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

Abstract

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.

Figure 1

Measurement of myocardial pO₂ using a short single probe resonator. A: baseline pO₂ of a mouse heart. B: pO₂ under several different conditions: baseline, ischemia and reperfusion. C: positioning of the external resonator over the large loop of the implantable resonator that was under the skin. D: a typical EPR spectrum (baseline pO₂). E: location of the resonator probe.

Figure 2

Repeated, long term pO₂ measurements with a 3-probe resonator implanted in a rat brain. The measurements were started the second day after implantation and continued for up to 180 days. A: 30% O₂ for baseline, B: 15% O₂ for hypoxia and C: carbogen for hyperoxia.

Figure 3

A four-probe implantable resonator used to measure dynamic responses in the rat brain to changes in inhaled oxygen levels. The resonator was inserted at the following coordinates: AP, 0 mm; ML, 2.0 mm and 3.5 mm to the left and right of midline, DV, 5.5 mm. Oxygen was delivered through a nose cone to the rat. Plots from left to right: brain pO₂ during change from 30% O₂ to air, from air to 15% O₂, and from air to carbogen.

Figure 4

pO₂ in rat F98 brain tumor measured with a single probe resonator. A: MRI images before and 19 days after F98 inoculation. Arrows indicate the probe locations in the brain. B: pO₂ measured the day before, 5, 15, 20 and 25 days after F98 inoculation in 3 rats with tumors and 2 rats with sham injections. C: Carbogen challenge at day 25 after baseline 30% pO₂.

Figure 5

pO₂ in skeletal muscle measured with a single probe resonator. The measurement was done four days after the implantation. As shown, the resonator measured the gradual changes of the pO₂ when oxygen in the breathing gas was changed from 30% to 15% (A), from 15% to 30% (B) and from 30% to Carbogen (C).