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DOI: 10.1148/radiol.2363041611
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(Radiology 2005;236:1059-1066.)
© RSNA, 2005

Thoracic Imaging

Silicosis: Expiratory Thin-Section CT Assessment of Airway Obstruction1

Hiroaki Arakawa, MD, Pierre Alain Gevenois, MD, Yoshiaki Saito, MD, Hisao Shida, MD, Viviane De Maertelaer, PhD, Hiroshi Morikubo, MD and Mutsuhisa Fujioka, MD

1 From the Department of Radiology, Dokkyo University School of Medicine, 880 Kita-Kobayashi, Mibu, Tochigi 321-0293, Japan (H.A., M.F.); Department of Radiology (P.A.G.) and Statistical Unit, Institute of Interdisciplinary Research in Human and Molecular Biology, Hôpital Erasme, Université Libre de Bruxelles, Belgium (V.D.M.); and Departments of Radiology (H.S., H.M.) and Respiratory Medicine (Y.S.), Labour Welfare Hospital for Silicosis, Tochigi, Japan. Received September 17, 2004; revision requested November 24; revision received December 6; accepted January 17, 2005. Address correspondence to H.A. (e-mail: arakawa{at}dokkyomed.ac.jp).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
PURPOSE: To prospectively evaluate if findings on paired inspiratory and expiratory thin-section computed tomographic (CT) scans in patients with silicosis correlate with pulmonary function test results.

MATERIALS AND METHODS: Institutional review board approval and patient consent were obtained. Thirty-seven men (mean age, 71 years; range, 53–88 years) with silicosis were included. All patients had undergone inspiratory and expiratory thin-section CT and spirometry. Silicotic nodules, large opacity, emphysema, reticular opacities, bronchiectasis, and air trapping were graded subjectively on CT images. Emphysema was quantified on these images with built-in software. CT numbers were correlated with spirometric findings by using Spearman rank correlation analyses. Ten healthy volunteers (three men and seven women; mean age, 58 years) served as control subjects.

RESULTS: After exclusion of three patients with inadequate image quality, 34 patients (mean age, 70 years; range, 53–88 years) were enrolled in the study group. Spirometric values did not differ significantly between patients with simple (n = 20) and patients with complicated (n = 14) silicosis but were significantly lower in patients than in control subjects. CT findings included air trapping (n = 33), emphysema (n = 26), nodules (n = 32), bronchiectasis (n = 22), large opacity (n = 19), and reticulation (n = 5). The extent of both air trapping and emphysema correlated negatively with spirometric values; the air trapping score showed the strongest correlation (ratio of forced expiratory volume in 1 second to forced vital capacity [FVC]: {rho} = –0.632, P < .001; forced expiratory flow at 50% of the FVC: {rho} = –0.576, P = .001). Silicotic nodule, large opacity, and bronchiectasis scores did not correlate with obstructive functional impairments.

CONCLUSION: In comparison with the spirometric value, the extent of air trapping proved the best CT index in the assessment of obstructive derangement in workers with exposure to silica dust.

© RSNA, 2005


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
In individuals occupationally exposed to silica, airway obstruction occurs in relation to the amount of dust to which the individual is exposed, and it occurs even before the development of radiographic appearance of silicosis (1,2). Small and large airways are affected, and the bronchiolar pathologic finding is called mineral dust airway disease (3). This disease induces the formation of silicotic nodules and a large opacity, which lead to progressive airway obstruction and a restrictive functional deficit (4,5). Factors contributing to the obstructive functional impairment at this period include airway disease and emphysema, the latter of which is caused by the synergetic effect of chronic dust inhalation and cigarette smoking (6–8). Investigators disagree about whether emphysema (9) or airway disease (7) is the predominant cause of airflow limitation in silicosis.

