Non-invasive methods for studying brain energy metabolism: what they show and what it means

This review summarises the ways in which magnetic resonance spectroscopy (MRS) and related methods can be used as windows on brain energy metabolism in vivo. (31)P-MRS can measure acute changes in non-oxidative ATP synthesis in transient states, and at steady state reflects the balance of ATP demand and mitochondrial function. (13)C-MRS labelling methods can measure a variety of carbon fluxes. The few (31)P- and (13)C-MRS studies of the response to functional activation suggest quite large increases in oxidative metabolism. Functional magnetic resonance imaging measures the hyperoxygenation that results from increase in cerebral blood flow in excess of glucose oxidation, attenuated somewhat by a smaller increase in oxygen consumption. Previous positron emission tomography studies disagree on the size of activation response. These are powerful but demanding techniques, valuable in understanding both normal physiology and pathophysiology. However, discrepancies remain to be reconciled, and this will require increasing sophistication of both techniques and analytical models.