HIGH-RESOLUTION COMPUTED TOMOGRAPHY IN THE EVALUATION OF FIBROSING ALVEOLITIS
Section snippets
TECHNICAL CONSIDERATIONS
The key determinants that contribute to the fine detail available on HRCT images are narrow beam collimation (section thickness) and a high spatial reconstruction algorithm.51 An understanding of these factors is useful in interpreting CT scans of patients with interstitial lung disease. A CT section, although apparently two-dimensional, has a third dimension of depth. The depth, or section thickness, is determined by the width of the slit through which the x-ray beam passes (beam collimation).
PATHOLOGIC ASPECTS OF FIBROSING ALVEOLITIS
Understanding of fibrosing lung disease has been hindered by confusing pathologic terminology and imprecision in clinical diagnosis. Idiopathic pulmonary fibrosis and cryptogenic fibrosing alveolitis can be regarded as synonymous and these umbrella terms encompass the many subtypes of the so-called interstitial pneumonias, which are in a constant state of reclassification. UIP is the most common subtype. Specific histopathologic criteria need to be satisfied for the term UIP to be appropriately
HIGH-RESOLUTION COMPUTED TOMOGRAPHY FEATURES OF FIBROSING ALVEOLITIS AND INTERPRETIVE CONSIDERATIONS
The typical radiographic pattern of established fibrosing alveolitis consists of reticulonodular shadowing concentrated at the lung bases, with the reticular component becoming more pronounced as the disease progresses. The most distinctive radiographic feature of fibrosing alveolitis is its predilection for the subpleural and lower zones of the lungs. In a minority of patients, the predominant pattern at presentation is ground-glass opacification, which may reflect a more cellular (DIP end of
ROLE OF HIGH-RESOLUTION COMPUTED TOMOGRAPHY IN THE DIAGNOSIS OF FIBROSING ALVEOLITIS
In terms of sensitivity and specificity, HRCT represents a significant advance over chest radiography for the diagnosis of diffuse interstitial lung disease. 24, 49, 58, 85, 97 In the specific context of fibrosing alveolitis, however, the diagnostic advantages of HRCT over chest radiography may not be as great as some of these studies suggest. The overall diagnostic sensitivity of CT for the diagnosis of fibrosing alveolitis can be extracted from five studies7, 24, 49, 64, 85 that comprise a
EVALUATION OF DISEASE REVERSIBILITY AND PROGNOSIS WITH HIGH-RESOLUTION COMPUTED TOMOGRAPHY
As a result of the early pathologic–HRCT correlative studies, the idea that the macroscopic patterns seen on HRCT might reflect the histopathologic morphology emerged. In general terms, a reticular pattern on HRCT almost invariably reflects significant disease (in the case of fibrosing alveolitis, established fibrosis) whereas ground-glass opacification, with some important caveats, usually represents reversible disease. 28, 42, 58, 92, 99 Historically, histopathologic examination of lung
FUNCTIONAL AND PATHOPHYSIOLOGIC INSIGHTS DERIVED FROM HIGH-RESOLUTION COMPUTED TOMOGRAPHY
Many recent investigations have relied on the ability of HRCT to map accurately the extent of patterns of fibrosing lung disease. The most common technique for quantitation of disease extent is by subjective visual estimation expressed either as the percentage of lung involvement or in terms of a categorical scoring system (at either a lobar or zonal level). 21, 23, 98 Although observer variation and other sources of error may be a problem, such a subjective approach is quick and simple.
CONCLUSION
The introduction of HRCT has had an impact on the diagnosis and evaluation of disease reversibility in fibrosing alveolitis. The added confidence that HRCT brings to a clinical diagnosis of many diffuse lung diseases, including fibrosing alveolitis, has resulted in fewer patients being submitted for confirmatory lung biopsy in many centers. Nevertheless, HRCT should not necessarily be regarded as the final diagnostic arbiter and an understanding of the limitations of HRCT should result in its
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Address reprint requests to: David M. Hansell, MD, FRCP, FRCR, Department of Radiology, Royal Brompton Hospital, Sydney Street, London SW 6NP, England, e-mail: [email protected]
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Department of Radiology, Royal Brompton Hospital, London, England