DOI: 10.1148/radiol.2361040190
(Radiology 2005;236:318-325.)
© RSNA, 2005
Multi–Detector Row CT Pulmonary Angiography: Comparison of Standard-Dose and Simulated Low-Dose Techniques1
Denis Tack, MD, PhD,
Viviane De Maertelaer, PhD,
Wendy Petit, MD,
Pierre Scillia, MD,
Patrick Muller, MD,
Christoph Suess, PhD and
Pierre Alain Gevenois, MD, PhD
1 From the Department of Radiology, Centre Hospitalier Universitaire de Charleroi, Charleroi, Belgium (D.T., P.M.); Statistical Unit, Institut de Recherche Interdisciplinaire en Biologie Humaine et Moléculaire, Université Libre de Bruxelles, Brussels, Belgium (V.D.M.); Department of Radiology, Hôpital Erasme, Université Libre de Bruxelles, Brussels, Belgium (W.P., P.S., P.A.G.); and Siemens Medical Solutions, Forchheim, Germany (C.S.). Received February 1, 2004; revision requested April 12; final revision received July 14; accepted September 25.
Address correspondence to D.T., Department of Radiology, RHMS Baudour, Rue Louis Caty 136, B-7331 Baudour, Belgium (e-mail: denis.tack{at}skynet.be).
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ABSTRACT
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PURPOSE: To compare standard-dose and simulated low-dose multi–detector row computed tomography (CT) pulmonary angiography.
MATERIALS AND METHODS: The institutional review board approved the study protocol and waived patient informed consent because the study was based on existing data. Raw data from 21 CT scans obtained at 90 mAs (effective) in 11 women and 10 men aged 25–74 years (mean, 52 years) that showed at least one filling defect within a pulmonary artery were used to simulate CT pulmonary angiography with reduced radiation doses, at 60, 40, 20, and 10 mAs. Three independent readers coded each central and segmental pulmonary artery twice as positive, negative, or inconclusive for presence of a filling defect. The second reading of images obtained with 90 mAs was considered the reference standard. The potential dependence of results on reader, radiation dose, and/or pulmonary artery segment was investigated with analysis of variance. Positive and negative consistent values were calculated for standard-dose scans and simulated low-dose scans in the first reading session. The branching order of the artery with the most distal filling defect was recorded. The quality of intravascular contrast at each tube current–time product setting was scored on a five-point scale. Interreader agreement was investigated with 
statistics.
RESULTS: The frequencies of positive and inconclusive results (P = .21 and .08, respectively), positive and negative consistent values (P = .19 and .34, respectively), and branching order of the most distal artery with a filling defect (P = .41) did not depend on the radiation dose. Values for inter- and intrareader agreement were higher for central arterial segments than for branch arteries but were not influenced by dose reduction, regardless of arterial segment. The quality of intravascular contrast was not significantly changed when the tube current–time product was reduced from 90 to 40 mAs (P = .10 to >.99).
CONCLUSION: The evaluated parameters remained stable when tube current–time product was reduced from 90 (effective) to 10 (simulated) mAs at multi–detector row CT pulmonary angiography.
© RSNA, 2005
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INTRODUCTION
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Computed tomography (CT) accounts for more than one-half of the collective radiation exposure delivered by diagnostic imaging procedures (1,2). During the 1990s, it was shown that naturally existing contrast within the lungs is high, and this discovery enabled a reduction in the radiation dose delivered at unenhanced CT, including thin-section CT (3), single-section CT (4), and helical CT (5). Also during that decade, helical CT pulmonary angiography became a major diagnostic imaging procedure in patients in whom pulmonary embolism was suspected (6). The radiation dose delivered at CT pulmonary angiography is lower than that at conventional pulmonary angiography (7,8), but, because CT pulmonary angiography is performed in almost all patients with suspected pulmonary embolism, the collective radiation dose delivered while ruling out pulmonary embolism is increasing (9). Furthermore, researchers in several studies have shown that the actual prevalence of pulmonary embolism among patients evaluated with CT pulmonary angiography is only 9%–35% (9–11). Given the negative test results in the majority of patients examined with CT angiography for pulmonary embolism, there is an urgent need to reduce the radiation dose, particularly in young female patients, who represent approximately 20% of the patients examined (11). The purpose of our study, therefore, was to compare standard-dose and simulated low-dose multi–detector row CT pulmonary angiography.
