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Cardiac Imaging |
1 From the Department of Radiology, Sifa Medical Center, Fevzipasa Boulevard No. 172/2, 35340 Basmane Izmir, Turkey. Received January 14, 2007; revision requested March 15; revision received March 23; accepted April 25; final version accepted June 11. Address correspondence to D.O. (e-mail: dilekoncel{at}hotmail.com).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEIMPLICATIONS FOR PATIENT CAREReferences |
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Materials and Methods: Institutional Review Board approval and informed consent were obtained. Fifteen consecutive patients (nine men, six women; mean age, 58.47 years) were examined. Image quality (good, moderate, or poor) and significant stenosis (>50%) were evaluated by two radiologists blinded to the conventional coronary angiography results. Sensitivity, specificity, positive predictive value (PPV), and negative predictive values (NPV) were calculated. McNemar test was used to search for any significant difference between dual-source CT and conventional coronary angiography in helping detect coronary stenosis. 
statistics were calculated for intermodality and interobserver agreement.
Results: Sixteen segments by reader 1 and 13 segments by reader 2 were considered as poor image quality and rejected for further analysis. All segments with good image quality were correctly diagnosed. The respective overall sensitivity, specificity, PPV, and NPV values were 87%, 98%, 77%, and 99% for reader 1 and 80%, 99%, 80%, and 99% for reader 2. No significant difference between dual-source CT and conventional coronary angiography was found in helping detect significant stenosis. statistics demonstrated good intermodality and excellent interobserver agreement.
Conclusion: Dual-source CT technology provides a temporal resolution that allows CT coronary angiography at higher heart rates and even with AF.
© RSNA, 2007
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEIMPLICATIONS FOR PATIENT CAREReferences |
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Apart from high heart rates, arrhythmia remained as a contraindication for CT angiography applications owing to between-beat variations, which led to inappropriate data sampling and resulted in severe motion artifacts (3–8). With the newly introduced dual-source CT, the main improvement is the increase in temporal resolution. The system combines two arrays, each consisting of an x-ray tube and a 64–section detector. With a rotation speed of 330 msec, a temporal resolution of 83 msec (one-fourth rotation) can be achieved independent of heart rate (9–11). This higher temporal resolution is expected to decrease motion artifacts in patients with high heart rates as well as patients with arrhythmia (10,11).
Atrial fibrillation (AF) is the most common type of arrhythmia and the incidence increases markedly with advancing age (12,13). AF is often associated with structural heart disease. Before initiating therapy, management of precipitating or reversible causes of AF is recommended (13). Coronary artery disease (CAD) is a cardiovascular condition associated with AF. Also, AF patients may manifest symptoms mimicking CAD. Therefore, the demonstration of CAD is an important issue in the management of AF patients (13).
As motion artifacts, owing to high heart rates, particularly arrhythmia, impair image quality to a great extent, AF has remained as a contraindication for CT angiography applications. Multidetector CT angiography was not considered as a diagnostic tool in the clinical workflow of AF, but the improved temporal resolution achieved with dual-source CT may improve the visualization of coronary vessels in patients with AF. Thus, the purpose of our study was to evaluate the sensitivity and specificity of dual-source CT for significant coronary stenosis (>50% narrowing) in patients with AF, by using conventional coronary angiography as the reference standard.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEIMPLICATIONS FOR PATIENT CAREReferences |
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Fifteen consecutive patients (nine men, six women; mean age ± standard deviation, 58.47 years ± 9.07; range, 45–74 years) with AF were examined with dual-source CT between September 2006 and January 2007. All patients were suspected of having coexisting CAD and were scheduled to undergo conventional angiography.
The exclusion criteria for CT were as follows: unstable clinical condition, previous allergy to iodinated contrast agents, elevated serum creatinine levels (>1.5 mg/dL, >132.6 µmol/L), previous bypass surgery or stenting, and failure to follow breath-hold commands. No patients were excluded on the basis of these criteria.
Conventional Coronary Angiography as Reference Standard
All patients underwent conventional coronary angiography with standard techniques by an experienced cardiologist (A.T., with 10 years coronary angiography experience) 1 day after CT examination. The cardiologist who evaluated angiograms was blinded to the results of CT angiography. The coronary arteries were segmented according to the guidelines of the American Heart Association (14). The vessel segments were evaluated by using the quantitative coronary analysis method (15). Lesions with a diameter reduction of 50% or more were considered to be hemodynamically significant lesions. All coronary vessel segments were included.
