Analytical methodology in quantitative digital subtraction radiography: analyses of the aluminum reference wedge

An aluminum wedge can be used to quantify volumetric change in crestal alveolar bone which has been identified through subtraction radiography. This study compares two methods for using an aluminum wedge for this purpose: 1) the aluminum wedge is present in both radiographs and used to determine the aluminum equivalent bone density present in the region of change for each radiograph; and 2) the aluminum wedge is present in only one radiograph and the difference image of the wedge in the subtraction image is used to calculate the aluminum equivalent change in bone density. Pairs of standardized x-rays were taken with synthetic bone chips of known weights placed when the second x-ray was taken. The volumetric change produced by the bone chips was calculated by the two methods. The relationship of the calculated volumes to the chip weights had a higher r2 value (P < 0.05) for the two-wedge method than for the one-wedge method. The two-wedge method is recommended for volumetric quantification of alveolar bone change. The influence of the clarity of the aluminum wedge image in the x-ray pairs was also examined in this study and found to be significant. Attention must be given to factors which affect the aluminum wedge image characteristics to achieve optimal results.