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Thoracic Imaging |
1 From the Department of Radiology, University of Michigan Health System, 1500 E Medical Center Dr, TC2910, Ann Arbor, MI 48109-0326. From the 1999 RSNA scientific assembly. Received July 2, 2001; revision requested August 20; revision received August 1, 2002; accepted September 26. Address correspondence to S.P. (e-mail: smitap@umich.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Sixty patients were examined with one of three techniques (20 patients each). Group 1 was examined with single–detector row CT with 3-mm collimation and 1.3–1.6 pitch; groups 2 and 3, with multi–detector row CT with 2.5- and 1.25-mm collimation, respectively. Three thoracic radiologists independently reviewed examination findings to determine if each main, lobar, segmental, and subsegmental artery was well visualized for presence of pulmonary embolism. 
2 tests were performed. For well-visualized vessels, the presence and/or absence of pulmonary embolism was recorded and 
statistic was determined.
RESULTS: Reader 1 scored 95% (114 of 120), 96% (115 of 120), and 99% (119 of 120) of lobar arteries (P > .05); 76% (304 of 400), 86% (346 of 400), and 91% (363 of 400) of segmental arteries (P < .001); and 37% (300 of 800), 56% (448 of 800), and 76% (608 of 800) of subsegmental arteries as well visualized (P < .001) using techniques 1, 2, and 3, respectively. Reader 2 scored 97% (116 of 120), 95% (114 of 120), and 99% (119 of 120) of lobar arteries (P > .05); 77% (308 of 400), 87% (349 of 400), and 93% (371 of 400) of segmental arteries (P < .001); and 39% (310 of 800), 53% (422 of 800), and 78% (621 of 800) of subsegmental arteries (P < .001) as well visualized using techniques 1, 2, and 3, respectively. Reader 3 scored 86% (103 of 120), 82% (98 of 120), and 91% (109 of 120) of lobar arteries (P > .05); 63% (252 of 400), 70% (280 of 400), and 85% (339 of 400) of segmental arteries (P < .001); and 39% (310 of 800), 56% (451 of 800), and 71% (572 of 800) of subsegmental arteries (P < .001) as well visualized using techniques 1, 2, and 3, respectively. Sixteen patients had pulmonary embolism. Interobserver agreement for detection of pulmonary embolism was significantly better for segmental and subsegmental arteries for all readers with technique 3 (segmental, = 0.79–0.80; subsegmental, = 0.71–0.76) than that with technique 1 (segmental, = 0.47–0.75; subsegmental, = 0.28–0.54).
CONCLUSION: Multi–detector row CT at 1.25-mm collimation significantly improves visualization of segmental and subsegmental arteries and interobserver agreement in detection of pulmonary embolism.
© RSNA, 2003
Index terms: Computed tomography (CT), angiography, 944.12916 • Computed tomography (CT), comparative studies • Computed tomography (CT), technology, 944.12912, 944.12914, 944.12915, 944.12916, 944.12919 • Embolism, pulmonary, 60.72, 944.77 • Pulmonary arteries, CT, 944.12912, 944.12914, 944.12915, 944.12916, 944.12919
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Newer and faster helical computed tomographic (CT) scanners have greatly improved the noninvasive visualization of the pulmonary arteries for patients suspected of having acute pulmonary thromboembolic disease (10–15). In a recent study in which V-Q scanning was compared with helical CT angiography in 179 patients, Blachere et al (16) suggested that CT angiography could replace V-Q scanning as the initial diagnostic imaging test for suspected pulmonary embolism, demonstrating statistically significant greater accuracy of CT angiography compared with that of V-Q scanning. An additional advantage of CT over V-Q scanning is the detection of diseases, such as acute pneumonia, pleuritis with a pleural effusion, pericardial effusion, interstitial pulmonary fibrosis, and malignancy, that mimic the signs and symptoms of pulmonary embolism in 40%–50% of patients undergoing CT (15–20).
