Chest
Volume 94, Issue 4, October 1988, Pages 782-787
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“Density Mask”: An Objective Method to Quantitate Emphysema Using Computed Tomography

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We used a computed tomography (CT) scanner program (“density mask”) that highlights voxels within a given density range to quantitate emphysema by defining areas of abnormally low attenuation. We compared different density masks, mean lung attenuation, visual assessment of emphysema and the pathologic grade of emphysema in 28 patients undergoing lung resection for tumor. In each patient, a single representative CT image was compared with corresponding pathologic specimens of tissue. There was good correlation between the extent of emphysema as assessed by the density mask and the pathologic grade of emphysema. The optimal attenuation level to define areas of emphysema may vary in different scanners, but, once determined for a particular scanner, the density mask accurately assesses the extent of emphysema and eliminates interobserver and intraobserver variability. It has the added advantage of determining the exact percentage of lung parenchyma showing changes consistent with emphysema. (Chest 1988; 94:782-87)

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MATERIALS AND METHODS

Twenty-eight patients (20 men and eight women) who had undergone CT scanning prior to thoracotomy for lung tumor were studied. The age of the patients ranged from 33 to 77 years (mean, 59 years; SD 10). These included 14 upper lobectomies, five lower lobectomies, and nine pneumonectomies. In each patient, a single representative CT image and a corresponding pathologic specimen of tissue were analyzed. The CT scans were obtained on a 9800 scanner (General Electric, Milwaukee) at 10-mm intervals

STATISTICAL ANALYSIS

The extent of emphysema as determined by the three different density masks, the mean lung attenuation, and the visual estimate of emphysema by both observers were compared with the pathologic grade using regression analysis. Interobserver and intraobserver variability was assessed using the intraclass correlation coefficient.14

RESULTS

The pathologic scores for emphysema ranged from 0 to 100 (Table 1). Visual comparison of the CT image and the corresponding transverse lung specimen showed remarkable similarity, particularly for areas of destruction greater than 0.5 cm (Fig 1). Mild panacinar emphysema and smaller areas of centriacinar emphysema could not be discerned by CT (Fig 2). The correlation between the mean of the four visual scores by the two independent observers and the pathologic score of emphysema was r = 0.90

DISCUSSION

Emphysema is a condition characterized by enlargement of the airspaces distal to the terminal bronchioles due to destruction of the airspace walls.1 This is detected pathologically by the presence of nonuniform enlargement of the respiratory airspaces or simply stated, by the presence of abnormal holes in the lung parenchyma. These holes are detected on CT by the presence of areas of abnormally low attenuation.5, 6, 7, 8, 9 As emphysema progresses, it may lead to disruption of the vascular

ACKNOWLEDGEMENTS

We thank Drs. Kenneth G. Evans and Bill Nelems for their referral of patients and Ms. Anne Dy Buncio for performing the statistical analysis.

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Supported in part by a grant from the British Columbia Lung Association.

revision accepted March 14.

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