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Original Research |
1 Department of Clinical Radiology, University of Munich, Grosshadern Campus,
Marchioninistraße 15, 81377 Munich, Germany.
2 Department of Cardiology, Medical Clinic I, University of Munich, Grosshadern
Campus, 81377 Munich, Germany.
Received July 5, 2005;
accepted after revision December 7, 2005.
Address correspondence to T. R. C. Johnson
(thorsten.johnson{at}med.uni-muenchen.de).
Abstract
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SUBJECTS AND METHODS. In 55 patients with acute chest pain, ECG-gated CT angiography was performed with a CT system in which 64 slices per gantry rotation were acquired. Density measurement and visual assessment of motion artifacts were performed to evaluate image quality. CT findings were correlated with results of laboratory tests and clinical follow-up. For 20 patients, two independent blinded reviewers compared findings on CT angiography with those on X-ray coronary angiography.
RESULTS. Adequate contrast enhancement of the pulmonary vessels, coronary arteries, and aorta was achieved in all cases. Regarding image quality of the coronary arteries, there was minor blurring in seven patients, and in one examination the images did not provide enough information for diagnosis. The average image quality rating was 1.2 on a scale in which 1 indicated no artifacts; 2, minor motion artifacts; and 3, image insufficient for diagnosis. The cause of chest pain was correctly identified with MDCT in 37 patients. The diagnoses included pulmonary embolism (n = 10), coronary stenosis (n = 9), and aortic dissection (n = 1). In four patients, additional diagnoses were found with other examinations.
CONCLUSION. With current techniques, ECG-gated CT angiography of the entire chest has very good image quality. The protocol proved helpful in the differential diagnosis of acute chest pain.
Keywords: aorta • cardiopulmonary imaging • CT coronary arteriography • embolism • emergency radiology
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These guidelines and the American College of Radiology Appropriateness Criteria on Acute Chest Pain [3] call for use of ECG and serum cardiac markers as first diagnostic measures and, depending on the patient's history, chest radiography, ventilation-perfusion scanning, resting myocardial perfusion scanning, echocardiography, CT, and aortic imaging or pulmonary angiography as further diagnostic steps. However, the initial symptoms of individual patients often are not characteristic [4, 5], and the various examinations needed for diagnosis can be time consuming and expensive. Therefore the initial focus should be on the possibility of the presence of acute life-threatening conditions, including acute ischemic heart disease, aortic dissection, and pulmonary embolism [6].
Although MDCT angiography is widely accepted and routinely used as a primary tool in the assessment of pulmonary embolism and aortic dissection, there is also evidence that with current techniques, this technique can be used in many cases to rule out clinically significant coronary artery disease [7-9]. Until recently, the scan times of CT angiography (CTA) required protocols for achieving adequate vascular contrast that differed for evaluations of pulmonary embolism, coronary stenosis and occlusion, and aortic dissection. Two studies have shown that "ECG-gated MDCT appears to be logistically feasible and shows promise as a comprehensive method for evaluating cardiac and noncardiac chest pain in stable emergency department patients" [10] and that "MSCT provides diagnostic operating characteristics suitable for triage of patients with ACS in the ED" [11]. Both reports, however, state that further hardware and software improvements are desirable for the integration of CT into the clinical practice of chest pain management. With the advent of new CT scanners with faster rotation times, additional detector rows, and more powerful tubes, sufficiently fast volume coverage has become available for ECG-gated CT of the entire chest within a single breath-hold. We sought to evaluate the diagnostic value of a protocol for the use of state-of-the-art techniques for CTA of the complete thoracic vasculature in a single ECG-gated scan.
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In the acute setting, relative contraindications were arrhythmia, history of severe allergic reaction to iodinated contrast media (i.e., anaphylactic reaction or shortness of breath at previous use of contrast material), and renal insufficiency. Young age (< 55 years) was considered a relative contraindication because of radiation exposure concerns. ß-blockers were not administered in preparation for the scan. Patients were examined in the supine position at inspiratory breath-hold.