End-expiratory (hereafter, expiratory) thin-section computed-tomography (CT) can depict airway abnormalities by showing air trapping in the areas of airway obstruction in various obstructive lung diseases (10,11). The extent of air trapping has been shown to correlate well with measures of functional obstruction (10,12). Although patients with silicosis show obstructive functional impairment, we know of no study in which correlation between expiratory thin-section CT and such impairment was shown. Thus, the purpose of the present study was to prospectively evaluate if findings on paired inspiratory and expiratory thin-section CT scans in patients with silicosis correlate with pulmonary function test results.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Our institutional review board approved our study, and informed consent from all study subjects was obtained.

Study Population
Thirty-seven men (mean age, 71 years; range, 53–88 years) with history of occupational dust exposure, exclusively to silica, underwent optional chest CT in June 2002. These men underwent annual follow-up examinations for worker's compensation according to the law of the Japanese Ministry of Health, Labor, and Welfare. Physical examination, chest radiography, spirometry, and sputum cytology were performed, and chest CT study was optional. Because lung cancer often complicates pulmonary silicosis (13), in Japan patients with silicosis are encouraged to undergo chest CT study as a cancer screening. Patients who participated in this optional examination were enrolled in our study. We had excluded patients who could not tolerate repeated forced inspiratory and expiratory effort during CT scanning, who had undergone thoracic surgery, and who showed obvious pleuropulmonary changes consistent with pulmonary tuberculosis (ie, diffuse pleural calcification, parenchymal distortion, or segmental bronchiectasis in apical and posterior segments of upper lobes). Thirteen patients were current smokers, 19 patients were ex-smokers, and five patients had never smoked. Ten healthy volunteers (three men and seven women; mean age, 58 years) who had no history of occupational dust exposure served as control subjects. The control subjects had no respiratory symptoms or history of respiratory diseases. These subjects underwent paired inspiratory and expiratory thin-section CT and pulmonary function tests.

Thin-Section CT Technique
Paired inspiratory and expiratory thin-section CT (nonenhanced) was performed in both patients and control subjects with Somatom Plus 4 or Volume Zoom (Siemens Medical Systems, Forchheim, Germany) scanners. The scans were obtained with all patients in a supine position and with full inspiration and expiration. The inspiratory scan was obtained from the apex to the base of the lungs with 2-cm intervals at full-suspended inspiration. The expiratory scan was obtained after forced exhalation at three preselected levels (at the mid-aortic arch, below the tracheal carina, and just above the right hemidiaphragm). All CT scans were obtained with 1- or 2-mm collimation, 120 kVp, 240 mA, and 0.75 second per gantry rotation and were reconstructed with a high-frequency algorithm. Images were displayed and photographed with a window width of 1200–1500 HU and window level of –650 HU.

Image Analysis
A posteroanterior chest radiograph obtained at 120–130 kVp 1 month before the CT study was available for all patients. Each chest radiograph was independently evaluated by two of the authors (a radiologist [H.S.] with more than 30 years of experience in pneumoconiosis and a chest radiologist [H.A.] with 12 years of experience) according to the International Labor Organization, or ILO, classification (14). They were not certified B readers, but B-reader certification is not required in Japan for official assessment of pneumoconiosis. The major categories of small opacities and their profusion, as well as the presence and extent of the large opacity, were assessed. Large opacity was defined as a large pneumoconiotic opacity more than 1 cm in diameter (14). Patients were divided into complicated silicosis and simple silicosis groups on the basis of the presence of large opacity; those with large opacity were placed in the complicated silicosis group, and those without large opacity were placed in the simple silicosis group. For statistical analysis, the small opacities were categorized (ILO classification) as either rounded (p, q, r) or irregular (s, t, u). Profusion was graded by using a 12-point ILO classification scale (score of 1, 0/–; score of 12, 3/+). Large opacity was classified as either A (the sum of the longest dimensions of all large opacities not exceeding 5 cm), B (the sum of the longest dimensions of all large opacities exceeding 5 cm but not exceeding the equivalent area of the right upper zone), or C (the sum of the longest dimensions of all large opacities exceeding the equivalent area of the right upper zone). Any disagreement with regard to categorization (p, q, r, s, t, u) of small opacities or large opacity was resolved by consensus, and the final profusion was derived by averaging the scores of the two reviewers. Chest radiographs were not reviewed for control subjects.