CT Coronary Angiography: Scan Protocol and Reconstruction
All CT examinations were performed with a dual-source CT scanner (Somatom Definition; Siemens, Forchheim, Germany).
CT scans were obtained with collimation, 64 sections of 0.6-mm thickness; rotation time, 0.33 msec; tube voltage 120 kV; effective charge, 900 mAs; pitch, 0.26–0.45 (depending on heart rate). The scan time was about 5.7–8.1 seconds for a single breath hold and the scan was performed in a craniocaudal direction. The patients did not receive any additional drugs prior to the CT examination for heart rate regulation.
The CT angiography was triggered automatically by the arrival of the main contrast material–enhanced bolus (automatic bolus tracking). We injected 70 mL nonionic contrast medium (Ultravist 350/mL; Iopromidum, Schering, Berlin, Germany) at a flow rate of 6 mL/sec. This was followed by a 50-mL saline chaser bolus (5 mL/sec) to washout contrast from the right ventricle.
During the scan, electrocardiography was recorded simultaneously. The retrospective reconstructions were done in all cardiac phases with 50-msec intervals. For the reconstruction of transverse images, we used a section thickness of 0.75 mm and section width of 0.5 mm; medium soft-tissue reconstruction kernels (B26f) were used. The reconstruction interval with the fewest motion artifacts was chosen and used for further analysis.
Prospective electrocardiographic tube current modulation (electrocardiographic pulsing) was not used to provide optimal radiation dose throughout the whole cardiac cycle.
Dual-source CT angiography was performed successfully without any complication in all 15 patients; all patients underwent conventional coronary angiography.
CT Coronary Angiography Analysis
The evaluation of the images was performed by two radiologists (D.O., G.O., with 5 years cardiac CT experience each), who were blinded to the results of conventional coronary angiography.
For the evaluation of coronary artery stenosis, original transverse images, multiplanar reformations, curved multiplanar reformations, maximum intensity projections, and volume-rendered images were used. For image postprocessing, a Leonardo (Siemens) workstation was utilized.
The segmental evaluation of the coronary arteries was performed according to the American Heart Association 15-segment classification. All segments were included for the assessment.
Image quality was evaluated qualitatively for each coronary artery segment by using a three-point grading system: grade 1, good image quality with no motion artifacts or minor artifacts not impairing the diagnostic quality; grade 2, moderate image quality with moderate artifacts and/or blurring but adequate and acceptable for clinical diagnosis; and grade 3, poor or nonevaluable image quality with severe artifacts that made vessel delineation impossible. The segments with grade 3 were not considered for further analysis. The reason for decreased image quality was considered for each segment.
The images were evaluated for the presence of significant stenosis. Similar to conventional coronary angiography, significant stenosis was defined as a narrowing of the coronary lumen exceeding 50%; all vessel segments were included in the analysis. Each vessel was analyzed on at least two planes, one parallel and one perpendicular to the course of the vessel. The vessel diameters were measured on reconstructions oriented perpendicular to the vessel course.
Comparisons and Evaluations
The results of CT coronary angiography to help detect significant stenoses (lesions >50%) were compared with the results of conventional coronary angiography according to: (a) per-segment analysis, comparing each segment in every vessel; (b) per-vessel analysis, examining the presence of significant lesions in each of the major coronary vessels; and (c) per-patient analysis, evaluating the presence of any significant lesion in a given patient. Also, the diagnostic performance of dual-source CT coronary angiography to help detect significant stenosis in patients with AF was also evaluated with respect to image quality scores.
We also evaluated the patient dose. The dose-length product was displayed by the dual-source CT system itself and the dose-length product was then converted into effective dose values by means of a conversion factor of 0.017 mSv/mGy · cm, according to the Commission of the European Communities guidelines on quality criteria for CT (16).
Statistical Analysis
We did not perform power analysis to determine the number of patients included in the study because this represented our initial experience. Statistical analysis was performed with statistical software (SPSS, version 12.0 for Windows; SPSS, Chicago, Ill). Sensitivity, specificity, PPV, and NPV were calculated. These diagnostic parameters were expressed with a 95% confidence interval (CI). Conventional coronary angiography was regarded as the reference standard.