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MATERIALS AND METHODS
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Subjects
From March to July 2002, raw image data sets from CT pulmonary angiography in 21 consecutive adult patients (11 women and 10 men) aged 25–74 years (mean, 52 years), which showed at least one pulmonary embolus detected at standard-dose CT pulmonary angiography, were included in our study. The mean body mass index of each patient, calculated from data available in the medical chart (12), was 24.8 kg/m2 ± 0.4 (standard deviation) and the range was 20.7–28.3 kg/m2. The institutional review board approved our research protocol and agreed to waive the requirement of patient informed consent because the study was based on existing data. No additional interaction with patients or additional radiation exposure to patients was necessary.
CT Examinations
Scans were obtained by using a commercially available multi–detector row CT scanner with four detector rows (Somatom Volume Zoom; Siemens Medical Solutions, Forchheim, Germany). Patients were examined while in a supine position with both arms fully extended above the head. A frontal 51-cm scout view was obtained first, at 80 kVp and 50 mA. Standard-dose CT scanning then was performed caudocephalad, from the level of the posterior costophrenic angles to the lung apices, after an intravenous injection of 100 mL of a contrast agent containing 350 mg/mL iodine (iobitridol, Xenetix 350; Guerbet, Aulnay-sous-Bois, France) at 3 mL per second by using a power injector (CT 9000; Liebel-Flarsheim, Cincinnati, Ohio). The scanning delay was automatically determined with bolus tracking in the main pulmonary artery, and scanning began when a predetermined threshold of 100 HU was reached. CT was performed with 4 x 1-mm collimation at 120 kVp and 90 (effective) mAs. No tube current modulation was applied during scanning. As defined by Mahesh et al (13), the effective radiation dose corresponds to the tube current–time product divided by the pitch, with pitch defined as in Silverman et al (14), as the ratio of the table feed per rotation to the x-ray beam width. Table feed was 7 mm per 0.5-second rotation (14 mm/sec). These parameters resulted in a pitch of 1.75.
Simulated Low-Dose CT Pulmonary Angiography
We used a computer-assisted method to generate CT pulmonary angiographic images at simulated reduced dose levels. This process involved the superimposition of computer-calculated noise on the original raw CT data. To simulate scans obtained with tube current–time product values of 60, 40, 20, and 10 mAs, the amount of added noise was increased stepwise so that its magnitude was proportional to the square root of the measured x-ray attenuation for each detector channel (15). The resultant 105 raw data sets (21 with original data and 84 with simulated data) were transferred to a CT postprocessing workstation and reconstructed with 1.25-mm section thickness, 0.80-mm increment, and a soft-tissue algorithm (kernel B20).
Image Analysis
Patient information and the simulated tube current–time product value were erased from images that were randomly renumbered by using random tables from Fisher and Yates (16). Three different readers independently read each 1.25-mm-thick scan at a clinical workstation with three-dimensional functionalities (Wizard; Siemens Medical Solutions). The readers comprised a general radiologist with 18 years of experience in reading CT scans (P.M., reader 1), a chest radiologist with 12 years of experience in all thoracic imaging techniques (P.S., reader 2), and a medical student who was nearing completion of medical school but who had no training in radiology or experience in medical imaging (W.P., reader 3). None of the readers were involved in selecting patients, conducting the CT examinations at which pulmonary embolism was detected, or preparing the image data sets, but they knew that at least one filling defect was present in each patient. Images were analyzed twice by each reader, in two separate and independent reading sessions separated by an interval of at least 2 months. Each reading session was completed within 2 weeks. Readers were asked to record filling defects in the central and lobar pulmonary arteries, which included the main, right, and left pulmonary arteries; the right upper-lobe pulmonary artery (anterior trunk); and the interlobar trunks of the right and left pulmonary arteries. For segmental arteries, we used the nomenclature outlined by Remy-Jardin et al (17), as adapted by Ghaye et al (20). This nomenclature is based on standard descriptions by Jackson and Huber (18) and Boyden (19). Segmental arteries in the right lung were labeled as RA1–RA10, and those in the left lung, as LA1–LA10. If a filling defect was detected in a location more distal than a segmental pulmonary artery, the corresponding segmental artery was coded as 1'. Each artery was rated on a three-point scale (0, no filling defect; 1, at least one filling defect; 2, inconclusive). Readers also were asked to record the branching order of the artery with the most distal filling defect detected and to grade the quality of intravascular contrast on a five-point scale. A contrast difference between blood clots and enhanced normal vascular content was scored 0 if it was very difficult to see, 1 if it was difficult to see, 2 if it was clearly depicted, 3 if it was very clearly depicted, and 4 if it was exceptionally well depicted. Two weeks before the first reading session, readers were familiarized with the scoring system in a training session with standard-dose CT scans obtained in 20 patients who were not included in the study group.