CT images obtained from both patients and control subjects were analyzed independently by the same chest radiologist who interpreted the chest radiographs and by another chest radiologist (P.A.G.) with 15 years of experience. The CT image analyses were performed at a different session from that of chest radiography and in random order.

The presence and extent of silicotic nodules, large opacity, emphysema, reticular opacity consistent with diffuse interstitial fibrosis, and bronchiectasis were assessed on the inspiratory images. The nodules included all measurable, well-defined, rounded opacities of less than 10 mm in diameter, whereas large opacity included opacity larger than 10 mm. On a CT scan, area of emphysema was defined as area with attenuation similar to air attenuation, lacking vascular or bronchial structures, and without visible walls. Bronchiectasis was considered present when normal tapering of the bronchial lumen was lost. Air trapping was assessed on expiratory images. Air trapping was considered to be present when lung regions on the expiratory CT images failed to increase in attenuation and/or decrease in volume in comparison with those on the corresponding inspiratory images.

After the presence of each abnormality was assessed, its extent and severity were evaluated subjectively in each of the three lung zones for each lung. The three lung zones consisted of upper, middle, and lower lung zones. The border between the upper and middle lung zones was set at the level of the tracheal carina, while that between the middle and lower lung zones was set at the level of the confluence of the inferior pulmonary vein. Silicotic nodules, large opacity, emphysema, reticular opacities, and bronchiectasis were scored on the inspiratory images.

The extent and severity of silicotic nodules were graded with the following four-point scale: score of 0, no nodule; score of 1, mild, small opacities definitely present but few in number; score of 2, moderate, numerous small opacities; and score of 3, severe, very numerous small opacities, poor visibility of normal anatomical lung structures.

The extent of emphysema, reticular opacities, and air trapping was graded with the following five-point scale: score of 0, no such area; score of 1, 1%–25% of the cross-sectional area of the lung affected; score of 2, 26%–50% of the lung affected; score of 3, 51%–75% of the lung affected; and score of 4, 76%–100% of the lung affected. The scores for each abnormality were totaled, yielding the final score. The maximum possible score was 18 for a silicotic nodule and 24 each for emphysema, air trapping, and reticular opacities. We excluded subpleural bullous areas from assessment of emphysema because subpleural bullae have little effect on pulmonary function (15).

The degree of bronchiectasis was graded according to the number of segments that showed bronchiectasis. The right lung consisted of 10 segments, and the left lung consisted of eight segments. Large opacity was scored with the following five-point scale: score of 0, no large nodules; score of 1, the total area of all large opacities did not exceed one-fourth of the cross-sectional area of the right upper lung zone at the level of the tracheal carina; score of 2, the total area of large opacities was between one-fourth and one-half of the cross-sectional area; score of 3, the total area of large opacities was between one-half and three-fourths of cross-sectional area; and score of 4, the total area of large opacities was more than three-fourths of the cross-sectional area.

We also quantitatively measured the areas of emphysema in the 29 patients for whom the original CT data were available and in all control subjects. The percentage of lung area occupied by pixels with attenuation values lower than a predefined threshold was calculated at all scanning levels, and the average was expressed. These values were calculated by means of software built into the scanner (Pulmo-CT, Siemens Medical System). We used –950 HU as the threshold for emphysema on the inspiratory images and –500 HU as the threshold for extracting lung parenchyma (16,17).

Pulmonary Function Tests
Within 1 month before or after the CT examination, pulmonary function tests were performed with the use of computerized pulmonary function instruments (FUDAC 70, Fukuda Denshi, Tokyo, Japan; or CHESTAC 11, Chest, Tokyo, Japan). Forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1), forced expiratory flow (FEF) at 50% of FVC (FEF50%), FEF at 25% of FVC (FEF25%), and FEF at 75% of FVC (FEF75%) were determined with spirometry. Each result was expressed as a percentage of the predicted value based on the patient's sex, age, and height (18). Diffusion capacity and other functional parameters were not measured for patients with pneumoconiosis because the government does not require these for compensation. However, pulmonary function tests for nonpneumoconiotic control subjects included spirometry and assessment of diffusion capacity for carbon monoxide.

