EXPIRATORY HIGH-RESOLUTION CT SCAN

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High-resolution CT (HRCT) is invaluable in the diagnosis and management of patients with diffuse lung disease, providing anatomic detail comparable with gross pathology. Although HRCT has proved most useful in the diagnosis of diffuse infiltrative lung diseases, its application to the diagnosis of airway and obstructive lung diseases has recently been emphasized.68, 102

Expiratory HRCT techniques as an adjunct to inspiratory are particularly useful in assessing patients with obstructive lung diseases, providing physiologic information in regard to regional lung function. In patients with obstructive lung diseases, morphologic abnormalities visible on inspiratory scans can be subtle or nonspecific in some patients. CT or HRCT obtained during forced exhalation (dynamic expiratory CT),93, 103 during suspended respiration after forced exhalation (postexpiratory CT), or at a user-selected respiratory level controlled during exhalation using a spirometer (spirometrically triggered expiratory CT)41, 42, 49 have all been shown to be useful in the diagnosis of diseases characterized by airflow limitation or air-trapping. Focal, multifocal, or diffuse air-trapping visible using these expiratory or postexpiratory CT techniques as areas of abnormally low attenuation can confirm the presence of obstructive physiology in patients with airway abnormalities visible on inspiratory scans, allow the diagnosis of obstructive abnormalities in some patients with normal inspiratory scans, and help in distinguishing between obstructive disease and infiltrative disease as a cause of inhomogeneous lung opacity seen on inspiratory scans. Air-trapping on expiratory CT has been recognized in patients with various obstructive or airway diseases, such as emphysema,44, 46, 93 asthma,74 constrictive bronchiolitis (CB),6, 26, 60, 64, 76, 93, 97 and bronchiectasis.36, 93

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Dynamic Expiratory HRCT

Scans obtained dynamically during forced expiration can be obtained using an electron-beam scanner or a helical scanner. Dynamic scanning with an electron-beam scanner has been termed dynamic ultrafast high-resolution CT (DUHRCT).92, 93

DUHRCT is performed using a scanner capable of obtaining a series of images with a 100-millisecond scan time (500-millisecond interscan delay, 3-mm collimation, 150 kVp, 650 mA).53, 92, 93, 94 In general, when using this technique, a series of 10 scans is

Lung Attenuation

In normals, lung attenuation at full inspiration appears relatively homogeneous, ranging from −700 to −900 H.2, 100 An attenuation gradient is typically present, however, with the most dependent lung regions being the densest, and the most nondependent lung regions being the least dense. This gradient is largely due to regional differences in blood and gas volume that, in turn, are determined by gravity, mechanical stresses on the lung, and intrapleural pressures.70, 100 Differences in

Lung Attenuation

Areas of air-trapping are seen as relatively low in attenuation on expiratory scans. Areas of air-trapping can be patchy and nonanatomic; can correspond to individual secondary pulmonary lobules, segments, lobes (see Fig. 2); or may involve an entire lung.80, 92 Pulmonary vessels within the low-attenuation areas of air-trapping often appear small relative to vessels in the more opaque normal lung (see Fig. 1).92

Although abnormal areas can often be identified subjectively because of their

EXPIRATORY CT FINDINGS IN SPECIFIC LUNG DISEASES

Expiratory CT has been shown to be valuable in the diagnosis of a variety of obstructive diseases. It can allow a diagnosis of air-trapping in some patients with normal inspiratory scans; this is most often the result of CB or asthma. In patients with evidence of airway abnormalities on inspiratory CT, expiratory CT can reveal the presence of obstructive physiology, and the extent of such abnormalities correlates with the degree of ventilatory impairment.

EXPIRATORY CT IN THE DIAGNOSIS OF INHOMOGENEOUS LUNG OPACITY

Inhomogeneous lung opacity visible on inspiratory HRCT may represent (1) ground-glass opacity, (2) mosaic perfusion resulting from air-trapping, (3) mosaic perfusion resulting from vascular obstruction, or (4) a combination of these. In the presence of mosaic perfusion, vessel caliber is often reduced in areas of low attenuation, whereas this is not the case in the presence of ground-glass opacity.

Expiratory CT can also help to discriminate these causes of inhomogeneous lung opacity seen on

References (106)

  • N.L. Müller et al.

    “Density mask”: An objective method to quantitate emphysema using computed tomography

    Chest

    (1988)
  • J.L. Myers et al.

    Pathologic manifestations of bronchiolitis, constrictive bronchiolitis, cryptogenic organizing pneumonia and diffuse panbronchiolitis

    Clin Chest Med

    (1993)
  • K.B. Newman et al.

    Quantitative computed tomography detects air trapping due to asthma

    Chest

    (1994)
  • S.P.G. Padley et al.

    Bronchiolitis obliterans: High-resolution CT findings and correlation with pulmonary function tests

    Clin Radiol

    (1993)
  • G. Santis et al.

    High resolution computed tomography in adult cystic fibrosis patients with mild lung disease

    Clin Radiol

    (1991)
  • M.C. Sweatman et al.

    Computed tomography in adult obliterative bronchiolitis

    Clin Radiol

    (1990)
  • J.F. Tomashefski et al.

