Perfluorocarbon temperature measurements using ¹⁹F NMR

Measuring the T₁ of the fluorine resonances of perfluorocarbons (PFC) is a unique method for monitoring oxygen tension in vivo. However, because T₁ is also temperature sensitive, error in the pO₂ determination due to animal‐to‐animal temperature variation may arise. Pathophysiologic conditions, such as ischemia, where temperature is not known a priori may also introduce error. Thus, measuring the PFC temperature is clearly desirable in order to correct for tissue temperature variations during the pO₂ determination. Because the fluorine chemical shift of various fluorinated compounds has a significant temperature dependence, we evaluated the effect of temperature on the chemical shift of the fluorine resonances of perfluorotributylamine (FTBA). A linear relationship was found between chemical shift and temperature in vitro. In addition, the relative FTBA chemical shifts were essentially independent of pO₂. Chemical shift temperature measurements in vivo, obtained from a 10 μL FTBA bubble in the preretinal vitreous space of the rabbit eye, were in good agreement (±0.5 °C) with thermocouple measurements from the same location. Good agreement between the NMR determined temperature and core body temperature was also found. The implication of such temperature measurements for the ultimate accuracy of the pO₂ determination based on PFC T₁ measurements is discussed. To the best of our knowledge, this report describes the first absolute temperature measurement in vivo by NMR.