McNemar test was used to search for a significant difference between dual-source CT angiography and conventional coronary angiography to help detect coronary stenosis. A P value of less than .05 was considered to indicate a significant difference. To avoid bias caused by clustering effect resulting from multiple observations in the same patients, the P value is corrected with an intracluster correlation factor (17). Also, the results were checked with generalized estimating equation methods (SAS/STAT, Release 8.2; SAS Institute, Cary, NC).
Intermodality agreement between dual-source CT and conventional coronary angiography and interobserver agreement were determined by calculating statistics. According to Landis and Koch (18), = 0 indicated poor agreement, = 0.01–0.20 indicated slight agreement, = 0.21–0.40 indicated fair agreement, = 0.41–0.60 indicated moderate agreement, = 0.61–0.80 indicated good agreement, and = 0.81–1.00 indicated excellent agreement.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEIMPLICATIONS FOR PATIENT CAREReferences |
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In 15 patients, a total of 225 segments were evaluated for image quality with respect to vessel visibility and existing artifacts. Reader 1 (D.O.) evaluated 101 (45%) segments as good, 108 (48%) as moderate, and 16 (7%) as poor image quality; whereas reader 2 (G.O.) evaluated 103 (46%) segments as good, 109 (48%) as moderate, and 13 (6%) as poor image quality (Table 1). The segments with poor image quality were not considered for further analysis.
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Motion artifacts resulted in blurred or doubled vessel contours. Most motion artifacts affected the middle section of the RCA and the left circumflex coronary artery. The left main coronary artery and proximal left anterior descending artery were the least affected segments with the best image quality.
Regarding the false-positive and false-negative results, in all of the misdiagnoses, the image quality was moderate. No segment with good image quality was misdiagnosed by both readers. Sensitivity, specificity, PPV, and NPV of dual-source CT angiography to help detect stenoses in patients with AF were all 100% when the image quality was good ( Figs 3, 4).
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The respective overall sensitivity, specificity, PPV, and NPV were 87% (13 of 15, 95% CI: 70%, 100%), 98% (190 of 194, 95% CI: 96%, 99%), 77% (13 of 17, 95% CI: 56%, 96%), and 99% (190 of 192, 95% CI: 98%, 100%) for reader 1 and 80% (12 of 15, 95% CI: 60%, 100%), 99% (194 of 197, 95% CI: 97%, 100%), 80% (12 of 15, 95% CI: 60%, 100%), and 99% (194 of 197, 95% CI: 97%, 100%) for reader 2.
Per-segment analysis.—Reader 1 correctly identified 13 of 15 significant lesions with a diameter reduction of 50% or greater. Two stenoses were missed (RCA segment 3 and LAD segment 7). Four lesions were detected at dual-source CT only and were not confirmed with conventional coronary angiography and were thus considered as false-positives (RCA segment 2, LAD segments 8 and 9, and left circumflex artery segment 13).
Reader 2 correctly diagnosed 12 of 15 important lesions. Three stenoses were misdiagnosed as normal (RCA segments 2 and 3 and left circumflex artery segment 12) and three normal segments were misdiagnosed as stenoses (RCA segments 2 and 3 and RCA segment 3).
Per-vessel analysis.—Reader 1 misdiagnosed two vessels without CAD as diseased (false-positive) and two vessels with stenoses as normal (false-negative).
Reader 2 misdiagnosed two vessels without CAD as diseased (false-positive) and three vessels with stenoses as normal (false-negative).
Per-patient analysis.—Reader 1 misdiagnosed single-vessel CAD in one patient as normal (false-negative) and absence of CAD as single-vessel CAD (false-positive) in one patient.
Reader 2 misdiagnosed single-vessel CAD (false-positive) in two normal patients. CAD was not misdiagnosed as normal in any patient.
The sensitivity, specificity, PPV, and NPV values of dual-source CT angiography for segment-, vessel-, and patient-based analysis for both readers are given in Table 2, while Table 3 demonstrated the diagnostic performance for each coronary artery and segment separately.
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Additionally, statistics demonstrated good intermodality agreement for both readers (reader 1, = 0.79 and reader 2, = 0.78).