Statistical Analysis
Interobserver agreement was assessed with Cohen {kappa} statistics and weighted {kappa} statistics for unranked and ranked scores, respectively. A {kappa} value lower than 0.20 indicated poor agreement; 0.21–0.40, fair agreement; 0.41–0.60, moderate agreement; 0.61–0.80, good agreement; and 0.81–1.00, excellent agreement (19). For quantitative data, interobserver agreement was assessed by using Spearman rank correlation coefficient, and differences were evaluated with Bland-Altman plots (20). Differences in CT scores and results of pulmonary function tests between patients with simple silicosis and patients with complicated silicosis and between control subjects and the patients were analyzed with the Student t test. Differences between the air trapping score and emphysema score were assessed for each reviewer with a Wilcoxon signed rank test for paired samples. Spearman rank correlation analyses were performed to correlate functional values and CT scores and also radiographic scores and CT scores. Statistical analyses were performed with SPSS (version 12.0; SPSS, Tokyo, Japan) and StatXact (version 5.0.3; Cytel Software Corporation, Cambridge, Mass).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Three of 37 patients were excluded because of motion artifacts caused by insufficient breath holding during expiratory CT scanning. Thus, the final study group comprised 34 patients (mean age, 70 years; range, 53–88 years). Eleven patients were current smokers, 18 patients were ex-smokers, and five patients had never smoked.

Chest Radiographs
Radiographic data are given in Table 1; data for patients with simple (n = 20) and complicated (n = 14) silicosis are shown separately and in combination. Twelve of the patients with complicated silicosis had category A large opacities, one had category B large opacity, and one had category C large opacity. All patients had nodules on chest radiographs or CT scans that were considered consistent with silicosis.


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  		TABLE 1. Radiographic Data

 
Correlation coefficients (Spearman {rho}) for interobserver agreement for radiographic profusion of small opacities and CT scores for air trapping, emphysema, bronchiectasis, and silicotic nodules were 0.755, 0.912, 0.849, 0726, and 0.871, respectively (P < .001). Bland-Altman plots were calculated for radiographic profusion of small opacities and for CT scores for silicotic nodules, air trapping, emphysema, and bronchiectasis; some are shown in Figures 1–3. The plots show good interobserver agreement for all scores. The interobserver agreement for both radiographic categorization of small opacities and for the extent of the large opacity on CT images was excellent ({kappa} = 1.000, P < .001 and weighted {kappa} = 0.807 ± 0.101, P < .001, respectively), and agreement for reticular opacity scores was good (weighted {kappa} = 0.686 ± 0.240, P = .004).



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  		Figure 1. Bland-Altman plot of CT scores for air trapping. The mean difference was –0.2, and the limits of agreement between the two observers in each patient ranged from –4.5 to +4, which was within the mean ± 1.96 standard deviation (SD).

 


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  		Figure 2. Bland-Altman plot of CT scores for emphysema. The mean difference was –0.1, and the limits of agreement between the two observers in each patient ranged from –4.2 to 4.0, which was within the mean ± 1.96 standard deviations (SD).

 


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  		Figure 3. Bland-Altman plot of CT scores for silicotic nodules. The mean difference was 0, and the limits of agreement between the two observers in each patient ranged from –2.1 to 2.2, which was within the mean ± 1.96 standard deviations (SD).

 
Chest CT Scans and Spirometry
The following patterns were observed on all thin-section CT scans: air trapping (n = 33), nodules (n = 32), emphysema (n = 26), bronchiectasis (n = 22), large opacity (n = 19), and reticular opacities (n = 5). Results of pulmonary function tests for control subjects and patients and the mean CT scores for patients derived from two readings are given in Table 2. All spirometric values except FEF75% were significantly lower in patients than in control subjects, but the mean FVC was 80.8% in patients, which was within normal range. In contrast, the mean FEV1/FVC was 67.6% in patients and was significantly impaired. Spirometric values did not differ significantly between patients with simple silicosis and patients with complicated silicosis. CT scores differed significantly between patients and control subjects except in the score of reticular opacities (P = .295). CT scores did not differ significantly between patients with simple versus patients with complicated silicosis except in the score of large opacity (P < .001).


