Towards Human Oxygen Images with Electron Paramagnetic Resonance Imaging

Abstract

Electron paramagnetic resonance imaging (EPRI) has been used to noninvasively provide 3D images of absolute oxygen concentration (pO₂) in small animals. These oxygen images are well resolved both spatially (~1 mm) and in pO₂ (1–3 mmHg). EPRI preclinical images of pO₂ have demonstrated extremely promising results for various applications investigating oxygen related physiologic and biologic processes as well as the dependence of various disease states on pO₂, such as the role of hypoxia in cancer.

Recent developments have been made that help to progress EPRI towards the eventual goal of human application. For example, a bimodal crossed-wire surface coil has been developed. Very preliminary tests demonstrated a 20 dB isolation between transmit and receive for this coil, with an anticipated additional 20 dB achievable. This could potentially be used to image local pO₂ in human subjects with superficial tumors with EPRI. Local excitation and detection will reduce the specific absorption rate limitations on images and eliminate any possible power deposition concerns. Additionally, a large 9 mT EPRI magnet has been constructed which can fit and provide static main and gradient fields for imaging local anatomy in an entire human. One potential obstacle that must be overcome in order to use EPRI to image humans is the approved use of the requisite EPRI spin probe imaging agent (trityl). While nontoxic, EPRI trityl spin probes have been injected intravenously when imaging small animals, and require relatively high total body injection doses that would not be suitable for human imaging applications. Work has been done demonstrating the alternative use of intratumoral (IT) injections, which can reduce the amount of trityl required for imaging by a factor of 2000- relative to a whole body intravenous injection.

The development of a large magnet that can accommodate human subjects, the design of a surface coil for imaging of superficial pO₂, and the reduction of required spin probe using IT injections all are crucial steps towards the eventual use of EPRI to image pO₂ in human subjects. In the future this can help investigate the oxygenation status of superficial tumors (e.g., breast tumors). The ability to image pO₂ in humans has many other potential applications to diseases such as peripheral vascular disease, heart disease, and stroke.