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DOI:10.2214/AJR.04.1773
AJR 2006; 186:S395-S400
© American Roentgen Ray Society

Clinical Observations

Early Postoperative Assessment of Coronary Artery Bypass Graft Patency and Anatomy: Value of Contrast-Enhanced 16-MDCT with Retrospectively ECG-Gated Reconstructions

Hélène Vernhet-Kovacsik1, Pascal Battistella2, Roland Demaria2, Jean Luc Pasquie3, Claudine Bousquet1, Georges Dogas1, Florence Leclercq3, Bernard Albat2 and Jean Paul Senac1

1 Department of Imaging, CHU Montpellier, 391 Avenue du Doyen Giraud, Montpellier 34295, France.
2 Department of Cardiovascular Surgery, CHU Montpellier, Montpellier 34295, France.
3 Department of Cardiology, CHU Montpellier, Montpellier 34295, France.

Received November 16, 2004; accepted after revision March 8, 2005.

 
Address correspondence to H. Vernhet-Kovacsik (h-vernhet{at}chu-montpellier.fr).


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The objective of our study was to assess early postoperative patency and anatomy of off-pump coronary artery bypass grafts (CABGs) using retrospectively ECG-gated MDCT.

CONCLUSION. Retrospectively ECG-gated MDCT is a promising noninvasive technique with which to assess early postoperative patency and anatomy of CABGs.

Keywords: cardiac imaging • coronary angiography • coronary artery bypass surgery • CT • MDCT


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Coronary artery bypass grafting (CABG) is the most often performed cardiac surgery in the industrialized countries. Both immediate and follow-up controls of bypass grafts' patency are not systematically performed because coronary angiography, the gold standard method, is an invasive and potentially complicated technique [1, 2]. However, some early and late common postoperative conditions, such as ECG changes or thoracic pain, may be difficult to analyze [3]. Unfortunately, when comparing CABG surgery with peripheral vascular surgery, there is no noninvasive and currently available imaging technique, such as Doppler examination, with which to systematically assess graft patency before the patient is discharged from the hospital. Regarding clinical mid- and long-term follow-up examinations, knowledge of postoperative myocardial perfusion supplied by grafts is an important prognostic factor [4, 5]. Because of major technologic improvements in CT techniques, 16-MDCT with ECG-gated reconstructions can depict coronary stenosis with a high negative predictive value in the native coronary artery [6, 7].

Recently, the high diagnostic value of MDCT for the depiction of both native coronary artery and bypass graft obstructions in symptomatic patients who have undergone CABG surgery has been reported [8-10]. In addition, a new surgical technique, "off-pump" CABG surgery, that avoids cardiac arrest and cardiopulmonary bypass has been introduced. Improvement in clinical results of high-risk patients who undergo the off-pump CABG surgery when compared with clinical results of those who undergo the traditional technique has been reported [11].

Because of some technical difficulties and limitations related to the "beating heart" conditions [12], systematic postoperative coronary angiography to evaluate CABG patency is a standard procedure in our institution. The aim of our study was to assess the feasibility of using MDCT with retrospectively ECG-gated reconstructions for the early postoperative evaluation of arterial and venous bypass graft patency after off-pump CABG surgery.


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Patients
Between April 2003 and October 2004, 19 consecutive patients who underwent off-pump coronary bypass grafting (14 men, five women; mean age, 72 years; age range, 62-76 years) were included in the study. All the patients gave written informed consent to participate in the study. A total of 29 bypasses were examined using both coronary angiography and MDCT: left internal mammary artery (LIMA) grafts, n = 19; right internal mammary artery grafts, n = 4; saphenous vein grafts, n = 5; and radial artery graft, n = 1. The grafts were anastomosed to the left anterior descending artery (n = 19), the diagonal branch (n = 1), the left marginal branches (n = 5), or the right coronary branch (n = 4).

Patients with allergies to iodine-containing contrast media, renal failure (serum creatinine level, > 100 µmol/L), or substantial respiratory or cardiac failure were excluded from the study. No additional ß-blocker therapy was used in this population. The mean delay between the CABG surgery and the imaging protocol was 8 days (range, 5-14 days). All MDCT examinations were performed before coronary angiography, and the interval between the respective procedures was 48 hr.