Imaging Technique
The patients were examined with an MDCT scanner (Somatom Sensation 64,
Siemens Medical Solutions) in which 64 slices per gantry rotation were
acquired at a gantry rotation time of 330 milliseconds. First, a coronal
topogram was acquired to determine scan volume, which included the entire
chest. A dual-head power injector (Stellant D, Medrad) was used for injection
of contrast medium. Iopromide (Ultravist 300, Schering) was used as the
contrast agent in all cases. To adapt the volume of contrast medium to the
individual scan range for various scan times (21.4 ± 3.2 seconds),
volume was adapted to scan length according to
Table 1, in accordance with
previous experience. Mean contrast volume was 136.9 ± 17.6 mL. Flow
rate also was adjusted to scan length
(Table 1) for a mean of 4.6
± 0.5 mL/s. A saline chaser bolus of 50 mL was injected at the same
flow rate. An antecubital vein was used for IV access in 47 patients. Seven
patients had central venous lines, which were used for injection.
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A bolus-tracking technique (CARE bolus, Siemens) in the ascending aorta was used for timing. After a delay of 4 seconds, axial images at the level of the ascending aorta were acquired with an interval of 2 seconds between subsequent images. As soon as a density of more than 100 H was detected in the region of interest in the ascending aorta, the scan was initiated with a delay of 6 seconds.
For scanning in the craniocaudal direction, collimation of 32 x 0.6 mm was used at a gantry rotation time of 0.33 seconds. Double z sampling resulted in 64 reconstructed slices per gantry rotation and an increased spatial resolution of 0.4 mm. Tube voltage was 120 kV; effective tube current, 750 mAs; and table feed, 11.6 mm/s, corresponding to a pitch of 0.2. For four obese patients, tube current was increased to 850-900 mAs. Total scanning time was 21.4 ± 3.2 seconds for the entire chest, depending on volume covered. ECG was recorded during acquisition.
Image Reconstruction
Depending on the patient's heart rate, retrospective ECG gating with single
half-segment reconstruction or, for heart rates greater than 75 beats/min,
two-segment reconstruction (Adaptive Cardiac Volume, Siemens) was used. For
optimal, motion-free image quality of the coronary arteries, data sets were
preferentially reconstructed in mid-diastole. A stack of axial images
containing the volume of the heart and the aortic root was reconstructed with
a slice thickness of 0.75 mm and an increment of 0.5 mm. In addition, axial
images of the entire chest were reconstructed with a slice thickness and an
increment of 3 mm. Additional coronal reconstructions and volume-rendered
images were available to the radiologist on request.
Image Analysis
The images were interpreted on a PACS workstation by a resident and an
attending physician or fellow immediately after the examination and later
interpreted by a third reviewer on a 3D workstation (Leonardo, Siemens) for
final diagnosis, image quality, and density measurements. The following
three-point grading system was used for evaluation of pulsation artifacts: 1,
depiction of the coronary arteries without motion artifacts; 2, minor motion
artifacts but sufficient depiction of coronary arteries for diagnostic
purposes; 3, insufficient image quality for diagnostic assessment.
In 20 cases, X-ray coronary angiograms were available for correlation. Ten patients had undergone coronary angiography within 1 year before CTA. The other 10 patients underwent coronary angiography within 4 days after CTA for confirmation of or intervention in lesions detected with CTA. The interpreting radiologists were unaware of the results of previous X-ray coronary angiography. Both reviewers assessed the CTA images of the coronary arteries and indicated potentially hemodynamically significant (i.e., occlusion or stenosis > 50%) and insignificant lesions in the individual segments according to the American Heart Association 15-segment model [12]. The results of consensus interpretation were compared with the findings on conventional coronary angiography.
Clinical Follow-Up
For follow-up until August 2005 (i.e., a period of at least 5 months), the
files of all patients were reviewed with a focus on additional diagnoses and
on creatinine levels to detect contrast-induced nephropathy. Cardiac enzyme
levels were reviewed for detection of hemodynamically significant stenosis or
occlusion of the coronary arteries.
