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The quantitative relation between T1-weighted and T2-weighted MRI of normal gray matter and iron concentration
J Vymazal1, M Hajek, N Patronas
1Neuroimaging Branch, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA.
Abstract:
A retrospective analysis of 158 T1-weighted and T2-weighted MRI scans of normal brains at 0.5 and 1.5 Tesla was performed. Signal intensities in the frontal cortex, caudate nucleus, putamen, and globus pallidus were divided by those of frontal white matter; and these gray/white ratios were correlated with iron concentration, estimated from the anatomical region and age of the patient. Intraregional plots were also made of gray/white ratio versus age for the 1.5 Tesla scans. The changes in both T1-weighted and T2-weighted ratios were consistent with the hypothesis that 1/T1 and 1/T2 vary linearly with iron concentration, and the corresponding coefficients, determined separately from the interregional and intraregional plots, were generally in agreement. Furthermore, the variability of the MRI ratios at 1.5 Tesla was consistent with expected iron variability except for the cortex, in which partial volume errors due to sulci and white matter caused increased variations. The MRI results agreed well with in vitro data on T1 and T2 in tissue specimens and with other MRI studies. When compared with T1 and T2 in ferritin solution, a significant "tissue relaxation enhancement" was found, attributable to slower diffusion and clustering of ferritin in tissue.
Insights
This study correlates brain MRI signal intensities with iron concentration, revealing that T1 and T2 relaxation times linearly depend on iron levels. Results support iron
Area of Science:
- Neuroimaging
- Biophysics
- Medical Physics
Background:
- Iron accumulation in the brain is linked to aging and neurological disorders.
- Magnetic Resonance Imaging (MRI) is sensitive to tissue properties, including iron content.
- Quantifying brain iron non-invasively is crucial for understanding neurodegeneration.
Purpose of the Study:
- To investigate the relationship between MRI signal intensities (T1 and T2 relaxation times) and iron concentration in normal human brains.
- To validate a quantitative MRI method for estimating brain iron levels.
- To assess the impact of age and magnetic field strength (0.5T and 1.5T) on MRI-based iron quantification.
Main Methods:
- Retrospective analysis of 158 T1-weighted and T2-weighted MRI scans from normal brains.
- Calculation of gray/white matter signal intensity ratios in specific brain regions (frontal cortex, caudate nucleus, putamen, globus pallidus).
- Correlation of MRI ratios with estimated iron concentration and patient age, using both interregional and intraregional analyses.
Main Results:
- T1 and T2 relaxation rates (1/T1, 1/T2) showed a linear correlation with iron concentration across different brain regions.
- MRI-derived coefficients for iron concentration were consistent between interregional and intraregional analyses.
- Variability in MRI ratios at 1.5 Tesla generally matched expected iron variability, with exceptions in the cortex due to partial volume effects.
Conclusions:
- MRI signal intensities provide a reliable, non-invasive method for estimating brain iron concentration.
- The observed relationship supports the hypothesis that T1 and T2 relaxation times are linearly dependent on iron levels.
- A "tissue relaxation enhancement" effect was observed, attributed to ferritin clustering and diffusion within brain tissue, distinct from simple ferritin solutions.