Macrophage infiltration after nerve injury and in certain neurological diseases is closely related to pathogenic and restorative processes. So, an important problem in the field is to understand the kinetics of infiltration. In a recent paper, Bendszus and Stoll report on a method to monitor macrophage migration into injury sites in vivo using magnetic resonance imaging (MRI).

The method depends on the injection of superparamagnetic iron oxide particles, which are routinely used in people to diagnose liver tumours. As macrophages take up these particles, the local accumulation of 'loaded' cells 24 h after injection of the particles results in alterations of the MRI signal from the surrounding areas, owing to the strong paramagnetic effect.

Using this approach, the authors discovered that crushing the sciatic nerve was followed by a loss of the local MRI signal 1–8 days later. This loss was transient and correlated with the immunocytochemical detection of macrophages in the lesion site, providing support for the validity of the method. In addition, by manipulating the interval between particle injection and detection, Bendszus and Stoll found evidence that macrophages did not move together with the receding stump of the degenerating nerve, but remained stationary after invading the lesion site.

Last, the method can also differentiate between macrophage and microglial responses. As the two cell populations undergo phagocytic transformation and express similar markers after activation, it is difficult to tell them apart in tissue sections. Bendszus and Stoll observed that crushing the optic nerve, a site in which there is no macrophage infiltration but a strong microglial response, did not result in the local accumulation of the particles. This ability to differentiate between the two cell types might be useful in research and perhaps in the clinic.