MDCT Acquisition
CT was performed on a 16-MDCT unit (Light-Speed, GE Healthcare). Patients received nasal oxygen (3 L/min) for 5 min before image acquisition. A short session of instructed hyperventilation was performed before scanning; the duration of the apnea ranged from 28 to 32 sec. Heart rate was recorded during the scanning time. No ß-receptor blocker medication was used to slow the heart rate. The scanning parameters were 0.6-mm slice thickness, a rotation time of 0.5 sec, 120 kV, 380 mA, and a field of view of 25 cm. A fast localization CT scan was obtained to determine the scanning range. The acquisition volume started at the mid part of the aortic button and ended 3 cm below the cardiac apex. One hundred forty milliliters of nonionic iodine contrast medium (iomeprol [Iomeron 300, Bracco]) was injected at a rate of 4 mL/sec through an 18-gauge needle into a right antecubital vein. The injection was coupled to the acquisition using bolus-tracking software.


Figure 1
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  		Fig. 1A —72-year-old man. MDCT multiplanar volume-rendering image shows internal mammary artery-left descending artery graft with clipped side branches that project over graft.

 


Figure 2
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  		Fig. 1B —72-year-old man. Using linear reconstructions of graft lumen and views from two different angles offers different projections of metallic clips located close to graft so that entire lumen is shown.

 


Figure 3
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  		Fig. 1C —72-year-old man. Using linear reconstructions of graft lumen and views from two different angles offers different projections of metallic clips located close to graft so that entire lumen is shown.

 
Serial scan images were acquired each second at the level of the main pulmonary artery, and the acquisition started when the continuously measured density into the ascending aorta reached 200 H. The pitch was adapted to the scanning protocol, which depended on the heart rate. The two-sector reconstruction algorithm, with a 125-msec temporal reconstruction window, was used in patients with a heart rate below or equal to 80 beats per minute (bpm), and the four-sector reconstruction algorithm, with a 65-msec temporal reconstruction window, was used in patients with a heart rate more than 80 bpm. Retrospectively ECG-gated reconstructions were obtained from 40% to 90% of the R-R interval in 10% increments.

Image Processing and Data Analysis
The reconstructed images were transferred to a workstation (Advantage 3.1, GE Healthcare). A stack of approximately 1,500-2,000 transverse CT sections was available per patient. Three-dimensional surface shaded display reconstructions, multiplanar volumetric reconstructions with maximum intensity projections (Fig. 1A), and linear and curved images from the enhanced lumen, which was displayed by dedicated software (Vessel Analysis, GE Healthcare), were applied to each examination (Figs. 1B and 1C). All bypass graft segments and anastomoses were independently evaluated by two radiologists experienced in cardiovascular imaging. Radiologists were aware of each anatomic type of coronary artery bypass in each patient.

Motion artifacts were analyzed. Radiologists searched for respiratory motion artifacts on axial images visualized using lung window settings (level, 2,000 H; width, -400 H) and for cardiac motion artifacts on 2D reconstructions obtained in the coronal plane, with each set of reconstructions at different percentages of the R-R interval. Both respiratory and cardiac artifacts were scored 0 (no artifact), 1 (minor artifacts with sufficient image quality to allow confident data analysis), or 2 (major artifacts with insufficient image quality to allow confident data analysis). Data analyzed on MDCT native images and the 2D and 3D reconstructions were anatomic type of CABG (i.e., recipient artery), patency of the bypass graft, and stenosis greater than 50%. Two-dimensional planar image reconstructions with a 0.6-mm section thickness that were perpendicular to the long axis of the studied vessel were performed to directly measure the diameters of the bypass graft and recipient coronary artery and the related percentage stenosis. The highest percentage of stenosis found for each R-R interval temporal image reconstruction was considered as the percentage of stenosis. Results were compared with coronary angiography. Bypass anatomy was established using both cine loop display of axial images and 3D surface shaded display image reconstruction analysis.

Conventional Coronary Angiography
Conventional coronary angiography was performed with 4-French Judkins coronary catheters via the femoral artery using a digital angiography system (V5000 Integris, Philips Medical Systems). Selective conventional catheterizations of the bypass grafts were performed. Quantitative coronary angiography was performed by experienced interventional cardiologists using the quantitative coronary angiography software.

Statistical Analysis
The observed sensitivity (Se), specificity (Sp), positive predictive value (PPV), and negative predictive value (NPV) of MDCT findings for stenosis of more than 50% were calculated as follows:

Formula

Formula

Formula
and

Formula
The 95% confidence intervals (CIs) were calculated using binomial proportions. The interobserver agreement for anatomic findings, patency, and stenosis of more than 50% at MDCT was evaluated using the kappa test.


