MRI Of Tissue pH Makes For Diagnostic Tool

Because changes in tissue pH can indicate problems such as cancer and inflammation, a clinical method for imaging pH in living systems could be a powerful diagnostic tool. A team of researchers led by Kevin M. Brindle at the University of Cambridge generates maps of tissue pH by magnetic resonance imaging (MRI) of hyperpolarized ¹³C-labeled bicarbonate (Nature, DOI: 10.1038/nature07017). Bicarbonate is the main extracellular buffer in living systems. Tissue pH can be calculated from the ratio of bicarbonate to carbon dioxide by using the Henderson-Hasselbach equation, pH = pK_a + log₁₀([HCO₃^-]/[CO₂]). The low natural abundance of ¹³C means that clinically relevant MRI measurements of H¹³CO_3- and ¹³CO₂ require large boosts in sensitivity. The researchers get the needed boost by using bicarbonate that has been labeled with hyperpolarized ¹³C, which dramatically increases its sensitivity to MRI detection. They inject the bicarbonate into mice with tumors and map the calculated pH values. The main drawback of the method is the rapid hyperpolarization decay. Nevertheless, because it uses an endogenous buffer that is already safely administered to humans, the method should be easily transferred to the clinic, the team notes.