Longitudinal In Vivo Diffusion Magnetic Resonance Imaging Remote from the Lesion Site in Rat Spinal Cord Injury

Differential Trajectory of Diffusion and Perfusion Magnetic Resonance Imaging of Rat Spinal Cord Injury

Traumatic spinal cord injury causes rapid neuronal and vascular injury, and predictive biomarkers are needed to facilitate acute patient management. This study examined the progression of magnetic resonance imaging (MRI) biomarkers after spinal cord injury and their ability to predict long-term neurological outcomes in a rodent model, with an emphasis on diffusion-weighted imaging (DWI) markers of axonal injury and perfusion-weighted imaging of spinal cord blood flow (SCBF). Adult Sprague-Dawley rats received a cervical contusion injury of varying severity (injured = 30, sham = 9). MRI at 4 h, 48-h, and 12-weeks post-injury included T1, T2, perfusion, and DWI. Locomotor outcome was assessed up to 12 weeks post-injury. At 4 h, the deficit in SCBF was larger than the DWI lesion, and although SCBF partially recovered by 48 h, the DWI lesion expanded. At 4 h, the volume of the SCBF deficit (R2 = 0.56, padj < 0.01) was significantly correlated with 12-week locomotor outcome, whereas DWI (R2 = 0.30, padj < 0.01) was less predictive of outcome. At 48 h, SCBF (R2 = 0.41, padj < 0.01) became less associated with outcome, and DWI (R2 = 0.38, padj < 0.01) lesion volume became more closely related to outcome. Spinal cord perfusion has unique spatiotemporal dynamics compared with diffusion measures of axonal damage and highlights the importance of acute perfusion abnormalities. Perfusion and diffusion offer complementary and clinically relevant insight into physiological and structural abnormalities following spinal cord injury beyond those afforded by T1 or T2 contrasts.

Ultra-high field (10.5T) diffusion-weighted MRI of the macaque brain

Diffu0sion-weighted magnetic resonance imaging (dMRI) is a non-invasive imaging technique that provides information about the barriers to the diffusion of water molecules in tissue. In the brain, this information can be used in several important ways, including to examine tissue abnormalities associated with brain disorders and to infer anatomical connectivity and the organization of white matter bundles through the use of tractography algorithms. However, dMRI also presents certain challenges. For example, historically, the biological validation of tractography models has shown only moderate correlations with anatomical connectivity as determined through invasive tract-tracing studies. Some of the factors contributing to such issues are low spatial resolution, low signal-to-noise ratios, and long scan times required for high-quality data, along with modeling challenges like complex fiber crossing patterns. Leveraging the capabilities provided by an ultra-high field scanner combined with denoising, we have acquired whole-brain, 0.58 mm isotropic resolution dMRI with a 2D-single shot echo planar imaging sequence on a 10.5 Tesla scanner in anesthetized macaques. These data produced high-quality tractograms and maps of scalar diffusion metrics in white matter. This work demonstrates the feasibility and motivation for in-vivo dMRI studies seeking to benefit from ultra-high fields.

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

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.

Diffusion tensor magnetic resonance imaging of the postoperative spine with metallic implants

There has been a growing need to understand the mechanism of development of acute spinal cord injury (SCI) and to optimize treatment. The paramagnetic nature of metallic implants has hampered the application of diffusion tensor imaging (DTI) in postsurgical SCI monitoring. We describe here a successful implementation of spinal DTI in postsurgical SCI patients. Data were acquired using a single-shot turbo-spin-echo sequence, where an extra gradient is applied before the refocusing pulse train to eliminate contributions from the non-Carr-Purcell-Meiboom-Gill components following a diffusion preparation block where a single-spin echo scheme is deployed. The DTI images were acquired in axial orientation with a 2 x 2 x 4 mm³ resolution and a total of 18 slices. Diffusion gradients were applied in six directions with b values of 0 and 600 seconds/mm² . The whole scan took ~10 minutes. The sequence was compared with SENSE-DW-EPI and ZOOM-DW-EPI on a phantom, eight patients with either anterior or posterior titanium alloy implants, and a pork loin with a similar implant. The protocol resulted in dramatically reduced geometric distortions compared with routine imaging sequences, however, the SNR efficiency was compromised. The spinal cord signal displacement was 0.68±1.00 mm (mean±SD, n = 8) for the proposed protocol, and 5.14±3.07 and 2.82±1.60 mm for the SENSE-DW-EPI and ZOOM-DW-EPI sequences, respectively. Fiber tracking was achieved in the presence of implants, which in one case was accompanied by central spinal cord caviation. Mathematical analysis concluded that the proposed protocol would be generally applicable in the spinal cord when the titanium alloy implant is ~15 mm away (<0.5 kHz B₀ field drift). The protocol described is capable of DTI in postsurgery SCI patients with metallic implants at sufficient resolution and SNR.

Diffusion MR imaging acquisition and analytics for microstructural delineation in preclinical models of TBI

Significant progress has been made toward improving both the acquisition of clinical diffusion-weighted imaging (DWI) data and its analysis in the uninjured brain, through various techniques including a large number of model-based solutions that have been proposed to fit for multiple tissue compartments, and multiple fibers per voxel. While some of these techniques have been applied to clinical traumatic brain injury (TBI) research, the majority of these technological enhancements have yet to be fully implemented in the preclinical arena of TBI animal model-based research. In this review, we describe the requirement for preclinical, MRI-based efforts to provide systematic confirmation of the applicability of some of these models as indicators of tissue pathology within the injured brain. We review how current DWI techniques are currently being used in animal TBI models, and describe how both acquisition and analytic techniques could be extended to leverage the progress made in clinical work. Finally, we highlight remaining gaps in the preclinical pipeline from data acquisition to final analysis that currently have no real, preclinical-based correlate.