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Thoracic Imaging |
1 From the Department of Radiology, Université Pierre-et-Marie-Curie, Hôpital de la Pitié-Salpêtrière, 47 boulevard de l’Hôpital, 75651 Paris cedex 13, France (O.L., P.A.G., M.C., I.M.G., P.C.); Institut National de la Santé et de la Recherche Médicale, Paris, France (P.A.G.); and Cyclotron Biomedical de Caen, Caen, France (K.B.). From the 1997 RSNA scientific assembly. Received March 29, 1999; revision requested May 10; final revision received January 7, 2000; accepted January 12. Address correspondence to O.L. (e-mail: olivier.lucidarme@psl.ap_hop_paris.fr).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Forty-nine patients with an airway disease were examined with suspended-end-expiration CT after a 6–8-second expiratory maneuver, which was followed with continuous-expiration CT during a 10-second expiratory maneuver. The extent of expiratory air-trapping areas was calculated by two observers by using a semiquantitative grid score. The relative decrease in attenuation in the areas of air trapping was evaluated with a visual continuous-scale score.
RESULTS: Air trapping was noted in 36 and 35 patients with continuous-expiration CT and with suspended-end-inspiration CT, respectively. The extents of and relative attenuation decreases in air-trapping areas in patients with air-trapping areas on at least one expiratory CT scan increased significantly in scans obtained with continuous-expiration CT compared with those obtained with suspended-end-expiration CT, respectively, with mean extent scores of 0.24 ± 0.20 (SD) and 0.18 ± 0.20 (paired t test, P = .001) respectively, and with mean relative contrast decrease scores of 0.35 ± 0.23 and 0.27 ± 0.23 (paired t test, P = .007), respectively.
CONCLUSION: When suspended-end-expiration CT images are ambiguous, complementary continuous-expiration CT can be used to improve the conspicuity and apparent extent of air trapping.
Index terms: Lung, abnormalities, 60.26, 60.751, 60.754, 60.755 • Lung, CT, 60.12111, 60.12115, 60.12118 • Lung, function • Lung, ventilation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Thin-section CT during suspended end expiration is the most widely used technique to visualize expiratory air trapping. This technique has been used to demonstrate air trapping in patients with emphysema (4), asthma (5,6), bronchiectasis (7), hypersensitivity pneumonitis (8,9), sarcoidosis (10), or Langerhans cell histiocytosis (11) and in asymptomatic patients who smoke (12). Furthermore, in a recent study, Arakawa et al (13) showed that the use of suspended-end-expiration scanning significantly improved diagnostic accuracy in patients with inhomogeneous areas of attenuation on inspiratory scans. CT during continuous expiration (1,2), which can be used to collect data at a fixed level during expiration, is the second technique available. Electron-beam CT initially was performed, with a scanning time of 100 msec per image, to assess the dynamic changes in lung attenuation and in architecture during expiration, with minimal motion artifacts. This technique has been used to demonstrate air trapping in patients with constrictive bronchiolitis (2) and in healthy men (1).
More recently, a dynamic expiratory maneuver performed during helical CT was described in a small number of patients, with good results in spite of a longer scanning time per image (14,15). To the best of our knowledge, no comparison between continuous-expiration CT and suspended-end-expiration CT to assess air trapping has been reported in the literature. The purpose of this study was to evaluate the reliability of continuous-expiration CT performed with a helical scanner and to compare it with that of suspended-end-expiration CT for the assessment of air trapping.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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CT Scanning
All CT scans were obtained with a Tomoscan SR 7000 scanner (Philips, Eindhoven, the Netherlands), with a scanning time of 1 second. First, full-inspiration thin-section CT was performed by using 1.5-mm collimation every 10 mm from the lung apex to the diaphragm. Then, during the same session, two expiratory thin-section CT examinations were performed successively. The first included five, six, or seven sections of 1.5-mm collimation obtained every 30 or 40 mm at the end of expiration, from the lung apex to the diaphragm (suspended-end-expiration CT).
The second examination was based on a volumetric acquisition during expiration. It was a standard examination at the Pitié Salpêtrière Hospital between November 1994 and November 1996 for evaluation of the change in the total cross-sectional lung area by using a cine loop. It consisted of a 15-mm-thick lung volume obtained above the bronchus intermedius that was acquired with 1.5-mm collimation and with a pitch of 1 in a caudocranial direction. A 180° linear interpolation reconstruction algorithm was used. Ten sections were obtained during a 10-second period as the patient performed an expiratory maneuver (continuous-expiration CT).
