Diffusion Tensor Imaging (DTI) has emerged as a key tool for in-vivo investigation of the central nervous system (i.e. brain and spinal cord (SC)) integrity and white matter (WM) connectivity mapping 1,2. It has the potential of probing changes in neural system microstructure with certain pathology by assessing the diffusion properties of water molecules inside the biological tissues in multiple directions, providing insights into the organization and orientation of structures 3. DTI-derived metrics, i.e., Fractional Anisotropy (FA), Mean Diffusivity (MD), Radial Diffusivity (RD), and Axial Diffusivity (AD), could be computed and used as a reliable imaging biomarker for describing the SC microstructure changes with certain pathology 4. In fact, numerous studies have demonstrated that there is a correlation between the computed metrics and the standard scales used for assessing the severity of physical disability such as the Japanese Orthopaedic Association (JOA) or the modified JOA (mJOA) score 5,6 as well the INSCSCI score.
Spinal Cord disorders such as spinal cord injury (SCI) and Degenerative Spondylotic Myelopathy (DSM) can affect the entire nervous system and lead to tissue/axonal damage, i.e., demyelination, transection, and atrophy, resulting in serious clinical complication including motor and/or sensory systems dysfunction, partial or complete paralysis, etc. In this context, the microstructural damages such as demyelination and axonal loss associated with SCI cause changes in the diffusivity of water in the spinal cord, and fiber bundles density, exhibiting changes in DTI-derived metrics according to the level and severity of the damage 7–9. Surgical intervention for SCI treatment entail implantation of metallic hardware (Anterior or Posterior) for maintaining SC stabilization and avoid further short-term or long-term complications 10.
Structural MRI techniques, i.e., T1- and T2-weighted imaging, and DTI have gained popularity in clinical practice compared to conventional radiology modalities (CT, X-Ray) for preoperative diagnosis and assess of SCI. Kara et al have demonstrated the feasibility of using DTI-derived indices as robust biomarkers for the early detection of DSM in patients with normal-appearing T2-weighted images 11. A recent study has shown that the change of DTI-metrics correlates with the change in the mJOA score as well as with the mJOA recovery rate, showing evidence that preoperative DTI has prognostic potential in predicting surgical outcomes 12. However, DTI is hampered by its intrinsic low-sensitivity, as well as the low-spatial resolution and high-sensitivity to motion, leading to image distortions and a weak signal-to-noise ratio (SNR). In addition, performing DTI on SC is technically challenging mainly due to the small dimensions of the SC, the physiological motion (e.g., heart, lungs, and throat in its proximity) and the susceptibility-induced distortions.
Even with these new improvements, postsurgical metallic implants typically induce dramatic magnetic field inhomogeneities, leading to severe image distortions. The metal-induced artifact depends on the implant hardware (material, size, shape), the magnetic field B0 strength, and MRI sequence 13. Moreover, DTI is often performed using the single-shot Echo Planar Imaging pulse sequence (SS-EPI) due to its fast acquisition speed. However, this acquisition method is prone to various limitations, including Eddy-current and high-sensitivity to magnetic field inhomogeneity, inducing artifacts dramatically affecting the image quality. Phase-segmented EPI (PS-EPI) and readout-segmented EPI (RS-EPI) pulse sequences have been introduced recently for performing high-resolution and distortion reduced DTI on the brain 14–16. Though, the application of these techniques for SC diffusion imaging remains limited due to their high sensitivity to motion.
Therefore, given these technical challenges limiting the potential of getting images near the metal hardware, the use of DTI for evaluating postoperative clinical outcomes remain an unexplored field and the post-surgery SCI assessment is still heavily based on structural MRI techniques and a surgeon’s skills. Studies have attempted various acquisition approaches to demonstrate the feasibility of performing metal-artifacts reduction for DTI scan SC near metal implant 17–19. Despite these recent technical developments, the potential to effectively suppress metal-induced artifacts for the diffusion-MRI scan is still unreached. This is mainly due to the limitations of the proposed techniques, including through-plane distortion, image blurring, and low SNR. In addition, this can be due to the high B0 field inhomogeneity in proximity to the metal, specifically at ultra-high field (UHF) such as 3 or 7 Tesla.
In this study, we propose a reduced-Field-Of-View phase-segmented EPI (rFOV-PS-EPI) diffusion weighted MR pulse sequence to address geometric distortions near the metal in DTI scan of SC at 3T. High-resolution distortion reduced diffusion-weighted images were collected on a custom-build cervical spine phantom model with a metal implant. The efficacy of the proposed pulse sequence in reduction of metal-artifacts was evaluated through the comparison with the rFOV-SS-EPI pulse sequence as well as the full-FOV approaches: SS-EPI, PS-EPI, and RS-EPI. To date, this is the first implementation of a rFOV-PS-EPI DTI sequence used to image spinal cord phantom model in the presence of metal-based spinal hardware currently used by surgeons.