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Thoracic Imaging |
1 From the Department of Radiology (A.A.B.) and Department of Cardio-Thoracic Surgery, Lung Transplantation Unit (P.J., W.K.), Medical University of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria; Department of Radiology, University of Amsterdam, Amsterdam, the Netherlands (C.S.); Statistical Unit, Institute of Interdisciplinary Research in Human and Molecular Biology (V.D.M.), and Department of Radiology, University Erasme Hospital (P.A.G.), Université Libre de Bruxelles, Brussels, Belgium; and Department of Radiology, RHMS Clinic Louis Caty, Baudour, Belgium (D.T.). Received February 1, 2006; revision requested March 29; revision received April 6; final version accepted June 1. Address correspondence to A.A.B. (e-mail: alexander.bankier{at}meduniwien.ac.at).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEReferences |
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Materials and Methods: In this ethical committee–approved study, 27 lung transplant recipients (12 women, 15 men; mean age, 54 years ± 2 [standard error of the mean]) underwent expiratory thin-section CT at 140 kVp and 80 mAs (effective). All patients gave written informed consent. Dose reduction corresponding to 60, 40, and 20 mAs (effective) was simulated. The extent of air trapping in both original and dose-reduced studies was scored by three independent readers. The effects of tube current–time product, reader, reading session, and body mass index on average air trapping scores were assessed with analysis of variance. Agreements between and within observers were assessed with a weighted 
statistic. Subjective scores for diagnostic confidence were attributed (3 = high, 2 = medium, 1 = low), and their means were calculated for each tube current–time product value.
Results: No significant effect on average air trapping scores as a result of tube current–time product (P = .222), reader (P = .217), reading session (P = .705), or body mass index (P = .505) could be detected. At 80 mAs, agreement between readers was excellent; agreement decreased to good or moderate at lower tube current settings. Agreement within readers decreased with a decrease in dose but remained good even at 20 mAs. Confidence also decreased, with mean scores decreasing from 2.33 ± 0.73 (standard deviation) to 1.04 ± 0.19 when dose decreased.
Conclusion: At 140 kVp, the tube current–time product can be reduced from 80 to 20 mAs without impairing the visual quantification of air trapping at expiratory thin-section CT and with acceptable decreases in agreement between and within readers and in reader confidence.
© RSNA, 2007
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INTRODUCTION |
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Expiratory thin-section CT does expose patients to additional radiation, and multi–detector row technology can further increase the delivered dose by up to 300% (4). This is of special concern in patients with bronchiolitis because they are often young and—despite their relatively favorable prognosis—have a high risk of recurrence, resulting in repeated follow-up examinations and repeated exposure to CT radiation (1,2,5).
Results of previous studies have shown that a reduced radiation dose for CT examinations can be used when looking for abnormalities that present a high contrast to normal lung areas, such as pulmonary nodules or consolidations (6–12). However, when abnormalities present low contrast, as is the case with air trapping, the effect of dose reduction is not known. The purpose of our study, therefore, was to prospectively investigate the effect of radiation dose reduction on the visual quantification of air trapping at expiratory thin-section CT.
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MATERIALS AND METHODS |
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Pulmonary Function Testing
Pulmonary function testing was performed within 2 and 12 hours before the CT examinations. Measurements of functional residual capacity, total lung capacity, and residual volume were obtained with the patient seated in a constant-volume body plethysmograph (Body Test; Jaeger, Würzburg, Germany). Measurements of forced vital capacity and forced expiratory volume in 1 second (FEV1) were obtained by using a spirometer (Sensormedics 2400; Sensormedics, Anaheim, Calif) according to the guidelines of the American Thoracic Society (16). Predicted values for static and dynamic lung volumes were derived from the literature (16,17).
