Asian Cardiovasc Thorac Ann 2008;16:444-449
© 2008 Asia Publishing EXchange Ltd
Can 64-Row Computed Tomography Replace Angiography After Coronary Bypass?
Hirosato Doi, MD,
Ryuji Koshima, MD,
Masato Suzuki, MD,
Ken Takahashi, MD,
Hiroichi Yokoyama, PhD1,
Naoya Yoshida, BA1
Department of Cardiovascular Surgery
1 Department of Diagnostic Radiology, Cardiovascular Center, Hokkaido Ohno Hospital, Hokkaido, Japan
For reprint information contact: Ryuji Koshima, MD Tel: 81 11 665 0020 Fax: 81 11 665 0242 Email: koushima{at}cvc-ohno.or.jp, Cardiovascular Center, Hokkaido Ohno Hospital, 4-1-1-30 Nishino, Nishiku, Sapporo, Hokkaido 063-0034, Japan.
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ABSTRACT
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Multi-detector (64-row) computed tomography has become an alternative to coronary angiography to diagnose graft occlusion and stenosis after coronary artery bypass. We compared the power of evaluation of multi-detector computed tomography with that of conventional coronary angiography in 60 patients who underwent coronary artery bypass with 135 grafts and 210 graft anastomoses. The diagnostic power of multi-detector computed tomography for graft occlusion was: 100% (2/2) sensitivity, 98.5% (131/133) specificity, 50% (2/4) positive predictive value, and 100% (133/133) negative predictive value; there were no significant differences in rates of occlusion among the different types of graft. The diagnostic power of multi-detector computed tomography for stenosis of the graft anastomosis was: 100% (2/2) sensitivity, 95.1% (194/204) specificity, 16.6% (2/12) positive predictive value, and 100% (194/194) negative predictive value, with no significant differences among grafts. Multi-detector computed tomography permits evaluation of bypass grafts and is much less invasive for the patients.
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INTRODUCTION
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Postoperative assessment of bypass conduits and anastomoses after coronary artery bypass grafting (CABG) is important to evaluate surgical technique.1,2 Coronary angiography (CAG) is the current gold standard for the evaluation of bypass graft patency and stenosis. However, CAG is invasive and associated with certain risks and complications, such as arrhythmia, graft dissection, myocardial infarction, and embolic events. These complications account for mortality rates of 0.14%–0.28% and morbidity rates of 0.2%–2.1%.3–5 Numerous studies comparing the accuracy of multi-detector (64-row) computed tomography (64-MDCT) assessment of coronary graft patency and stenosis with that of CAG have shown that 64-MDCT is a reliable diagnostic tool and less invasive than CAG.6–11 In our institution, CAG is carried out in the first 2 days after CABG because it is easy to reopen the mediastinum without severe adhesions and repair graft problems such as occlusion, stenosis, and kinking. We investigated the accuracy of 64-MDCT as a substitute for CAG in post-CABG evaluation.
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PATIENTS AND METHODS
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We studied 60 patients from December 2006 to December 2007 (Table 1
). All patients gave written informed consent, and the study protocol was approved by the Institutional Review Board. There were 60 left internal thoracic artery (LITA) grafts, 15 right internal thoracic artery (RITA) grafts, 30 right gastroepiploic artery (RGEA) grafts, 30 saphenous vein grafts (SVG), and 210 graft anastomoses (including 30 proximal aortic anastomoses and 1 anastomosis of a composite graft) assessed by 64-MDCT and CAG. Patients with severe heart failure, unstable hemodynamics, significant renal dysfunction (serum creatinine > 2.5 mg·dL–1), or tachycardia (> 90 beats·min–1) were excluded from the study. CABG was performed through a median sternotomy. LITA, RITA, RGEA, and saphenous vein were harvested and skeletonized, and the patients were heparinized to maintain an activated clotting time > 250 sec. A suction-type mechanical stabilizer was used to immobilize the target coronary artery, and a heart positioner to elevate the heart in the vicinity of the circumflex and right coronary arteries.
Invasive CAG was performed via a right femoral approach using a 4F catheter. Images were acquired at 12.5 frames per second. Selective imaging of bypass grafts was performed in 30° right anterior oblique and 60° left anterior oblique views. Graft occlusion was defined as absence of contrast material along the course of the graft, through the graft anastomosis to the native distal artery, or to the next graft and the native artery in sequential anastomoses. In sequential anastomoses, each distal anastomosis was counted as an individual graft. Significant stenosis of the graft anastomosis in a patent graft was determined if there was 
70% reduction in the mean diameter.
Multi-detector computed tomographic images were obtained using a 64-row scanner (Aquilion 64 Super Heart; Toshiba Medical Systems Corp., Japan) with retrospective electrocardiography gating. The scanning parameters were: 0.5-mm slice collimation pitch 11.2, 135 kV and 380 mA. We continuously injected 80–100 mL (according to the scanning area) of nonionic contrast material (iopamidol; 370 mg iodine/mL) at a rate of 5 mL·sec–1, followed by 50 mL saline solution. The CT scan started automatic bolus tracking in conjunction with ascending aortic enhancement of 100 HU. The reconstructed multiphase images were transferred to a workstation (ZIO Station; ZIO Soft, Tokyo, Japan) and the best cardiac phase was selected. Graft occlusion was defined as absence of contrast material along the course of the graft, through the graft anastomosis to the native distal artery, or to the next graft and the native artery in sequential anastomoses. In sequential anastomoses, each distal anastomosis was counted as a bypass graft. Significant stenosis of the graft anastomosis in a patent graft was defined as 50% reduction of luminal diameter. All patients received 20–60 mg oral metoprolol tartrate according to body weight and blood pressure 1 h before the scan if the heart rate was > 80 beats·min–1, and 0.3 mg nitroglycerin was given just before the scan when systolic blood pressure was > 100 mm Hg.
