A growing trend in neuroimaging research is to combine standard imaging techniques with other methods that might allow us to get additional information from the activation signal. A good example is the use of magnetoencephalography (MEG) — the detection of the small magnetic fields that are generated by neuronal activity — in combination with imaging methods, usually positron emission tomography (PET). The underlying idea is that the high temporal resolution that MEG affords is a perfect complement for the relatively good spatial resolution of PET. But a recent paper reports on the development of magnetic-source magnetic resonance imaging (msMRI) as a powerful alternative to reach the same goal in a simpler way.

The theoretical foundation of msMRI is relatively simple. The basic MRI signal depends on the use of a strong magnetic field to induce phase-coherent nuclear spins, the relaxation of which is subsequently detected. But if these spins are exposed to the intrinsic magnetic field that is generated by neuronal activity, their coherence will decrease, leading to a reduction in the magnitude of the MRI signal. Xiang et al. set out to establish whether detecting such a decrease was possible, using a simple visuomotor task that elicits well-characterized activations of the visual, motor and premotor cortices. msMRI allowed them to detect activations with the same spatial resolution (3 mm) and of the same strength (1% of the baseline signal) as functional MRI, but with a much higher temporal resolution (100 ms), comparable to what is obtained with electrophysiological methods.

In addition to increasing the temporal resolution of current imaging methods without compromising on spatial resolution, msMRI offers other advantages. In contrast to PET and functional MRI, msMRI does not depend on the haemodynamic brain response and its complex relation to neuronal activity. And in contrast to MEG, msMRI does not depend on the detection of neuronal magnetic fields at the scalp, but allows us to look directly at their source in the brain parenchyma. Owing to this combination of attributes, msMRI should be received as a welcome and powerful addition to the imaging arsenal.