Factors influencing quantitative CT measurements of solitary pulmonary nodules

Quantitative computed tomographic (CT) measurement of pulmonary nodules has not been widely applied despite favorable reports. Its usefulness has been questioned by some investigators. A series of experiments on six different scanners was undertaken to study the factors that affect the applicability of this technique. The type of reconstruction algorithm, the design of the CT system, the true slice thickness, and the beam kilovoltage were the most important factors identified. These factors can produce large variations in the CT numbers of pulmonary nodules, preventing direct comparison of results from scanner to scanner. Before undertaking studies of pulmonary nodules, the effect of these variables in each individual scanner should be evaluated. Despite the current lack of standardization, reliable CT number measurements using a specific kilovoltage and slice thickness should be possible on every modern scanner provided it is positionally uniform and gives a linear response to varying densities. We propose that the CT number above which a pulmonary nodule can be considered benign should be the representative CT number of a 1 cm diameter syringe filled with a hydrous calcium chloride solution of 40 mg/ml of water and scanned in air. A better understanding of the physics of lung nodule densitometry is necessary for the proper application of this technique in the management of patients with solitary pulmonary nodules.