The severity of BOS was graded according to the revised recommendations of the International Society for Heart and Lung Transplantation (18). Accordingly, a BOS score of 0 indicated an FEV1 of more than 90% of the best postoperative baseline value and a forced expiratory flow, midexpiratory phase (FEF25%–75%) of more than 75% of the postoperative baseline value (and thus indicated that BOS was not present); a BOS score of 0p (potential BOS) indicated a sustained decline in FEV1 between 81% and 90% of the best postoperative baseline value and/or a decline in FEF25%–75% less than or equal to 75% of the postoperative baseline value; a BOS score of 1 indicated a sustained decline in FEV1 to between 80% and 66% of the best postoperative value; a BOS score of 2 indicated a sustained decline in FEV1 to between 65% and 50% of the best postoperative value; and a BOS score of 3 indicated a sustained decline in FEV1 of less than 50% of the best postoperative value.
At the time our study was performed, 12 (44%) of the 27 patients had a BOS score of 0, eight (30%) had a BOS grade of 0p, four (15%) had a BOS grade of 1, two (7%) had a BOS grade of 2, and one (4%) had a BOS grade of 3.
Image Acquisition
All CT images were obtained with a commercially available multi–detector row scanner (Somatom Sensation 16/Navigator, Software VA 70C; Siemens Medical Solutions, Forchheim, Germany). The tube current was systematically set at 140 kVp and 80 mAs (effective), which is the standard tube current setting at our institution. As defined by Mahesh et al (19), effective tube current corresponds to the milliampere-second value divided by the pitch, whereby pitch is defined by Silverman et al (20) as the ratio between the table feed per rotation and the x-ray beam width. Beam collimation was 16 x 0.75 mm, and tube rotation time was 0.75 second. To avoid a potential confounding factor by software components for automated dose reduction, these components were disabled. Patients were examined in the supine position, and none received contrast material. Examinations were performed from the apex to the base of the lungs during breath holding at full suspended inspiration and full suspended expiration.
Breath holding at both lung volumes was rehearsed with each patient prior to the CT examination. After acquisition was completed, raw data were transferred for processing with Siemens Modality Store (Siemens Medical Solutions, Forchheim, Germany) to a three-dimensional multimodality workstation (Leonardo; Siemens Medical Solutions).
Image Processing
After the raw data acquired at 80 mAs (effective) were loaded, images were reconstructed with a section thickness of 1 mm and a section increment of 10 mm. The size of the image matrix was 512 x 512, and a high-spatial-resolution reconstruction algorithm (B60) was used. The display window was set at a center of –650 HU and a width of 1500 HU. The resulting image set closely resembled thin-section CT studies from the pre-multi–detector row CT era (21).
After reconstruction of the original image set, three additional image sets were reconstructed. For these additional image sets, all technical parameters were kept identical to those used for the original image set. Computer-calculated noise was superimposed on each of the three additional image sets by using the software tools of the reconstruction console. This software obviates repeated CT examinations with varying tube currents by adding noise to images acquired at a particular tube current and thereby simulating images acquired at a lower tube current (11,22). The resulting images can then be used for the systematic evaluation of radiation dose reduction (5,11,22,23). For the three sets of additional images, noise corresponding to 60, 40, and 20 mAs (effective), respectively, was added. Consequently, four CT studies per patient were available for analysis: the examination with the original tube current settings (140 kVp, 80 mAs [effective]), and three examinations with simulated dose reduction (140 kVp and 60, 40, and 20 mAs [effective]). All four CT examinations were then coded such that the patient's name and the tube current settings were not visible. For the reading sessions, the CT studies were finally transferred to a picture archiving and communication workstation that complied with the Digital Imaging and Communications in Medicine standard (Impax 4.1.SP2; Agfa Gevaert Healthcare Informatics, Waterloo, Ontario, Canada).