Statistical Analysis
Continuous variables were presented as mean ± SD. The diagnostic
accuracy of CTA in the detection of coronary artery stenosis was evaluated
with quantitative X-ray coronary angiography as the standard of reference.
Sensitivity, specificity, negative predictive value, and positive predictive
value were calculated. Interobserver agreement for the two reviewers was
quantified with kappa value (MedCalc software version 7.0.0.1, 2002). Kappa
values were interpreted as follows: < 0.20, 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, very good agreement. For all statistical tests, p <
0.05 was considered statistically significant.
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Patient Findings
In 37 patients, the cause of chest pain was identified in the CT
examination. Table 3 shows a
list of diagnoses found. In 14 patients, CTA findings did not explain the
chest pain adequately and were in accordance with the clinical follow-up
findings, which also did not reveal a diagnosis. In four patients, follow-up
findings revealed causes of chest pain that had not been identified on CTA. In
one patient, pulmonary congestion was evident on the CT scan. The diagnosis
was high-grade insufficiency of the mitral valve, and the patient underwent
valve replacement. Another patient had intermittent arrhythmia and received a
pacemaker. The condition of another patient was diagnosed as endocarditis. In
one patient chest pain was caused by stenosis of the circumflex branch of the
left coronary artery, as proved with conventional X-ray coronary angiography,
and the stenosis was dilated and stented. CTA depicted this lesion, but the
degree of stenosis was underestimated.
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In follow-up of creatinine levels, there was no evidence of contrast-induced nephropathy in any of the patients. In one patient renal failure developed 2 weeks after CTA but probably was the result of prolonged resuscitation after brainstem ischemia.
Blood serum levels of cardiac enzymes on the day of the CT examination were available for comparison in 32 patients. Three of these patients had abnormal findings. Two patients had elevated muscle-brain type creatine kinase and troponin levels, which corresponded to two newly diagnosed cases of high-grade stenosis of the coronary arteries. One patient had only moderately elevated levels of muscle-brain type creatine kinase; her condition was diagnosed as a cardiac mass in the right ventricle. Continued follow-up after discharge revealed readmission of one patient for pneumonia, of which there had been no evidence 3 months earlier on the initial CT scan.
If the findings for the 14 patients whose clinical follow-up did not show diagnoses explaining chest pain are regarded as true-negative results, the sensitivity of CTA for identification of the origin of chest pain can be estimated at 92.7%. Calculation of specificity may be somewhat arbitrary for diagnoses such as pneumonic consolidation, atelectasis, and aneurysm, which can but may not in all cases be the cause of chest pain.
Coronary Arteries
Despite the high heart rates of our patients (71 ± 15 beats/min;
range, 51-123 beats/min), only one examination of the right coronary artery
resulted in image quality insufficient for diagnosis (grade 3). Apart from
that, only minor motion artifacts were visible in 10 of the patients (grade 2,
18.5%; heart rate, 63-118 beats/min). In two patients, motion artifacts
affected only the right coronary artery, and in one patient, only the left
coronary system. These artifacts caused only minor impairment of image
quality, and occlusion and high-grade stenosis were ruled out. Thus, overall
the image quality rating was 1.2 ± 0.4.
Twenty-four patients had signs of severe atherosclerosis of the coronary arteries. High-grade coronary stenosis or occlusion (Figs. 2A and 2B) was found in nine patients. Six of the lesions had been depicted on coronary angiograms obtained within 1 year before the CT examination. The other three were newly diagnosed and were confirmed at subsequent X-ray angiography. One of the patients had in-stent restenosis of the left circumflex coronary artery (segment 11) and underwent bypass surgery. One patient had high-grade stenosis of the right coronary artery, which was dilated and stented. The third patient had stenosis in the left anterior descending coronary artery (segments 7 and 8) and in the first diagonal branch (segment 9), and these segments were dilated.
