EXPIRATORY HIGH-RESOLUTION CT SCAN
Section snippets
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
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Address reprint requests to W. Richard Webb, MD, Department of Radiology, University of California–San Francisco, 505 Parnassus Avenue, M–396, San Francisco, CA 94143–0628
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Department of Radiology, University of California–San Francisco, San Francisco, California