Theory, Instrumentation, and Applications of Electron Paramagnetic Resonance Oximetry

Oxygen is the third most abundant element in the universe by mass after hydrogen and helium. It has an atomic number 8 and is represented by the symbol O. At standard temperature and pressure, two atoms of the element bind to form molecular oxygen (O2) which is a colorless, odorless, and tasteless diatomic gas, occupying 20.9% of air volume. Since its discovery1 in the 1770s, its properties, chemistry, and relevance to life have intrigued generations of scholars and scientists. A multitude of scientific evidence2–4 builds a compelling case for the vital role of oxygen in the evolution of life on earth. Aerobic organisms consume molecular oxygen to generate chemical energy, required for biologic processes, in the form of adenine triphosphate. Also, O2 serves as a regulatory molecule for important physiologic processes. In mammals, including humans, inhaled oxygen from the lungs is carried to the target tissue by the oxygen-carrying protein, hemoglobin. At the target tissue, where the cells actively engage themselves in respiration (oxidative phosphorylation), oxygen is released from the oxygen-bound hemoglobin (oxyhemoglobin), and the released oxygen is available to the metabolically active tissues. Any imbalance in tissue oxygen levels, which may occur due to altered supply or utilization of oxygen, may affect metabolic homeostasis and lead to pathophysiological conditions5. In addition, the level of oxygen at specific sites may affect cell signaling6,7. Hence, a precise knowledge of the levels of oxygen in the tissue of interest will be of paramount importance in our ability to understand the mechanism of pathogenesis and to develop strategies to correct the imbalance. This would require methods capable of quantifying the levels of tissue oxygenation with good spatial and temporal resolution. The information gained will enable better understanding of various metabolic and disease states and will assist in making effective clinical decisions regarding treatment and therapy options. The chemical and physical properties of oxygen enable a wide variety of methods for measuring and mapping oxygen content in vivo. There are numerous reviews on various oxygen measurement techniques and their applications to specific organs and diseases8–10. For any particular application, the choice of an oximetry method is determined by its accuracy, measurement procedure, acquisition time, invasiveness, and relevance of the measured form of oxygen, which includes oxygen concentration, partial pressure of oxygen (pO2), or oxygen saturation. Electron paramagnetic resonance (EPR), also called electron spin resonance, is a magnetic resonance based technique capable of measuring oxygen levels in biological sampling, both in vitro and in vivo11. Over the past couple of decades, EPR oximetry technique has been continually refined to collect repetitive, minimally invasive, and accurate measurements of pO2 over an extended duration12,13. At the same time, the biological applications for EPR oximetry have been rapidly growing and now include monitoring tumor oxygenation for determining cancer-treatment efficacy14,15 and measuring tissue oxygen for estimating the extent of myocardial injury during both ischemia and subsequent reperfusion16. This article provides a brief survey of basic principle and novelty, instrumentation, measurement procedures, and a few promising applications of EPR oximetry for biological systems. Section 2 describes some commonly used experimental and clinical methods for tissue oxygen measurements. Section 3 discusses the basic principle of EPR oximetry and provides motivation by outlining its unique advantages. Section 4 provides the basic layout of a typical EPR spectrometer. Section 5 covers basics of data collection and processing procedures for EPR oximetry, and highlights some of the developments proposed to speed up the acquisition process. Section 6 describes spin probe development for EPR oximetry and encapsulation methods for particulate spin probes. Section 7 lists a few important, mainly in vivo, applications of EPR oximetry.