The breath-hold technique at full expiration and during the expiratory maneuver was rehearsed with each patient before the CT examination. For the first examination, the patients were instructed to exhale for 6–8 seconds to reach the residual volume and then to stop breathing. For the second examination, the patients were instructed to exhale completely for 6–8 seconds to reach the residual volume and then to continue their expiratory efforts until the end of the acquisition. The operator controlled these maneuvers visually during the procedure. All lung images were reconstructed and were laser printed with a high-frequency reconstruction algorithm. Window settings appropriate for the assessment of the bronchi and the lung parenchyma (level, -600 HU; width, 1,600 HU) and of the mediastinum (level, 30–50 HU; width, 300–450 HU) were used. No contrast medium was used.
Interpretation of CT Scans
For each patient, the 10 images obtained during the expiratory maneuver and the one image obtained at a comparable level with suspended-end-expiration CT were interpreted independently and in a random order by two radiologists (O.L., I.M.G.) who were not blinded to the type of expiratory scan (the 49 patients in the study were selected from among 59 patients because the two expiratory scans were obtained at the same level). They compared them with the corresponding full-inspiration image. The observers interpreted the images twice. The first reading was performed independently by the two observers to evaluate the presence, extent, and relative contrast of areas of air trapping compared with the surrounding lung parenchyma, whereas the second reading was performed with consensus for motion artifacts and for the presence of air trapping. We defined air trapping as all areas that failed to increase in attenuation after full expiration compared with full inspiration. This definition included areas of decreased attenuation that involved only a few secondary pulmonary lobules, which usually are considered to be physiologic, per lung (1). The spurious appearances of areas of decreased attenuation (rarefied zones in the vicinity of the minor fissure or the relatively low-attenuating apical segments of the lower lobes [1]) that were caused by beam hardening due to the effect of adjacent ribs all were disregarded.
The relative contrast, which represents the attenuation difference between the areas of air trapping and the surrounding lung parenchyma, was assessed semiquantitatively with a visual score by using a continuous 50-mm scale. The zero point represented the absence of areas of decreased attenuation; the 50-mm point corresponded to a theoretic maximum contrast between the area of decreased attenuation, which would produce no attenuation (black on the image), and the surrounding lung parenchyma, which would produce complete attenuation (white on the image). The sequence of 10 images obtained with continuous-expiration CT was accorded an overall score. When different scores coexisted on the same or successive images, only the highest value was retained. A relative contrast score was calculated by measuring the distance between the zero point and the point reported by each observer, which then was divided by the length of the scale. A mean relative contrast score was calculated by averaging the two observers’ scores.
The cross-sectional areas of air trapping were assessed semiquantitatively on the expiratory scans by superimposing a grid of 2 x 2-mm squares that corresponded to 0.3–0.7 cm2 on the CT image (according to the field of view used for image reconstruction) (3,16). The number of squares that contained an area of air trapping was counted, as was the number of squares that overlaid the parenchyma of both lungs. Furthermore, an air-trapping extent score was calculated by determining the ratio of the total number of squares that contained an area of decreased attenuation to the total number of squares that overlaid the lung parenchyma. Only the image in the series of 10 sequential continuous-expiration CT images for which the relative contrast score was considered to be at a maximum was retained to establish the continuous-expiration CT score. A mean air-trapping extent score was calculated by averaging the results from the two observers.
The change in the total cross-sectional lung area between the inspiration section and each corresponding expiration section (degree of expiration) also was calculated independently by each radiologist by using DE = 1 - (Ne/Ni), where DE represents degree of expiration, Ne represents the number of squares that overlaid the lung parenchyma on the expiration image (as above, only the continued-expiration CT image for which the relative contrast score was considered to be at a maximum was counted), and Ni represents the number of squares that overlaid the lung parenchyma during full inspiration. A mean degree of expiration was calculated by averaging the two observers’ results.
Statistical Analyses
Results were expressed as means (± SD) or as percentages. To assess the interobserver variability of the nominal CT findings (the presence or absence of air trapping), the 
coefficient of agreement was computed (17,18). Its significance was interpreted according to its z score value. Furthermore, values were interpreted subjectively, as recommended by Altman (19) ( < 0.20, poor; = 0.21–0.40, fair; = 0.41–0.60, moderate; = 0.61–0.80, good; = 0.81–1, very good).
Interobserver agreements on the quantitative parameters (air-trapping extent score, relative contrast score, degree of expiration) were assessed for the patients considered by both observers to have decreased-attenuation areas to minimize the underestimation that resulted from interobserver disagreement for the presence or absence of air trapping. This was done by using the intraclass correlation coefficient of agreement (ri) (16,20). Values obtained were interpreted as follows: 0.30 as poor; 0.31–0.50, low; 0.51–0.70, moderate; 0.71–0.90, good; and 0.91–1, very good interobserver agreement (20).
