Quantitation of Emphysema by Computed Tomography Using a “Density Mask” Program: DISCUSSION part 2
The percentage of lung with emphysema by CT “density mask” quantitation correlated inversely with indices of airflow FEV1 FEV./FVC (Fig 4), FEF50%, and with diffusing capacity, expressed both as Dsb percent predicted and as a Dsb/Ул ratio percent predicted. Our findings agree with those in a previously reported study from our institution by Morrison and co-workers who found correlations of —0.50 to — 0.55 between Dsb and Dsbco/VA and the extent of emphysema determined either by pathologic assessment of resected lung or by a visual CT score. We found abetter correlation between FEV, and FEF50% and the extent of emphysema measured by the CT density mask (r= -0.51 to -0.56) than Morrison and colleagues when they assessed the extent of emphysema by a visual CT score (r= —0.34). This may be because the visual overall CT score used by Morrison and co-workers may not quantitate as precisely mild degrees of emphysema as the “density mask.” There were no significant differences in the correlations between FEV\ and FEF50% and the extent of emphysema when assessed by the CT “density mask” in this study, when compared with the correlations (— 0.54 to — 0.50) obtained with pathologic assessment of emphysema.
There were 11 patients in our study with an abnormal Dsb who had no significant emphysema (<1 percent) by CT “density mask” (Table 4); however, three of these patients had a normal Dsb/VA. This suggests that a Dsb/VA ratio discriminates better between the presence or absence of emphysema than Dsb. In these three patients, the single-breath alveolar volume during the Dsb test was significantly reduced resulting in a low Dsb with a normal Dsb/VA ratio and could explain the abnormal Dsb in these patients with no significant emphysema on CT “density mask.” However, there were still eight subjects with abnormally low Dsb and Dsb/VA, which may be due to the CT underestimating or missing mild emphysema. Careful visual inspection of the CT revealed that all these patients had less than 1 percent emphysema which was correctly indicated by the “density mask.” M tiller et al pointed out that CT may miss mild emphysema; this is not a deficiency of the “density mask” program itself since the mild emphysema in these cases was also missed by careful visual assessment of the CT slices by two independent observers.
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Localized areas of emphysema less than 0.5 cm in diameter often cannot be seen on CT. None of these subjects had prior chest radiation, chemotherapy, congestive heart failure, tuberculosis or evidence of interstitial lung disease, pneumonia or pleural disease, which could affect Dsb. If the patient was anemic, then Dsb was corrected for the effect of anemia by the method of Cotes and colleagues. Although some of our patients were current smokers, they refrained from smoking prior to the tests, so it is unlikely that the reduced Dsb could be due to the effects of carboxyhemoglobin which reduces Dsb by about 1 percent for each 1 percent carboxyhemoglobin. Thus, it is unlikely that the reduced Dsb is due to other causes. Therefore, we conclude that the low Dsb, in eight of our subjects with < 1 percent emphysema, is probably due to emphysema being missed or underestimated by CT.
In a recent report Sakai and co-workers correlated pulmonary function tests with the extent of emphysema using a CT score determined both by direct observation and by a grid method in 30 subjects. Their correlations between the extent of emphysema and pulmonary function are similar but a little higher than those in the present study. This is perhaps due to their use of a four-point scale combining the extent and severity of emphysema, whereas we assessed the extent of emphysema based on the actual percentage of lung involved (ie, a 100-point scale). They also appear to have had a larger proportion of subjects with more severe emphysema than in our study. Ten of their 30 subjects (33 percent) had “moderate” emphysema, and six of 30 (20 percent) had “severe” emphysema as assessed by CT, while 17 of 85 (20 percent) of the patients in the present study had > 10 percent of the lung involved by emphysema, and nine of 85 (11 percent) had > 20 percent emphysema. They also did not have any subjects with 0 score of emphysema, whereas 25 of 85 (29 percent) of our patients had emphysema involving < 0.5 percent of the overall lung area, which probably roughly corresponds to an emphysema score of 0 on visual assessment.
It was shown previously that the “density mask” is an accurate method for quantitating the extent of emphysema. This study shows that the parameters of lung function usually affected by the presence of emphysema are correlated with the CT “density mask” extent of emphysema, showing increasing abnormality with increasing extent of emphysema on CT. Most studies comparing lung function with the extent of emphysema rely on a pathologic assessment of emphysema. Pathologic evaluation remains the gold standard for assessment of emphysema since emphysema is defined in pathologic terms. However, in studies of physiologic-pathologic correlations performed in patients undergoing lung resection, pathologic assessment of emphysema is based on one lung or often on one lobe, neither of which may accurately reflect the overall extent of emphysema in both lungs. The CT “density mask” program, on the other hand, gives an objective quantitative assessment of the overall extent of emphysema in both lungs. Despite missing mild degrees of emphysema, it provides a useful technique for quantitating the overall extent of emphysema in life.
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