Differential diagnosis of pulmonary emphysema using the CT index: LL%w

We measured the computed tomography (CT) index, LL%w, in 81 patients of chronic obstructive pulmonary disease (COPD) and asthma. In this study we defined LL%w as the mean value of the proportion of the low density area under -950 Hounsfield units in the six lung fields: upper, middle and lower lung fields bilaterally, at deep expiration. To examine the usefulness of LL%w in differentiating pulmomary emphysema (PE) from bronchial asthma (BA) and chronic bronchitis (CB), we excluded the overlapped cases of each disease. Mean value (± standard deviation) of LL%w in PE was 24.6±20.2% (n=40), whereas it was 0.5±0.8% (n=27) in BA and 0.2±0.3% (n=14) in CB respectively. There were clear statistically differences in the values of LL%w between clinically diagnosed emphysema and others. We considered that the value of LL%w within 1% would be observed nonspecifically, because the frequent existence of low density areas originated in bronchial tangents and/or motion artifacts mainly in the left lower lung field. Thus we judged that cases with over 1% of LL%w had abnormal CT findings. The relationship between clinically diagnosed emphysema and CT abnormality (LL%w > 1%) was significant in the analysis of the four-fold table. The CT sensitivity for diagnosing PE was 100%, the CT specificity was 87.8%, and CT accuracy was 93.8%. When cases of LL%w > 1% were shown in BA or CB, it would be better to consider the existence of complicated emphysema or the presence of air trapping or air spaces of any origin. We compared three groups (A', E', C') selected from groups BA, PE and CB, respectively. The groups consisted of patients showing almost the same mean values of FEV1.0/VC(%). The value of the LL%w of E', selected from PE, also showed a significantly higher value than those from BA or CB. (J.P.N)