On the correlation between the water diffusion coefficient and oxygen tension in RIF-1 tumors

Water diffusion-coefficient mapping was used in conjunction with 19F inversion-recovery echo-planar imaging (IR-EPI) of a sequestered perfluorocarbon (PFC) emulsion to investigate the spatial correlation between the diffusion coefficient of water and the tissue oxygen tension (pO2) in radiation-induced fibrosarcoma (RIF-1) tumors (n = 11). The diffusion-time-dependent apparent diffusion coefficient, D(t), was determined by acquiring diffusion coefficient maps at 20 different diffusion times. Maps at four representative time points in different regions of the D(t) curve were selected for final analysis. An intravenously administered PFC emulsion, perfluoro-15-crown-5-ether, was used to generate the PO2 maps. D(t) and PO2 data were acquired with the animal breathing either air or carbogen (95% O2 - 5% CO2) to investigate the effects of increased tumor pO2 on D(t). The average increase in tumor pO2 was 22 torr when the breathing gas was changed from air to carbogen. Correlation plots generated from pixel data for D(t) (air breathing) vs D(t) (carbogen breathing) show little deviation from a slope of unity. Correlation plots of D(t) vs PO2 indicate that no correlation is present between these two parameters. This study also confirmed that necrotic tissue was best differentiated from viable tumor tissue based on D(t) maps at long diffusion times.