Statistical Methods
Inter- and intrareader agreement was investigated with the calculation of Cohen statistics and asymptotic standard errors (21). Interreader agreement was assessed for both reading sessions. The hypothesis of no agreement between the two readers was tested, and the associated P values were calculated (22). All values were interpreted as proposed in the literature (23). A value of 0.20 or less indicated poor agreement; 0.21–0.40, fair agreement; 0.41–0.60, moderate agreement; 0.61–0.80, good agreement; and 0.81–1.00, excellent agreement.
To compare diagnosis achieved with simulated low-dose CT pulmonary angiography with that achieved with standard-dose CT pulmonary angiography in comparable artery segments, we pooled pulmonary arteries into the following four groups: central and lobar pulmonary arteries (the main, right, and left pulmonary arteries; right upper-lobe pulmonary artery; and right and left interlobar pulmonary arteries); upper-lobe segmental pulmonary arteries (segments RA1–RA3 and LA1–LA3); middle-lobe and lingular segmental arteries (segments RA4, RA5, LA4, and LA5); and lower-lobe segmental arteries (segments RA6–RA10 and LA6–LA10).
The possible dependence of the number of inconclusive results on the reader, on the radiation dose (ie, the tube current–time product), and/or on the artery group was investigated by using an analysis of variance with two repeated factors (three reader levels and five tube current–time product levels), one group factor (four artery group levels), and two-way interactions between those factors. Because images in routine practice are usually read only once, only data from the first reading session were included in this analysis. The same analysis was performed for the number of positive results.
To eliminate the potential for reader memorization of scans from one session to another, we separated the reading sessions with an interval of at least 2 months. In addition, changes in tube current–time product settings produced two variables: different radiation doses and subjectivity in reader interpretation of the image data set at the second reading. To overcome the variable of reader subjectivity, we used the results from the second reading session as a reference for the interpretation of results from the first session. Two other dynamics are also important to bear in mind: (a) The more images a reader reviews, the more adept he or she becomes at correctly interpreting noisy images, and (b) in clinical practice, there is only one reading. Consequently, we investigated the performance of standard- and simulated low-dose CT at the first reading session in comparison with that at the second reading session, with reading of images obtained with 90 mAs used as the reference. Because there was thus no way to account for true-positive and true-negative findings, we considered positive and negative consistent values instead of positive and negative predictive values. For each reader and each artery group, the positive and negative consistent values were compared between different tube current–time product settings by using the Fisher exact test.
The mean branching order of the arteries with the most distal filling defect, as well as the intravascular contrast quality scores, were compared among tube current–time products for the first reading session and for each reader by using the Friedman test and, when a statistically significant difference was found, the Wilcoxon signed rank test for paired data.
Statistical significance for all tests was set at a P value of less than .05. Statistical software (SPSS for Windows, release 11.0, SPSS, Chicago, Ill; StatXact 3, Cytel, Cambridge, Mass) was used.
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RESULTS
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CT pulmonary angiographic images acquired with an effective tube current–time product of 90 mAs and with simulated values of 60, 40, 20, and 10 mAs are shown in Figure 1. The numbers of inconclusive and positive results for each reading session, each reader, and each tube current–time product are given in Tables 1 and 2, respectively. The number of inconclusive results varied according to the reader (P < .001) but not the tube current–time product (P = .08) or the artery group (P = .25). The number of positive results did not vary according to the reader (P = .54), the tube current–time product (P = .21), or the artery group (P = .51).