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  		TABLE 2. Results of Pulmonary Function Tests and CT Scores

 
Correlation coefficients for the relation between chest radiographic and CT scores are shown in Table 3. Positive correlations were noted between radiographic and CT scores for small and large opacities ({rho} = 0.620, P < .001 and {rho} = 0.352, P = .045, respectively). The CT air trapping score was significantly higher than the CT emphysema score (P = .001) (Fig 4) and correlated positively with the relative lung areas with attenuation value of less than –950 HU (RA950) ({rho} = 0.523, P = .004) and with the emphysema score ({rho} = 0.575, P < .001). The CT nodular profusion score correlated positively with the large opacity CT score ({rho} = 0.377, P = .028) and negatively with the visual and quantitative emphysema scores ({rho} = –0.593, P < .001 and {rho} = –0.448, P = .015, respectively).


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  		TABLE 3. Spearman Rank Correlation between Chest Radiographic and CT Scores

 


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  		Figure 4. Scatterplot of the CT scores for air trapping and emphysema in each patient with silicosis. The air trapping score was equal or higher than that of emphysema in most patients, and the difference was significant (Wilcoxon signed rank test, P = .001).

 
Spearman rank correlation coefficients for CT scores and pulmonary function test results are shown in Table 4. The air trapping score and the visual and quantitative emphysema scores showed significant negative correlation with spirometric values (eg, FEV1/FVC: P < .001, P = .001, and P = .003, respectively). The reticular opacities score showed a weak positive correlation with FEF75% ({rho} = 0.367, P = .033), FEF50% ({rho} = 0.381, P = .026), and FEV1 ({rho} = 0.369, P = .032) but not with FEV1/FVC ({rho} = 0.323, P = .062). The scores for silicotic nodules, bronchiectasis, and large opacity did not correlate with spirometric values. The air trapping score consistently showed the strongest correlation with all the spirometric measures of airflow limitation (Fig 5). For example, relations between CT scores for air trapping, emphysema, and RA950 with FEV1/FVC are shown in Figures 6 –8.


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  		TABLE 4. Spearman Rank Correlation between the Extent of Individual CT Patterns and Spirometric Values

 


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  		Figure 5a. Transverse thin-section CT scans in a 65-year-old man with complicated silicosis. (a) Inspiratory scan at upper lung zone shows large opacities (arrows) against background of sparse small opacities. There is no substantial amount of emphysema on this image. However, FEV1/FVC, FEF25%, and FEF50% were 58.4%, 16.4%, and 24.3%, respectively, and the values were significantly reduced relative to those of control subjects. (b) Expiratory scan at same level as a shows a loss of the normal increase in lung attenuation in both lungs, especially around large opacities, which indicates air trapping.

 


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  		Figure 5b. Transverse thin-section CT scans in a 65-year-old man with complicated silicosis. (a) Inspiratory scan at upper lung zone shows large opacities (arrows) against background of sparse small opacities. There is no substantial amount of emphysema on this image. However, FEV1/FVC, FEF25%, and FEF50% were 58.4%, 16.4%, and 24.3%, respectively, and the values were significantly reduced relative to those of control subjects. (b) Expiratory scan at same level as a shows a loss of the normal increase in lung attenuation in both lungs, especially around large opacities, which indicates air trapping.

 


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  		Figure 6. Scatterplot shows significant negative correlation between air trapping score and FEV1/FVC ({rho} = –0.632, P < .001).

 


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  		Figure 7. Scatterplot shows significant negative correlation between emphysema score and FEV1/FVC ({rho} = –0.547, P = .001).

