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  • Quantitation of Emphysema by Computed Tomography Using a “Density Mask” Program: DISCUSSION

Density Mask" Program: DISCUSSION

In this study we used the CT “density mask” program to quantitate areas with attenuation values less than —910 HU which were taken to represent areas of emphysema, and to provide an overall percentage of lung involvement by emphysema. The CT quantitation of emphysema was correlated with mean lung density, overall lung volume and with commonly performed pulmonary function tests that are affected in emphysema.

The mean lung density was found to correlate well both with the total CT lung volume and with the percentage of lung with emphysema. Lung size in­creases in the presence of both emphysema and hyperinflation. Pathologic-physiologic correlative studies have shown a good correlation between path­ologic extent of emphysema and lung size as indicated by TLC. Thus, one would expect a good correlation between the extent of emphysema on CT and CT lung volume. As the extent of emphysema increases, the total lung size increases due to overinflation and due to lung destruction with dilatation of terminal air­spaces, resulting in a decrease in lung density (ie, attenuation values closer to that of air which is — 1,000 HU). Mean lung density is dependent not only on the presence of emphysema, but also on the presence of airflow obstruction with hyperinflation which would result in a reduction in overall lung density even in the absence of emphysema. Goddard and co-workersalso found that mean lung density decreased with hyperinflation in the absence of emphysema. The presence of a mass with high attenuation values did not significantly affect mean density since the mean density of slices with masses greater than 2 cm was recalculated to exclude the effect of the mass on the recorded mean attenuation. We excluded patients with masses greater than 7 cm in diameter which would have a volume of about 180 ml for a spherical lesion, an insignificant volume compared with the volume of both lungs.

While there is a good correlation between mean density and extent of emphysema (r = 0.55, p<0.001), there is a large scatter about the regression line. Mean lung density did not distinguish clearly cases without significant emphysema from cases with >= 5 percent emphysema. Our finding that mean lung density is not a good indicator of the presence or absence of emphysema is in agreement with previous investiga­tors, who were not successful in attempting to assess emphysema by measuring the mean attenuation of the lung parenchyma. Extensive emphysema must be present to decrease mean lung density significantly. cialis professional

There are several limitations to the measurement of density on CT which have been reviewed by Zerhouni and colleagues and summarized by Mtiller and co­workers. Measurements of density are affected by patient size, location and environment of the area being assessed, type of CT scanner, kilovoltage and reconstruction algorithm. However, in spite of these theoretical limitations, M tiller et al found that using an attenuation value to diagnose emphysema was as reliable as the visual assessment. This is not surprising because the visual measurement of emphysema is based on the presence of localized areas of low attenuation which differ considerably from the re­maining parenchyma and therefore can be reliably quantitated by the “density mask” program. Use of the threshold value also has the advantages of elimi­nating inter- and intra-observer variability and allow­ing accurate determination of the percentage of lung parenchyma with emphysema.

As expected, there was a good correlation between the CT lung volume and pulmonary function measure­ments oflung volume. The best correlation was with FRC (in liters) by either the helium dilution method (r = 0.81, p<0.001, n = 57), or the plethysmographic method (r = 0.79, p<0.001, n = 46). This is because the CT was obtained with the patients supine after submaximal inspiration, while helium dilution and plethysmographic lung volumes were obtained with patients in the sitting position. Because of the decrease in FRC with the supine position, and the submaximal inspiration used during CT, lung volume using CT would be expected to correspond more closely with FRC rather than with TLC with the sitting position. Since CT lung volume was not adjusted for differences in body size, it correlated better with absolute lung volumes than volumes expressed as percent predicted. The CT overall lung volume was found to correlate with tests of airflow (FEV„ FEV/FVC, FEF50%), with CT lung volume increasing as flow rates de­creased. This would be expected since reduction in airflow is known to be accompanied by hyperinflation and an increase in lung volumes on pulmonary function tests.

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