Self-Diffusion of Water in Multicellular Spheroids Measured by Magnetic Resonance Microimaging

Abstract Nuclear magnetic resonance microimaging measurements of the self-diffusion coefficient of water in large (>2 mm) EMT-6 multicellular spheroids were performed in order to elucidate diffusion mechanisms in tumors. Pulsed gradient spin echo-imaging methods were developed for measuring diffusion in an intravoxel multicompartment system. The self-diffusion coefficient (at 22°C) for water in the medium (D m ) consisted of only a single diffusion compartment [D m = 1.99 ± 0.03 (SE) × 10 -5 cm 2 /s]. Similarly, the spheroid necrotic center showed a single water diffusion compartment with a self-diffusion coefficient (D c ) significantly lower than that of the medium (D c = 1.54 ± 0.05 × 10 -5 cm 2 /s). The spheroid viable rim region showed two distinct compartments of approximately equal volume, one with a large diffusion coefficient (1.70 ± 0.12 × 10 -5 cm 2 /s) and a second with a significantly smaller diffusion coefficient (0.25 ± 0.01 × 10 -5 cm 2 s). We propose that these two experimentally distinguishable compartments correspond to the extra- and intracellular regions, respectively, of the viable rim of the spheroid. Although the diffusion coefficients were significantly different in the medium, the necrotic center, and the viable rim, the activation energy for diffusion was the same in the three regions (0.20 eV). Studies of perfused spheroids at 37°C show the same dependence of the diffusion coefficients on the diffusion filter as observed for unperfused spheroids at 22°C. These results demonstrate the ability of nuclear magnetic resonance microimaging to investigate diffusion at the cellular level, which will lead to a better understanding of microenvironmental regulation in tumors.