2D sodium MRI of the human calf using half-sinc excitation pulses and compressed sensing

PURPOSE

Sodium MRI can be used to quantify tissue sodium concentration (TSC) in vivo; however, UTE sequences are required to capture the rapidly decaying signal. 2D MRI enables high in-plane resolution but typically has long TEs. Half-sinc excitation may enable UTE; however, twice as many readouts are necessary. Scan time can be minimized by reducing the number of signal averages (NSAs), but at a cost to SNR. We propose using compressed sensing (CS) to accelerate 2D half-sinc acquisitions while maintaining SNR and TSC.

METHODS

Ex vivo and in vivo TSC were compared between 2D spiral sequences with full-sinc (TE = 0.73 ms, scan time ≈ 5 min) and half-sinc excitation (TE = 0.23 ms, scan time ≈ 10 min), with 150 NSAs. Ex vivo, these were compared to a reference 3D sequence (TE = 0.22 ms, scan time ≈ 24 min). To investigate shortening 2D scan times, half-sinc data was retrospectively reconstructed with fewer NSAs, comparing a nonuniform fast Fourier transform to CS. Resultant TSC and image quality were compared to reference 150 NSAs nonuniform fast Fourier transform images.

RESULTS

TSC was significantly higher from half-sinc than from full-sinc acquisitions, ex vivo and in vivo. Ex vivo, half-sinc data more closely matched the reference 3D sequence, indicating improved accuracy. In silico modeling confirmed this was due to shorter TEs minimizing bias caused by relaxation differences between phantoms and tissue. CS was successfully applied to in vivo, half-sinc data, maintaining TSC and image quality (estimated SNR, edge sharpness, and qualitative metrics) with ≥50 NSAs.

CONCLUSION

2D sodium MRI with half-sinc excitation and CS was validated, enabling TSC quantification with 2.25 × 2.25 mm2 resolution and scan times of ≤5 mins.

Dynamic induction of the myelin-associated growth inhibitor Nogo-A in perilesional plasticity regions after human spinal cord injury

The myelin-associated inhibitor Nogo-A (Reticulon 4, RTN4) restricts axonal outgrowth, plasticity, and neural circuitry formation in experimental models of spinal cord injury (SCI) and is targeted in clinical interventions starting treatment within 4 weeks post-SCI. Specifically, Nogo-A expressed by oligodendroglia restricts compensatory neurite sprouting. To interrogate the hypothesis of an inducible, lesion reactive Nogo-A expression over time, we analyzed the spatiotemporal Nogo-A expression at the spinal lesion core (region of tissue necrosis and axonal damage/pruning) and perilesional rim (region of plasticity formation). Spinal cord specimens of SCI subjects (n = 22) were compared to neuropathologically unaltered controls (n = 9). Nogo-A expression was investigated ranging from acute (0-3 days), early subacute (4-21 days), late subacute (22-90 days) to early chronic-chronic (91 days to 1.5 years after SCI) stages after SCI. Nogo-A expression in controls is confined to motoneurons in the anterior horn and to oligodendrocytes in gray and white matter. After SCI, the number of Nogo-A+ and TPPP/p25+ oligodendrocytes (i) inclined at the organizing perilesional rim specifically, (ii) increased further over time, and (iii) peaked at chronic stages after SCI. By contrast, at the lesion core, the number of Nogo-A+ and TPPP/p25+ oligodendrocytes did not increase. Increasing numbers of Nogo-A+ oligodendrocytes coincided with oligodendrogenesis corroborated by Nogo-A coexpression of Ki67+ , TPPP/p25+ proliferating oligodendrocytes. Nogo-A oligodendrocyte expression emerges at perilesional (plasticity) regions over time and suggests an extended therapeutical window for anti-Nogo-A pathway targeting interventions beyond 4 weeks in patients after SCI.

Altered Circulating Immune Cell Distribution in Traumatic Spinal Cord Injury Patients in Relation to Clinical Parameters

Following a spinal cord injury (SCI), an inflammatory immune reaction is triggered which results in advanced secondary tissue damage. The systemic post-SCI immune response is poorly understood. This study aimed to extensively analyse the circulating immune cell composition in traumatic SCI patients in relation to clinical parameters. High-dimensional flow cytometry was performed on peripheral blood mononuclear cells of 18 traumatic SCI patients and 18 healthy controls to determine immune cell subsets. SCI blood samples were collected at multiple time points in the (sub)acute (0 days to 3 weeks post-SCI, (s)aSCI) and chronic (6 to >18 weeks post-SCI, cSCI) disease phase. Total and CD4+ T cell frequencies were increased in cSCI patients. Both CD4+ T cells and B cells were shifted towards memory phenotypes in (s)aSCI patients and cSCI patients, respectively. Most profound changes were observed in the B cell compartment. Decreased immunoglobulin (Ig)G+ and increased IgM+ B cell frequencies reflected disease severity, as these correlated with American Spinal Injury Association (ASIA) impairment scale (AIS) scores. Post-SCI B cell responses consisted of an increased frequency of CD74+ cells and CD74 expression level within total B cells and B cell subsets. Findings from this study suggest that post-SCI inflammation is driven by memory immune cell subsets. The increased CD74 expression on post-SCI B cells could suggest the involvement of CD74-related pathways in neuroinflammation following SCI. In addition, the clinical and prognostic value of monitoring circulating IgM+ and IgG+ B cell levels in SCI patients should be further evaluated.

Motor and higher-order functions topography of the human dentate nuclei identified with tractography and clustering methods

Deep gray matter nuclei are the synaptic relays, responsible to route signals between specific brain areas. Dentate nuclei (DNs) represent the main output channel of the cerebellum and yet are often unexplored especially in humans. We developed a multimodal MRI approach to identify DNs topography on the basis of their connectivity as well as their microstructural features. Based on results, we defined DN parcellations deputed to motor and to higher-order functions in humans in vivo. Whole-brain probabilistic tractography was performed on 25 healthy subjects from the Human Connectome Project to infer DN parcellations based on their connectivity with either the cerebral or the cerebellar cortex, in turn. A third DN atlas was created inputting microstructural diffusion-derived metrics in an unsupervised fuzzy c-means classification algorithm. All analyses were performed in native space, with probability atlas maps generated in standard space. Cerebellar lobule-specific connectivity identified one motor parcellation, accounting for about 30% of the DN volume, and two non-motor parcellations, one cognitive and one sensory, which occupied the remaining volume. The other two approaches provided overlapping results in terms of geometrical distribution with those identified with cerebellar lobule-specific connectivity, although with some differences in volumes. A gender effect was observed with respect to motor areas and higher-order function representations. This is the first study that indicates that more than half of the DN volumes is involved in non-motor functions and that connectivity-based and microstructure-based atlases provide complementary information. These results represent a step-ahead for the interpretation of pathological conditions involving cerebro-cerebellar circuits.