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 pathology induced changes in neural system microstructure 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 Fractional Anisotropy (FA), Mean Diffusivity (MD), Radial Diffusivity (RD), and Axial Diffusivity (AD), can be computed and used as a reliable imaging biomarkers for describing the SC microstructure changes with certain pathologies [4]. In fact, numerous studies have demonstrated that there is a correlation between the computed metrics and the standard clinical 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, and partial or complete paralysis. In this context, the microstructural damage such as demyelination and axonal loss associated with SCI cause changes in the diffusivity of water in the spinal cord, and fiber bundles density, resulting in changes in DTI-derived metrics according to the level and severity of the damage [[7], [8], [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 pre-operative diagnosis and assessment 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 pre-operative DTI has prognostic potential in predicting surgical outcomes [12,13]. 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. However as described below, the last decade has seen numerous developments in newer pulse sequences coupled with spine specific post processing capabilities to overcome these barriers enabling reproducible and reliable data collection of the human spinal cord.

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-currents and high-sensitivity to magnetic field inhomogeneity, inducing artifacts dramatically affecting the image quality. To address some of these technical issues, two dimension radiofrequency (2DRF) pulses have been introduced and coupled with the SS-EPI readout for collecting reduced Field-Of-View (rFOV) dMRI data of the SC [[14], [15], [16]]. The rFOV-SS-EPI acquisition technique has been shown to provide high-resolution, reduced-distortion dMRI of SC and has been implemented on most vendors' MR scanners, namely ZOOMIT (Siemens), IZOOM (Philips), and FOCUS (GE). In addition, 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 [[17], [18], [19]]. However, the application of these techniques for SC diffusion imaging remains limited due to their high sensitivity to motion.

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 [20]. Given these technical challenges limiting the ability of collecting images near the metal hardware, the use of DTI for evaluating post-operative 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 near metal implant [[21], [22], [23]]. 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 combine the rFOV technique with the PS-EPI pulse sequence, so called (rFOV-PS-EPI), to address geometric distortions near the metal in DTI scan of SC at 3 T. Distortion-reduced DTI data were collected on a custom-built cervical spine phantom model with a metal implant configuration commonly used in spine surgery. The efficacy of the proposed pulse sequence in reduction of metal artifacts was visually and quantitatively evaluated compared to the gold standard rFOV-SS-EPI pulse sequence as well as the full-FOV approaches: SS-EPI, PS-EPI, and RS-EPI.