Figure 6: Tumor acidosis can be interrogated with noninvasive imaging. A) The pH low-insertion peptide (pHLIP) inserts itself in the membranes of cells in low pH microenvironments. B) Coupled with a 64Cu contrast agent, the pHLIP protein can be used to measure tumor pHe with positron emission topography. C) Using an FDA approved contrast agent, acidoCEST MRI measures tumor extracellular pH before and after drug treatment. D) Hyperpolarized 13C MRI can be used to measure tumor pHe by measuring the ratios of hyperpolarized bicarbonate to hyperpolarized carbon dioxide. Reproduced with permission from references 96, 99, 102, and 103.