Expiratory CT scan in patients with normal inspiratory CT scan: a finding of obliterative bronchiolitis and other causes of bronchiolar obstruction
© Gaeta et al.; licensee BioMed Central Ltd. 2013
Received: 24 March 2013
Accepted: 31 May 2013
Published: 9 July 2013
Expiratory CT scan is usually obtained as supplement to normal inspiratory CT scan to recognize air-trapping, which is expression of small airways obstruction. In some patients the air-trapping may be the only sign of an early-stage small airways disease in an otherwise normal lung.
The purpose of this article is to illustrate pathologic conditions, namely obliterative bronchiolitis, in which expiratory CT scan can be abnormal despite normal inspiratory CT examination, and to highlight indications for this technique in patients with clinical and functional suspect of bronchiolar obstruction.
KeywordsAir trapping CT Expiratory CT scan Inspiratory CT scan Airways disease
Expiratory CT scan is sensitive for the detection of air-trapping, which is a definitive sign of airway obstruction in various airway disease, including emphysema, bronchiolitis obliterans, bronchial asthma, Swyer-James syndrome, cystic fibrosis, sarcoidosis, hypersensitivity pneumonitis [1, 2]. In many of such patients abnormal findings (i.e. areas of emphysema, bronchiectasis, ground-glass opacity, tree-in-bud) are usually depicted by inspiratory scan that permits a correct diagnosis. However, frequently, the air-trapping may be the only finding of a pulmonary disease in patients with a normal-appearing inspiratory CT scan . According to Fleischner Society glossary , “air-trapping is seen on end-expiration CT scans as parenchymal lung areas with less than normal increase in attenuation and lack of volume reduction”.
Although some authors recommend routine use of paired inspiratory and expiratory CT scans in patients suspected of having diffuse lung disease, this approach is questionable, especially considering the delivered radiation. This is of special concern in young patients or in subjects undergoing repeated exposures .
The purpose of this article, which is based on more than 100 consecutive patients who underwent expiratory CT scan after a normal inspiratory CT examination, is to illustrate diseases which may demonstrate abnormalities on expiratory CT scan despite normal inspiratory CT scan, as obliterative bronchiolitis and less usual causes of bronchiolar obstruction. Furthermore, we have highlighted the indications for expiratory CT scan in patients with clinical and functional suspect of bronchiolar obstruction.
CT scan techniques
Inspiratory and expiratory CT scans are typically obtained at the end of full inspiration and at the end of forced expiration. Expiratory CT scan can be performed with a volumetric or an incremental technique (a limited number of slices at different levels with a section thickness of 1-mm and a table increment of 10-mm). Moreover, it is possible to modulate the radiation dose burden using a low-dose acquisition by reducing the tube current. One study demonstrated that it is possible to reduce the tube current-time product up to 20 mAs without impairing the visualization of air-trapping .
Before expiratory scan, patients are usually instructed: “Take a deep breath, blow out hard, and do not breathe in again for 10 seconds.” It is useful that each patient practices this breathing instructions several times before scanning begin.
Both inspiratory and expiratory scans are performed with the patient in the supine position from the apex to the base of the lungs. No contrast medium administration is necessary.
Inspiratory and expiratory CT images are reconstructed by using a high-spatial-resolution (bone) algorithm at a display window width of 1,600 Hounsfield Units (HU) and a window center of −600 HU.
In recent years, several quantitative analyses for air trapping evaluation are used [6–10]. The most widely explored quantitative CT methods are density-based measures: a) expiratory to inspiratory ratio of mean lung density; b) expiratory to inspiratory relative volume change of voxels with attenuation values between −860 and −950 HU and c) percentage of voxels below −856 HU in expiratory CT scan [6, 7]. In a recent paper, the first of the above mentioned measures performed significantly better than the others in early detection of small airways disease on low-dose CT .
Moreover, it has been demonstrated that lung volume collapsibility, represented by the ratio of expiratory to inspiratory lung CT computed volume, correlates significantly with pulmonary function tests, tissue density-based measures and disease severity in chronic obstructive pulmonary disease [9, 10].
Normal findings on expiratory CT scan
Finally, normal lung tissue increases homogeneously in CT attenuation from inspiration to expiration (Figure 1) because the volume of air in the lung being scanned is reduced.
Air-trapping in healthy subjects
Pulmonary function tests
Interval asthma and chronic bronchitis
Obliterative bronchiolitis is defined histologically as concentric luminal narrowing of the membranous and respiratory bronchioles secondary to submucosal and peribronchiolar inflammation and fibrosis without any intraluminal granulation tissue or polyps. Obliterative bronchiolitis can be cryptogenic, postinfectious (mostly, secondary to prior viral or Mycoplasma infection), or secondary to noxious fume inhalation, graft-versus-host disease, lung transplantation, rheumatoid arthritis, inflammatory bowel disease, and penicillamine therapy [25, 26].
Hypersensitivity pneumonitis is a diffuse granulomatous interstitial lung disease caused by inhalation of various antigenic organic particles. Hypersensitivity pneumonitis is often insidious to diagnose because the clinical manifestations are nonspecific and the radiological and histological patterns can mimic those of other interstitial and small airway diseases. Early diagnosis is mandatory since patients may develop UIP/NSIP lung fibrosis patterns .
Sarcoidosis is a multisystem disorder that is characterized by non-caseous epithelioid cell granulomas, which may affect almost any organ. Pulmonary sarcoidosis is a disease of the interstitium and occurs in approximately 90% of patients. Usually advanced pulmonary sarcoidosis causes a restrictive functional deficit due to fibrosis. On the other hand, the granulomas developing in centrilobular and peribronchiolar lymphatics frequently involve small airways; thus, evidence of air-trapping is considered a common feature of the disease .
Every pneumologist and radiologist should be aware that the air-trapping may be the only finding of a pulmonary disease in patients with a normal-appearing inspiratory CT scan. The knowledge of the possible underlying disorders is the key which permits to suspect the potential diagnoses. Final diagnosis can be reached by means of one or more of these approaches: transbronchial biopsy, open lung biopsy, bronchoscopy, bronchioloalveolar lavage, laboratory tests, response to therapy on follow-up.
patients with respiratory tests showing obstructive pattern, particularly patients showing a small airways obstruction pattern;
patients with chronic cough and/or wheezing;
patients with exertional dyspnea;
patients with demonstrated or suspected conditions associated with small airways diseases, namely sarcoidosis, hypersensitivity pneumonitis and diseases that may cause bronchiolitis obliterans.
Finally, it is worth of attention that MR imaging of the lung, whose main advantage is absence of radiation, is an emerging tool in diagnosis of pulmonary diseases; namely, in evaluating disease activity in chronic lung diseases , in evaluating mucus-containing lung lesions  and in diagnosing invasive mucinous adenocarcinoma (formerly known as mucinous bronchioloalveolar carcinoma) [33–35]. In patients with small airway obstruction, MR imaging with hyperpolarized Helium is an interesting diagnostic option which allows a functional and dynamic evaluation of pulmonary ventilation [36, 37]; however, today it is not widely disposable for clinical use since it is expensive and difficult to perform. In the future a combined use of CT and MR imaging could enhance our capacity to detect more specific patterns of obstructive pulmonary diseases.
Written informed consent was obtained from the patients for publication of this report and any accompanying images.
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