A new approach to characterize cardiac sodium storage by combining fluorescence photometry and magnetic resonance imaging in small animal research

Figures

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  Figure 1. Compartment model of an imaging voxel. Each voxel in the cardiac tissue is constituted by intra- and extracellular space in different proportions. Each compartment contains free and bound sodium ions. One can assume a fixed concentration of extracellular free sodium ions (157 mmol/l), as this compartment is in equilibrium with the blood serum sodium. TSC is quantified in vivo by 23Na-MRI measurements and after sacrificing the animals, intracellular sodium concentration is measured by fluorescence microscopy. The fraction of intracellular bound sodium is below 1% and thus neglected. Other models additionally consider a ”fat compartment”, which is relevant in e.g. breast tissue or skin, but not in the myocardium. This simplifies calculation of [Na+]e. TSC total sodium concentration (per Voxel), IC Intracellular, ICV Intracellular Volume, EC extracellular, ECV extracellular Volume.

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  Figure 2. Cardiac functional parameters measured by MRI. MR measurements of left ventricular ejection fraction (LVEF), left ventricular end diastolic volume (LVEDV), and left ventricular mass (LVMass) are presented. TAC and MI animals showed reduced LVEF (A). LVEDV was largest in the MI group (B), whereas LVmass was highest in the TAC group. (C) The grey area represents the 95% confidence interval of values from the SHAM group. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001.

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  Figure 3. T1 and ECV values. (A) is a representation of typical ROI placement in the septum for MRI data analyses. The native T1 values determined in the respective ROI for the individual groups are shown in (B). The corresponding ECV results can be found in (C). Of note, ECV data is only available from 6 SHAM mice, as contrast agent injection failed in one mouse. ns: p = not statistically significant.

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  Figure 4. Total myocardial sodium content and intracellular sodium levels. (A) Total tissue sodium levels are determined by sodium MRI in the myocardial septum, where the TSC in MI is significantly higher compared to TAC and SHAM. (B) Intracellular sodium levels measured via SBFI fluorescence was elevated in TAC and MI compared to SHAM, when stimulated with 2 Hz. For (B) significance is reported as results from a nested one-way ANOVA. *p < 0.05; **p < 0.01; ****p < 0.0001; ns: p not statistically significant.

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  Figure 5. Comparing SHAM and MI proton and sodium short axis images. This panel shows the respective midventricular short axis slice acquired as proton image (left column) and the corresponding sodium image (middle column) as well as the fusion of both (right column). In the proton MI SAX, the thinned myocardium and the dilated left ventricle. In the sodium images the two reference vials (100 mmol/l—the tube with the higher signal, and 50 mmol/l—the round slightly weaker signal above the 100 mmol/l tube) are clearly distinguishable.

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  Figure 6. Comparison of TSC against intracellular sodium levels and representation of the calculated intra- and extracellular fraction of the TSC signal. (A) shows the ratio [Na+]intra/TSC, with a higher value representing a greater dependence of TSC on intracellular sodium. A lower ratio shows a greater dependence on extracellular sodium content. (B) shows the calculated contribution of the intra- or extracellular sodium content (in mmol/l) to the total TSC signal per voxel.