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Interobserver agreement for the two independent blinded reviewers regarding presence or absence of clinically significant stenosis of the coronary arteries in each patient showed a kappa value of 0.81, indicating very good agreement. Agreement regarding number of significant stenoses per patient showed a kappa value of 0.62, corresponding to good agreement. Regarding wall irregularities with stenosis < 50%, the kappa value was 0.43 (moderate agreement). These vessel wall changes were not consistently described in conventional angiography reports, and therefore correlation was not possible.
Pulmonary Arteries
Pulmonary embolism was diagnosed in 10 patients (Figs.
1C and
3) and ruled out to the
subsegmental level in the others. ECG gating proved useful in avoiding
blurring of pulmonary arteries due to cardiac motion, enabling the ruling out
of subsegmental embolism in the paracardiac pulmonary arteries. Minor motion
artifacts of the paracardiac subsegmental pulmonary arteries adjacent to the
left ventricle were found in two patients in whom there were also motion
artifacts of the coronary arteries. One patient had secondary pulmonary
hypertension due to chronic recurrent thromboembolism, which was correctly
diagnosed with CTA and confirmed with conventional angiography.
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Gated CTA can be a useful clinical tool in the context of evaluation of acute chest pain, enabling exclusion of significant coronary stenosis and occlusion, especially in patients with inconclusive ECG findings or no signs of ischemia or infarction. If the symptoms are not characteristic of a specific cause, an ECGgated scan covering the entire chest allows simultaneous assessment of many additional life-threatening differential diagnoses, such as pulmonary embolism, aortic dissection, spontaneous pneumothorax, and pneumonic consolidation. The focus of this study was to evaluate a protocol that allows simultaneous and fast assessment of the most important vascular regions of the chest in one scan and to determine whether sufficient image quality and diagnostic accuracy can be achieved in patients with acute chest pain.
Until recently, the scan times required for ECG-gated high-resolution CTA of the entire chest required breath-holds lasting more than 30 seconds. With CT scanners with faster gantry rotation times and additional detector rows, scanning time has decreased significantly, making ECG-gated examination of the entire chest feasible even for patients with moderate dyspnea. Our study showed that sufficient contrast enhancement of the pulmonary vessels, coronary arteries, and aorta can be achieved without excessive doses of contrast medium.
A major problem arising from faster volume coverage is contrast administration. As scan times shorten, higher injection rates are needed to achieve sufficient contrast with acceptable volumes of contrast material. We chose the ascending aorta as the region of interest for bolus tracking to ensure sufficient opacification of the coronary arteries [21]. Although it did not occur in our study, insufficient opacification of the pulmonary arteries can happen in patients with severe pulmonary congestion [22]. To address this problem, it may be helpful to use a test bolus for simultaneous evaluation of arrival and transition times of contrast medium in regions of interest in the pulmonary trunk and ascending aorta. It would be possible to adjust delay and amount of contrast agent to ensure sufficient opacification of both the pulmonary vessels and the aorta.
The radiation exposure of retrospectively ECG-gated CTA scans is higher than that of nongated chest protocols. Therefore this protocol should be used only if there is a clinical indication for imaging of the pulmonary vessels and the coronary arteries, aorta, or both. On the other hand, our protocol allows evaluation of the entire chest in one scan and may therefore help to avoid additional diagnostic procedures. In our experience, a specific protocol with an increased pitch factor of 0.3 (compared with 0.2 in a standard CTA protocol for depiction of the coronary arteries) seems helpful for reducing radiation exposure, scanning time, and volume of contrast medium to approximately two thirds those of a standard CTA protocol without significant changes in image quality. Given the average scan range of approximately 248 mm, the estimated equivalent dose is approximately 6.9 mSv for this protocol (at a pitch of 0.3 and with ECG pulsing). According to the literature [23, 24], equivalent doses for alternative examinations can be estimated at approximately 3.2 mSv for pulmonary angiography, 1.2 mSv for ventilation-perfusion scintigraphy, and approximately 1.6-7 mSv (CT dose index volume, 19.4 mGy) for nongated CT pulmonary angiography. Roos et al. [25] cited doses of 3.65, 8.85, and 4.50 mSv for prospectively triggered, retrospectively gated, and nongated CTA of the aorta, respectively, covering a range of 15 cm. Equivalent doses of 5-7 mSv for retrospectively gated CTA examinations of the coronary arteries have been described [26], whereas conventional angiography requires an average dose of approximately 5 mSv [27]. Thus the dose required for the combined examination protocol is within the range of CT examinations of the individual vascular territories. This feature may contribute to reduced radiation exposure.
