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Thoracic Imaging |
1 From the Departments of Radiology (H.T.W.M., J.R., M.S.J., R.D.T., H.S., J.N., D.C., S.G.J., J.Y., S.O.T., K.K.K.) and Medicine, Division of Pulmonary, Critical Care, and Occupational Medicine (M.D.W.), Indiana University School of Medicine, Indianapolis. Received October 29, 2003; revision requested January 20, 2004; final revision received April 15; accepted June 15. Supported by a research grant from Philips Medical Systems, Cleveland, Ohio. Address correspondence to H.T.W.M., 11224 Clarkston Rd, Zionsville, IN 46077 (e-mail: hwinermu@iupui.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Patients referred for PA to assess suspected PE were eligible. Institutional review board approval and written informed consent were obtained. All patients underwent CT and PA within a 48-hour period. For CT, 4 x 2.5-mm collimation was used. Three readers independently evaluated each study for PE presence. PE status, vessel level, and lobar location were determined by means of majority rule, and interobserver agreement (
) was calculated for PE status, as assessed with each modality. Sensitivity and specificity of CT were calculated by using PA as the reference standard. Two radiologists later reviewed false-positive CT studies.
RESULTS: The study group comprised 93 patients (median age, 56 years; range, 19–88 years). Sensitivity, specificity, and accuracy of CT were 100%, 89%, and 91%, respectively. values were 0.71 and 0.83 for CT and PA, respectively, and were not significantly different between modalities. At PA, 18 patients (19%) had PE at 50 vessel levels (five main and/or interlobar, 24 segmental, and 21 subsegmental), 17 (94%) of which had PE at multiple sites. At CT, 26 patients (28%) had PE at 71 vessel levels (24 main and/or interlobar, 33 segmental, and 14 subsegmental). Twenty patients (77%) had PE at multiple sites. Review of eight false-positive CT studies showed an appearance highly suggestive of acute PE in three patients, chronic PE in one, and no PE in three; one study was inconclusive. CT better demonstrated large-level vessel involvement (P < .01), while PA better demonstrated small-level vessel involvement (P < .01).
CONCLUSION: Multi–detector row CT has an accuracy of 91% in the depiction of suspected acute PE when conventional PA is used as the reference standard.
© RSNA, 2004
Index terms: Angiography, comparative studies, 60.1241 • Computed tomography (CT), multi–detector row, 60.12118 • Embolism, pulmonary, 60.72 • Lung, CT, 60.12118
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Few studies, however, have compared the results of CT and PA (4–9). For single–detector row helical CT, sensitivity and specificity of CT in the detection of PE varies from 53% to 91%, and 78% to 97%, respectively. Drucker et al (6) performed a prospective study of 47 patients who underwent both single–detector row CT and PA within 24 hours. Two readers interpreted the results obtained with each modality; sensitivity, specificity, and accuracy were 53%–60%, 81%–97%, and 75%–83%, respectively. The authors concluded that the suboptimal sensitivity of CT for the diagnosis of PE potentially limits the clinical applicability of this modality. The development of multi–detector row CT has made it possible to image the entire chest with high in-plane (x- and y-axis) and improved through-plane (z-axis) resolution in a single breath hold (10); however, we are aware of only one clinical study in which the diagnostic accuracy of multi–detector row CT was compared with that of PA (7). In that study of 157 patients, the sensitivity and specificity of dual-section CT were 90% and 94%, respectively.
The purpose of our study was to determine the diagnostic accuracy of multi–detector row CT in emergency room and inpatient populations suspected of having acute PE who prospectively underwent imaging with both CT and PA.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Patients who were suspected of having acute PE on the basis of clinical presentation were eligible for the study. Exclusion criteria included age of less than 18 years, serum creatinine levels of more than 1.5 mg/dL (132.6 µmol/L) within the previous 24 hours (unless the patient was undergoing hemodialysis for chronic renal failure), history of severe allergic reaction to iodinated contrast material, pregnancy or possibility of pregnancy, and recent lower-extremity US study that demonstrated deep venous thrombosis.
