Magnetic resonance (MR) and ionophoresis are two experimental methods that provide measurements of molecular diffusion in living tissue. Typical experimental settings yield MR studies that are sensitive to mean molecular displacements of ∼5 μm, and ionophoresis experiments to displacements of ≥100 μm. An assessment of the correspondence between the methods is hampered by the fact that no common probe molecule has been used. One of the most frequently utilized probe molecules in ionophoresis measurements is the tetramethylammonium (TMA) ion. In the current work the diffusion properties of TMA were studied in rat brain in vivo with localized 1H MR spectroscopy (MRS). Standard treatment of the MR data yielded a 3.6-fold lower apparent diffusion coefficient (ADC) compared to ionophoresis. To explore the source of this discrepancy, a separate data processing scheme was applied to the MR data to monitor individual elapsed displacement-distance subpopulations of TMA molecules. This analysis revealed a dependence of the ADC estimation on a given subpopulation's elapsed displacement distance. The MR-derived ADC approached the ionophoresis-derived value as the elapsed displacement distance increased to 15 μm. These observations demonstrate that MR and ionophoresis studies provide complementary information, and that ADC estimates obtained from the two techniques are sensitive to different biophysical determinants.
- Extracellular space