Implantable resonators--a technique for repeated measurement of oxygen at multiple deep sites with in vivo EPR

EPR oximetry using implantable resonators allows measurements at much deeper sites than are possible with surface resonators (> 80 vs. 10 mm) and achieves greater sensitivity at any depth. We report here the development of an improved technique that enables us to obtain the information from multiple sites and at a variety of depths. The measurements from the various sites are resolved using a simple magnetic field gradient. In the rat brain multi-probe implanted resonators measured pO(2) at several sites simultaneously for over 6 months under normoxic, hypoxic, and hyperoxic conditions. This technique also facilitates measurements in moving parts of the animal such as the heart, because the orientation of the paramagnetic material relative to the sensing loop is not altered by the motion. The measured response is fast, enabling measurements in real time of physiological and pathological changes such as experimental cardiac ischemia in the mouse heart. The technique also is quite useful for following changes in tumor pO(2), including applications with simultaneous measurements in tumors and adjacent normal tissues.