Problems around the in vitro and in vivo application of quantitative digital subtraction radiography

The validity of a quantitative digital subtraction technique was investigated in vitro when influenced by the following aspects: the material enveloping the aluminium reference wedge, the radiation quality used to produce the radiographs and the effect of differences in image geometry between repeated radiographs. The test object consisted of a dry mandible in which small test objects made of aluminium with known volumes were introduced. By means of the quantitative digital subtraction technique, the aluminium volumes of these test objects were determined. The best agreement between the measured volumes and the actual volumes, was found when the aluminium reference wedge was embedded in polymethylmethacrylate and exposure conditions of 50 kVp, 15 mAs were used. An increase in the differences in image geometry between radiographs led to a decrease in the validity and accuracy of the measurements. For the in vivo application of this method, serial radiographs of 4 patients were taken to register the periodontal bone changes over a 5-8 month period. Differences in the approximal bone between radiographs were quantified in aluminium equivalent volumes (AEV's). In 23% of the measured sites, changes in the mineral content could be detected. The changed sites were found in only 2 patients. One patient showed 3 sites with remineralization, while the other patient showed 2 sites with demineralization. The differences detected ranged from -1.54 to +0.38 mm3 aluminium equivalent.