The CT attenuation histogram may be used to quantify lung fibrosis. In the lung parenchyma, CT attenuation, measured in Hounsfield units, is determined by the relative amounts of air, soft tissue, and blood in each volume element (voxel). The attenuation values for each pixel can be expressed as a histogram. The CT attenuation histogram of normal lung deviates from the Gaussian normal distribution in that it is sharply peaked at approximately − 800 Hounsfield units, and is markedly skewed to the left. Since lung fibrosis or inflammation cause an increase in the amount of soft tissue in the lung, it will increase mean lung attenuation, and will decrease the sharpness of the histogram peak (kurtosis), and the degree of leftward skewness of the curve. Mean lung attenuation, skewness, and kurtosis can therefore be used as measures of the extent of lung fibrosis. In a study of 24 subjects with idiopathic pulmonary fibrosis and 60 subjects with asbestosis, Hartley et al8found that the mean and median lung attenuation derived from high-resolution CT images were independently associated with the presence of moderate-to-severe dyspnea, a higher profusion of chest radiograph abnormalities, a lower FVC, and abnormalities on BAL. More recently, a study9 of 144 patients with idiopathic pulmonary fibrosis showed that measures of mean lung attenuation, skewness, and kurtosis derived from the CT density histogram correlate with evidence of physiologic impairment.