Dual energy film subtraction technique for detecting calcification in solitary pulmonary nodules

A simple film subtraction technique has been devised that isolates calcium image contrast and mutes tissue image contrast. Two exposures are required. The first is made on XL film using a 65 kVp beam filtered with 2 mm aluminum. The second is made on OG (high contrast) film using a 130 kVp beam filtered with 2 mm copper and 2 mm aluminum. The effective energies of these two beams are approximately 45 keV and 83 keV, respectively. A subtraction image is made, using the low energy image for the mask. With this technique it is possible to detect concentrations of 125 mg/cm3 of diffuse calcification in a chest nodule 1 cm in diameter. If the presence of diffuse calcification is found to be an indicator of benignancy in solitary pulmonary nodules, this technique may have diagnostic value for the detection of such calcification. Computerized tomographic findings are discussed and related to this technique.