A limitation of helical CT has been sensitivity at the subsegmental pulmonary artery level (14,21). For instance, Remy-Jardin et al (22) reported that only 37% of subsegmental arteries are well seen with the 3-mm collimation helical CT technique, which is important since most reports comparing helical CT angiography with catheter pulmonary angiography have used the single–detector row CT 3- or 5-mm collimation technique (12). The recent introduction of multi–detector row CT scanners, which permit even faster scanning at thinner collimation over a greater scanning volume, should improve the visualization of subsegmental pulmonary arteries. The purpose of our study was to compare the frequency of well-visualized pulmonary arteries according to anatomic level by using different collimation with single– and multi–detector row CT scanners in patients undergoing CT for suspected acute pulmonary embolism.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Scanning Techniques
The following three scanning techniques were used: Group 1 was examined with single–detector row helical CT (HiSpeed CT/I; GE Medical Systems, Milwaukee, Wis) at 3-mm collimation and 1.3–1.6 pitch adjusted for patient size and breath-holding capacity; group 2, with multi–detector row helical CT (LightSpeed QX/I; GE Medical Systems) at 2.5-mm collimation (high-speed mode); and group 3, with multi–detector row helical CT (LightSpeed QX/I; GE Medical Systems) at 1.25-mm collimation (high-speed mode). Overlapping reconstruction at 50% scanning collimation was performed on all data sets. All studies were performed with 150 mL of iohexol 300 (Omnipaque-300; Nycomed, Princeton, NJ) administered at a rate of 4 mL/sec with an automated injector device (Liebel-Flarsheim; Mallinckrodt, Cincinnati, Ohio) through a 20-gauge peripheral intravenous catheter located in the antecubital vein. The scanning delay time was 20 seconds in group 1, in which single–detector row CT was used, and 25 seconds in groups 2 and 3, in which multi–detector row CT was used. The mean acquisition time was 40 seconds in group 1 and 18 seconds in groups 2 and 3. CT scans were obtained from the caudal to cranial direction during suspended inspiration.
Image Interpretation
All 60 CT scans were loaded onto a workstation (GE Advantage Windows software, version 3.1; GE Medical Systems) for interpretation. Three thoracic radiologists (S.P., E.A.K., P.N.C.) blinded to all clinical data and the CT report of a positive or negative scan of pulmonary embolism independently reviewed each CT scan on the computer workstation. Cases were reviewed by using random ordering of all 60 cases. The readers were allowed to manipulate the window width and level settings and to use the scrolling mode for interpretation. Readers scored the main, lobar, segmental, and subsegmental arteries as well visualized, suboptimally visualized, or not visualized at all. An artery was defined as well visualized when there was a motion-free vessel with uniform intravenous contrast enhancement to confidently diagnose presence or absence of a clot, and it was suboptimally visualized when the artery was found but was not seen adequately for readers to confidently diagnose presence or absence of a clot due to motion artifact, streak artifact, partial-volume averaging, or poor contrast enhancement. If the arteries were well visualized, readers recorded the presence or absence of a pulmonary embolus. The arteries were named according to the standard nomenclature from Boyden (23) and Jackson and Huber (24). By using a scoring sheet similar to that used by Remy Jardin et al (22), a total of five lobar, 20 segmental, and 40 subsegmental arteries were evaluated in each patient. When a pulmonary artery was not visualized, readers were asked to indicate a possible reason for nonvisualization, such as insufficient vascular opacification with intravenous contrast material, patient motion, or lung abnormality.
Statistical Analysis
The proportion of arteries that were well visualized at the lobar, segmental, and subsegmental levels for each of the three CT techniques was compared with each reader by using a 3 x 2 2 test. Comparison across readers and scanning techniques was also made by calculation of the percentage of well-visualized arteries with 95% CIs. The interobserver agreement for the presence or absence of pulmonary embolism was calculated at the segmental and subsegmental levels with different CT techniques by using a statistic. The value is an adjustment of the percentage of agreement to accommodate for chance agreement. The following values have been suggested to define interobserver agreement: poor (<0), slight (0–0.200), fair (0.210–0.400), moderate (0.410–0.600), substantial (0.610–0.800), and excellent (0.810–1.000) (25).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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2 test) with techniques 1, 2, and 3, respectively. Reader 2 identified 97%, 96%, and 99% of lobar pulmonary arteries (P > .05, 2 test) with techniques 1, 2, and 3, respectively. Reader 3 identified 86%, 82%, and 91% of lobar pulmonary arteries (P > .05, 2 test) with techniques 1, 2, and 3, respectively. All readers demonstrated incremental improvement in visualization of the segmental and subsegmental pulmonary arteries with decrease in scanning collimation. The respective percentages of nonvisualized pulmonary arteries at the lobar, segmental, and subsegmental levels by reader 1 were 0%, 3%, and 25% with technique 1; 0%, 4%, and 14% with technique 2; and 0%, 1%, and 6% with technique 3. The respective percentages of nonvisualized pulmonary arteries at the lobar, segmental, and subsegmental levels by reader 2 were 0%, 2%, and 24% with technique 1; 0%, 4%, and 16% with technique 2; and 1%, 0.5%, and 5% with technique 3. The respective percentages of nonvisualized pulmonary arteries at the lobar, segmental, and subsegmental levels by reader 3 were 0%, 5.5%, and 19% with technique 1; 1%, 2%, and 9% with technique 2; and 1%, 2%, and 7% with technique 3.
