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Thoracic Imaging |
1 From the Mallinckrodt Institute of Radiology (D.S.G., T.K.P., L.C., R.M.S., K.T.B.) and Divisions of Pulmonary and Critical Care Medicine (R.D.Y., S.S.L.), General Medical Sciences (R.D.Y.), and Cardiothoracic Surgery (J.D.C.), Washington University School of Medicine, Barnes-Jewish Hospital, 216 S Kingshighway Blvd, St Louis, MO 63110. From the 2000 RSNA scientific assembly. Received September 13, 2000; revision requested October 27; revision received February 23, 2001; accepted March 16. Supported in part by the American Lung Association of Eastern Missouri. R.D.Y. supported in part by the National Heart, Lung, and Blood Institute of the National Institutes of Health (1 K23 HL04236-01). Address correspondence to D.S.G. (e-mail: gieradad@mir.wustl.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Initial and same-day repeat routine inspiratory spiral chest CT studies were performed in 29 LVRS candidates (group 1, routine study vs repeat study). In a separate cohort of 29 LVRS candidates, spiral chest CT studies were performed both without and with spirometric gating by using a spirometer to trigger scanning at 90% of vital capacity (group 2, spirometric gating study). In each study, Pearson and intraclass correlation coefficients were calculated to determine the agreement between multiple pairs of whole-lung quantitative CT indexes of emphysema, and mean values were compared with two-tailed paired t tests.
RESULTS: Pearson and intraclass correlation coefficients were high for all quantitative CT indexes (all 
0.92). No significant differences were found between mean values of quantitative CT indexes in group 1. Variation in quantitative CT results was small but more prominent in group 2 than in group 1. The variation in quantitative CT results was primarily related to differences in lung volume (r2 as great as 0.83).
CONCLUSION: Repeatability of quantitative CT test results in LVRS candidates is high and unlikely to improve by using spirometric gating.
Index terms: Emphysema, pulmonary, 60.751 • Lung, CT, 60.12115, 60.12119 • Lung, ventilation, 60.91
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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All patients presenting for a clinically scheduled CT examination as part of their on-site LVRS evaluation were potential candidates for the study. Patients were recruited at the time of arrival in the radiology department according to the availability of the research technologist (L.C.). The study was performed with the approval of our institutional review board. Written informed consent was obtained from all patients in whom extra CT was performed for this study.
CT Technique
All CT scans (Somatom Plus 4; Siemens Medical Systems, Iselin, NJ) were obtained by using the spiral technique with 8-mm collimation, 16 mm/sec table speed, 120–159 kV, 130–159 mA, and 0.75-second scanning time, without intravenous administration of contrast material. Routine inspiratory scans were obtained during full inspiration, with patients instructed to take in a deep breath and hold it. The mean scanning time for the entire chest among all patients in the study was 13 seconds ± 2 (SD; range, 10–19 seconds). To minimize motion artifacts in patients unable to hold their breath for the entire study, the lungs were scanned in the inferior to superior direction.
Routine Study versus Repeat Study: Group 1
Routine (without spirometric gating of lung volume) inspiratory and repeat routine CT scans were obtained in 30 LVRS candidates. After the routine study, patients briefly got up from the scanner table before the repeat study was performed. The mean time between the end of the first study and the beginning of the repeat study was 15 minutes ± 9 (range, 5–54 minutes). One patient was excluded from analysis because the scanning protocol was not followed correctly. Thus, 29 of 30 patients were available for analysis in this part of the study.
Spirometric Volume Gating Study: Group 2
The inspiratory level of 90% of vital capacity used for gating scans in the spirometric volume gating study described later was based on the results of a preliminary study designed to quantify the average inspiratory effort of patients when given routine breath-hold instructions for inspiratory CT. In this preliminary study, a series of spirometric measurements was obtained in a separate group of 25 LVRS candidates (Table 1) who did not undergo CT or analysis for this investigation. These subjects were drawn from a referred population of patients being evaluated for LVRS between September 1997 and March 1998. The patients were recruited for spirometric testing in the hospital pulmonary rehabilitation clinic prior to their scheduled exercise sessions, according to the availability of one of the authors (D.S.G.).
