal gradient field inhomogeneities as one of the main factors contributing to inaccuracies in the estimated apparent diffusion coefficient (ADC). Biases of up to 10% in diffusion measures derived from diffusion tensor imaging (DTI) and diffusion kurtosis imaging (DKI) resulting from gradient field nonlinearities have been reported (Bammer et al, 2003;Mesri, David, Viergever, & Leemans, 2020.…”

“…The effect of gradient nonlinearities in diffusion MRI and their corrections have been investigated in several studies (Bammer et al, 2003; Jovicich et al, 2006; Malyarenko et al, 2014; Mohammadi et al, 2012; Nagy et al, 2007; Setsompop et al, 2013). Water phantom studies (Nagy et al, 2009; Rogers et al, 2018, 2017) reveal gradient field inhomogeneities as one of the main factors contributing to inaccuracies in the estimated apparent diffusion coefficient (ADC). Biases of up to 10% in diffusion measures derived from diffusion tensor imaging (DTI) and diffusion kurtosis imaging (DKI) resulting from gradient field nonlinearities have been reported (Bammer et al, 2003; Mesri, David, Viergever, & Leemans, 2020, 2018).…”

Empirical field mapping for gradient nonlinearity correction of multi-site diffusion weighted MRI

“…To remove the need for the manufacturer supplied specifications, we demonstrate an empirical field-mapping procedure which can be universally applied across platforms as defined by Rogers et al [29, 30]. At two scanners (scanner A and scanner B), a large oil-filled phantom is used to measure the magnetic field produced by each gradient coil.…”