(Circulation. 1997;96:2785-2788.)
© 1997 American Heart Association, Inc.
Articles |
Detection of High-Grade Restenosis After PTCA Using Contrast-Enhanced Electron Beam CT
From the Department of Internal Medicine II, University of Erlangen-Nürnberg (Germany).
Correspondence to Dr med Stephan Achenbach, Medizinische Klinik II, Universität Erlangen-Nürnberg, Östliche Stadtmauerstr 29, 91054 Erlangen, Germany. E-mail stephan.achenbach{at}stud.uni-erlangen.de
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Abstract |
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Background Contrast-enhanced electron beam computed tomography (EBCT) has been shown to permit noninvasive visualization of the coronary arteries. We determined the value of EBCT to noninvasively detect high-grade restenosis after percutaneous transluminal coronary angioplasty (PTCA).
Methods and Results Fifty patients (37 to 68 years of age), were investigated by EBCT at a mean interval of 9 months after PTCA of coronary artery stenoses. Forty axial cross-sections of the heart (3-mm slice thickness, 1-mm overlap) were acquired triggered to the ECG after intravenous injection of contrast agent. Three-dimensional reconstructions of the coronary arteries were rendered with a lower threshold of 80 HU to selectively visualize the contrast-enhanced vessel lumen. EBCT results were compared with conventional quantitative coronary angiography (QCA) performed within 1 week. In 6 patients, the PTCA segment could not be evaluated because of impaired image quality. Sixteen of the remaining 44 patients had high-grade restenoses in QCA (>70% diameter reduction), which was correctly detected by EBCT in 15 cases (94% sensitivity). There were 5 false-positive EBCT results of high-grade restenosis (82% specificity).
Conclusions EBCT with intravenous injection of contrast agent permits the noninvasive diagnosis of restenosis after PTCA, with high sensitivity and sufficient specificity.
Key Words: angioplasty • computed tomography • follow-up studies • imaging • stenosis
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Introduction |
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Annually, an estimated 900 000 percutaneous transluminal coronary angioplasty (PTCA) procedures are performed worldwide.1 Despite a high primary success rate of
90%, the rate of restenosis within 1 to 6 months is
30%.1 2 3 Because contrast-enhanced electron beam
computed tomography (EBCT) has been shown to permit noninvasive imaging
of the coronary arteries,4 5 6 7 we investigated the
value of EBCT to detect high-grade restenoses after angioplasty
by blinded comparison to quantitative coronary angiography
(QCA). |
Methods |
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Patients
Fifty patients (8 women, 42 men) with a mean age of 55 years (37 to 68 years) were included in the study. Percutaneous balloon angioplasty (PTCA) of coronary artery stenoses had been performed 1 to 60 months (mean, 8.8 months) before the EBCT investigation. The site of angioplasty was the left anterior descending coronary artery (LAD) in 27 patients, the left circumflex coronary artery (LCx) in 12, and the right coronary artery (RCA) in 11 patients. Twenty seven (54%) of the patients were symptomatic at the time of investigation. Exclusion criteria for the study were implanted coronary artery stents, atrial fibrillation, and an unstable clinical condition. All patients had given their written consent after the nature of the investigation had been fully explained.
Electron Beam Computed Tomography
The technical scanner characteristics and the investigation
protocol used for coronary artery visualization have been
published in detail elsewhere.4 5 8 9 The patients were
scanned with an EVOLUTION scanner (Imatron). Before the investigation,
0.8 mg of nitroglycerin was given to the patients for
vasodilation.
After we determined the heart position using 8 transaxial cross-sections of the chest, we determined the contrast agent transit time by measuring the time from injection of a 10-mL bolus of contrast agent into a peripheral vein to maximum contrast enhancement of the ascending aorta in 10 cross-sections acquired at the same level over a time period of 20 heartbeats.5 The volume dataset of the heart that served to visualize the coronary vessels consisted of 40 transaxial cross-sections. The first image was positioned at the level of the aortic root, and the following were added in a caudal direction. Slice thickness was 3 mm, with a table feed of 2 mm to generate overlapping slices. The acquisition time was 100 ms per section. Image acquisition was done in inspiratory breathhold, triggered to the ECG with one acquisition after every QRS complex at 80% of the RR interval. Contrast agent (120 to 160 mL iohexol, Ultravist370, Schering) was injected into the cubital vein at a rate of 4 mL/s. After the initiation of contrast injection, the start of image acquisition was delayed according to the individually determined contrast agent transit time.
Data Evaluation
The EBCT images were evaluated without knowledge of the
patients' current coronary angiograms. The site of prior
angioplasty, however, was known to the investigators. To facilitate
evaluation of the coronary arteries, three-dimensional
reconstructions of the heart and coronary arteries were
generated with commercially available software (MagicView, Siemens).