Figure 4
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  		Fig. 2A —73-year-old man who underwent off-pump left internal mammary artery-left descending artery bypass graft surgery 12 days earlier. Surface shaded 3D CT image shows that recipient artery is diagonal artery.

 


Figure 5
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  		Fig. 2B —73-year-old man who underwent off-pump left internal mammary artery-left descending artery bypass graft surgery 12 days earlier. Diagonal artery was also assessed on conventional coronary angiography.

 

Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Image Quality
Patient heart rate ranged from 69 to 98 bpm, and one patient had an atrial fibrillation. The two-sector reconstruction algorithm was used in 13 patients with a heart rate below or equal to 80 bpm, and the four-sector reconstruction algorithm was used in six patients with a heart rate of more than 80 bpm.

Concerning respiratory artifacts, among the 29 bypass grafts, two bypass grafts in two patients were scored 2, one graft was scored 1, and the 26 remaining grafts were scored 0. Concerning cardiac motion artifacts, two bypass grafts in one patient (left descending artery [LDA], n = 1; right coronary artery [RCA], n = 1) with atrial fibrillation were scored 2, 13 CABGs were scored 1 (LDA, n = 6; RCA, n = 2; marginal artery, n = 5), and 14 CABGs were scored zero (LDA, n = 12; RCA, n = 1; diagonal artery, n = 1).

Coronary Angiography
Catheterization of one LIMA-LDA bypass graft failed in one patient.

Bypass Anatomy
On both MDCT and conventional coronary angiography, the recipient artery was found to be a diagonal artery instead of the LDA in one patient (Figs. 2A and 2B). The remaining occlusion of an LDA was successfully managed by a second aorta-LDA bypass graft using a saphenous vein 2 weeks later. Two saphenous bypass grafts were twisted on MDCT without any significant stenosis seen on both MDCT and conventional coronary angiography. The remaining CABGs were correctly identified by both MDCT and angiography.


Figure 6
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  		Fig. 3 —MDCT multiplanar volume-rendering image of 69-year-old woman shows thrombosis of radial artery-diagonal artery graft. Graft could not be catheterized and enhanced at angiography.

 
Patency and Stenosis Rate
One radial artery-diagonal artery bypass graft was occluded on both MDCT and conventional coronary angiography (Fig. 3). Two patients had stenosis of more than 50% at the distal anastomosis of LIMA-LDA bypass grafts on both MDCT and conventional coronary angiography (Figs. 4A and 4B). In one patient with a LIMA-LDA bypass graft, the LDA was dissected at the distal anastomosis with a stenosis rate of more than 50% that was detected on coronary angiography but missed on MDCT (Figs. 5A, 5B, and 5C). The remaining CABGs were patent without stenosis of greater than 50% on both MDCT and angiography. The overall sensitivity, specificity, PPV, and NPV of MDCT for the diagnosis of stenosis of greater than 50% were 66.67% (95% CI, 9.43-99.16%), 100% (83.89-100%), 100% (15.81-100%), and 95.45% (77.16-99.88%), respectively.


Figure 7
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  		Fig. 4A —75-year-old man who had off-pump left internal mammary artery-left descending artery bypass graft surgery 1 week earlier. Both multiplanar volume-rendering CT image (A) and conventional angiography image (B) show stenosis that is greater than 50% located at distal anastomosis.

 

Figure 8
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  		Fig. 4B —75-year-old man who had off-pump left internal mammary artery-left descending artery bypass graft surgery 1 week earlier. Both multiplanar volume-rendering CT image (A) and conventional angiography image (B) show stenosis that is greater than 50% located at distal anastomosis.

 

Figure 9
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  		Fig. 5A —70-year-old man who had off-left internal mammary artery-left descending artery (LDA) bypass graft surgery 10 days earlier. Coronary angiography (A) shows dissection flap on recipient LDA that was well documented on cranial view (B) but that was missed at MDCT on multiplanar volume-rendering image (C).

 

Figure 10
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  		Fig. 5B —70-year-old man who had off-left internal mammary artery-left descending artery (LDA) bypass graft surgery 10 days earlier. Coronary angiography (A) shows dissection flap on recipient LDA that was well documented on cranial view (B) but that was missed at MDCT on multiplanar volume-rendering image (C).

