Emphysema is a pathologic abnormality of the lungs defined by enlargement of terminal airspaces and destruction of airspace walls, Though spirometry is used to diagnose COPD, it is a relatively inaccurate means of assessing for emphysema. Conventional CT is a highly accurate way to assess the severity of emphysema, which can be quantified by the decrease in x-ray attenuation of the lungs that results from airspace enlargement and alveolar destruction. However, CT is performed using relatively high doses of ionizing radiation, which limits its acceptability as a screening and follow-up test, particularly in early or mild disease. Low dose CT scanning, currently under evaluation as a means of screening for lung cancer, allows depiction of substantial lung detail at up to one-tenth of the radiation dose of conventional CT. Increased image noise is commonly recognized as a trade-off of using low dose CT technique, but the effect of this on lung attenuation values and the quantification of emphysema is unknown. We hypothesize that emphysema quantitation using low dose CT will have accuracy comparable to conventional CT scanning methods. In this project, we propose to: 1. Determine whether measurements of emphysema severity on low dose CT scans differ from those obtained on usual dose CT scans. 2. Determine whether any quantitative differences on low dose scans compared to usual dose scans can be corrected by changing the attenuation threshold used for defining emphysema. 2. See whether low dose NLST CT scans reveal an increase in severity of emphysema over time. The first two aims will be addressed by comparing attenuation histogram data from low dose NLST CT scans with attenuation histogram data from follow-up clinical CT scans that were obtained with usual radiation dose. The second aim will be addressed by comparing attenuation histogram data on T0 and T1 scans.