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Figure 1a. Transverse views of lower lobe of right lung, obtained with multi–detector row CT pulmonary angiography. (a) Image acquired at 90 mAs. (b–e) Images obtained with simulated tube current–time products of(b) 60 mAs, (c) 40 mAs, (d) 20 mAs, and (e) 10 mAs. A filling defect (arrow in a) is seen in arterial segment R10A at all radiation dose levels. Noise in the chest wall is more visible at 20 and 10 mAs than at 90 mAs.
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Figure 1b. Transverse views of lower lobe of right lung, obtained with multi–detector row CT pulmonary angiography. (a) Image acquired at 90 mAs. (b–e) Images obtained with simulated tube current–time products of(b) 60 mAs, (c) 40 mAs, (d) 20 mAs, and (e) 10 mAs. A filling defect (arrow in a) is seen in arterial segment R10A at all radiation dose levels. Noise in the chest wall is more visible at 20 and 10 mAs than at 90 mAs.
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Figure 1c. Transverse views of lower lobe of right lung, obtained with multi–detector row CT pulmonary angiography. (a) Image acquired at 90 mAs. (b–e) Images obtained with simulated tube current–time products of(b) 60 mAs, (c) 40 mAs, (d) 20 mAs, and (e) 10 mAs. A filling defect (arrow in a) is seen in arterial segment R10A at all radiation dose levels. Noise in the chest wall is more visible at 20 and 10 mAs than at 90 mAs.
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Figure 1d. Transverse views of lower lobe of right lung, obtained with multi–detector row CT pulmonary angiography. (a) Image acquired at 90 mAs. (b–e) Images obtained with simulated tube current–time products of(b) 60 mAs, (c) 40 mAs, (d) 20 mAs, and (e) 10 mAs. A filling defect (arrow in a) is seen in arterial segment R10A at all radiation dose levels. Noise in the chest wall is more visible at 20 and 10 mAs than at 90 mAs.
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Figure 1e. Transverse views of lower lobe of right lung, obtained with multi–detector row CT pulmonary angiography. (a) Image acquired at 90 mAs. (b–e) Images obtained with simulated tube current–time products of(b) 60 mAs, (c) 40 mAs, (d) 20 mAs, and (e) 10 mAs. A filling defect (arrow in a) is seen in arterial segment R10A at all radiation dose levels. Noise in the chest wall is more visible at 20 and 10 mAs than at 90 mAs.
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Representative values at 90, 40, and 10 mAs are shown in Figures 2 and 3 for intrareader and interreader agreement, respectively. For display simplicity, values for agreement at 60 and 20 mAs are not shown, but they were similar to those at 90, 40, and 10 mAs. Values for intrareader agreement reached statistical significance, except for that for reader 3 at evaluation of arterial segments RA2 and RA4. Interreader agreement was higher between readers 1 and 2 than between either one and reader 3; values generally did not reach statistical significance when reader 3 was involved. However, values at reduced tube current–time product settings were within the same range as those at the reference level.
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Figure 2. Plot shows intrareader agreement for readers 1 ( ), 2 ( ), and 3 ( ) and for all pulmonary arterial segments evaluated. For each arterial segment, vertical lines link the values of the three readers. Vertical lines within each triplet, from left to right, show values for intrareader agreement at 90, 40, and 10 mAs, respectively. Vertical lines in almost all triplets indicate the same approximate range of values for each tube current–time product setting. LA1–LA10 = left segmental pulmonary artery, LILPA = left interlobar pulmonary artery, LPA = left pulmonary artery, MPA = main pulmonary artery, RA1–RA10 = right segmental pulmonary artery, RILPA = right interlobar pulmonary artery, RPA = right pulmonary artery, RULPA = right upper-lobe pulmonary artery.