After manual editing of the EBCT images by a physician to remove
superimposed structures such as the chest wall and parts of the
pulmonary trunk, shaded surface display reconstructions of the
heart and coronary arteries were rendered with a lower
threshold of 80 HU to selectively visualize the contrast-enhanced
coronary artery lumen.4 Two investigators
evaluated the reconstructions, using three categories to rate the
former PTCA segment by visual estimation: presence of high-grade
restenosis (>70% diameter reduction, according to
restenosis class NHLBI II),2 absence of high-grade
restenosis, or impaired image quality that made
analysis impossible. In the case of disagreement between
observers, consent was achieved in a joint reading.
Conventional coronary angiograms were obtained in all patients 1 to 6 days after the EBCT study. They were evaluated by QCA with an off-line system (CAAS, Pie Medical Equipment). A high-grade restenosis was assumed if the diameter reduction in the former PTCA segment exceeded 70% (NHLBI class II).
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Results |
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In 6 patients,the PTCA segment (1 LAD, 4 LCx, 1 RCA) could not be evaluated as to the presence or absence of restenosis because of degraded image quality of the EBCT reconstructions. Reasons for impaired image quality were movement or respiration artifacts (3 patients), severe calcifications of the former PTCA segment (1 patient), and insufficient contrast enhancement in 2 overweight patients with reduced left ventricular function. Out of these 6 patients, 2 had a high-grade restenosis in QCA (1 LAD, 1 LCx). Of the remaining 44 patients, 16 had high-grade restenosis in the conventional coronary angiogram as measured by QCA, which was correctly detected by EBCT in 15 cases (8 LAD, 3 LCx, 4 RCA) (Fig 1
). One
high-grade restenosis of the LAD was not detected in the EBCT
reconstruction because of calcification of the PTCA segment. Out of 28
patients with an angiographic diameter reduction of <70% in the
former PTCA segment, the absence of restenosis was correctly
detected in 23 cases by EBCT (Fig 2).
There were 5 false-positive EBCT results (3 LAD, 1 LCx, 1 RCA). While
in these 5 patients a high-grade restenosis had been diagnosed
in the EBCT reconstructions, the diameter reduction was 49% to 66% in
QCA.
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These results correspond to a sensitivity of 94% and specificity of
82% for the detection of restenosis by EBCT. The positive
predictive value was 75% and the negative predictive value was 96%.
At a confidence level of 0.975, McNemar's 
2 test
yielded no difference for the detection of restenosis in EBCT
and conventional angiography. Interobserver agreement was achieved in
45 of the 50 patients (90%). Disagreement mainly concerned image
quality: In 4 of the 5 patients in which the observers disagreed, one
observer considered the image quality too poor for evaluation. Cohen's
was 0.84, indicating close interobserver
agreement.10
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Restenosis, a common problem after coronary angioplasty, has an incidence rate of
30%.1 2 3 Because
clinical symptoms are not predictive,11 12 there have been
numerous attempts to use noninvasive methods for the detection of
restenosis, including stress ECG, nuclear perfusion studies,
and stress echocardiography. However, none of these
diagnostic tests have proven clinically useful for the
reliable detection of restenosis (see
Table).13 14 15 16 17 18 19 20 In contrast to
these methods, which rely on the demonstration of functional
consequences of impaired coronary flow, contrast-enhanced EBCT
has been shown to permit direct visualization of the coronary
artery lumen.4 5 6 7 In our study, EBCT had a sensitivity of
94% and specificity of 82% for the detection of high-grade
restenosis after PTCA.
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Among the drawbacks of the method is the fact that the reconstructions could not be evaluated because of impaired image quality in 6 out of 50 patients (12%), mainly because of movement and respiration artifacts. Also, the injection of contrast agent is necessary, and EBCT requires expensive technical equipment. Three-dimensional reconstructions, while facilitating image evaluation, have several disadvantages: Manual segmentation is a time-consuming process and requires a careful operator with detailed knowledge of cross-sectional coronary anatomy. Dedicated software could aid in the reconstruction process, reducing both the amount of user interaction and reconstruction time. Also, the obtained reconstructions are heavily influenced by the chosen threshold. Even though a value of 80 HU has proven useful to separate contrast-enhanced vessel lumen from the surrounding connective tissue,4 the use of one fixed threshold in all patients may have contributed to the rate of false-positive results from overestimation of stenoses. The use of an individually defined threshold might be more appropriate. Also, the accuracy of EBCT coronary angiography is reduced in the mid and especially distal segments of the coronary arteries.4 5 Small vessel diameters can lead to an overestimation of stenoses. Finally, even though not included in our study, coronary metal stents cause artifacts in the three-dimensional reconstructions that make analysis of the respective segment impossible. Schmermund et al21 therefore have proposed a different approach for the investigation of stents, based on the analysis of time-density curves within the coronary artery lumen measured before and after the placement of the stent.
Regarding the fact that EBCT is a noninvasive investigation that can be performed on an outpatient basis and that the investigation protocol permits simple and rapid image acquisition, EBCT seems to be a promising tool in the follow-up after coronary interventions, with potential cost saving by replacing a large number of invasive diagnostic procedures.
Received June 24, 1997; revision received August 21, 1997; accepted August 27, 1997.
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