Image Analysis
Image analysis was performed independently by three board-certified chest radiologists who had 13 years (reader 1, A.A.B.), 20 years (reader 2, P.A.G.), and 15 years (reader 3, C.S.) of experience in interpreting thoracic CT studies. Two of the three radiologists (reader 1 and reader 2) had additional expertise in the interpretation of CT studies in lung transplant recipients and in the assessment of air trapping in this patient population. In two separate and independent sessions, each radiologist analyzed all 27 original inspiratory and expiratory CT examinations and all 81 (three times 27) electronically processed inspiratory and expiratory CT studies with simulated dose reduction. In both reading sessions, the CT studies were presented in random order. For each radiologist, the independent reading sessions were separated by a period of 7 days. The radiologists were unaware of all clinical and functional information and of all information related to the electronic processing that simulated dose reduction. Manipulation of the CT data and selection of the examinations to be reviewed was performed by a radiologic technologist not involved in image analysis. For each CT examination, all sections obtained above the level of the diaphragm were analyzed. Each section was assessed individually. In patients who had undergone single lung transplantation, only the transplanted lung was assessed. In patients who had undergone double lung transplantation, left and right lungs were not graded separately but received a common score.
CT at full suspended inspiration.—Because our study was focused exclusively on air trapping, CT images obtained at full suspended inspiration were used only for comparison with the corresponding CT images obtained at full suspended expiration. Bronchial and vascular abnormalities were not recorded. However, as detailed in the literature (24), bronchial and vascular abnormalities were used to overcome potentially confounding features of airway and vascular diseases.
CT at full suspended expiration.—Air trapping was considered to be present on the expiratory CT images when lung regions failed to increase in attenuation and/or failed to decrease in volume, when compared with the corresponding inspiratory images (1,25). The diagnosis of air trapping was based on subjective visual assessment. Objective measurements were not obtained. The scoring system used to assess the extent of air trapping was adapted from prior studies (25–27). For a more subtle grading of air trapping, the number of possible scores was increased from five to six. A score of 0 was assigned if there was no abnormality; a score of 1, if less than 20% of the parenchyma in a CT section showed air trapping; a score of 2, if 20%–39% of the parenchyma showed air trapping; a score of 3, if 40%–59% of the parenchyma showed air trapping; a score of 4, if 60%–79% of the parenchyma showed air trapping; and a score of 5, if 80% or more of the parenchyma showed air trapping.
To obtain the average air trapping score for a given CT examination and for each of the three readers, the sum of scores for all CT sections from one patient was divided by the number of acquired sections for this patient. The resulting score was expressed as a percentage of the maximum possible score—that is, the score that theoretically would be obtained if all sections in a patient were given a score of 5.
Readers were also asked to score their confidence in the quantification of air trapping. Confidence was a purely subjective parameter that denoted the level of confidence that the individual reader had in his diagnosis. For quantification of subjective reader confidence, a score of 3, which indicated high confidence ("sure of the diagnosis"), a score of 2, which indicated medium confidence ("slightly unsure about the diagnosis"), or a score of 1, which indicated low confidence ("unsure of the diagnosis"), was attributed to each study by each of the three readers. On the basis of the confidence scores for the individual examinations, a mean confidence score was then calculated for each of the four predefined tube current settings.
Effective Radiation Dose
The effective dose was simulated on a personal computer by using commercially available software (CT-Expo; Medizinische Hochschule, Hannover, Germany). This software does not require phantom measurements. CT acquisition parameters, patient sex, and the scanned region, as represented on a graph of the Monte Carlo phantom model, were entered into the program. For each tube current–time product value, the program calculated effective doses by taking into account scanner parameters as reported by Nagel (28) and conversion factors as reported by Zankl et al (29,30). The calculated effective doses were expressed according to the International Committee on Radiation Protection and Measurements Publication 60 (31) recommendations.
Statistical Analysis
All statistical analyses were performed with a personal computer by using commercially available software packages (Statistica, version 5.0, StatSoft, Tulsa, Oklahoma; and SPSS, version 12.0, SPSS, Chicago, Illinois). Normally distributed data were expressed as means (±1 standard error of the mean), and skewed data were expressed as medians with 25% and 75% quartiles. The normality of the distribution of the data was evaluated by using the Kolmogorov-Smirnov test (with SPSS, version 12.0). Statistical significance for all tests was set at P < .05.
To analyze the potential effect of radiation dose—that is, the tube current–time product, the individual readers, the reading session, or the BMI of our patients—on the average air trapping scores, we used an analysis of variance with the four radiation doses, the three readers, and the two reading sessions as repeated factors, and with the BMI of our patients as a covariate. Because in routine practice images are usually read only once, the analysis of variance was restricted to data from the first reading session.