With the advent of MDCT, CTA has become a primary imaging tool for the examination of patients with suspected pulmonary embolism [28]. Higher resolution and faster acquisition times improve visualization of subsegmental pulmonary arteries [29-33]. Artifacts from cardiac motion compromise image quality, however, doubling or blurring paracardiac pulmonary arteries and bronchi and hindering assessment of emboli. Previous evaluations of ECG-gated chest scans for pulmonary embolism have been limited by prolonged scan times. To some authors [24], the additional information has not seemed worth the increase in scanning time and radiation exposure. In the evaluation of unclear chest pain, however, simultaneous assessment of coronary arteries and the ascending aorta may justify the disadvantages, especially because scanning time is only moderately prolonged with current techniques.
Patients with typical angina and known coronary artery disease should be examined with appropriate techniques such as X-ray coronary angiography. Our protocol, however, may help to exclude significant coronary stenosis in patients with atypical chest pain and normal creatine kinase and troponin levels. Studies with 16-MDCT have shown a high negative predictive value of MDCT angiography [7-9]. A problem of ECG-gated image acquisition in previous studies was the reduced image quality in patients with heart rates greater than 80 beats/min, because the diastolic data acquisition window became too short in approximately 25% of cases [34, 35]. However, with the shortened gantry rotation time of 0.33 seconds and an inherent temporal resolution of 165 and 83 milliseconds for single half-segment reconstruction and dual multisegment reconstruction, respectively, we obtained sufficient quality of images of the coronary arteries in almost all of our patients, although we did not use ß-blockers to lower heart rates. Further studies are needed to evaluate the effect of faster rotation on coronary CTA of patients with higher heart rates.
Another frequent diagnostic problem in nongated CTA is assessment of aortic dissection, especially of the ascending aorta. This problem can be eliminated with ECG gating. We found no motion artifacts in the aortic root in our study. Roos et al. [25] found the advantages of ECG gating for this purpose, and with dedicated multiphase reconstruction, the aortic and mitral valves can be evaluated [36], which we did not address in this study. Some authors advocate use of an unenhanced scan for more reliable identification of intramural hematoma and hyperdense acute thrombi in the false lumen of dissected vessels. To limit radiation exposure, we chose not to acquire an unenhanced scan. If the clinical presentation of a patient suggests aortic dissection, an additional unenhanced scan may be appropriate.
Aortic aneurysm is the cause of acute chest pain in some patients but is an incidental finding in others. Evaluation of the dilated aortic wall and reliable measurement of the diameter profit from the absence of motion artifacts. The clinical significance of this finding has to be considered individually.
A limitation of this study was the inclusion criteria. We examined only patients referred by the emergency department for unclear chest pain, and this aspect of the study implies preselection of patients. For example, a patient with signs of myocardial infarction would have been referred immediately for invasive coronary angiography without undergoing CTA. This factor may explain the relatively high prevalence of pulmonary embolism in our patients. Another limitation was the lack of correlation with X-ray angiography for all patients and the long interval between invasive angiography and CTA in one half of our patients. It would be desirable to correlate the CTA findings with those of the respective standard examinations, such as ventilation-perfusion scintigraphy, aortic MRI, and quantitative coronary angiography. Doing so, however, would have been unethical because of radiation exposure, patient encroachment, and time.
In our experience, the protocol described in this study proved helpful in patients with acute chest pain of unclear cause. The three vascular territories (pulmonary arteries, coronary arteries, and aorta) were evaluated without limitation compared with dedicated protocols for the individual territories. Moreover, because of ECG gating, evaluation of the paracardiac pulmonary arteries and ascending aorta was improved by the absence of pulsation artifacts.
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