Novel 1H NMR approach to quantitative tissue oximetry using hexamethyldisiloxane

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Abstract
19F NMR spin-lattice relaxometry of hexafluorobenzene (HFB) has been shown to be a highly sensitive indicator of tumor oxygenation. In this study hexamethyldisiloxane (HMDSO) was identified as a proton NMR analog, and its potential as a probe for investigating dynamic changes in tissue oxygen tension (pO2) was evaluated. HMDSO has a single proton resonance (δ= –0.3 ppm) and the spin-lattice relaxation rate, Rl (= 1/T1) exhibits a linear dependence on pO2: R1 (s–1) = 0.1126 + 0.0013* pO2 (torr) at 37°C. To demonstrate application in vivo, HMDSO was administered into healthy rat thigh muscle (100 μl) and tumors (50 μl). Local pO2 was determined by using pulse-burst saturation recovery (PBSR) 1H NMR spectroscopy to assess R1. Water and fat signals were effectively suppressed by frequency-selective excitation of the HMDSO resonance. Rat thigh muscle had a mean baseline pO2 of 35 ± 11 torr, with a typical stability of ±3 torr over 20 min, when the rats breathed air. Altering the inhaled gas to oxygen produced a significant increase in pO2 to 100–200 torr. In tumors, altering the inspired gas also produced significant (albeit generally smaller) changes. This new pO2 reporter molecule offers a potentially valuable new tool for investigating pO2 in vivo. Magn Reson Med, 2006.