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values (Table 3). The value is an adjustment of the percentage of agreement to accommodate for chance agreement. With 3-mm collimation, the values between paired readers were 0.47–0.75 at the segmental level and 0.28–0.54 at the subsegmental level. Consistently higher values were obtained by all readers at both the segmental ( = 0.79–0.80) and the subsegmental ( = 0.71–0.76) levels by using 1.25-mm collimation.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Pulmonary angiography has been considered the standard imaging modality for the diagnosis of pulmonary embolism in vivo. It allows direct imaging of the pulmonary arteries and is relatively safe, with a procedure mortality rate of 0.5% and a major complication rate of 1% (29,30). However, given the underutilization of pulmonary angiography, there is a need for a noninvasive test for the diagnosis of pulmonary embolism. Furthermore, pulmonary angiography is an invasive procedure that is consistently accurate at the lobar and segmental levels but has wide interobserver variability at the subsegmental arterial level. The rate of overall interobserver variability can approach 10%–15% with pulmonary angiography and is higher for smaller vessels (30–33). For instance, in the Prospective Investigation of Pulmonary Embolism Diagnosis, or PIOPED, study (6), 6% of patients had pulmonary embolism limited to subsegmental pulmonary arteries. The co-positivity among expert angiographers for pulmonary embolism limited to subsegmental arteries was 66% (6,30). Quinn et al (33) reported an agreement at the subsegmental level in only 13% of cases and Diffin et al (32), in 17%. In the latter group, the initial interobserver agreement was 45%, with a unanimous consensus agreement in 79% of patients who had isolated subsegmental pulmonary embolism. The detection of small subsegmental emboli may be worse with commonly used digital subtraction angiography because of cardiac pulsation artifact.
In the past few years, CT angiography and MR angiography have been pursued for noninvasive visualization of the pulmonary arteries. Helical CT has had a tremendous effect on the evaluation of patients suspected of having pulmonary embolism (7,10–12,15,22,34). By comparing helical CT with pulmonary angiography, consistent accuracy has been demonstrated at the lobar level, with a wider range of accuracy reported at the segmental arterial level. Accuracy at the subsegmental level has been consistently poor with CT, pulmonary angiography, and magnetic resonance (MR) imaging. For example, Goodman et al (12) performed helical CT of the chest and selective pulmonary angiography in patients with an unresolved suspicion for pulmonary embolism after V-Q scanning with 5-mm collimation, pitch of 1, and overlapping 3-mm reconstructions. The subsegmental arteries were difficult to identify at CT, with isolated subsegmental emboli at pulmonary angiography identified at CT in only one of four patients. The sensitivity of helical CT for pulmonary embolism was 86% at the lobar and segmental levels, and the specificity was 92%. However, when the lobar, segmental, and subsegmental arteries were considered together, the sensitivity decreased to 63% and specificity remained high at 89%. In another series, similar results were found by using a collimation of 3 mm and pitch of 2:1 (35).
In the series by Oser et al (36), 17% of patients had isolated subsegmental emboli. The authors concluded that if helical CT can depict emboli only in the segmental and lobar arteries, then subsegmental emboli in 30% of patients would be missed. They suggested that evolving and newer CT techniques might be a more accurate diagnostic tool for pulmonary embolism detection at the subsegmental level. Ghaye et al (37) investigated the best possible visualization of small arteries using multi–detector row CT with optimal contrast enhancement and no pleural or parenchymal disease in a subset of 30 patients who were selected out of a group of 130 clinical patients suspected of having pulmonary embolism. In this ideal patient group, 94% of the subsegmental (fourth-order) and 74% of the fifth-order pulmonary arteries were analyzable with images reconstructed at 1.25-mm collimation, compared with 82% and 47%, respectively, of arteries being analyzable with images reconstructed at 3-mm collimation.
