Magnetic resonance imaging for in vivo assessment of tissue oxygen concentration

Magnetic resonance imaging (MRI) provides high-resolution morphological images useful in diagnostic radiology to differentiate normal from abnormal/pathological states. More recently, emerging developments in MRI seek to add a functional/physiological dimension to the anatomic images to provide better understanding of the physiology of pathological conditions. Three MRI methods offer the promise of providing important physiologic information, such as oxygen status and redox capability of tissues, and these are discussed in the context of their potential usefulness to radiation oncology. The techniques include blood oxygen level-dependent (BOLD) MRI, Overhauser enhanced MRI (OMRI), and electron paramagnetic resonance imaging (EPRI). BOLD MRI provides information of tumor oxygen status by using the differences in MRI images from tumors obtained when breathing air or carbogen. Deoxyhemoglobin serves as an endogenous BOLD MRI contrast agent. OMRI utilizes the enhancement of proton MRI images by a nontoxic free radical contrast agent. The advantages of this technique are the very low magnetic fields used and its capability to provide quantitative information of tissue oxygen concentration. EPRI also uses free radical contrast agents and can provide redox and oxygen status differences between tumor and normal tissues. Some of the contrast agents used in EPRI have been identified as radiation protectors. The images obtained from each of the technologies may ultimately be used to overlay their respective views (containing spatial tissue physiology information) onto detailed anatomic maps.