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The spirometric volume gating study was performed by using the scanner manufacturer’s optional equipment (Pulmo; Siemens Medical Systems), which consists of a handheld spirometer with electronic scanner interface. This equipment allows measurement of vital capacity, then triggers closure of the spirometer valve and initiates scanning when the subject subsequently inhales to the user-selected percentage of the vital capacity.
The subjects of the spirometric gating study were 32 LVRS candidates different from the 29 in group 1. The ungated study was performed first, prior to the measurement of vital capacity and the volume-gated study, to avoid any learning effects and tendency to reproduce the effort of the gated study. For the ungated studies, patients were instructed to take in a deep breath and hold it, without any additional coaching. At completion of the ungated studies, the patients remained on the scanner table, vital capacity was measured, and scanning was performed with lung volume gated at 90% of vital capacity. The mean time between the end of the ungated study and the beginning of the gated study was 20 minutes ± 7 (range, 12–42 minutes). If the patient used supplemental oxygen at rest, the nasal cannula was removed for the gated study only and replaced immediately at completion of the scanning. Three patients were excluded from analysis because the scanning protocol was not followed correctly, for a total of 29 patients in this portion of the study, as for group 1. Table 2 lists the clinical characteristics of groups 1 and 2.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Routine Study versus Repeat Study: Group 1
Agreement as measured by means of the Pearson and intraclass correlation coefficients was excellent, with all coefficients 0.98 or higher (Table 4). No significant differences were found between mean values of the routine (ungated) and those of the repeat routine study data for each of the quantitative CT indexes of emphysema (Table 4). Mild disagreement occurred in a few cases (Fig 2). There was no obvious tendency toward bias related to the magnitude of the measurement.
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The vital capacity measured by using the CT spirometer with the patients supine was smaller than the postbronchodilator measurements obtained with the patients seated in the pulmonary function laboratory in 22 of the 29 patients. The mean difference between the CT spirometric and pulmonary function laboratory measurements was 0.41 L ± 0.59 (range, -0.32 to 1.9 L). The CT-measured supine vital capacity was thus 84% ± 22 (range, 31%–126%) of the vital capacity measured in the pulmonary function laboratory.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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We found excellent agreement between quantitative CT results obtained from the initial and those obtained from the repeat routine inspiratory studies. This provides evidence that the test results are highly repeatable in candidates for LVRS. Quantitative CT results obtained without and those obtained with spirometric volume gating also were in close agreement and thus, lack of volume gating does not appear to adversely affect the test results. Although statistical differences were found between mean values for many of the indexes in the spirometric gating study (group 2), this finding is of doubtful potential clinical importance, for several reasons. The sizes of the differences were small relative to the magnitude of the means. Since these were paired comparisons, the statistical significance of these small differences is likely related to the observation that the emphysema indexes were slightly higher when measured from the ungated studies in three-fourths of the patients. These differences were small for a majority of patients, and consequently the 95% CIs were small (Table 4). No relation to the magnitude of the measurement was seen.
Differences in lung volume and quantitative CT measurements at different inspiratory breath holds in LVRS candidates are likely small because of reduced inspiratory capacity. Data from previous studies (8,9) in which scans were obtained over a wide range of lung volumes show that the volume of air in the lungs at the time of scanning can have a substantial effect on quantitative CT measurements. Our results are consistent with these considerations. The differences in quantitative CT results in both groups were small, and most of the variability in the indexes of emphysema severity was explained by means of variation in lung volume. Thus, active encouragement of a maximal inspiratory effort for scanning, which was not performed in our study, may help to further improve the precision of quantitative CT testing.
In group 2, the severity of airflow obstruction, as measured by means of the percentage of predicted FEV1, and the percentage of predicted forced vital capacity were weakly related to the variation in the percentage of severe emphysema in the lower lungs. This might relate in some way to the relative preservation of ventilation in the lower lobes that is a typical feature of smoking-related centrilobular emphysema, such as through a greater potential for variation in lower lobe volume with different breath holds. Regardless, it raises the possibility that quantitative CT measurements of the percentage of severe emphysema in the lower lungs may become less reliable than other quantitative CT indexes as airflow obstruction increases. However, the value of this particular index as a predictor of outcome in LVRS is unknown.
