Sodium ion apparent diffusion coefficient in living rat brain

✍ Scribed by James A. Goodman; Christopher D. Kroenke; G. Larry Bretthorst; Joseph J. H. Ackerman; Jeffrey J. Neil

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: English
• Weight: 202 KB
• Volume: 53
• Category: Article
• ISSN: 0740-3194
• DOI: 10.1002/mrm.20444

✦ Synopsis

Abstract

The apparent diffusion coefficient (ADC) of Na^+^ was determined in live rat brain. The brain extracellular‐to‐intracellular Na^+^ content ratio is ∼8:2, which is the inverse of that for water in these spaces. Consequently, the ADC of Na^+^ is primarily affected by motion in the extracellular space, and Na^+^ can be viewed as a reporter molecule for motion in that space. Likewise, water ADC is dominated by intracellular motion. The brain Na^+^ ADC was 1.15 ± 0.09 μm^2^/ms, which is 61% of the aqueous Na^+^ free diffusion coefficient (D^free^) at 37°C (1.9 μm^2^/ms), while the ADC for brain water is 28% of the water D^free^ at 37°C (3 μm^2^/ms). This suggests that the ADC of molecular species within the extracellular space is roughly twofold that within the intracellular space. In postmortem brain, both Na^+^ and water decrease to 17% of the respective D^free^ values. These results are consistent with Na^+^ and water ADC values sharing the same biophysical determinants in postmortem brain. The observed difference between Na^+^ and water ADC/D^free^ ratios in living brain tissue may be attributable to the extracellular environment hindering molecular displacements twofold less than the intracellular environment. Magn Reson Med 53:1040–1045, 2005. © 2005 Wiley‐Liss, Inc.