Magnetic Resonance Imaging of the Tumor Microenvironment in Radiotherapy: Perfusion, Hypoxia, and Metabolism

The tumor microenvironment is characterized by hypoxia, low pH, and high interstitial fluid pressure. Hypoxic regions in tumors with low partial pressure of oxygen (pO2) levels can result in resistance to radiotherapy, thus causing local failure. Therefore, it would be desirable to noninvasively measure pO2 levels in the tumor before, during, and after treatment to better customize therapy and follow treatment response. Several techniques used in preclinical and clinical studies to obtain the pO2 status of tissue, such as dynamic contrast-enhanced magnetic resonance imaging, blood oxygen level–dependent imaging, and electron paramagnetic resonance imaging, are reviewed. Furthermore, the ability to hyperpolarize specific metabolic substrates that are isotopically labeled with 13C coupled with magnetic resonance spectroscopy enables noninvasive imaging of tissue metabolism, such as glycolysis.