Twelve patients with chronic airflow limitation and 12 patients with a histological diagnosis of fibrosing alveolitis were studied. The calculated mean (SD) tissue volume of a single lung at total lung capacity was 467 (91) ml in the patients with alveolitis, which was 43% (14%) more than predicted for healthy people of the same age, sex, and height. The tissue volume of a single lung at total lung capacity was 436 (82) ml in the patients with chronic airflow limitation, which was 26% (21%) more than predicted. At residual volumes the tissue contents of the fibrotic and the obstructed lungs changed very little (to 407 (84) ml and 433 (84) ml respectively). This allowed tissue volume to be used as a marker of position within the lung, to match inspiratory and expiratory slices and to calculate regional ventilation. In both groups local ventilation was diminished and more variable than in healthy lungs-that is, in the mid 70% of lung volume the local residual volume to total gas volume ratios (RV/TGV) were 32% (10%) in the fibrotic group and 66% (14%) in the group with chronic airflow limitation, compared with 23% (5%) in healthy subjects. As expected, the fibrotic lungs were much denser (0-246 (0036) g/ml) and the lungs with chronic airflow obstruction were less dense (0-114 (0-026) g/ml) than were healthy lungs (0-126 (0-017) g/ml).We have previously shown that computed tomography could measure the total gas and tissue volumes and the regional vital capacities and residual volumes of the lungs in healthy men.' The aim of the present study was to discover how well computed tomography could recover the same volumes in patients with lung disease. MethodsWe studied 12 patients with chronic airflow limitation and 12 patients with fibrosing alveolitis. They were chosen from larger groups of patients with probable emphysema or fibrosis in whom computed tomography had been performed for clinical reasons. Their scans, which were stored on magnetic tape, were selected for more detailed study retrospectively, on the basis of clinical, radiographic, functional, and (in the case of fibrosing alveolitis) histological confirmation of the diagnosis.

This study set out to determine whether quantitative features of lung computed tomography scans could be identified that would lead to a tightly defined normal range for use in assessing patients. Fourteen normal subjects with apparently healthy lungs were studied. A technique was developed for rapid and automatic extraction of lung field data from the computed tomography scans. The Hounsfield unit histograms were constructed and, when normalised for predicted lung volumes, shown to be consistent in shape for all the subjects. A three dimensional presentation of the data in the form ofa "net plot" was devised, and from this a logarithmic relationship between the area of each lung slice and its mean density was derived (r = 0-9, n = 545, p < 0O0001