 


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  		Figure 8. Scatterplot shows significant negative correlation between RA950 and FEV1/FVC ({rho} = –0.526, P = .003).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
We evaluated 34 patients with silicosis by using paired inspiratory and expiratory thin-section CT and spirometry and correlated the CT findings with spirometric values to find the CT index of airflow limitation. All of our patients had radiographic abnormalities consistent with silicosis; however, the disease was mild to moderate or at an early stage in most cases. Even the patients with large opacities showed mostly category A opacities, and comparison of the CT scores with spirometric values revealed no significant differences between simple and complicated silicosis. The lack of a difference in the results of pulmonary function tests between simple and complicated silicosis was attributable, at least in part, to the fact that large opacities were category A and the functional derangement had not yet advanced. The pulmonary function test results confirmed the diagnosis of relatively mild or early-stage disease. The average FVC was 80.8%, which is in the normal range. The FEV1/FVC, FEF50%, and FEF25% were significantly reduced, but FEF75% did not differ significantly between the patients and the control subjects.

We found that the air trapping and emphysema scores were the only consistent and significant indicators of airflow limitation in our patients. The air trapping score was higher than the visual emphysema score and showed consistently stronger correlation with spirometric measures of obstruction in comparison with both the visual and quantitative emphysema scores. Pathologically, airflow limitation in dust-exposed workers is attributed to hyperresponsive airways, mineral dust airway disease, and emphysema (21). Usually the extent of emphysema can be evaluated on a thin-section CT scan obtained with deep inspiration. However, small airway disease cannot be evaluated with ordinary imaging methods. The expiratory thin-section CT scan can depict areas distal to obstructed airways and is considered sensitive in the detection of small airway obstruction (10). Because areas of emphysema usually remain lucent on expiratory CT scans, and these areas were included in our definition of air trapping, it is reasonable that in our study the air trapping areas tended to be broader than the areas of emphysema alone. The air trapping areas not related to emphysema can be considered areas with airway disease, and they contributed to the increased amount of air trapping in comparison to emphysema in our series.

In a series of surgically resected lung specimens from 89 patients, functional airflow limitation correlated more strongly with areas of air trapping on expiratory CT scans than with pathologically determined areas of emphysema or RA950 (22). Although the air trapping score showed stronger correlation with spirometric values in comparison to the emphysema score, the differences were not as large as might be expected if mineral dust airway disease is the main cause of airflow limitation in silicosis. We suspect that this was partly because we used spirometry to evaluate airflow limitation. Spirometry is not sensitive to abnormalities in peripheral airways and does not reflect all abnormalities in the airways (23), whereas air trapping on CT images results from airway obstruction at any segment.

The functional impairments of patients with silicosis reportedly depend on several factors, including the presence of a large opacity (24,25), the degree of emphysema (5,25–27), and the duration or total amount of dust exposure (1,27,28). For example, in an autopsy series of 706 South African gold miners with insignificant or mild degrees of silicosis, emphysema was the main determinant of airflow impairment (7). In another study that included 76 patients with advanced silicosis (58 of whom had large opacity), CT grades of a large opacity and emphysema were the independent determinants of airflow limitation (25). The presence of a large opacity causes substantial scarring of lung parenchyma that results in the development of cystic spaces, which further reduce lung function. In that study (25), the extent of large opacity was scored at CT by using an 18-point scale, and it ranged from 1 to 18 (mean, 6.5). On the other hand, we scored the extent of the large opacity by using a five-point scale and found the mean CT score of only 1.2 ± 0.5 (standard deviation). The contrast between the findings of these previous studies and ours clearly results from the differences in methodology and patient selection.

Airway disease has also been suggested as an important factor in the functional impairment associated with silicosis, especially in mild cases (3,29). In the examination of pathologic specimens from workers exposed to a variety of nonasbestos mineral dusts, Churg and Wright (3) reported fibrosis and pigmentation in the walls of respiratory bronchioles, which they believe served as specific markers of mineral dust exposure in their subjects. Patients with these pathologic findings showed lower FEV1, FEF50%, and nitrogen washout values in comparison to age and smoking-matched control subjects, and the authors regarded the lesions as a more important contributor than emphysema to airflow limitation in dust-exposed subjects (30).

