Composite spin probes with adjustable oxygen sensitivity for pulse electron paramagnetic resonance imaging.

Purpose: Solid crystalline spin probes, such as lithium phthalocyanine (LiPc) and lithium octa-n-butoxynaphthalocyanine (LiNc-BuO), allow repeated oxygen measurement using electron paramagnetic resonance (EPR). Due to their short relaxation times, their use for pulse EPR oxygen imaging is limited. In this study, we developed and tested a new class of solid composite spin probes that modified the relaxation rates R₁ and R₂ of LiPc or LiNc-BuO probes, which allowed pO₂ measurements in the full dynamic (0-760 torr) range.

Methods: The composite probes were developed by embedding LiPc or LiNc-BuO with bonewax, beeswax, or petroleum jelly. All experiments were performed using a 25-mT EPR imager, JIVA-25®. A selected composite probe, LiPc-BW5 (LiPc + 5-times bonewax), was tested for its in vivo stability and robustness using oxygen-enhanced EPR oxygen imaging. As another application, a LiPc-BW5-based marker was tested to outline the SCC7 tumor in a C3H mouse.

Results: The composite probes showed an increased oxygen measurement range compared with unaltered probes. The in vivo experiments with LiPc-BW5 showed the stability of the probes for repeated oxygen imaging up to 4 weeks of measurements. The in vivo pO₂ at the subcutaneous site changed from 26.1 ± 1.9 torr to 118.9 ± 1.9 torr when the breathing gas was changed from 21% O₂ to 100% O₂. The use of LiPc-BW5 as a fiducial marker showed its use in outlining the tumor boundaries.

Conclusions: We developed a new class of robust and versatile solid composite probes with adjustable oxygen sensitivity that allows pO₂ imaging in the broad dynamic range.