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Figure 3. Plot shows interreader agreement for the first reading session. Vertical lines within each triplet, from left to right, show values at 90, 40, and 10 mAs, respectively, for agreement between readers 1 and 2 ( ), 2 and 3 ( ), and 1 and 3 ( ). Vertical lines in almost all triplets indicate the same approximate range of values for each tube current–time product setting. LA1–LA10 = left segmental pulmonary artery, LILPA = left interlobar pulmonary artery, LPA = left pulmonary artery, MPA = main pulmonary artery, RA1–RA10 = right segmental pulmonary artery, RPA = right pulmonary artery, RILPA = right interlobar pulmonary artery, RULPA = right upper-lobe pulmonary artery.
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The relationship between negative and positive consistent values at the various tube current–time product settings is shown for each reader in Figures 4 and 5. When the tube current–time product was reduced from 90 to 10 mAs, we observed no significant difference in consistent values for each reader and each artery group (P = .19 to >.99 and .34 to >.99, respectively, for positive and negative consistent values).
The relationship of the branching order of arteries with the most distally located filling defect and with the tube current–time product settings is shown for each reader in Figure 6. When the tube current–time product was reduced from 90 to 10 mAs, we did not observe any significant difference in branching order (P = .41 to .55) from reader to reader.
The relationship between intravascular contrast quality and the tube current–time product settings is shown for each reader in Figure 7. We did not observe any significant difference in the quality score from 90 to 40 mAs for any reader (P = .10 to >.99), but we did observe a significant difference at 20 and 10 mAs for reader 1 (P = .005), and at 10 mAs for reader 2 (P = .02) and reader 3 (P = .003).
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Figure 7. Graph shows variations in quality score of intravascular contrast as a function of tube current–time product. Significant reductions in the quality score were observed for reader 1 at settings lower than 40 mAs (P = .005) and for readers 2 and 3 at settings lower than 20 mAs (P = .02 and P = .003, respectively).
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DISCUSSION
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The results of our study show that positive and negative consistent values of CT pulmonary angiography for the diagnosis of pulmonary embolism, as well as values for inter- and intrareader agreement, remain stable when the tube current–time product is reduced from 90 to 10 mAs. In addition, identification of the branching order of the artery with the most distal filling defect is not influenced by the consequent dose reduction. However, intravascular contrast quality decreased, at least for one reader, when the tube current–time product setting was lower than 40 mAs. Our study results show that although the level of intrareader agreement depends on the reader's experience and the location of the emboli, it is not influenced by radiation dose. Thus, the reduction of the tube current–time product setting to 40 mAs to achieve a reduced radiation dose at CT pulmonary angiography appears acceptable.
We observed levels of intra- and interreader agreement that were lower than those usually reported. These lower levels of agreement are likely due to the fact that our readers were required to identify emboli in more distal arterial segments than is usual (24,25) and to record the specific location of each embolus rather than merely to classify the examination result as positive or negative. One might expect that readers with perfect intrareader agreement would have achieved consistent values of 100% between the first reading session with 90 mAs and the second reading session with the same tube current–time product. Nonetheless, the corresponding consistent values for experienced readers ranged from 77% to 95%, with a difference of 5%–23% between the values achieved and those expected. For decreases in tube current–time product from 90 to 10 mAs, however, consistent values did not vary significantly. In other words, the repetition of the reading had a much greater influence on consistent values than did the reduction of the radiation dose.
The number of inconclusive results was higher for reader 3, a medical student with no training in radiology or experience in medical imaging, than for readers 1 and 2, who were experienced radiologists. This number was not influenced, however, by dose reductions. In addition, none of the radiation dose reductions in our study resulted in the filling defect becoming undetectable; we cannot, however, conclude that a tube current–time product of 10 mAs would be adequate for imaging in patients of any body habitus, as data from obese patients were not included in our study (the body mass index ranged from 20.7 to 28.3 kg/m2).
Obtaining a true-negative or true-positive diagnosis of pulmonary embolism is the primary objective for imaging evaluation of patients in whom pulmonary embolism is suspected; risk from radiation dose can, in principle, be regarded as a less important consideration. However, since CT pulmonary angiography is now often the first imaging technique used in these patients and since a high number of patients prove not to have pulmonary embolism, the risk-benefit ratio of radiation exposure is becoming a more important issue. Dose reduction (in accordance with the patient's body size) should, therefore, be recommended. A recent survey of practices and policies for the use of CT pulmonary angiography (26) in pregnant women has revealed that the method most often used for reducing the radiation dose consists of a reduction in z-axis coverage. On the basis of the results of our study, we recommend reducing the tube current–time product, as well.
