CT measurements of lung attenuation have been shown to correlate with the amount of emphysema. Using CT, emphysema is usually quantified as the percentage of the lung having an attenuation below a specific threshold value. Studies using single detector scanners have found that the optimal attenuation threshold for defining emphysema depends on the slice thickness (Muller et al, 1988; Gevenois et al, 1995). Lung attenuation values also vary depending on the reconstruction algorithm applied to the raw data (Boedecker et al, 2004s). Thus, quantitative CT measurements of emphysema may be significantly influenced by how a study was reconstructed. The purpose of this study is to define the relationships between the different reconstruction algorithms and attenuation values in lung tissue on multidetector CT scans. The results in turn may be useful for comparing emphysema measurements on scans obtained using different reconstruction methods. In this study, we will analyze and compare NLST CT scans using multiple slice thicknesses and reconstruction filters in order to: 1. Understand the impact of different reconstruction algorithms (i.e., slice thickness and reconstruction filter) on the attenuation values of lung tissue. 2. Develop a regression model that predicts lung attenuation values from various slice thicknesses and reconstruction filters, thereby determining conversion factors for comparing emphysema measurements obtained using different reconstruction parameters. 3. Utilize the relationships between the reconstruction parameters and various attenuation thresholds for defining emphysema, to determine how scanner reconstruction algorithms may impact the classification of emphysema severity. 4. Determine the prevalence of emphysema in the Washington University NLST population.