The interobserver agreement in detecting significant stenosis was calculated as = 0.865, corresponding to excellent agreement.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEIMPLICATIONS FOR PATIENT CAREReferences |
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On the other hand, at end-systole, the temporal variation in each cardiac cycle is small; therefore, the image quality is better. This results in superior image quality of the images reconstructed at end systole rather than those reconstructed at mid diastole in patients with AF. This is also why absolute intervals are preferred for reconstructions instead of relative intervals.
Despite the high temporal resolution and appropriate selection of the reconstruction intervals, the main source of image-degrading artifacts was residual cardiac motion. The RCA appears particularly vulnerable because of its extensive motion radius and short motion-free period (21,22). In our study, the middle sections of the RCA and the left circumflex artery were the most affected segments. The left main coronary artery and proximal LAD were the least affected segments with highest image quality, possibly owing to their horizontal course.
Apart from blurring or double-contour appearance owing to residual heart motion, some vessel discontinuity also resulted. However, this discontinuity was not overly severe in most cases and did not complicate image interpretation to a great extent. No breath-holding problems occurred, owing to short scan times.
Extensive vessel wall calcifications also impaired image quality and made interpretation difficult, especially when associated with residual cardiac motion artifacts. In the case of calcifications, we used multiplanar reformation and curved multiplanar reformation images for evaluation. Although the presence of calcifications made the evaluation difficult, they did not lead to false results.
Considering the false-positive and false-negative results, no segment with good image quality was misdiagnosed by both readers. Therefore, when image quality was good, the performance of dual-source CT to help detect coronary stenosis, even in patients with AF, was excellent.
Prospective electrocardiographically gated tube current modulation (electrocardiographic pulsing), which is applied to minimize the patient dose by limiting the optimal radiation dose to a certain period of the cardiac cycle (usually to diastolic phase) (23,24), was not used in our study to allow maximal flexibility in reconstruction intervals. With faster heart rates, the optimal timepoint for image reconstruction becomes more difficult to predict and frequently occurs during the end-systolic phase of total myocardial contraction (25–28). In AF patients the R-R intervals demonstrate great between-beat variability. This increased the patient dose accordingly, but heart rate–adaptive table feed settings helped decrease the patient dose to some extent by shortening acquisition time. This property also helped obtain more motion-free images at high heart rates (10,11,29).
Although promising results were obtained, the main limitation of our study was the small number of patients included in our study group. However, this represented our initial experience and this low number of patients is not enough to draw conclusions. For example, only one false-negative evaluation resulted in a low NPV for reader 1 in per-patient analysis. The small number of patients resulted in a small number of true-negative evaluations (n = 5, NPV = five of six).
Also, the results were influenced by the high prevalence of CAD in our patient group because they were all suspected of having CAD and were scheduled to undergo conventional coronary angiography. Thus, the sensitivity and specificity values may differ in AF patients with a lower prevalence of CAD. Our study also did not include semiquantitative analysis of coronary CT angiography to provide more objective criteria.
In conclusion, the recently introduced dual-source CT technology provides better temporal resolution and minimizes motion artifacts, which allows coronary angiography at higher heart rates, even in cases of arrhythmia. The high NPV may be useful for obviating invasive coronary angiography in patients with AF whose symptoms or abnormal stress test results make it necessary to rule out the presence of coronary artery stenosis to guide therapy. Therefore, dual-source CT has the potential to make noninvasive coronary angiography effective in a significantly increased number of patients and in a wider spectrum of clinical situations as compared with earlier scanners. Larger studies will be needed to determine if our results are reproducible.
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ADVANCE IN KNOWLEDGE |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEIMPLICATIONS FOR PATIENT CAREReferences |
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IMPLICATIONS FOR PATIENT CARE |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEIMPLICATIONS FOR PATIENT CAREReferences |
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FOOTNOTES |
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Abbreviations: AF = atrial fibrillation • CAD = coronary artery disease • CI = confidence interval • LAD = left anterior descending artery • NPV = negative predictive value • PPV = positive predictive value • RCA = right coronary artery
Guarantors of integrity of entire study, D.O., G.O.; 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, D.O., G.O.; clinical studies, all authors; statistical analysis, D.O.; and manuscript editing, D.O., G.O.
Authors stated no financial relationship to disclose.
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References |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONADVANCE IN KNOWLEDGEIMPLICATIONS FOR PATIENT CAREReferences |
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