Model for understanding the link between radiographic and magnetic resonance imaging (MRI) erosion. (a) A diagrammatic 'chessboard' model of a normal joint. The white squares depict the calcium-containing tissues on radiography. The black squares depict the soft tissues that are 'invisible' on radiography but that are visible on MRI due to their hydrogen atom content (fat or water on TI-weighted images and water on fat suppression images). (b) Radiographic erosion where bone cortex and trabecular bone are lost, hence the x-ray appearance of a 'hole' in the bone. Though not visible on x-ray, the erosion may be filled with stromal tissue. (c) Bone oedema on a fat suppression MRI. In this pre-erosive stage, the bone trabecular network is invisible to MRI (but is nevertheless present). The marrow soft tissues have an increased water content due to the osteitis that is seen as a high signal on fat suppression MRI (grey squares), as shown in (b). In the early stages, this will be associated with minimal bone trabecular destruction since it takes time from the inflammatory insult to end in osteoclast-mediated joint destruction. So the MRI pre-erosion lesion will not be evident on radiography. Because this is essentially inflammatory tissue, the resultant MRI lesions may appear to shrink or heal following therapy. Unlike radiographic erosions, this represents regression of inflammation rather than true bone repair. Nevertheless, the consequence of suppressing MRI erosions is that future radiographic damage may be prevented.