2620
Investigation of metabolic changes in the hippocampus following spinal cord injury applying a metabolite-cycling semi-LASER technique
Sandra Zimmermann1, Dario Pfyffer1, Roland Kreis2,3, Kadir Simsek2,3, Patrick Freund1,4, and Maryam Seif1,4
1Spinal Cord Injury Center, Balgrist University Hospital, Zurich, Switzerland, 2Magnetic Resonance Methodology, Institute of Diagnostic and Interventional Neuroradiology, University of Bern, Bern, Switzerland, 3Translational Imaging Center, sitem-insel, Bern, Switzerland, 4Neurophysics Department, Max Plank Institute, Leipzig, Germany
1Spinal Cord Injury Center, Balgrist University Hospital, Zurich, Switzerland, 2Magnetic Resonance Methodology, Institute of Diagnostic and Interventional Neuroradiology, University of Bern, Bern, Switzerland, 3Translational Imaging Center, sitem-insel, Bern, Switzerland, 4Neurophysics Department, Max Plank Institute, Leipzig, Germany
Synopsis
This work uses magnetic resonance spectroscopy data obtained in the hippocampus of 28 patients with traumatic spinal cord injury (SCI) to investigate SCI-induced metabolic changes. Structural magnetic resonance imaging was performed for hippocampal volumetric assessment. Eighteen healthy controls underwent the same imaging protocol. Study participants underwent a functional assessment by testing the visuospatial and verbal memory performance to check for cognitive impairments. In SCI patients without cognitive impairments, hippocampal metabolites did not differ from healthy controls. This study does not support evidence of degeneration and inflammation in the hippocampus of animal models of SCI that show impaired spatial memory performance.
Introduction
Traumatic spinal cord injury (SCI) is a life-changing event and results in neurodegeneration at the lesion site and remote from the lesion site1-4. Previous animal studies have reported SCI-induced changes in the hippocampus5-10, such as chronic inflammation9,10 and neurodegeneration9,10. Moreover, spatial memory performance, for which the right hippocampus plays a critical role11, was impaired in rodents with SCI9,10. However, pathological mechanisms within the human hippocampus after SCI are understudied. Magnetic resonance spectroscopy (MRS) is a suitable technique to quantitively investigate the underlying molecular mechanisms of pathophysiological changes in the hippocampus12,13. However, applying MRS in the hippocampus is challenging due to its small size and deep location in the brain14. Using non-water-suppressed MRS based on the metabolite-cycling (MC) technique has shown substantial benefit for MRS of small volumes15. In this work, semi-LASER was thus combined with MC for optimal sensitivity in recording MRS data in the hippocampus of a cohort of SCI and age- and sex-matched healthy controls to assess metabolic changes after SCI. A potential relationship between metabolite changes, hippocampal volume and changes in memory performance was also investigated.Methods
Twenty-eight patients with chronic traumatic SCI (> 6 months post-injury, age [mean ± SD] 48 ± 13 years, 18 tetraplegics, 10 paraplegics, 12 injuries classified as AIS A, 16 injuries classified as AIS B-D) were compared to 18 healthy controls (age [mean ± SD] 49 ± 13 years) that were matched regarding age, sex, and education (Tab. 1). 1H MRS and magnetic resonance imaging (MRI) measurements were carried out on a Prisma 3T scanner (Siemens Healthcare, Erlangen, Germany) with a 64-channel receive head and neck coil. Single-voxel MRS was performed in the right hippocampus (dimensions: 25x12x8 mm3, 2.4 mL) and in a reference region in the posterior parietal lobe (20x14x22 mm3, 6.2 mL) (Fig. 1) with a custom-made semi-LASER16,17 MC technique (TE = 35 ms, TR = 2500 ms). The reference region served as an internal control to identify hippocampus-specific metabolite changes after SCI. In total, 256 acquisitions were acquired per subject for both voxels of interest. The acquired spectra were preprocessed according to a motion compensation (MoCom) scheme18 and fitted with the software FitAID19 using a linear combination model. Absolute metabolite concentrations were quantified with parenchymal water signal as an internal reference20, which was obtained from the identical voxels with a water-unsuppressed echo series (TE = 35, 1000, 50, 400, 200, 75, and 140 ms, TR = 6000 ms). Analyzed metabolites included total N-acetylaspartate (tNAA, N-acetylaspartate plus N-acetylaspartylglutamate), myo-inositol, total choline (tCho, glycerophosphorylcholine plus phosphocholine), total creatine (tCr, creatine plus phosphocreatine), and Glx (glutamate plus glutamine). Mean metabolite concentrations were compared between SCI patients and healthy controls. MPRAGE whole brain and upper cervical cord images were acquired for macrostructural analysis using voxel-based morphometry (VBM) with voxel-wise statistical analysis21 in the right hippocampus. For the functional assessment, visuospatial and verbal memory performance were assessed with the visuospatial and verbal memory test (VVM)22 at immediate and intermediate recall (~90 min after memorizing). Mean memory performance for both subtests at both points in time was compared between SCI patients and healthy controls. The threshold for statistical significance was set to p<0.05 for all performed statistical analyses.Results & Discussion
