Abstract
Saline solutions are commonly employed as a vehicle for drugs administered intramuscularly. In this study, in vivo measurements of spinโspin relaxation (T~2~) processes by magnetic resonance imaging (MRI) were performed to investigate the distribution of water in rat masseter muscle tissue after intramuscular injection of saline solutions of varying tonicity. Prior to saline injection, imageโbased T~2~ relaxation decay of muscle was monoexponential. After injection of saline, the T~2~ relaxation decay became multiexponential. Nonโnegative least squares (NNLS) analysis of the decay curves revealed two relaxation components: a fast component (T~2~ = 20โ40 ms) and a slow component (T~2~ = 150โ400 ms), which are assigned to intraโ and extracellular water protons, respectively. Injection of hypertonic saline solutions significantly increased the extracellular water component in muscle tissue compared to isotonic saline solutions, an effect which lasted for more than 60 min. These findings suggest that MRI techniques may be useful to investigate the effect of hyperโ or hypotonic solutions on muscle tissue in vivo. Magn Reson Med 46:592โ599, 2001. ยฉ 2001 WileyโLiss, Inc.