Proton imaging of siloxanes to map tissue oxygenation levels (PISTOL): A tool for quantitative tissue oximetry

Hexamethyldisiloxane (HMDSO) has been identified as a sensitive proton NMR indicator of tissue oxygenation (pO₂) based on spectroscopic spin-lattice relaxometry. A rapid MRI approach has now been designed, implemented, and tested. The technique, proton imaging of siloxanes to map tissue oxygenation levels (PISTOL), utilizes frequency-selective excitation of the HMDSO resonance and chemical-shift selective suppression of residual water signal to effectively eliminate water and fat signals and pulse-burst saturation recovery ¹H echo planar imaging to map T₁ of HMDSO and hence pO₂. PISTOL was used here to obtain maps of pO₂ in rat thigh muscle and Dunning prostate R3327 MAT-Lu tumor-implanted rats. Measurements were repeated to assess baseline stability and response to breathing of hyperoxic gas. Each pO₂ map was obtained in 3 1/2 min, facilitating dynamic measurements of response to oxygen intervention. Altering the inhaled gas to oxygen produced a significant increase in mean pO₂ from 55 Torr to 238 Torr in thigh muscle and a smaller, but significant, increase in mean pO₂ from 17 Torr to 78 Torr in MAT-Lu tumors. Thus, PISTOL enabled mapping of tissue pO₂ at multiple locations and dynamic changes in pO₂ in response to intervention. This new method offers a potentially valuable new tool to image pO₂ in vivo for any healthy or diseased state by ¹H MRI.