Influence of 7T GRE-MRI Signal Compartment Model Choice on Tissue Parameters

Altitude-related features and prognosis in patients with reversible splenial lesion syndrome

INTRODUCTION

RESLES (Reversible splenial lesion syndrome) can be observed secondary to various diseases, and intramyelinic edema may play a crucial role in the pathogenesis of SCC (Splenium of the corpus callosum). Some studies have suggested that hypoxic-ischaemic encephalopathy may constitute a risk factor for SCC lesions. However, the potential impact of high-altitude environments on SCC, especially during chronic exposure, remain obscure.

METHODS

Our study included 19 patients who satisfied the diagnostic criteria of RESLES at high altitudes. Ten low-altitude patients with RESLES were included as controls. All participants received MRI (Magnetic resonance imaging) scans twice. Routine blood tests, liver, kidney and thyroid function, coagulation function, electrolytes and vitamins were detected during hospitalization and before discharge. In addition, the patients were followed up in May 2023.

RESULTS

Hypoxic environments at high altitudes may increase the risk of RESLES. The two groups showed different clinical symptoms. High-altitude patients had significantly higher CRP levels than low-altitude patients. The lesion size in high-altitude patients showed a positive correlation with SaO2 levels. However, the patients at low altitudes had positive correlation trends between lesion size and several inflammatory markers (WBC, NEU and CRP). All patients had a benign prognosis that may not be affected by the use of prednisone acetate.

CONCLUSIONS

Hypoxic environments at high altitudes may play a role in the aetiology of RESLES. Additionally, RESLES is a reversible disease and the administration of glucocorticoids may be dispensable for its treatment.

Towards in vivo g-ratio mapping using MRI: Unifying myelin and diffusion imaging

BACKGROUND

The g-ratio, quantifying the comparative thickness of the myelin sheath encasing an axon, is a geometrical invariant that has high functional relevance because of its importance in determining neuronal conduction velocity. Advances in MRI data acquisition and signal modelling have put in vivo mapping of the g-ratio, across the entire white matter, within our reach. This capacity would greatly increase our knowledge of the nervous system: how it functions, and how it is impacted by disease.

NEW METHOD

This is the second review on the topic of g-ratio mapping using MRI.

RESULTS

This review summarizes the most recent developments in the field, while also providing methodological background pertinent to aggregate g-ratio weighted mapping, and discussing pitfalls associated with these approaches.

COMPARISON WITH EXISTING METHODS

Using simulations based on recently published data, this review reveals caveats to the state-of-the-art calibration methods that have been used for in vivo g-ratio mapping. It highlights the need to estimate both the slope and offset of the relationship between these MRI-based markers and the true myelin volume fraction if we are really to achieve the goal of precise, high sensitivity g-ratio mapping in vivo. Other challenges discussed in this review further evidence the need for gold standard measurements of human brain tissue from ex vivo histology.

CONCLUSIONS

We conclude that the quest to find the most appropriate MRI biomarkers to enable in vivo g-ratio mapping is ongoing, with the full potential of many novel techniques yet to be investigated.

Field strength influences on gradient recalled echo MRI signal compartment frequency shifts

Different trends of echo time dependent gradient recalled echo MRI signals in different brain regions have been attributed to signal compartments in image voxels. It remains unclear how variations in gradient recalled echo MRI signals change as a function of MRI field strength, and how data processing may impact signal compartment parameters. We used two popular quantitative susceptibility mapping methods of processing raw phase images (Laplacian and path-based unwrapping with V-SHARP) and expressed values in the form of induced frequency shifts (in Hz) in six specific brain regions at 3T and 7T. We found the frequency shift curves to vary with echo time, and a good overlap between 3T and 7T mean frequency shift curves was present. However, the amount of variation across participants was greater at 3T, and we were able to obtain better compartment model fits of the signal at 7T. We also found the temporal trends in the signal and compartment frequency shifts to change with the method used to process images. The inter-participant averaged trends were consistent between 3T and 7T for each quantitative susceptibility pipeline. However, signal compartment frequency shifts generated using different pipelines may not be comparable.