Diffusion-prepared fast spin echo for artifact-free spinal cord imaging

Abstract

Purpose: Diffusion MRI provides unique contrast important for the detection and examination of pathophysiology after acute neurologic insults, including spinal cord injury. Diffusion weighted imaging of the rodent spinal cord has typically been evaluated with axial EPI readout. However, Diffusion weighted imaging is prone to motion artifacts, whereas EPI is prone to susceptibility artifacts. In the context of acute spinal cord injury, diffusion filtering has previously been shown to improve detection of injury by minimizing the confounding effects of edema. We propose a diffusion-preparation module combined with a rapid acquisition with relaxation enhancement readout to minimize artifacts for sagittal imaging.

Methods: Sprague-Dawley rats with cervical contusion spinal cord injury were scanned at 9.4 Tesla. The sequence optimization included the evaluation of motion-compensated encoding diffusion gradients, gating strategy, and different spinal cord-specific diffusion-weighting schemes.

Results: A diffusion-prepared rapid acquisition with relaxation enhancement achieved high-quality images free from susceptibility artifacts with both second-order motion-compensated encoding and gating necessary for reduction of motion artifacts. Axial diffusivity obtained from the filtered diffusion-encoding scheme had greater lesion-to-healthy tissue contrast (52%) compared to the similar metric from DTI (25%).

Conclusion: This work demonstrated the feasibility of high-quality diffusion sagittal imaging in the rodent cervical cord with diffusion-prepared relaxation enhancement. The sequence and results are expected to improve injury detection and evaluation in acute spinal cord injury.

Keywords: artifact reduction; diffusion imaging; motion preparation; spinal cord.

Figure 1

Pulse sequence diagram of diffusion prepared RARE (DW_prep-RARE). Diffusion encoding is initiated after a cardiac post delay. A four-segment adiabatic B1 insensitive rotation (BIR-4) pulse was applied in combination with diffusion gradients (gray trapezoid) having either zeroth-order (m0), first-order (m1), or second-order (m2) moment compensation, depicted only on one axis for clarity. The duration of the diffusion preparation (TE_prep) for a b-value of 2000 s/mm₂ is indicated and varied with the degree of compensation. A magnitude stabilizer (red) before the final 90° pulse in the DW_prep reduced ghost artifact and eddy currents and was incorporated into the spoiler gradients of the RARE readout.

Figure 2

Image quality of sagittal DW_prep-RARE in the spinal cord. In the images without diffusion weighting (b₀, A-B), prominent artifacts exist in the EPI image (A) that are not apparent in the RARE (B), noting these are from different animals. With b_⊥=2000 s/mm² (C-G), subtle artifacts were evident in both respiratory (C) and dual (D) gating conditions with both m1 (E) and m2 (F) compensation eliminating the artifacts. Higher order motion compensation slightly reduced tSNR (G). Data shown for n=4 animals obtained from ROIs within the spinal cord as shown (H) exhibit trends consistent with those seen qualitatively in the images.

Figure 3

Diffusion encoding schemes specific to the spinal cord. DTI uses a rotationally invariant single b-value with 6 of 20 directions shown. The other three filtered DWI schemes used a diffusion filter gradient perpendicular to the cord of 2000 s/mm² with different sets of 3D, 2D, or 1D sampling. 3D filtered DWI used the same set of directions as DTI but offset in the left-right (perpendicular) direction from a rotationally invariant shell. 2D filtered DWI was similar but employed in-plane sampling, whereas 1D employed single-axis sampling along the main cord axis (rostral-caudal).

Figure 4

DW_prep-RARE parameter maps in healthy (n=3) and injured (n=3) spinal cords. Compared to DTI, contrast in the 2D or 1D schemes were generally similar, although the 3D-fDWI condition had greater noise in the estimated maps. In the injured cord, the fDWI images better revealed the lesion compared to DTI, primarily in both AD and MD maps. Note that 2D and 1D conditions had less noise compared to 3D-fDWI. Compared to DTI, maps of FA derived from fDWI had lower quality. Across all animals and contrasts, fADC_|| measured from fDWI had larger lesion to healthy cord contrast. CSF=cerebrospinal fluid, WM=white matter, GM=gray matter

Figure 5

Orientation dependence of diffusion schemes. Initial images were aligned with the spinal cord axis and diffusion directions aligned with the image orientation. To assess the dependence on orientation, the imaging orientation was rotated counter-clockwise in 20 ° increments (A). Maps of AD for an injured animal demonstrate the sensitivity of each scheme to orientation (B), with 1D-fDWI having high sensitivity to the angle, as expected, compared to DTI or 2D-fDWI. In DTI, the lower cervical cord was contaminated by wraparound aliasing artifact. Across 3 animals, the 2D-fDWI showed the most consistent reduction in AD in the injury site as well as the highest contrast to the uninjured cord (C). Diffusivity units are in μm²/ms.