Unwarping of unidirectionally distorted EPI images

Abstract

Abstract-Echo-planar imaging (EPI) is a fast nuclear magnetic resonance imaging (MRI) method. Unfortunately, local magnetic field inhomogeneities induced mainly by the subject's presence cause significant geometrical distortion, predominantly along the phase-encoding direction, which must be undone to allow for meaningful further processing. So far, this aspect has been too often neglected.In this paper, we suggest a new approach using an algorithm specifically developed for the automatic registration of distorted EPI images with corresponding anatomically correct MRI images. We model the deformation field with splines, which gives us a great deal of flexibility, while comprising the affine transform as a special case. The registration criterion is least squares. Interestingly, the complexity of its evaluation does not depend on the resolution of the control grid. The spline model gives us good accuracy thanks to its high approximation order. The short support of splines leads to a fast algorithm. A multiresolution approach yields robustness and additional speedup.The algorithm was tested on real as well as synthetic data, and the results were compared with a manual method. A wavelet-based Sobolev-type random deformation generator was developed for testing purposes. A blind test indicates that the proposed automatic method is faster, more reliable, and more precise than the manual one.

Assessment of a commercially available automatic deformable registration system

“…Intra‐modality deformable registration validation using known transformations is common in inter‐subject brain studies, ( 26 , 41 , 42 , 43 , 44 ) but has had limited use in applications directly relevant to radiation therapy. By registering multiple B‐spline warped images to original thoracic CT images, we quantified the commercial system's performance over a range of potential initial clinical deformations.…”

Geometric distortion correction for echo planar images using nonrigid registration with spatially varying scale

“…Thus, the displacement at location (x, y, z) can be written as (1) where Δy(x, y, z) is the geometric distortion at the original position (x, y, z) and is calculated by: (2) Here, γ̶ is the gyromagnetic ratio, ΔB is the perturbation of the B 0 field, and FOV y and BW y are the Field of View and bandwidth in the phase encoding direction, respectively.…”

“…According to (2), the amount of geometric distortion is proportional to the amplitude of the B 0 field inhomogeneity ΔB. Such inhomogeneity is induced mainly by the magnetic susceptibility differences between substances within the subject inside the scanner.…”

Simulation-based evaluation of susceptibility distortion correction methods in diffusion MRI for connectivity analysis

“…Another approach referred to as reversed encoding-based (REB), acquires an extra EPI volume in the orthogonal or reversed phase encoding that can be combined to remove the geometric distortions [4,5]. The last set of methodologies uses an extra T2-weighted image as anatomical reference to seek for the deformation map through nonlinear registration [6,7]. These T2w registration-based (T2B) methods usually map the T2w image to the baseline volume or b0 of dMRI, as the latter exhibits a contrast very similar to the anatomical T2w.…”