Toward Noninvasively Imaging pH at the Surface of Implanted Orthopedic Devices in Live Rabbits Using X-ray Excited Luminescence Chemical Imaging.

X-ray excited luminescence chemical imaging (XELCI) and associated sensor surfaces are designed to noninvasively study, detect, and monitor local chemistry at the surface of modified implanted medical devices during infection. Implants are coated with polymer films containing scintillators and pH indicator dyes that together generate a pH-dependent luminescence when irradiated by X-rays. A focused X-ray beam provides high spatial resolution, while the pH indicator provides chemical sensitivity. A live rabbit pH-imaging study on a scintillator-coated implanted titanium plate evaluates the sensor performance with and without Staphylococcus aureus infection and biocompatibility through long-term histological examinations. 5000 cfu are sufficient to cause infection without fatality. XELCI images clearly shows dye and reference regions in live rabbits; no leaching is evident in titanium plates coated with polyethylene glycol (PEG) hydrogel up to 10 days, although dye leached fromvinyl-PEG film-coated implants. No toxicity is evident, and pH sensors remained stable after up to 3 months postimplantation. In these preliminary studies, acidosis is not observed in either infected or control legs in vivo or postmortem. The results demonstrate the feasibility of imaging pH and provide insights for optimizing the sensor and the imaging modality for subsequent studies on pH changes on implants during infection.