Chest
Volume 106, Issue 1, July 1994, Pages 105-109
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Clinical Investigations
Asthma COPD
Quantitative Computed Tomography Detects Air Trapping due to Asthma

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Objective: The purpose of this study was to prospectively see if quantitative computed tomography (QCT) could separate asthmatic patients from normal control subjects. The QCT results were also correlated with the pulmonary function tests (PFT) that were done on both the asthmatic patients and control subjects.

Subjects and methods: Eighteen adult nonsmoking asthmatics and 22 adult control subjects were entered into the study. Quantitative CT was performed at the level of the transverse aorta and just above the diaphragm at both end inspiration and end expiration in all patients and control subjects: 10-mm and 1.5-mm collimation using a high spatial frequency algorithm was used to obtain the QCT examinations. The percent of pixels below —900 Hounsfeld units, pixel index, in each of the QCT axial images of the lungs was calculated for each asthmatic and control subject in the study. Pulmonary function testing was performed on both the asthmatics and control subjects and included determination of FEV1, FVC, FRC, RV, and TLC. Unpaired Student's t test analysis of the QCT data was done to statistically compare the asthmatics with the control subjects. Linear regression analysis was done to compare the QCT results with PFT data on the asthmatics and control subjects.

Results: When scans were performed at end expiration, at a level immediately superior to the diaphragm, the mean pixel index was significantly higher in asthmatic subjects compared with normal individuals on both CT (mean for normal subjects 0.16 vs 4.45 for asthmatics, p<0.004) and high-resolution CT (HRCT) images (mean for normal subjects 1.04 vs 10.03 in asthmatics, p<0.0001) indicating more areas of low attenuation in asthmatics. The CT and HRCT images from the lower lung zones that were performed at end expiration provided the best separation between the groups. The pixel index on expiration correlated with the degree of air trapping and airflow limitation in the asthmatic group based on FEV1, FRC, RV, and to a lesser extent, FVC.

Conclusion: Expiratory QCT is a useful method to assess air trapping in asthmatic patients. The percent of abnormal lung in asthmatics as determined by QCT has a significant correlation with the PFTs that reflect air trapping in asthmatic patients. Quantitative CT may be helpful in assessing degrees of air trapping present in other diseases affecting the airways.

Section snippets

Study Population

We studied 18 adult asthmatic patients of whom 8 were men and 10 were women, none of whom ever smoked. Their ages ranged from 24 to 76 years (average age, 53.5 years). We defined asthma as (1) the presence of intermittent symptoms of wheezing, coughing, or chest tightness, plus (2) significant bronchial hyperresponsiveness. Bronchial hyperresponsiveness was considered significant if the patient's forced expiratory volume in 1 s (FEV1) rose at least 20 percent following the administration of a

Results

The results of the pulmonary function tests and the pixel indices for both groups are summarized in Table 1. As expected, asthmatic subjects had significantly more airflow limitation and hyperinflation compared with normal subjects. We observed a significant elevation in the mean pixel indices for asthmatic subjects on HRCT with both superior and inferior level expiratory scans, and on the conventional CT expiratory inferior level scan. Figure 1 demonstrates the pixel indices for the normal and

Discussion

The results of the current investigation indicate that expiratory CT scans through the lower lung zone can distinguish asthmatics with air trapping from normal subjects. Furthermore, the CT findings using the density mask techniques correlate well with the pulmonary physiologic abnormalities found in asthmatics.19,20 Both HRCT (1.5 mm) and conventional CT (10-mm collimation) scans proved useful in detection of these abnormalities in asthmatics.

To our knowledge, this is the first study to

Conclusion

In conclusion, by quantitating areas of low attenuation on CT and HRCT scans, it is possible to objectively differentiate asthmatics with air trapping from normal control subjects. The discrimination is optimized by obtaining a single expiratory HRCT scan above the level of the diaphragm. The CT findings are indicative of air trapping and airflow limitation. Future research should examine the value of this tool in assessing air trapping in other lung diseases and longitudinally in asthma.

ACKNOWLEDGMENTS

The writers thank Rick Orton, R.N., for his assistance in enrolling and testing volunteers, and Kristen Moore for secretarial assistance.

REFERENCES (26)

  • BerginC et al.

    The diagnosis of emphysema: a computed tomographic-pathologic correlation

    Am Rev Respir Dis

    (1986)
  • SandersC et al.

    Detection of emphysema with computed tomography: correlation with pulmonary function tests and chest radiography

    Invest Radiol Radioladiol

    (1988)
  • WatanabeK et al.

    Bullous lesions detected by computed tomography

    Radiat Med

    (1986)
  • Cited by (0)

    Supported in part by Physician Scientist Award ES-00173 from the US Public Health Service, grant ES-04843, and SCOR grant HL-27353.

    Manuscript revision accepted October 28.

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