Agreement between and within observers for the four predefined tube current settings was assessed by using a weighted statistic (32). The asymptotic standard errors and the 95% confidence intervals for the weighted values were calculated (32,33). All values were interpreted as recommended in the literature (32,34). A value of 0.20 or less indicated poor agreement; a value of 0.21–0.40, fair agreement; a value of 0.41–0.60, moderate agreement; a value of 0.61–0.80, good agreement; and a value of 0.81–1.00, excellent agreement.
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RESULTS |
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The mean average air trapping scores for two reading sessions, the three readers, and the four predefined tube current settings are shown in the Table. According to results of the Kolmogorov-Smirnov test, all subgroups of average air trapping scores were normally distributed.
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Effect of Tube Current Setting, Reader, Reading Session, and BMI
Representative examples of air trapping at 80, 60, 40, and 20 mAs (effective, simulated) are shown in Figures 1 and 2. The analysis of variance did not reveal any statistically significant effect of either the four tube current settings (P = .222), the three individual readers (P = .217), the two reading sessions (P = .705), or the BMI of our patients (P = .505) on the average air trapping scores. Neither could any statistically significant effect on the average air trapping scores be detected for the combinations of tube current settings and individual readers (P = .215), individual readers and reading session (P = .373), and tube current settings and reading sessions (P = .527). Finally, the analysis of variance did not reveal any statistically significant effect on the average air trapping scores of the combinations between BMI and tube current settings (P = .250), BMI and individual readers (P = .452), and BMI and reading session (P = .761).
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DISCUSSION |
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Our results also indicate that reduced tube current–time product is associated with a decrease in both agreement between and within readers and in their diagnostic confidence. As to the agreement between readers, a similar observation has previously been made for inspiratory thick-section CT (35). Our findings, based on expiratory thin-section CT, complement this observation by showing that the decrease in the agreement between readers depends not only on dose reduction but also on the experience of the individual reader. Even at reduced tube current settings, the agreement between the two readers with additional comparable experience in the assessment of air trapping was good to excellent, whereas the agreement could decrease to moderate for comparisons involving a reader without such experience. It is of note that there was a decrease in agreement between readers but little change in average scores. This suggests that as the simulated dose decreased, there was a wider spread in readings that still centered around the same mean. This could be a potential problem with use of lower dose expiratory images, in that there would be a greater chance that an individual reader would come up with a reading that is considerably high or low. On the other hand, the decrease in agreement within readers affected all three readers. Agreement within readers remained good to excellent for all three readers even at the lowest tube current–time product. This suggests that the agreement within readers is less sensitive to dose reduction than the agreement between readers. Finally, our readers' confidence systematically decreased with a reduction in radiation dose. Confidence in the diagnosis of air trapping certainly is a purely subjective parameter assessed in our study. This parameter encompasses a complex blend of factors that cannot be easily quantified. In addition to reader experience, these factors include the technical quality of the image and the visual appearance of the disease under investigation. Previous investigators (10,35) have found a correlation between a decrease in tube current–time product and a decrease in the subjective perception of image quality, and it is likely that this phenomenon also influenced the diagnostic confidence as assessed in our study. Although we did not analyze the relative contribution of each of the factors involved, our results confirm that there is a close relationship between dose reduction and diagnostic confidence.
Our study was conducted in a cohort of lung transplant recipients for two reasons. First, air trapping in lung transplant recipients occurs frequently, as a result of chronic allograft rejection, which is the most common complication in lung transplant recipients with long-term survival (26,36–39). This makes air trapping in lung transplant recipients a model of the radiologic manifestation of bronchiolitis, whose characteristics can likely be expanded to bronchiolitis of other origins. Second, the follow-up of lung transplant recipients is based on the intrapatient evolution over time of predefined test parameters measured at sequential examinations and compared with a postoperative baseline value (13,18,26,39,40). Repeated follow-up CT examinations result in a cumulative radiation exposure that increases with duration of follow-up (39,41,42).
