Reduction of eddy‐current‐induced distortion in diffusion MRI using a twice‐refocused spin echo

Abstract

Image distortion due to field gradient eddy currents can create image artifacts in diffusion-weighted MR images. These images, acquired by measuring the attenuation of NMR signal due to directionally dependent diffusion, have recently been shown to be useful in the diagnosis and assessment of acute stroke and in mapping of tissue structure. This work presents an improvement on the spin-echo (SE) diffusion sequence that displays less distortion and consequently improves image quality. Adding a second refocusing pulse provides better image quality with less distortion at no cost in scanning efficiency or effectiveness, and allows more flexible diffusion gradient timing. Multidirectional diffusion sequences (1-3) have recently been shown to be useful in the diagnosis and assessment of acute stroke and in mapping of tissue structure (4 -10). These methods apply gradient pulses at higher intensity and with longer duration than in any other well known MRI sequence, resulting in comparatively large and persistent eddy currents. Use of the spin-echo (SE) diffusion sequence with an echo planar (EP) readout combines atypically large eddy currents with an eddy current-sensitive EP readout, causing spatial distortion dependent on the direction of the applied diffusion gradient. Misregistration artifacts result when directional diffusion is calculated from multiple images with differing gradient directions.Each on and off field gradient transition produces eddy currents to some degree. If the eddy current (and its associated magnetic field) decays to an inconsequential value between the time of the applied field gradient transition and the image readout, a spatially dependent change in image phase will result with no discernible distortion. Since diffusion encoding relies on the attenuation of the image magnitude, a change in image phase does not change the diffusion measurement as long as the phase gradient per pixel is small (11). However, when the eddy current decays slowly, so that a residual field remains during the image readout, the field behaves like an additional spatial encoding gradient field and causes distortion of the image.While the usual SE diffusion sequence, introduced by Stejkal and Tanner (12), uses a single refocusing RF pulse, many SE diffusion sequence variants can be created using multiple refocusing pulses. SEs result from any combination of refocusing pulses that returns the spins' phase evolution to the origin in classical phase space (13). Using more than one refocusing pulse permits variable intervals between the pulses, requiring only that the spins' alternating defocusing and refocusing times sum equally at the time of the intended SE. This flexibility in timing adds utility when used for diffusion imaging.Since the on and off field gradient transitions produce equal and opposite eddy currents, the shorter the time between on and off transitions, the less decay of the residual fields during the gradient pulse and the more complete the fields' cancellation. Toward this end, a reduction of distor...

Brain microstructural correlates of visuospatial choice reaction time in children

“…T 2 -weighted images of the whole head were acquired using a 3D turbo spin echo sequence (TR = 3000 ms, TE= 354 ms, FOV = 282 × 216,matrix= 256 × 196,192 sagittal slices, 1.1 × 1.1 × 1.1 mm 3 voxels, acquisition time = 8:29). Whole brain diffusion-weighted (DW) images were acquired using a twice-refocused balanced spin echo sequence that minimised eddy current distortion (Reese et al, 2003). Ten non-DW images (b= 0) and 61 DW images (b= 1200 s/mm 2 ), encoded along independent collinear diffusion gradient orientations, were acquired (TR = 8200 ms, TE= 100 ms, FOV= 220 × 220, matrix= 96× 96, GRAPPA: factor = 2, 48 lines, 61 transverse slices with no gap, 2.3 × 2.3 × 2.3 mm 3 voxels, 14.0 ± 1.7 13.4 ± 2.0 13.7 ± 2.0 13.7 ± 1.9…”

Efficient gradient calibration based on diffusion MRI

“…A second limitation is that only one radiofrequency coil was used for calibration. Because different radiofrequency coils may give rise to different eddy current behavior, it could be beneficial to use pulse sequences such as the twice‐refocused SE 29 that minimize eddy currents. In the present study, eddy currents were mitigated by using volume transmit coils with shields built from overlapping slits 30, and were not found to be a major issue.…”

Diffusion anisotropy indexes are sensitive to selecting the EPI readout-encoding bandwidth at high-field MRI

“…Eddy currents can produce geometric distortions of contraction or dilation, shift and shear in diffusion-weighted images [9,18]. This artifact can be reduced by using a double refocusing SE sequence [19], by image postprocessing [20][21][22] or by using stimulated echo sequences [23]. Another type of artifact generated during diffusion studies is the susceptibility artifact caused by an inhomogeneous magnetic field when an EPI sequence is used to obtain data.…”