Baile et al (38) demonstrated that there was no difference in pulmonary embolism detection between spiral CT pulmonary angiography and pulmonary angiography at the subsegmental level when methacrylate beads were injected into the pulmonary arterial tree of 16 pigs. Sensitivity for detecting pulmonary emboli at 3-mm collimation, 1-mm collimation at CT pulmonary angiography, and at pulmonary angiography was 82%, 87%, and 87%, respectively, and the specificity was 94%, 81%, and 88%, respectively. They concluded that spiral CT is comparable with pulmonary angiography in detection of pulmonary embolism (38). Qanadli et al (39) compared dual-section helical CT at 2.7-mm effective section thickness with pulmonary arteriography in 158 patients and found that helical CT had a high sensitivity and specificity in the detection of pulmonary embolism. Selective pulmonary angiography was used as a standard of reference. In their study, 92 subsegmental emboli were depicted with CT, compared with only 56 depicted at pulmonary angiography; interobserver agreement was slightly better with CT ( = 0.78–0.94) than it was with pulmonary angiography ( = 0.67–0.89). They concluded that in the majority of patients, helical CT could replace pulmonary angiography in the depiction of pulmonary embolism (39).
The subsegmental pulmonary arteries are also not well assessed with gadolinium-enhanced MR angiography (40). Authors of a recent study using MR angiography and comparing it with digital subtraction angiography found a high accuracy of MR angiography in depicting the lobar and segmental emboli, but it was unable to depict four of the five subsegmental emboli, resulting in a sensitivity of 68% and a specificity of 99%. When the small subsegmental emboli were ignored, the sensitivity increased to 87% and the specificity remained high at 100%. Gupta et al (41) compared MR angiography with digital subtraction angiography in 36 patients and also concluded that MR angiography had a high accuracy of depicting lobar and segmental pulmonary embolism, but it missed four of the five subsegmental emboli.
Using a single–detector helical CT, Remy Jardin et al (22) compared a 3-mm collimation technique with a 2-mm collimation technique in a total of 40 patients; 20 were examined with each technique. The percentage of analyzable well-visualized segmental and subsegmental arteries improved from 85% and 37%, respectively, with the 3-mm collimation technique, to 93% and 61%, respectively, with the 2-mm collimation technique. However, patients had to meet strict entry criteria. Patients with a history of thoracic surgery, lung distortion, or parenchymal infiltration and suspected or proven primary or secondary pulmonary hypertension were excluded. Moreover, their scans had to be technically acceptable, and the acquisition had to be performed during strict inspiratory apnea with a high or at least sufficient enhancement of the pulmonary arteries. These strict criteria likely contributed to an overestimate of analyzable well-visualized segmental and subsegmental arteries than would be seen in patients suspected of having pulmonary embolism, many of whom have lung or pleural disease and have difficulty holding their breath. Recent advances in CT technology, specifically with the advent of multi–detector four-row CT, allow faster scanning of larger volumes at thinner collimation without tube-cooling problems. These advantages should translate into improvements in the evaluation of segmental and subsegmental pulmonary artery visualization, with improved accuracy at identifying thrombi in these arteries.
If pulmonary thromboemboli are to be detected, the arteries must be well visualized. We demonstrate a significant improvement in the proportion of well-visualized segmental and subsegmental pulmonary arteries between single–detector row CT at 3-mm collimation and multi–detector row CT at both 2.5- and 1.25-mm collimation, with the greatest gain with 1.25-mm collimation. These gains were seen for all three readers. Although our results at the subsegmental level are only slightly better than those of Remy-Jardin et al (22), they are likely a true representation of a more realistic population of consecutive patients examined for suspected acute pulmonary embolism, as we did not apply any technical exclusion criteria related to the examination quality, surgical history, or examination findings (Fig 2). The higher detection rate of subsegmental pulmonary embolism with multi–detector row CT is equivalent to that of pulmonary angiography. Furthermore, not only are the segmental and subsegmental arteries better seen with multi–detector row CT but there is better interobserver agreement for detection of pulmonary embolism with the 1.25-mm collimation technique. These results suggest that multi–detector row CT may be a noninvasive replacement for angiography in the setting of suspected acute pulmonary embolism.
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CT pulmonary angiography as a diagnostic tool for suspected pulmonary embolism is safe, readily available, and cost effective. Recent advances in CT technology with the proliferation of multi–detector row CT scanners allow faster scanning of larger volumes at thinner collimation, improving both the visualization of segmental and subsegmental pulmonary arteries and interobserver agreement about the presence or absence of pulmonary embolism. Since this study was completed, 16–detector row CT scanners are becoming increasingly prevalent. These results should translate into improved detection of pulmonary embolism and wider acceptance of CT angiography for suspected acute pulmonary embolism.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, S.P., E.A.K.; study concepts and design, S.P., E.A.K.; literature research, S.P., E.A.K.; clinical studies, S.P., E.A.K., P.N.C.; data acquisition and analysis/interpretation, S.P., E.A.K., P.N.C.; statistical analysis, E.A.K.; manuscript preparation, definition of intellectual content, editing, revision/review, and final version approval, S.P., E.A.K.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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