A paired t test was performed to compare the intersubject quantitative CT parameters of both CT scans. When the difference between the CT parameters had a very skewed distribution and the intergroup differences in variability were significant, the nonparametric paired Wilcoxon test was performed to compare continuous-expiration and suspended-end-expiration CT scores. The 
2 test with continuity correction or the Fisher exact test was used, as appropriate, to compare percentages. All P values were two-tailed and, when less than .05, were considered to indicate a significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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= 0.81, P < .001) and 85% ( = 0.71, P < .001), respectively. The 10th continuous-expiration CT image always was selected by each observer to quantify the air-trapping score and the degree of expiration. Good interobserver agreements were obtained for the degrees of expiration, with ri values of 0.82 for both techniques. The interobserver agreements were moderate for the extent of air trapping (continuous-expiration CT and suspended-end-expiration CT ri values of 0.61 and 0.69, respectively) and for the assessment of the relative contrast of air-trapping areas (continuous-expiration and suspended-end-expiration CT ri values of 0.64 and 0.62, respectively).
For the 39 (80%) of 49 patients with areas of air trapping on at least one expiratory CT scan at the consensual reading, the air-trapping extent and the relative contrast scores obtained with continuous-expiration CT were significantly higher than those obtained with suspended-end-expiration CT. The mean extent scores were 0.24 ± 0.20 and 0.18 ± 0.20 (paired t test, P = .001), and mean relative contrast scores were 0.35 ± 0.23 and 0.27 ± 0.23 (paired t test, P = .007) for continuous-expiration CT and suspended-end-expiration CT, respectively. The degrees of expiration obtained with continuous-expiration CT did not differ significantly from those obtained with suspended-end-expiration CT (paired t test, P = .07), with respective mean values of 0.25 ± 0.09 and 0.21 ± 0.10.
For the subgroup (25 [64%] of 39) of patients with higher mean air trapping extent scores obtained with continuous-expiration CT, the degree of expiration was increased significantly by, on average, 0.07 ± 0.05 with continuous-expiration CT compared with suspended-end-expiration CT (paired t test, P = .01) (Figure). Likewise, for the 28 (72%) patients with higher mean relative contrast scores obtained with continuous-expiration CT, the degree of expiration was increased significantly by, on average, 0.07 ± 0.06 (paired t test, P = .02) (Figure). For the 14 (36%) patients with lower mean air-trapping extents and for the 11 (28%) patients with lower mean relative contrast scores obtained with continuous-expiration CT, the degrees of expiration did not differ significantly between continuous-expiration CT and suspended-end-expiration CT (paired t test, P = .13).
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Motion-related artifacts were considered to be present on the continuous-expiration and suspended-end-expiration CT scans of 12 (24%) and seven (14%) patients (not significant, P = .31), respectively, but without spoiled scan interpretation.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Motion artifacts, which increase as temporal resolution decreases, represent the major limitation of continued-expiration CT. In our study, minor motion-related degradation of images acquired during active expiration but without spoiled scan interpretation was visible for 12 (24%) of the 49 cases, which was not significantly different from that acquired during suspended-end-expiration CT. This result could be explained by the use of a 180° linear interpolation algorithm with a 1-second rotation time, which represents a scanning period of about 500 msec, to reconstruct individual active expiratory scans. Furthermore, since the acquisition was made in a caudocranial direction during expiration to compensate for the rising diaphragm and the cephalad movement of the lungs, motion artifacts were minimized. Finally, motion artifacts were at a maximum during the early phase of expiration and were at a minimum during its late phase, which thereby allowed good visualization of lobular air trapping with helical CT.
Three potential limitations of our study were the subjective quantification of the intensity of air trapping by using the relative contrast score, the order of implementation of each expiratory technique, and the lack of blinding of the observers to the type of expiratory scan being read. Because it was a retrospective study, we could not use regions of interest to quantify the relative contrast between the areas of air trapping and the surrounding lung parenchyma. However, in standard clinical examinations, only subjective analysis is performed routinely (13). Since suspended-end-expiration CT always was performed before continued-expiration CT, a training bias might explain the higher degree of expiration with continued-expiration CT. On the other hand, since the residual volume was maintained passively at suspended-end-expiration CT and actively at continued-expiration CT, the two expiratory techniques were sufficiently different at the late phase of expiration that performance of the first technique did not really ensure the greater success of the second. Finally, since the two observers were not blinded to the type of expiratory scan, they might have been biased subconsciously toward the more experimental continued-expiration CT technique. However, this bias should have been marginal, as scans obtained with both expiratory techniques were interpreted independently.
In conclusion, when suspended-end-expiration CT images are ambiguous or when patients have difficulty performing the suspended-end-expiration maneuver adequately, complementary continued-expiration CT can be performed with a helical scanner to improve the conspicuity and the apparent extent of air trapping at a given anatomic level.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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