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

Area of Science:

• Biomedical Engineering
• Materials Science
• Medical Imaging

Background:

• Solid crystalline spin probes like lithium phthalocyanine (LiPc) are used for oxygen measurement via electron paramagnetic resonance (EPR).
• Existing probes have short relaxation times, limiting their application in pulse EPR oxygen imaging.
• A need exists for spin probes with improved performance for dynamic oxygen sensing.

Purpose of the Study:

• To develop and test a novel class of solid composite spin probes.
• To modify relaxation rates (R1 and R2) of LiPc or LiNc-BuO probes for enhanced oxygen measurement.
• To enable pO2 measurements across the full dynamic range (0-760 torr).

Main Methods:

• Composite probes were created by embedding LiPc or LiNc-BuO with bonewax, beeswax, or petroleum jelly.
• Experiments utilized a 25-mT EPR imager (JIVA-25®).
• In vivo stability and robustness of a selected probe (LiPc-BW5) were assessed using oxygen-enhanced EPR imaging; its utility as a tumor-marking agent was also evaluated.

Main Results:

• Composite probes demonstrated an expanded oxygen measurement range compared to unaltered probes.
• In vivo studies confirmed the stability of LiPc-BW5 for repeated oxygen imaging over 4 weeks.
• In vivo pO2 measurements showed significant changes with altered breathing gas (26.1 ± 1.9 torr to 118.9 ± 1.9 torr).
• LiPc-BW5 effectively served as a fiducial marker for outlining tumor boundaries.

Conclusions:

• A new class of robust, versatile solid composite probes was successfully developed.
• These probes offer adjustable oxygen sensitivity.
• The developed probes enable pO2 imaging across a broad dynamic range.