Figure 6. An example of 1.5 T versus 3 T MRI of the brain. It must be remembered that image acquisition is evolving along with image registration. With new imaging opportunities come new registration challenges. This pair of corresponding slices from rigidly registered brains acquired on two different scanners have exactly the same structure but appear subtly different, despite efforts to match the image acquisition schemes. The 3 T image has higher signal to noise ratio than the 1.5 T image but is also more prone to image artefacts, most obviously here in significant amounts of signal inhomogeneity across the brain (so-called "shading artefact"), and also flow artefacts from the carotid arteries. Registration algorithms driven by intensity information find it hard to differentiate between image differences caused by biological processes and those caused by details of the acquisition process. Image analysis studies that migrate from 1.5 T to 3 T scanners, or which involve aggregation of scans collected from scanners of different field strength are likely to have problems separating real effects from scanner-induced effects.