Efficient correction of inhomogeneous static magnetic field-induced distortion in Echo Planar Imaging

Abstract

Single-shot Echo Planar Imaging (EPI) is one of the most efficient magnetic resonance imaging (MRI) acquisition schemes, producing relatively high-definition images in 100 ms or less. These qualities make it desirable for Diffusion Tensor Imaging (DTI), functional MRI (fMRI), and Dynamic Susceptibility Contrast MRI (DSC-MRI). However, EPI suffers from severe spatial and intensity distortion due to B(0) field inhomogeneity induced by magnetic susceptibility variations. Anatomically accurate, undistorted images are essential for relating DTI and fMRI images with anatomical MRI scans, and for spatial registration with other modalities. We present here a fast, robust, and accurate procedure for correcting EPI images from such spatial and intensity distortions. The method involves acquisition of scans with opposite phase encoding polarities, resulting in opposite spatial distortion patterns, and alignment of the resulting images using a fast nonlinear registration procedure. We show that this method, requiring minimal additional scan time, provides superior accuracy relative to the more commonly used, and more time consuming, field mapping approach. This method is also highly computationally efficient, allowing for direct "real-time" implementation on the MRI scanner. We further demonstrate that the proposed method can be used to recover dropouts in gradient echo (BOLD and DSC-MRI) EPI images.

Figure 1

Forward (top) and reverse (bottom) spin-echo EPI at 3T. Left: uncorrected. Middle: field map corrected. Right: corrected as described here.

Figure 2

Complementary color comparison of forward and reverse spin-echo EPI. Top row shows 3T axial images corrected as described here: forward (red), reverse (cyan), and on the right their color-overlay sum (red+cyan=white). Middle row shows 3T sagittal color-overlay sum: left, uncorrected forward (red) and reverse (cyan); center, field map corrected forward and reverse; right, forward and reverse corrected as described here. Bottom row: same as middle row for a similar sagittal plane, but at 1.5T.

Figure 3

Corrected spin-echo EPI compared with T₁-weighted structural scan. Left: axial and sagittal views of the average forward and reverse corrected 3T EPI. Middle: 3T T₁-weighted structural. Right: overlay of corrected EPI on structural.

Figure 4

Axial slices from 3T gradient echo EPI, corrected using the displacement field calculated from spin echo EPI, showing complementary in-plane signal dropout patterns. Top row L to R: forward; reverse; root mean square of forward and reverse. Bottom row L to R: forward overlain with Jacobian of displacement field; reverse overlain with Jacobian of inverse displacement field; identical slice from average corrected forward and reverse spin echo EPI. For the overlays, red and blue indicate regions from which tissue was recorded as compressed or expanded, respectively, in the uncorrected images. Red thus indicates regions of recoverable dephasing dropout due to spin bunching.