In all patients, CT and PA were performed within 48 hours of each other. Imaging with either modality could be performed first; the order reflected availability of imaging equipment and/or personnel at the time the patient entered the study. Clinical treatment decisions relating to the presence or absence of PE were based solely on the PA interpretation at the time of the examination. The CT findings, except those related to the presence or absence of PE, were reported to the referring physician. At the time this study was performed (1999–2001), PA was considered to be the reference standard and was performed at our institution when a definitive diagnosis was necessary to initiate or stop treatment. The consent form specified that PA was used as the reference standard to determine if PE was present and that PA had been ordered by the patient’s physician. Furthermore, it was clearly stated that treatment would be based solely on the PA findings. The information about possible PE obtained with CT would not be provided to the patients’ doctors and would not be used for the patients’ medical care, since the accuracy of multi–detector row CT in showing PE was not known and was being tested. If the CT study provided additional information unrelated to PE, however, that information would be given to the patients’ physicians.
PA Technique
All studies were performed with either a DFP-2000A angiographic unit (Toshiba, Kawasaki, Japan) or an Integris V3000 angiographic unit (Philips Medical Systems, Eindhoven, the Netherlands). The machines varied in their maximum filming rate: The Integris V3000 unit could obtain 7.5 images per second while maintaining a 1024 x 1024 matrix, whereas the DFP-2000A unit could obtain 10 images per second while maintaining the same matrix.
Examinations were performed as follows: Each patient’s venous system was accessed via one of the common femoral or internal jugular veins, and a vascular sheath was introduced. A directional flush catheter, most commonly a 6-F catheter (GPC: Grollman pigtail catheter; Cook, Bloomington, Ind), was introduced through the sheath into either of the pulmonary arteries. The image intensifier was positioned in the ipsilateral anterior oblique projection and centered over the chest. Nonionic contrast material, either iopamidol (Isovue 370; Bracco Diagnostics, Princeton, NJ) or iohexol (Omnipaque 350; Amersham Health, Princeton, NJ), was injected at a rate of 25 mL/sec for 1 second. Images were acquired with a matrix of 1024 x 1024. Prior to the injection of contrast material, multiple mask images (four to six images) were obtained. Imaging was continued until the pulmonary veins were opacified, which usually occurred about 4 seconds after injection of contrast material. The image intensifier was then positioned to obtain posteroanterior images. These images were obtained by using the same dose and imaging parameters as those used to obtain oblique images. After images for one side were obtained, the catheter was repositioned in the contralateral pulmonary artery, and similar (eg, posteroanterior and ipsilateral oblique) images were obtained. Thus, in most patients, bilateral PA was completed by using less than 120 mL of contrast material. PA images were recorded on either videotapes or magneto-optical disks.
CT Technique
All CT examinations were performed by using a four-channel multi–detector row CT scanner (MX8000; Philips Medical Systems, Cleveland, Ohio). Scanning was performed in a caudocranial direction by using 10-mm nominal collimation (4 x 2.5 mm), with an effective section width of 3.2 mm, a gantry rotation speed of 0.5 seconds, a table speed of 20 mm/sec, a pitch of 1, a tube voltage of 120 kV, and a tube current of 200–300 mAs. With these imaging parameters, the entire chest could be imaged in 14–17 seconds. Each patient was instructed to try to hold his or her breath for the entire examination; if the patient was unable to do so, he or she was instructed to breathe lightly. Each examination was reconstructed at an increment of 1.3 mm, which yielded approximately 240 images. All images were saved to magneto-optical disks.
Scanning was performed during intravenous administration of the low-osmolar nonionic iodinated contrast material iopamidol (Isovue 300; Bracco Diagnostics). Contrast material was injected with a 20-gauge catheter into the antecubital vein with a monophasic technique at a rate of 4 mL/sec. The total amount of contrast material injected was 120 mL, with an average scan delay of 20 seconds (range, 17–22 seconds) from the start of injection to the start of scanning. Bolus tracking software was not available, and test bolus injections were not performed. Shorter scan delays were used in younger patients without a history of cardiac disease, and longer delays were used in older patients with a history of cardiac disease.
