R_1ρ dispersion and sodium imaging in human calf muscle

To evaluate the magnitude of chemical exchange effects and R_1ρ dispersion in muscle and their relationship to tissue sodium levels with aging.

Seven healthy volunteers (aged 24 to 87years, median age 47) underwent MRI to assess tissue sodium levels and water T_1ρ values at different spin-locking frequencies in calf muscles. T_1ρ 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_1ρ (=1/T_1ρ) rates were compared at different locking fields. In particular, the dispersion of R_1ρ (ΔR_1ρ=R_1ρ(0Hz)-R_1ρ(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_1ρ with age and sodium were analyzed by linear regression.

T_1ρ values and sodium content both increased with age. R_1ρ dispersion also increased with age and showed a strong linear correlation (correlation coefficient r=0.98, P=0.000578) with sodium content.

ΔR_1ρ 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_1ρ 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_1ρ dispersion and sodium imaging in the assessment of pathological changes in muscle such as fibrosis.