More bullets for PISTOL: linear and cyclic siloxane reporter probes for quantitative ¹H MR oximetry

Tissue oximetry can assist in diagnosis and prognosis of many diseases and enable personalized therapy. Previously, we reported the ability of hexamethyldisiloxane (HMDSO) for accurate measurements of tissue oxygen tension (pO₂) using Proton Imaging of Siloxanes to map Tissue Oxygenation Levels (PISTOL) magnetic resonance imaging. Here we report the feasibility of several commercially available linear and cyclic siloxanes (molecular weight 162-410 g/mol) as PISTOL-based oxygen reporters by characterizing their calibration constants. Further, field and temperature dependence of pO₂ calibration curves of HMDSO, octamethyltrisiloxane (OMTSO) and polydimethylsiloxane (PDMSO) were also studied. The spin-lattice relaxation rate R₁ of all siloxanes studied here exhibited a linear relationship with oxygenation (R₁ = A' + B'*pO₂) at all temperatures and field strengths evaluated here. The sensitivity index η( = B'/A') decreased with increasing molecular weight with values ranged from 4.7 × 10^-3-11.6 × 10^-3 torr^-1 at 4.7 T. No substantial change in the anoxic relaxation rate and a slight decrease in pO₂ sensitivity was observed at higher magnetic fields of 7 T and 9.4 T for HMDSO and OMTSO. Temperature dependence of calibration curves for HMDSO, OMTSO and PDMSO was small and simulated errors in pO₂ measurement were 1-2 torr/°C. In summary, we have demonstrated the feasibility of various linear and cyclic siloxanes as pO₂-reporters for PISTOL-based oximetry.