Innovations In ¹H MR Tissue Oximetry: New Probes And Pulses

Low oxygenation level (hypoxia) of tissue leads to cellular dysfunction, damage and poor response to therapy. Quantitative measurement of tissue oxygenation is critical not only for the study of tissue physiology, but could also help improve efficacy of therapeutic procedures. Our group has previously developed and implemented a novel approach to quantify oxygen tension in tissue using ¹H MRI, known as “PISTOL” (Proton Imaging of Siloxanes to map Tissue Oxygenation Levels). It exploits a linear relationship between the longitudinal relaxation rate (R1) and oxygen tension (pO₂) of a reporter molecule (probe). Due to close proximity of fat (~ 1 ppm) and probe (~0 ppm) resonances there is a possibility of contamination of probe signal by spurious fat signal causing errors in quantification. This study focuses on two aspects that are important to ¹H MR tissue oximetry 1) Characterization of new probes and 2) design and implementation of spatial spectral pulses for use with ¹H MR reporter molecules for improved lipid suppression. Several ¹H based pO₂ reporters and reporter molecule based nano emulsions (to facilitate systemic delivery) were characterized with respect to changes in oxygenation levels and temperature. This study included characterization of a) hexamethyldisiloxane (HMDSO) at 9.4 T and b) hexamethyldisiloxane (HMDSO) based nanoprobes c) octamethyltrisiloxane (OMTSO) at 4.7 T and 9.4 T. To further improve the PISTOL technique a single spatial spectral pulse (SPSP) has been designed and implemented that would enable selective examination of a particular slice and specified probe simultaneously without exciting signal from any other species in the slice. The work presented here has the potential to improve and extend the use of PISTOL for quantitative tissue oximetry.