Image Analysis
Three interventional radiologists (H.S., J.N., S.O.T.) with 10, 5, and 10 years of experience, respectively, in the acquisition and interpretation of PA images independently and retrospectively reviewed the PA studies. Unless the reader performed the examination, each reader was blinded to the clinical information and interpretation of the PA study at the time that the examination was performed. In 36 patients, one of the three PA readers performed the clinical procedure. Each reader was blinded to CT findings and interpretations. Six additional interventional radiologists with 1, 2, 3, 5, 8, and 10 years of experience performed PA procedures during the period of the study at our institution.
The stored images obtained with each PA examination were reviewed by each reader and assessed to determine if PE was present. Because image playback was available as a feature of the angiographic unit console, readers were able to process images as they desired (remask individual images and magnify portions of images). For each examination, each reader noted the following: (a) the presence or absence of acute PE, (b) the level of vessel involved with PE (eg, main and/or interlobar, segmental, or subsegmental), and (c) the lobar location of PE (eg, the right-upper, middle, and/or right-lower lobe and the left-upper, lingula, and/or left-lower lobe). Acute PE was defined as the presence of a filling defect in an arterial branch. The main and/or interlobar location included the main, right and left main, truncus anterior, and right and left interlobar arteries. As right and left segmental and subsegmental vessels were also evaluated, 10 vessel levels were analyzed in each patient. The lobar arteries, excluding the truncus anterior artery, were recorded together with segmental vessel involvement. If multiple same-level emboli were observed in one lobe, those were tabulated as one positive finding. For example, two segmental emboli in the left lower lobe were tabulated as one positive finding. Each reader completed a form that included the previously mentioned data.
Since the study was directed toward acute PE, patients who had only chronic PE (eg, organizing mural thrombus and/or recanalization of the lumen) were categorized as negative for PE. Findings suggestive of chronic PE were defined as dilatation of the central pulmonary arteries, areas of poor perfusion, and tortuous and pruned peripheral arteries with strictures, webs, or both. Those findings might be seen with pulmonary hypertension resulting from a cause other than emboli as well, but they are highly suggestive of chronic PE in a person with an appropriate history (eg, a history of chronic venous thromboembolic disease).
Three chest radiologists (H.T.W.M., R.D.T., D.C.) with more than 15 years of experience in the interpretation of chest CT studies independently and retrospectively reviewed the CT studies. Each study was displayed at an MxView workstation (Philips Medical Systems). Each reader could alter the window width and level settings and zoom into areas of interest. Each reader was unaware of the PA findings and interpretations and had no knowledge of clinical information, other than the fact that acute PE was suspected on the basis of clinical signs and symptoms.
For CT, the presence of acute PE was defined as a low-attenuation filling defect noted after enhancement of pulmonary arteries. For each CT scan, each reader noted whether PE was present, the vessel level involved, and the lobar location—in the same manner as was done with the PA images—by using an identical form. Each CT reader noted if contrast enhancement was suboptimal.
Since the study was directed toward acute PE, patients with only chronic PE (organizing mural thrombus and/or recanalization of the lumen) were categorized as negative for PE. Chronic PE was defined as an eccentric crescentic mural-adherent filling defect with irregular margination.
Statistical Analysis
For each modality, presence or absence of PE was determined by means of majority rule (eg, the diagnosis reached by at least two of the radiologists was designated as truth for that modality). Sensitivity and specificity were also computed by using majority rule. The statistic for multiple readers was calculated for each modality (11). An SAS (SAS Institute, Cary, NC) macro called "magree" was used to calculate the statistic for the three readers for each modality (12). The level of vessel involvement and lobar location were also determined with majority rule. Both vessel involvement and lobar location were analyzed by using generalized linear models (13). Generalized estimating equations were used to deal with the non-independence of the binary response in the models (14). The computations were based on the SAS Genmod procedure (SAS Institute). For each modality, we tested the differences of diagnosis of vessel level and lobar location among readers. Then, for each modality and on the basis of majority rule, we estimated the odds that a particular vessel level and/or lobar location was involved with PE. The two modalities were compared with odds ratios, and hypothesis tests were used to investigate if they were equivalent. The mean age of men and women was compared by using a two-tailed unpaired t test. For all tests, a P value of less than .05 was considered to indicate a significant difference.
