Quantification of Cardiac and Tissue Iron by Nuclear Magnetic Resonance Relaxometry in a Novel Murine Thalassemia-cardiac Iron Overload Model

Objective: To determine whether nuclear magnetic resonance (NMR) relaxation parameters can be used to quantify iron in tissues, the relationship between NMR spectrometric T2 relaxation measurements and tissue iron concentration were verified in a novel murine cardiac iron overload model.

Methods: Congenital heterozygous thalassemic mice and controls were injected with intraperitoneal iron or saline and were sacrificed at three weeks. Samples of liver, heart and peripheral muscle were subjected to NMR relaxation measurements and continuous distribution analysis. Tissue ferritin levels were determined with immunoadsorbance techniques, and elemental iron was assayed by flame atomic absorption. Tissues were analyzed pathologically with hematoxylin and eosin and Prussian blue staining to confirm the localization of iron.

Results: This murine iron loading model was uniquely successful in loading iron into the major organs, especially the heart, and produced significant reductions in T1 and T2 NMR relaxation values. There was a good correlation between soluble ferritin and total iron levels (r=0.92), indicating that there is a constant and significant fraction of total iron present in ferritin irrespective of absolute iron concentrations. Regression analysis between total iron content and T2 relaxivity showed a linear relationship (r=0.96), suggesting that the T2 relaxation parameter is related to tissue iron concentration. The regression relationship suggested that NMR can detect iron levels as low as 0.1 mg/g of tissue.

Conclusions: Parenteral iron loading in mice produces unique iron overload in major organs, including the heart. Local iron deposition is detectable by NMR relaxometry at 0.1 mg/g or higher. There is a linear relationship between iron concentration and T2 relaxivity. Thus, NMR may be an important and useful clinical tool to quantify iron excess in various pathobiological states of human disease due to iron overload, including heart disease.