By superimposing computer-calculated noise on the original raw CT data, the simulated dose reduction might have simulated attenuation heterogeneities of the lung parenchyma. These attenuation heterogeneities may be misinterpreted as areas of air trapping at expiratory thin-section CT. Twelve of our 27 patients had a BOS score of 0, indicating that they were not susceptible to pathologic air trapping. The lack of a statistically significant effect of simulated dose reduction on the air trapping scores indicates that no relevant attenuation heterogeneities were misinterpreted as areas of air trapping.
Our study had several limitations. First, images with a low tube current–time product that is simulated by adding random noise to the raw data might not correspond exactly to images actually acquired with a low tube current–time product. In a validation trial, however, experienced chest radiologists were unable to distinguish CT images with simulated dose reduction from CT images with a truly reduced dose (11). Furthermore there is no reason to assume that expiratory multi–detector row CT would produce results any different than inspiratory incremental CT. Second, only one obese patient was included in our study. This study might, therefore, have underestimated the effect of dose reduction on the visualization of air trapping in such patients. Because the effective dose is lower in obese than in thinner patients, the need for dose reduction appears less critical in them. Third, we could not identify a minimum tube current–time product below which the visualization of air trapping is measurably and consistently compromised. This, however, does not imply that such a minimum tube current–time product does not exist. As with all other tube current–time products, the lowest value in this study was chosen arbitrarily, and one could argue that by setting the dose even lower, statistically significant effects of dose reduction on the analyzed parameter could well have appeared. On the other hand, our study was embedded in a clinical context that makes a dose reduction below 20 mAs hardly conceivable because such a reduction could deteriorate the visualization of findings other than air trapping.
Fourth, the incidence of BOS in our study population was relatively low. This could have biased our population sample toward a low incidence of pathologic air trapping. Our CT scores nevertheless showed that the air trapping scores averaged around 35% to 40%, and this amount has been considered pathologic in several previous studies (26,36,39). Fifth, all three readers in this study had relatively high levels of experience. This could make it difficult to globally translate our results to readers with substantially less experience, such as students or residents.
In conclusion, our study results show that at 140 kVp, the tube current–time product can be reduced (by simulation) from 80 to 20 mAs without impairment of the visual quantification of air trapping on expiratory thin-section CT studies and with acceptable decreases in agreements between readers, within readers, and in a reader's confidence. Because its radiation dose approximates that of incremental thin-section CT with 10-mm section intervals performed with a standard dose, expiratory low-dose multi–detector row CT can be used in the assessment of air trapping in patients with suspected bronchiolitis.
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ADVANCE IN KNOWLEDGE |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEReferences |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Abbreviations: BMI = body mass index • BOS = bronchiolitis obliterans syndrome • FEF25%–75% = forced expiratory flow, midexpiratory phase • FEV1 = forced expiratory volume in 1 second
Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, A.A.B., V.D.M., P.A.G.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, A.A.B., C.S., V.D.M., D.T., P.A.G.; clinical studies, A.A.B., C.S., V.D.M., P.J., W.K., P.A.G.; statistical analysis, A.A.B., V.D.M., P.A.G.; and manuscript editing, all authors
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References |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEReferences |
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This article has been cited by other articles:
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  S. Matsuoka, Y. Kurihara, K. Yagihashi, M. Hoshino, and Y. Nakajima Airway Dimensions at Inspiratory and Expiratory Multisection CT in Chronic Obstructive Pulmonary Disease: Correlation with Airflow Limitation Radiology, September 1, 2008; 248(3): 1042 - 1049. [Abstract] [Full Text] [PDF]
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A. A. Bankier, S. Mehrain, D. Kienzl, M. Weber, M. Estenne, and P. A. Gevenois Regional Heterogeneity of Air Trapping at Expiratory Thin-Section CT of Patients with Bronchiolitis: Potential Implications for Dose Reduction and CT Protocol Planning Radiology, June 1, 2008; 247(3): 862 - 870. [Abstract] [Full Text] [PDF]
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