Consensus Analysis
After statistical analyses had been performed, two radiologists (H.T.W.M., J.R.) with 20 years of experience in interpreting CT scans worked together and re-reviewed the CT scans of each patient in whom the final diagnosis of PE differed between modalities. The CT appearance obtained with each examination was characterized as follows: (a) CT appearance that should be interpreted as positive for PE, (b) CT appearance that is inconclusive for PE, and (c) CT appearance that should be interpreted as negative for PE.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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, 0.83). Of these patients, 17 (94%) had PE at multiple sites. PE was detected in five main and/or interlobar, 24 segmental, and 21 subsegmental vessels (Table 2). There was no significant difference between the three readers in the interpretation of the level of vessel involvement (P = .08). A total of 57 PE were seen in lobar locations in 18 patients; 28 PE (49%) were located in the lower lobes, 16 (28%) in the upper lobes, eight (14%) in the middle lobe, and five (9%) in the lingula (Table 2). Thirty-one of the involved lobes were located on the right side of the body, and 26 were located on the left. There was a significant difference in the interpretations of lobar involvement between the three readers (P < .01).
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, 0.71), although it was not as high with CT as it was with PA (, 0.83); this difference was not significant (P = .65). Although three CT examinations were considered to have suboptimal contrast enhancement at majority rule, these examinations were included in the analysis.
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CT Consensus Analysis
There were eight patients whose condition was defined as false-positive for PE (CT findings were positive for PE, and PA findings were negative for PE). The lobar location and vessel level involved, as determined with CT, are shown in Table 5. Retrospective review of these CT examinations by two radiologists (H.T.W.M., J.R.) showed that three patients with false-positive findings were still considered to have a CT appearance similar to that in patients with acute PE (Figs 1–3). In all three patients, the abnormalities were in the right lower lobe in segmental and subsegmental vessels. Three patients were considered to not have any convincing evidence of acute PE (Figs 4–6). Findings in one patient remained inconclusive, even after review, because of bronchovascular interstitial thickening and adjacent parenchymal opacification (Fig 7). Findings were complex in another patient who had atrial septal defect and Eisenmenger complex; CT showed chronic intraarterial thromboses in markedly enlarged central pulmonary arteries. Two CT readers read this study as positive for PE, and one read it as negative for PE.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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There were eight patients in whom CT findings were considered false-positive for PE because PA studies were negative, and we defined PA as the reference standard prior to the study. On the basis of consensus review, however, the CT appearance in three of these patients was consistent with that in patients with acute PE. A reasonable argument can be made on the basis of the findings of this study that the sensitivity of CT may be higher than that of PA. If we believe that the CT findings in these three patients—which were originally considered false-positive—were indeed true-positive, then the sensitivity, specificity, and accuracy of CT are 100%, 93%, and 95%, respectively, and the sensitivity, specificity, and accuracy of PA are 86%, 100%, and 97%, respectively.
Some other interesting and unexpected findings were noted in this study. First, in central arteries, CT showed PE at significantly more vessel levels. One explanation may be that the interpretation of the extent of vessel involvement differed between the two modalities. For example, an embolus thought to be in the right main pulmonary and interlobar arteries with CT was considered to be in only the interlobar artery with PA. This occurred in only four of the 11 patients with central PE that was diagnosed with CT. Another explanation might be thrombus migration between studies; however, PA was performed first in eight of the 11 patients. Second, PA showed significantly more lobes involved with PE (right middle, left lower). Multi–detector row CT, as performed in this study, produced asymmetric spatial resolution (eg, 1 mm in the transverse plane and 3 mm in the longitudinal plane). Thus, emboli detection was certainly lower in vessels running in a transverse orientation. It should be noted that there was a significant difference in the lobes recorded as being positive for PE by the three PA readers.
While all patients with PE were identified with CT, CT failed to demonstrate subsegmental vessel involvement in 53% of patients with subsegmental PE at PA. This has been reported by other investigators (4–6,16). PA demonstrated isolated subsegmental embolus in only one patient in our study; CT showed both segmental and subsegmental emboli. Despite the almost unanimous acceptance of CT for evaluation of PE in clinical practice, many authors continue to describe the inability of CT to accurately depict small peripheral emboli.
