Quantitative microfocal radiography accurately detects joint changes in rheumatoid arthritis.

Microfocal radiography, producing x5 magnified images of the wrist and hands with a high spacial resolution (25 microns) in the film, permitted direct measurement of erosion area and joint space width in patients with rheumatoid arthritis. The magnitude of errors relating to direct measurement, repositioning the wrist and hand on successive x ray visits, repeated identification of erosions and their area calculation were assessed. The coefficients of variation for length and area measurements were 3.7% and 13% respectively, while the change in joint space width and erosion area in five patients over 18 months had average coefficients of variation of 12.7% and 42.0% respectively. The combined errors correspond approximately to the fifth percentile level for the total changes in size of these x ray features. The remaining alterations were due to the disease, which was markedly greater that that attributable to errors of measurement.