PURPOSE - To evaluate the magnitude of chemical exchange effects and R dispersion in muscle and their relationship to tissue sodium levels with aging.
METHODS - Seven healthy volunteers (aged 24 to 87years, median age 47) underwent MRI to assess tissue sodium levels and water T values at different spin-locking frequencies in calf muscles. T values at each locking field were computed based on a three-parameter mono-exponential model to fit signals obtained at different locking times, and R (=1/T) rates were compared at different locking fields. In particular, the dispersion of R (ΔR=R(0Hz)-R(500Hz)) was examined as a function of subject age. Muscle sodium content was calculated by comparing signal intensities between tissues and reference standards within the same image. The variations of ΔR with age and sodium were analyzed by linear regression.
RESULTS - T values and sodium content both increased with age. R dispersion also increased with age and showed a strong linear correlation (correlation coefficient r=0.98, P=0.000578) with sodium content.
CONCLUSION - ΔR reports on the contribution of labile protons such as hydroxyls which may be associated with macromolecule accumulation in the extracellular matrix (ECM). An increase of sodium signal suggests an enlarged ECM volume fraction and/or an increase in sodium concentration, which occurs during normal aging. The strong correlation between ΔR and sodium is likely the consequence of increased ECM and density of total charged sites within the matrix from molecules such as collagens and proteoglycans. The results from this study show the potential use of R dispersion and sodium imaging in the assessment of pathological changes in muscle such as fibrosis.
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