Volume 453 Issue 7197, 12 June 2008
A low tissue pH is often associated with disease - for example cancer, ischaemia and inflammation - so a technique that could image tissue pH would have considerable potential for disease diagnosis and for monitoring response to treatment. A new, non-invasive method of pH imaging has now been devised, and demonstrated by monitoring the extracellular pH in living mice. It combines magnetic resonance imaging (MRI) and the intravenous injection of carbon-13-labelled bicarbonate - made extremely sensitive to detection by the use of dynamic nuclear polarization. Bicarbonate is a natural buffer in mammalian tissues, resisting pH change via interconversion with carbon dioxide in the reaction catalysed by carbonic anhydrase. The ratio of labelled bicarbonate to carbon dioxide can be used to calculate pH from the Henderson-Hasselbalch equation. Images of mice with implanted tumours confirmed a reduced tumour pH, and suggest that clinical application of the new technique is a realistic proposition. On the cover, colur in the images represents pH. Cover graphic: Mikko Kettunen & Rebekah McLaughlin.
Editorial
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