Spinal cord atrophy in injured rodents: high-resolution MRI

The spatiotemporal spread of cervical spinal cord contusion injury pathology revealed by 3D in-line phase contrast synchrotron X-ray microtomography

Extensive structural changes occur within the spinal cord following traumatic injury. Acute tissue debris and necrotic tissue are broken down, proliferating local glia and infiltrating leukocytes remodel tissue biochemical and biophysical properties, and a chronic cavity surrounded by a scar forms at the injury epicentre. Serial-section 2D histology has traditionally assessed these features in experimental models of spinal cord injury (SCI) to measure the extent of tissue pathology and evaluate efficacy of novel therapies. However, this 2D snapshot approach overlooks slice intervening features, with accurate representation of tissue compromised by mechanical processing artefacts. 3D imaging avoids these caveats and allows full exploration of the injured tissue volume to characterise whole tissue pathology. Amongst 3D imaging modalities, Synchrotron Radiation X-ray microtomography (SRμCT) is advantageous for its speed, ability to cover large tissue volumes at high resolution, and need for minimal sample processing. Here we demonstrate how extended lengths of formalin-fixed, paraffin-embedded (FFPE) rat spinal cord can be completely imaged by SRμCT with micron resolution. Label-free contrast derived from X-ray phase interactions with low-density soft tissues, reveals spinal cord white matter, gray matter, tissue damage and vasculature, with tissue still viable for targeted 2D-histology after 3D imaging. We used SRμCT to quantify tissue pathology after a midline, cervical level (C6), 225 kDyne contusion injury over acute-to-chronic (24 h to 5 weeks) post injury time points. Quantification revealed acute tissue swelling prior to chronic atrophy across the whole imaged region (spanning 2 spinal segments above and below injury), along with rostro-caudal asymmetries in white and gray matter volume loss. 3D volumes revealed satellite damage in tissue far removed from the epicentre, and extensive rostro-caudal spread of damage through the base of the dorsal columns at 24 h post injury. This damage overlapped regions of vasogenic oedema, confirmed with subsequent histology. Tissue damage at later time points in border regions was most prominent in the dorsal columns, where it overlapped sites of damaged venous vasculature. Elaborating rostro-caudal and spatiotemporal asymmetries in reduced traumatic injury models centred on these regions may inform future treatments that seek to limit the spread of tissue pathology to these ‘at-risk’ regions.

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Whole rat spinal cord SRμCT tomograms (up to 20 mm length) with μm resolution

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Pathology of 3 SHAM and 24 acute-to-chronic C6 midline contusion SCIs quantified

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Rostro-caudal asymmetries in gray and white matter pathology progression

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Differences in ascending and descending dorsal column tract pathology

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Delayed rostral-caudal pathology associated with sites of venous vasculature

Automatic segmentation of rodent spinal cord diffusion MR images

MRI, is a key tool for noninvasive spinal cord lesion analysis; however, accurate, quantitative methods for this analysis are lacking. A new, multistep, multidimensional approach, utilizing the classification expectation maximization algorithm, is proposed for MRI segmentation of spinal cord tissues. Diffusion tensor imaging is used to generate multiple images of each spinal slice, with different diffusion direction weightings. The maximum likelihood tissue classifications are then jointly estimated to produce a binary classification image, corresponding to voxels containing either spinal cord or background. Edge detection is employed to find a nonparametric curve encapsulating the entire spinal cord. The algorithm is evaluated using data from in vivo diffusion tensor imaging of control and injured mouse spinal cords. The algorithm is shown to remain accurate for whole spinal cord, white matter, and hemorrhage segmentation in the presence of significant injury. The results of the method are shown to be at least on par with expert manual segmentation.

Neuronal and axonal degeneration in experimental spinal cord injury: in vivo proton magnetic resonance spectroscopy and histology

Longitudinal in vivo proton magnetic resonance spectroscopy (1H-MRS) and immunohistochemistry were performed to investigate the tissue degeneration in traumatically injured rat spinal cord rostral and caudal to the lesion epicenter. On 1H-MRS significant decreases in N-acetyl aspartate (NAA) and total creatine (Cr) levels in the rostral, epicenter, and caudal segments were observed by 14 days, and levels remained depressed up to 56 days post-injury (PI). In contrast, the total choline (Cho) levels increased significantly in all three segments by 14 days PI, but recovered in the epicenter and caudal, but not the rostral region, at 56 days PI. Immunohistochemistry demonstrated neuronal cell death in the gray matter, and reactive astrocytes and axonal degeneration in the dorsal, lateral, and ventral white-matter columns. These results suggest delayed tissue degeneration in regions both rostrally and caudally from the epicenter in the injured spinal cord tissue. A rostral-caudal asymmetry in tissue recovery was seen both on MRI-observed hyperintense lesion volume and the Cho, but not NAA and Cr, levels at 56 days PI. These studies suggest that dynamic metabolic changes take place in regions away from the epicenter in injured spinal cord.

Rapid, postmortem 9.4 T MRI of spinal cord injury: correlation with histology and survival times

High field magnetic resonance imaging (MRI) has been increasingly used to assess experimental spinal cord injury (SCI). In the present investigation, after partial spinal cord injury and excision of the whole spine, pathological changes of the spinal cord were studied in spinal cord-spine blocks, from the acute to the chronic state (24 h to 5 months). Using proton density (PD) weighted imaging parameters at a magnetic field strength of 9.4 tesla (T), acquisition times ranging from <1 to 10 h per specimen were used. High in-plane pixel resolution (68 and 38 microm, respectively) was obtained, as well as high signal-to-noise ratio (SNR), which is important for optimal contrast settings. The quality of the resulting MR images was demonstrated by comparison with histology. The cord and the lesion were shown in their anatomical surroundings, detecting cord swelling in the acute phase (24 h to 1 week) and cord atrophy at the chronic stage. Haemorrhage was detected as hypo-intense signal. Oedema, necrosis and scarring were hyper-intense but could not be distinguished. Histology confirmed that the anatomical delimitation of the lesion extent by MRI was precise, both with high and moderate resolution. The present investigation thus demonstrates the precision of spinal cord MRI at different survival delays after compressive partial SCI and establishes efficient imaging parameters for postmortem PD MRI.