    Pulmonary air cysts in cystic fibrosis: Relation of pathologic features to radiologic findings and history of pneumothorax

    Hum Pathol

    (1985)
  • D.R. Aberle et al.

    Lymphangiomyomatosis: CT, chest radiographic, and functional correlations

    Radiology

    (1990)
  • B.D. Adler et al.

    Chronic hypersensitivity pneumonitis: High-resolution CT and radiographic features in 16 patients

    Radiology

    (1992)
  • M. Akira et al.

    Summer-type hypersensitivity pneumonitis: Comparison of high-resolution CT and plain radiographic findings

    AJR Am J Roentgenol

    (1992)
  • American Thoracic Society

    Standards for the diagnosis and care of patients with chronic obstructive pulmonary disease (COPD) and asthma

    Am Rev Respir Dis

    (1987)
  • S.L. Aquino et al.

    Bronchiolitis obliterans associated with rheumatoid arthritis: Findings on HRCT and dynamic expiratory CT

    J Comput Assist Tomogr

    (1994)
  • Arakawa H, Webb WR, McCowin M, et al: Inhomogeneous lung opacity on thin collimation CT: Diagnostic value of expiratory...
  • M. Bhalla et al.

    Cystic fibrosis: Scoring system with thin-section CT

    Radiology

    (1991)
  • M.W. Brauner et al.

    Pulmonary sarcoidosis: Evaluation with high-resolution CT

    Radiology

    (1989)
  • M.W. Brauner et al.

    Pulmonary histiocytosis X: Evaluation with high resolution CT

    Radiology

    (1989)
  • C.D. Burger et al.

    Fast-CT evaluation of the effect of lung volume on upper airway size and function in normal men

    Am Rev Respir Dis

    (1992)
  • D.L. Buschman et al.

    Chronic hypersensitivity pneumonitis: Use of CT in diagnosis

    AJR Am J Roentgenol

    (1992)
  • C.B. Carrington et al.

    Lymphangioleiomyomatosis: Physiologic-pathologic-radiologic correlations

    Am Rev Respir Dis

    (1977)
  • C. Chryssanthopoulos et al.

    Hypersensitivity pneumonitis

    J Asthma

    (1983)
  • A. Coleman et al.

    Histologic diagnosis of extrinsic allergic alveolitis

    Am J Surg Pathol

    (1988)
  • B. Corrin et al.

    Pulmonary lymphangiomyomatosis: A review

    Am J Pathol

    (1975)
  • R.S. Crausman et al.

    Lymphangioleiomyomatosis: The pathophysiology of diminished exercise capacity

    Am J Respir Crit Care Med

    (1996)
  • D.M. Denison et al.

    Estimation of regional gas and tissue volumes of the lung in supine man using computed tomography

    Thorax

    (1986)
  • C.D. Eber et al.

    Bronchiolitis obliterans on high-resolution CT: A pattern of mosaic oligemia

    J Comput Assist Tomogr

    (1993)
  • E.E. Frey et al.

    Chronic airway obstruction in children: Evaluation with cine-CT

    AJR Am J Roentgenol

    (1987)
  • P.J. Friedman et al.

    Eosinophilic granuloma of lung: Clinical aspects of primary pulmonary histiocytosis in the adult

    Medicine

    (1981)
  • K. Garg et al.

    Proliferative and constrictive bronchiolitis: Classification and radiologic features

    AJR Am J Roentgenol

    (1994)
  • A.F. Gelb et al.

    Limited contribution of emphysema in advanced chronic obstructive pulmonary disease

    Am Rev Respir Dis

    (1993)
  • G.P. Genereux

    Computed tomography and the lung: Review of anatomic and densitometric features with their clinical application

    J Can Assoc Radiol

    (1985)
  • P.A. Gevenois et al.

    Pulmonary emphysema: Quantitative CT during expiration

    Radiology

    (1996)
  • F.V. Gleeson et al.

    Evidence of expiratory CT scans of small-airway obstruction in sarcoidosis

    AJR Am J Roentgenol

    (1996)
  • R. Gold et al.

    Adenoviral pneumonia and its complications in infancy and childhood

    J Can Assoc Radiol

    (1969)
  • P. Grenier et al.

    Bronchiectasis: Assessment by thin-section CT

    Radiology

    (1986)
  • P. Grenier et al.

    Abnormalities of the airways and lung parenchyma in asthmatics: CT observations in 50 patients and inter- and intraobserver variability

    Eur Radiol

    (1996)
  • D.M. Hansell et al.

    Hypersensitivity pneumonitis: Correlation of individual CT patterns with functional abnormalities

    Radiology

    (1996)
  • D.M. Hansell et al.

    Bronchiectasis: Functional significance of areas of decreased attenuation at expiratory CT

    Radiology

    (1994)
  • E.J. Hapke et al.

    Farmer's lung: A clinical, radiographic, functional and serologic correlation of acute and chronic stages

    Thorax

    (1968)
  • J. Ikezoe et al.

    Bronchopulmonary sequestration: CT assessment

    Radiology

    (1990)
  • K. Iwai et al.

    Intralobar pulmonary sequestration, with special reference to developmental pathology

    Am Rev Respir Dis

    (1973)
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    Department of Radiology, University of California–San Francisco, San Francisco, California

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