Very tight cohort data was obtained for the concentrations of the main metabolites in both groups of participants (Fig. 2). Cohort variance for all metabolites seems as good here with short TE MC semi-LASER with 256 acquisitions as in an earlier study14 using up to 9 times more averages. Nevertheless, between-group analysis showed no significant differences in the mean concentrations of all reported metabolites in the right hippocampus (Fig. 3). Therefore, metabolite concentrations appear largely unaffected in the right hippocampus in patients with chronic SCI. As markers of specific molecular processes, NAA can be considered a neuronal marker13 and myo-inositol often serves as a gliosis marker13. Therefore, the absence of a significant difference in all mean metabolite concentrations suggests that there is no ongoing neurodegeneration and/or neuroinflammation in the right hippocampus in chronic SCI. No significant differences were observed in the reference region either. Macrostructural analysis with VBM did also not result in a significant difference in the volume of the right hippocampus between the two cohorts indicating that there is no severe hippocampus atrophy. Finally, the functional tests (Fig. 4) did not show any significant differences in the mean performance for the visuospatial and verbal memory for immediate and intermediate recall. These results suggest that the spatial memory, for which the right hippocampus is important11, might not be impaired in patients with chronic SCI. Importantly, the presented results are consistent across the three approaches (metabolite concentrations, macrostructure, and cognitive function).Conclusion
• Short TE semi-LASER combined with MC is well suited for MRS of hippocampus reflected in tight cohort data, thus allowing relatively short acquisition times given the small volumes of interest.• No significant alterations were found for patients with chronic SCI, neither for metabolic, structural nor performance measures.
Acknowledgements
We would like to thank all study participants for their precious time to contribute in this study.References
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Figures
Tab. 1: Demographic table including spinal cord injury patients (n = 28). The age [mean ± SD] was 48 ± 13 years. 18 participants were tetraplegics (upper part) and 10 were paraplegics (lower part). 12 patients had a complete injury classified as American Spinal Injury Association Impairment Scale (AIS) A and 16 injuries were classified as incomplete (AIS B-D). Both subgroups are arranged from AIS grades A to D in the table. NLI = neurological level of injury.
Fig. 1: Representative spectroscopic voxel placements. (A) The voxel (25 mm x 12 mm x 8 mm, 2.4 mL) was placed in the right hippocampus parallel to its longitudinal axis and focusing on the body and tail. (B) Additionally, a voxel (20 mm x 14 mm x 22 mm, 6.2 mL) was placed in the posterior parietal lobe, which served as a reference region. Voxel placement was achieved according to T2-weighted brain images (TR = 5000 ms, TE = 96 ms, in-plane resolution 0.5 x 0.5 mm, flip angle 150°, slice thickness 1.6 mm).
Fig. 2: Example spectra and the corresponding voxel locations. Acquired spectra (light red) are illustrated for (A) a healthy control and (B) a spinal cord injury (SCI) patient. The fits are shown in red and the residuals in grey. The main resonances of the presented metabolites are indicated. tNAA = total N-acetylaspartate; mI = myo-inositol; tCr = total creatine; tCho = total choline; Glx = glutamine plus glutamate; ppm = parts-per-million.
Fig. 3: Metabolite concentrations in the hippocampus. The boxplots show the median and quartiles of the metabolite concentrations in the hippocampus for spinal cord injury (SCI) patients (in orange) and healthy controls (HC, in blue). Group comparisons (unpaired t-test or Mann-Whitney U test) did not reveal significant differences in the mean metabolite concentrations. ns = not significant; tNAA = total N-acetylaspartate; mI = myo-inositol; tCr = total creatine; tCho = total choline; Glx = glutamine plus glutamate.
Fig. 4: Visuospatial and verbal memory test performance. The boxplots show the median and quartiles of the memory performance for the visuospatial memory at (A) immediate and (B) intermediate (after ~90 min) recall and for the verbal memory at (C) immediate and (D) intermediate recall (higher score = better performance). Spinal cord injury (SCI) patients are indicated in orange and healthy controls (HC) in blue. Group comparisons of the mean performance did not show significant differences in any of the tests. ns = not significant.
DOI: https://doi.org/10.58530/2022/2620