We noted that the mean supine postbronchodilator vital capacity as measured by means of the CT spirometer was 84% of the postbronchodilator forced vital capacity measured in the pulmonary function laboratory and was smaller in 22 of 29 patients. This probably is due, at least in part, to differences in position, since vital capacity is lower in the supine than in the seated position (22). However, the large discrepancies seen in some patients (more than 30% difference between CT spirometric and pulmonary function laboratory test results in four patients) may not be related only to differences in positioning and could indicate either technologist error or inadequate effort by the patient during CT spirometry. A comparison of quantitative CT results from gated and from repeat gated studies would have been useful to assess this discrepancy but was not a part of this study. Nevertheless, this observation highlights the potential practical technical limitations of using spirometric gating for CT in a clinical setting in patients with severe emphysema.
There was greater variation and wider range in the individual differences between quantitative CT indexes and lung volume measured at quantitative CT in the spirometric gating study than in the routine and repeat studies. Since quantitative CT indexes vary with lung volume, this suggests that interindividual variation in lung volume between individuals on ungated studies (assessed in group 2 by means of comparison with the standardized gated CT-measured volumes) was higher than intraindividual variation in lung volume on ungated studies (assessed in group 1). This is not surprising, considering the degree of variation in inspiratory effort we found in the preliminary breath-hold study: although the average CT volume was 90% of the supine vital capacity, the range was 72%–104%, and it was less than 85% in six of 25 patients and greater than 95% in seven of 25 patients.
Would it be helpful to use volume gating during scanning for quantitative CT studies in LVRS candidates? Elimination of the interindividual variation in lung volume by means of spirometric gating during scanning theoretically ensures that the same quantitative CT results would be obtained in different patients with the same severity and distribution of emphysema and is a potential advantage of the technique. However, we found that agreement between gated and routine ungated inspiratory studies was good in a majority of patients. Furthermore, since the spirometric test results of patients can fluctuate even during the same day (22), there is no guarantee that spirometric gating will improve the accuracy of results. For these reasons, and because of the potential for technical difficulties noted earlier, we think that spirometric gating would not provide a clinically important advantage over the already high level of accuracy and precision that should be achievable if studies are performed with patients verbally coached to inhale maximally during scanning. A comparison of gated and repeat gated results might reveal a small improvement in repeatability, but the incremental gain is unlikely to be worth the additional effort required for performing gated studies.
A limitation of this study is that the repeatability assessment was restricted to a short period. Repeatability during longer intervals could be different, because the respiratory status of patients with chronic obstructive lung disease may change over time. Variability in quantitative CT measurements because of changes in respiratory status might be minimized if patients underwent scanning only after medical therapy has been optimized and if scanning were performed after bronchodilator administration. Our study also lacks a comparison with the repeatability of subjective reader assessment of emphysema severity and distribution. However, it would have been extremely difficult to discriminate intraobserver variation, which can be substantial (21), from variation due to the subtle differences between initial and repeat studies.
In summary, quantitative CT measurements of emphysema made on the basis of routine inspiratory chest CT results are highly repeatable, with no apparent advantage added by using spirometric gating. Intraindividual variation in quantitative CT results was predominantly explained by intraindividual differences in lung volume, which suggests that consistent coaching of patient inspiratory efforts will help to maximize test precision. Further studies relating preoperative quantitative CT indexes of emphysema to outcome after LVRS may allow development of patient selection guidelines that incorporate quantitative CT analysis in the clinical setting.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, D.S.G.; study concepts, D.S.G., T.K.P., K.T.B.; study design, D.S.G., K.T.B.; literature research, D.S.G., R.D.Y.; clinical studies, D.S.G., L.C.; data acquisition, D.S.G., L.C.; data analysis/interpretation, all authors; statistical analysis, D.S.G., R.D.Y., T.K.P.; manuscript preparation and definition of intellectual content, D.S.G.; manuscript editing, D.S.G., R.D.Y., L.C., R.M.S.; manuscript revision/review and final version approval, all authors.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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