In our series, the visual extent of emphysema was greater in cases of simple silicosis than cases of complicated silicosis, in part because subpleural bullous areas, which are often seen in complicated silicosis, were excluded from visual emphysema scoring. The fact that RA950, which included such subpleural bullae as emphysematous area, did not differ between the two groups supports this explanation.

We found no correlation between the profusion of nodules and any functional parameter. Pathologically, silicotic nodules are observed around respiratory bronchioles, pulmonary arterioles, and paraseptal and subpleural interstitium (31). Such nodules often obliterate the bronchioles and pulmonary arteries and can cause airway obstruction (31). However, the correlation between radiographic silicosis and measures of functional obstruction is controversial (8,9,24,32), and the effect of direct bronchiolar obliteration by a silicotic nodule remains to be determined. We suspect that obliteration of bronchioles by the silicotic nodules does not significantly contribute to functional impairment as determined with spirometry, unless the bronchiolar obliteration is extensive, because spirometry is relatively insensitive in the depiction of the peripheral airway obstruction (34).

Although radiographic and CT scores for silicotic nodules did not correlate with the air trapping CT score, the silicosis CT score correlated inversely with the visual ({rho} = –0.593, P < .001) and quantitative ({rho} = –0.448, P = .015) emphysema scores and correlated positively with the large opacity CT score ({rho} = 0.377, P = .028). The inverse correlation between CT findings of silicotic nodules and emphysema was also reported in the autopsy study of South African gold miners (7). However, authors of another study showed a positive correlation between thin-section CT scores of emphysema and silicosis (24). The relation of emphysema and silicosis varies because silica exposure alone does not cause significant emphysema (33), whereas the extent of silicosis and large opacity depends on the total amount of dust exposure (34,35).

Our study had several limitations. First, we did not measure the diffusion capacity in our patients and thus could not correlate this variable with CT findings. Diffusion capacity decreases in emphysema but does not decrease in airway disease. Diffusion capacity data could be expected to enhance our interpretation of the functional derangement depicted on CT images. Second, because spirometry is not sensitive in depicting peripheral airway disease, spirometric measures may cause underestimation of the extent of bronchiolar abnormalities (23). Because air trapping is considered to be sensitive for airway abnormalities, the correlation of CT measures and pulmonary functional parameters might improve if we perform specific pulmonary function tests such as the nitrogen washout test (21). Third, we obtained expiratory scans at only three preselected levels, which might not be enough to estimate the extent of air trapping in the entire lung. Fourth, a study group of 34 patients may not be sufficient to support extensive conclusions. Fifth, we excluded subpleural bullous areas from visual assessment of emphysema, which might have resulted in underestimation of the functional derangement in patients, especially when the bullae were prominent. However, quantitative assessment of emphysema with use of built-in CT software also yielded similar results in our series, and we suppose that subpleural bullous areas did not have significant effect on the results drawn from our study.

In conclusion, we examined paired inspiratory and expiratory thin-section CT scans obtained from patients with silicosis whose functional and radiographic abnormalities were mild to moderate. Air trapping was identified in most of the patients and to a consistently greater extent than emphysema. In comparison with the spirometric value, the extent of air trapping proved the best CT index in the assessment of obstructive derangement in dust-exposed workers.


   ACKNOWLEDGMENTS
 
The authors thank Kazunori Toki, RT, at Labor Welfare Hospital for Silicosis for his technical assistance. Mr Toki suffered a cerebral infarction just after this project was completed.


   FOOTNOTES
 

Abbreviations: FEF = forced expiratory flow • FEF50% = FEF at 50% of FVC • FEF75% = FEF at 75% of FVC • FEF25% = FEF at 25% of FVC • FEV1 = forced expiratory volume in 1 second • FVC = forced vital capacity • RA950 = relative lung areas with attenuation value of less than –950 HU

Authors stated no financial relationship to disclose.

Author contributions: Guarantor of integrity of entire study, H.A.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, H.A., P.A.G., H.M., M.F.; clinical studies, H.A., P.A.G., Y.S., H.S.; statistical analysis, H.A., V.D.M.; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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