The percentage of patients who have their largest emboli in subsegmental arteries is not known. It has been reported to vary from 6% to 30% (17,18). Some investigators have speculated that this may occur and be pertinent in patients with cardiopulmonary disease and that it could be a factor in the development of pulmonary hypertension in patients with chronic thromboembolic disease (10). The small emboli are indicators of deep venous thrombosis and may herald more severe embolic events (10). The clinical consequences of missing such an embolus are unclear. Routine use of smaller section widths and multiplanar reformatting may improve detection of PE in the fifth-order vessels (10,19). Ghaye et al (19) compared 1.25- and 3-mm section widths for the depiction of fifth- and sixth-order peripheral pulmonary vessels. There was a significant improvement for visualization of these vessels with the narrower section widths (19).
Acute PE is a potentially fatal disease; if death does not occur immediately, it may subsequently occur as a result of repeated events (20). In most radiologic practices, the number of patients suspected of having PE who undergo CT is increasing, while the number of patients who undergo scintigraphy and PA is decreasing (1). The logistic advantages of using CT in these patients are clear; CT is relatively easy to perform, and images can be acquired quickly. Once the patient is on the gantry table, an examination with multi–detector row CT takes approximately 20 seconds to perform with one breath hold. In addition, not only is the embolus directly visualized with CT, but other thoracic conditions that might be causing the patient’s symptoms can be evaluated.
We tried to the best of our ability to offer all patients who met entry criteria the opportunity to enroll in this study; however, we are certain that some patients were missed. We do not know how many patients either were eligible and not offered the opportunity to enroll or were offered the opportunity to enroll and refused to do so. Recruitment was difficult because referring physicians wanted to order only CT for their patients. The physicians had seen the exquisite images of PE in patients who underwent CT outside of this study, and they wanted to order only CT examinations for their patients suspected of having PE and not enroll them in a research study that required PA. Convincing the referring physicians to allow their patients to be recruited into the study was increasingly difficult as the study progressed over the course of 2 years. This was the reality of conducting a clinical study in an acute care setting where technology advanced faster than our ability to scientifically prove its accuracy.
Another limitation of the study was that the two examinations could have been performed as much as 48 hours apart. Unfortunately, we do not know whether any discrepancies between PA and CT findings were due to thrombolysis or embolus migration in the interval between studies; however, the mean and median interval between examinations was only 14 hours.
Since the three interventional radiologists were not excluded from the pool of nine clinical radiologists performing PA, there is a possibility of memory bias. In most patients, however, angiography was not performed by one of the three readers. Furthermore, each result required two of three readers to agree. A single reader’s knowledge of the clinical outcome could not in and of itself alter the interpretation of the PA study.
Use of 2.5-mm collimation rather than 1-mm collimation may be considered another limitation of the study. Since many of our patients were obese, we decided to use this collimation to produce images of better quality with less image noise. Furthermore, since most patients with PE are dyspneic, the shorter breath hold with 2.5-mm collimation may reduce image degradation from respiratory motion.
In summary, we have shown that four-channel multi–detector row CT is a sensitive and specific diagnostic test in patients suspected of having acute PE. CT has sufficient diagnostic accuracy to serve as the primary diagnostic modality in the exclusion of PE in most patients.
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FOOTNOTES |
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Abbreviations: PA = pulmonary arteriography, PE = pulmonary embolism
Author contributions: Guarantor of integrity of entire study, H.T.W.M.; study concepts, K.K.K., S.O.T.; study design, K.K.K., S.O.T., R.D.T.; literature research, H.T.W.M., K.K.K.; clinical studies, H.T.W.M., J.R., R.D.T., H.S., J.N., D.C., S.O.T.; data acquisition, S.G.J., M.D.W., K.K.K.; data analysis/interpretation, H.T.W.M., S.G.J., J.R., K.K.K.; statistical analysis, J.Y.; manuscript preparation, H.T.W.M., S.G.J., J.R., M.S.J.; manuscript definition of intellectual content, H.T.W.M., J.R., M.S.J., K.K.K.; manuscript editing, H.T.W.M., J.R., M.S.J., S.G.J., K.K.K., S.O.T.; manuscript revision/review, H.T.W.M., S.G.J., J.R., M.S.J., K.K.K.; manuscript final version approval, all authors
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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