Apparent diffusion coefficient (ADC) maps will be generated with vendor software using a monoexponential fit. ADC maps will be analysed using a linear region-of-interest (ROI) technique, as previously described.10 Two or three ROIs are drawn across both joints on each image slice. Each ROI measuring 14-16mm in length, but is subsequently 'cut' to a standard 14mm length using dedicated Matlab software. This procedure is repeated on the central four axial slices (each of which measures 8mm in thickness). A further ROI will be drawn on normal sacral bone to use as a reference. Normalised ADC values (nADC) will be calculated for each ROI as the mean ADC divided by the mean reference ADC.