Nonlinear Gradient Fields Research Articles

Extended field-of-view imaging with table translation and frequency sweeping.

A new method for MRI of an extended field of view (FOV) has been developed and validated. The method employs concurrent MR data acquisition and patient table motion. Table motion-induced image artifacts are minimized by sweeping the frequency of the receiver at a rate matching the table's speed. Multiple regional images are collected and combined to reconstruct the full FOV. The imaging parameters and table speed are chosen to ensure that each regional image of the subject is collected while the corresponding anatomy is in the useable imaging volume of the scanner. Additional strategies are applied to further reduce field inhomogeneity-induced artifacts, especially distortions due to gradient field nonlinearity. The method is robust and can be easily incorporated into most multislice 2D and volumetric 3D imaging pulse sequences. It is anticipated that this technique will be useful for a variety of applications, including angiographic runoffs, whole-body screening, and short-magnet imaging.

An efficient geometric image distortion correction method for a biplanar planar gradient coil.

Since the spatial field non-linearity of gradient coils translates into image geometric distortion in MRI, in many applications, such as cardiac function analysis and interventional MR-based device tracking/guidance, where the precise geometric information is needed, the presence of geometric image distortion can not be simply ignored. To address the concern for geometric image distortion, we have developed and validated a general and efficient numerical technique for parameterizing the global image distortion for a bi-planar gradient coil as well as accomplishing image restoration as a post-imaging processing. This image correction methodology is based on a global distortion coordinate mapping function which can be systematically defined directly from the gradient field non-linearity in 3-dimension (3D) of a given gradient coil. The image correction was carried out in two steps: (1) map each pixel of the corrected image representation onto its distorted image according to the distortion mapping; (2) interpolate the pixel intensity in the distorted image using its neighboring points via a bi-linear interpolation procedure. The results showed clearly that the distortion correction method was robust in term of the capability of reducing image geometric distortion dramatically. Also it is shown that the magnetic field non-linearity or the image distortion of a typical bi-planar gradient coil can be adequately parameterized using a finite Taylor series expansion based on its design parameters. Furthermore, this image distortion correction method is very efficient in practice for performing 3D correction for any image orientation since a compact parameterized field expression contains non-zero terms.

MRI geometric distortion: A simple approach to correcting the effects of non‐linear gradient fields

We present a method to correct intensity variations and voxel shifts caused by non-linear gradient fields in magnetic resonance images. The principal sources of distortion are briefly discussed, as well as the methods of correction currently in use. The implication of the gradient field non-linearities on the signal equations are described in a detailed way for the case of two- and three-dimensional Fourier imaging. A model of these non-linearities, derived from the geometry of the gradient coils, is proposed and then applied in post-processing to correct any images regardless of the acquisition sequence. Initial position errors, as large as 4 mm (i.e., four voxels of 1 × 1 × 1.4 mm3 ) before correction, are reduced to less than the voxel sizes after correction. J. Magn. Reson. Imaging 1999;9:821–831. © 1999 Wiley-Liss, Inc.

MRI geometric distortion: A simple approach to correcting the effects of non-linear gradient fields

We present a method to correct intensity variations and voxel shifts caused by non-linear gradient fields in magnetic resonance images. The principal sources of distortion are briefly discussed, as well as the methods of correction currently in use. The implication of the gradient field non-linearities on the signal equations are described in a detailed way for the case of two- and three-dimensional Fourier imaging. A model of these non-linearities, derived from the geometry of the gradient coils, is proposed and then applied in post-processing to correct any images regardless of the acquisition sequence. Initial position errors, as large as 4 mm (i.e., four voxels of 1 x 1 x 1.4 mm3) before correction, are reduced to less than the voxel sizes after correction.

Characterization of spatial distortion in magnetic resonance imaging and its implications for stereotactic surgery.

The different sources of spatial distortion in magnetic resonance images are reviewed from the point of view of stereotactic target localization. The extents of the two most complex sources of spatial distortion, gradient field nonlinearities and magnetic field inhomogeneities, are discussed both qualitatively and quantitatively. Several ways by which the spatial distortion resulting from these sources can be minimized are discussed. The clinical relevance of the spatial distortion along with some strategies to minimize the localization errors in magnetic resonance-guided stereotaxy are presented.

Reconstruction of magnetic resonance images by iterative methods

AbstractIn MRI based on the time‐varying gradient magnetic field, the image must be reconstructed from the sampled values which are distributed in k‐space with nonuniform intervals. If there exists any static field inhomogeneity, gradient field nonlinearity, or patient motion, the image must be reconstructed considering these effects, thereby preventing the deterioration of the reconstructed image.This paper proposes a new MRI image reconstruction method to cope with the forementioned problems in a unified way. The proposed method of reconstruction is based on the principle whereby the imaging equation representing the relation between the spin density distribution and FID signal is solved directly by an iterative least‐squares method. The proposed reconstruction method strictly considers that mathematically the measured data are the finite number of sampled values of the FID signal. The minimum norm solution is obtained as the reconstructed image. Also, a method is proposed which improves the convergence of the iterative least‐squares method by utilizing the information concerning the density of the sampling point distribution in k‐space. An image reconstruction experiment is shown assuming the spiral k‐trajectory.

NMR imaging in the presence of magnetic field inhomogeneities and gradient field nonlinearities.

The relative merits of the spin-warp method and the multiple-angle projection-reconstruction method are assessed for nuclear magnetic resonance imaging in the presence of magnetic field inhomogeneities. The results of computer modeling studies demonstrate the superiority of the spin-warp method under these conditions. Because the form of the point spread function for the spin-warp approach is insensitive to magnetic field irregularities, simple correction algorithms have been developed to reduce artifacts associated with bias field inhomogeneities. In addition, the formalism developed for magnetic field inhomogeneities can be easily extended to correct for gradient field nonlinearities.