Our study had some limitations. First, scans with a low tube current–time product simulated by adding random noise to the raw data may not correspond exactly to scans actually acquired in patients with a low tube current–time product. Nevertheless, in a validation trial of unenhanced CT, experienced chest radiologists were unable to distinguish CT scans obtained with a simulated reduced dose from those obtained with an actual reduced dose (13). Furthermore, there is no logical reason to believe that contrast-enhanced CT pulmonary angiography would differ from unenhanced CT with regard to simulated dose reductions. Second, the data to which we had access did not include data from obese patients. However, as the effective dose is lower in obese patients than in those with a lower body mass index (27), the need for reduction of the tube current–time product appears less critical in obese patients. Nonetheless, dose reduction should be investigated in such individuals. Third, as only patients who actually had a filling defect were included in our study sample and as we had no independent method of reference, we were unable to investigate the possible influence of dose reduction on the false-positive rate at CT pulmonary angiography. As opposed to the usual clinical setting, which is characterized by a majority of patients with no definite pulmonary embolism, our recruitment process may have biased the study results. Consequently, the results must be considered only with respect to the stability of findings at reduced radiation dose levels. Fourth, we did not address the possible influence of dose reduction on mixing artifacts. At our workstation, we differentiated intraarterial clots from mixing artifacts (ie, artifacts caused by inhomogeneous mixing of the contrast material and blood) by considering the contours of the hypoattenuated area on multiplanar reformatted images and by measuring the attenuation in that area. Because a reduction in radiation dose results in an increase in the standard deviation of the attenuation values calculated in the region of interest but not in an increase in the attenuation values themselves, dose reduction probably does not influence the ability to discriminate between these entities.
In conclusion, the parameters that we evaluated remained stable with reduction of the radiation dose from 90 (effective) to 10 (simulated) mAs for multi–detector row CT pulmonary angiography. Further research is needed to optimize tube current–time product settings in a larger group of patients in whom pulmonary embolism is suspected, including obese patients, with regard to improving both the differential diagnosis of pulmonary embolism and patient outcome.
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FOOTNOTES
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Author contributions: Guarantors of integrity of entire study, D.T., P.A.G.; study concepts and design, all authors; literature research, W.P., D.T., P.A.G., V.D.M.; clinical studies, D.T., W.P., P.S., P.M.; data acquisition, D.T., W.P., P.S., P.M., C.S.; data analysis/interpretation, D.T., V.D.M., W.P., P.A.G.; statistical analysis, V.D.M., P.A.G., D.T.; manuscript preparation, D.T., V.D.M., C.S., P.A.G.; manuscript editing, D.T., P.A.G.; manuscript definition of intellectual content, revision/review, and final version approval, all authors
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T. Kubo, P.-J. P. Lin, W. Stiller, M. Takahashi, H.-U. Kauczor, Y. Ohno, and H. Hatabu
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J. D. MacKenzie, J. Nazario-Larrieu, T. Cai, M. S. Ledbetter, M. A. Duran-Mendicuti, P. F. Judy, and F. J. Rybicki
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C. M. Kramer, M. J. Budoff, Z. A. Fayad, V. A. Ferrari, C. Goldman, J. R. Lesser, E. T. Martin, S. Rajagopalan, J. P. Reilly, G. P. Rodgers, et al.
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Vascular Medicine,
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C. M. Heyer, P. S. Mohr, S. P. Lemburg, S. A. Peters, and V. Nicolas
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C. M. Kramer, M. J. Budoff, Z. A. Fayad, V. A. Ferrari, C. Goldman, J. R. Lesser, E. T. Martin, S. Rajagopalan, J. P. Reilly, G. P. Rodgers, et al.
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A. Madani, V. De Maertelaer, J. Zanen, and P. A. Gevenois
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E. Coche, S. Vynckier, and M. Octave-Prignot
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TOP
ABSTRACT
INTRODUCTION
], upper-lobe segmental [
), reader 2 (