Rapid tissue oxygen tension mapping using 19F inversion‐recovery echo‐planar imaging of P erfluoro‐15 ‐crown‐5‐ether

Abstract

Fluorine-19 inversion-recovery, echo-planar imaging (IR-EPI) wais used in conjunction with a new PFC emulsion, perfluoro-15-crown-5-ether, to map the spatial distribution of oxygen tension in murine liver, spleen and radiation induced fibrosarcoma (RIF-1) tumors. Intravenously administered PFC emulsions were allowed to sequester in the liver, spleen, and tumor 3 to 7 days prior to imaging experiments. Seven, 64 × 64 IR-EPls were acquired with successively increasing inversion times (TI). A nonlinear least-squares regression algorithm was used to fit the seven two-dimensional matrices, on a pixel-by-pixel basis, to solve for the relaxation rate, R₁, of the sequestered PFC. From in vitro calibration curves, the oxygen tension (pO₂,) was calculated from the measured R₁. Oxygen tension maps were then murine liver and spleen were produced (in 2.5 min) to demonstrate the technique and changes in tissue oxygenation as a function of breathing gas (air and carbogen (95% O₂ - 5% CO₂)) are presented. Tissue pO₂ maps from RIF-1 tumors (n = 5) were obtained in less than 10 min and changes in tumor pO₂ were studied when the breathing gals was switched from air to carbogen. The results from tumor pO₂ maps were compared with ¹⁹F MR spectroscopy measurements to check for consistency. Histogram analysis yielded an average liver and spleen pO₂ of 43 torr and 26 torr for RIF-1 tumors when the animals were breathing air. Statistically significant changes in tumor oxygenation as a function of breathing gas were obtained from both pO₂ maps (6 ± 2 torr, P < 0.05) and ¹⁹F MR spectroscopy (13 ± 3 torr, P < 0.01) as evaluated using the Student's paired t test.