 

Figure 11
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  		Fig. 5C —70-year-old man who had off-left internal mammary artery-left descending artery (LDA) bypass graft surgery 10 days earlier. Coronary angiography (A) shows dissection flap on recipient LDA that was well documented on cranial view (B) but that was missed at MDCT on multiplanar volume-rendering image (C).

 
The interobserver agreement was a kappa value of 100% for anatomic findings, bypass graft patency, and depiction of stenosis of more than 50%.


Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
This study shows that the depiction of postoperative CABG failure is feasible using contrast-enhanced 16-MDCT with retrospectively ECG-gated reconstructions. The reported CABG patency rate at 3 months is lower for all territories after off-pump surgery than with the conventional technique: 88% versus 98%, respectively [13]. Therefore, a systematic postoperative control of graft patency after off-pump surgery is justified.

Impressive results have been reported with coronary MDCT angiography to diagnose coronary artery stenosis [6, 7]. There are, however, some limitations with this technique in the early postoperative period. The observed heart rate is higher in our patients than in the population previously evaluated for graft and coronary artery stenosis. Because of some limitations in medically slowing the heart with ß-blockers during the early postoperative period, all the patients in this study had a heart rate of more than 65 bpm and six of the 18 patients had a heart rate of more than 80 bpm. However, new algorithms dedicated to obtaining retrospectively reconstructed images using short temporal windows—up to 65 msec—allow sufficient image quality. Only two CABGs in one patient with atrial fibrillation and related cardiac motion artifacts could not be evaluated.

The rate of reported atrial fibrillation after CABG surgery is high, up to 39% but seems to decrease with the off-pump technique, 8% in the studies of Reston et al. [11] and Ascione et al. [14] and 5% in our study that included only 19 patients. This low rate is explained by the fact that patients were referred to MDCT the day before leaving the surgical department and, at that date, atrial fibrillation is usually medically controlled. On the other hand, according to Yoo et al. [9], cardiac motion is not so disabling for image quality: The main territory currently affected by graft anastomosis is the LDA, where left ventricular wall motion is less marked than in the circumflex artery or RCA territories. As a result, no cardiac artifact was present in 11 of the 19 LDA grafts and cardiac artifacts were found in all the five marginal artery grafts investigated in this study. The second limitation of MDCT angiography in the postoperative period is the need to inject high volumes of contrast medium. Patients with acute left ventricular failure cannot be safely evaluated at this time and have to wait for recovery of cardiac function to undergo MDCT coronary angiography.

The compromise between slice thickness and thoracic volume coverage is less of an issue since 16-MDCT scanners became available. Large volumes including the origin of the mammary arteries display fair anatomic information, but a slice of more than 1 mm is too thick to show subtle parietal abnormalities in small-caliber arteries. Because of the risk of respiratory motion artifacts, large length coverage associated with submillimeter slices should be avoided if breath-holding is for more than 30 sec. Sixteen-MDCT now allows exploration of the ascending aorta including both the proximal anastomosis of venous CABGs and the entire coverage of the heart with thin slabs, as reported by Dewey et al. [15].

Temporal resolution still remains a limitation of 16-MDCT. Moving abnormalities— for example, the intimal flap at the distal anastomosis of a LIMA graft on an LDA that was missed in our study—cannot be diagnosed even when using a 65-msec temporal reconstruction window. The lack of selective arterial enhancement is also a limitation in observing relative flow changes in the coronary vasculature, such as competitive flows. Another potential limitation in assessing graft patency using 16-MDCT is in cases of clipped side branches and grafts with clips that are too close to the anastomotic sites. Using image reconstruction software that provides linear reconstructions from the enhanced lumen and multiple views around the centerline of the vessel so that clips and their surrounding artifacts are projected outside the lumen consistently improves confidence in lumen analysis.

The need for a noninvasive imaging technique is emphasized.

The advantages of assessing graft failure before the patient is discharged from the hospital are multiple. First, appropriate medical therapies, interventional coronary procedures, or surgical corrections can be planned to avoid or to treat ultimate bypass graft thrombosis and subsequent myocardial infarction [16]. Finally, knowing the graft patency status when the patient is discharged from the hospital should improve mid- and long-term follow-up and management.

Sixteen-MDCT is the sole anatomically noninvasive, currently available, and reliable imaging technique with which to investigate CABGs. The number of patients included in this study is too few to draw any definitive conclusion. However, our study results suggest that its use to evaluate CABG patency and anatomy in the postoperative period should be considered.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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