For statistical analysis, all data were expressed as mean ± standard deviation. The reliability of 64-MDCT was evaluated by calculating sensitivity, specificity, positive and negative predictive values. The percentage and 95% confidence interval were calculated for each value.
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RESULTS
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Coronary angiography and 64-MDCT were performed without complications in any patient. Of 135 bypass grafts, including 1 composite graft, CAG showed 133 were patent and 2 were occluded (one LITA distal anastomosis to a native coronary artery in sequential anastomosis, and one SVG distal anastomosis to a native coronary artery; 98.5% patency rate). Compared with CAG, 64-MDCT correctly showed 131 of 133 patent grafts (Figure 1) and both occluded grafts (Figure 2). Of the 2 grafts for which 64-MDCT failed to show the correct diagnosis, one (LITA) resulted from sequential anastomosis to very small native coronary artery (diameter 
1.0 mm) undetectable by 64-MDCT (Figure 3), and one poor-quality image was caused by insufficient contrast material that was placed proximal to the site of a RGEA graft. Sensitivity, specificity, positive and negative predictive values are given in Table 2. Of the 131 patent grafts evaluated by 64-MDCT, there were 206 anastomoses including 176 distal and 30 proximal aortic anastomoses. Coronary angiography demonstrated significant stenosis of only 2 graft anastomoses (LITA and RITA), whereas 64-MDCT showed no significant graft anastomosis stenosis in 194 of the remaining 204, and confirmed significant stenosis in the 2 demonstrated by CAG (Figure 4). Of 10 graft anastomoses that 64-MDCT failed to correctly diagnose (2 LITA, 1 RITA, 2 RGEA, 5 SVG), there were 7 metallic clip artifacts (2 at a proximal site, 5 at a distal site), 2 motion artifacts (heart rate > 80 beats·min–1), and 1 diameter mismatch between the graft and the native coronary artery, which was over-indicated by 64-MDCT (Figure 5). The sensitivity, specificity, positive and negative predictive values are given in Table 3.
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Figure 1. (A) Coronary angiography in a 65-year-old man with sequential grafting of the left internal thoracic artery (LITA) to the left anterior descending artery (LAD) and to the 1st diagonal branch (D1), showing all grafts were patent and no graft stenosis. (B) Multi-detector computed tomography confirming all grafts were patent and no graft stenosis.
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Figure 2. (A) Coronary angiography in a 70-year-old woman with sequential grafting of the left internal thoracic artery (LITA) to the left anterior descending artery (LAD) and to the 1st diagonal branch (D1), showing that the LITA-to-LAD graft was patent and no significant stenosis of the graft anastomosis, but the LITA-to-D1 graft had 100% occlusion (black arrows). (B) Multi-detector computed tomography showing the LITA-to-LAD graft was patent, but there was no significant stenosis of the graft anastomosis.
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Figure 3. (A) Coronary angiography in a 78-year-old man with sequential grafting of the left internal thoracic artery (LITA) to the left anterior descending artery (LAD) and to the 1st diagonal branch (D1), showing all grafts were patent and no stenosis of the graft anastomosis (black arrows). (B) Multi-detector computed tomography showing that the LITA-to-LAD graft was patent, no significant stenosis of the graft anastomosis, but the LITA-to-D1 graft was 100% occluded (white arrows).
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Figure 4. (A) Coronary angiography in a 72-year-old man, showing a patent right internal thoracic artery (RITA)-to-left circumflex artery (LCx) graft and significant stenosis of the graft anastomosis (white arrow). (B) Multi-detector computed tomography confirmed a patent RITA-to-LCx graft and significant stenosis of the graft anastomosis (white arrow).
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Figure 5. (A) Coronary angiography in a 68-year-old woman, showing a patent saphenous vein graft (SVG) to the left circumflex artery (LCx) and no significant stenosis of the graft anastomosis (white arrows). (B) Multi-detector confirmed a patent SVG-to-LCx graft, but there was significant stenosis of the graft anastomosis (white arrows).
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DISCUSSION
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Tortuosity or spasm of arteries proximal to grafts sometimes precludes their visualization by postoperative CAG, particularly RGEA grafts. Because CAG was used as the reference standard in this study, grafts not evaluated by CAG were not included. A recent study of 64-MDCT and angiography reported sensitivity of 97% and specificity of 98% for graft occlusion. However, requirements such as minimum heart rate, long breath-holding time to cover scan length, and limited spatial resolution caused by thick collimation, reduced the ability to evaluate stenosis of patent grafts (evaluation of possible grafts, 62%).7 64-MDCT has shown high-quality images and satisfactory accuracy in evaluating coronary artery stenosis and bypass graft patency.12 Left internal thoracic artery and RITA are often used for CABG. Currently, RGEA is also a key graft for total arterial revascularization, so LITA or RITA and RGEA are often used simultaneously. 64-MDCT was able to visualize from the LITA or RITA proximal site to the RGEA proximal site in one photograph, this is impossible with fewer MDCT channels (Figure 6). Furthermore, 64-MDCT visualizes all grafts at once and evaluates all arterial diseases such as arteriosclerosis obliterans and aortitis syndrome. Several drawbacks of CAG regarding the amount of contrast medium and native artery status (tortuosity, stenosis, occlusion) were avoided by 64-MDCT.
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Figure 6. Multi-detector computed tomography in a 62-year-old man with coronary artery grafting (LITA to LAD, RGEA to 4th posterior descending branch), allowing visualization from the proximal left anterior descending artery (LITA) to the proximal right gastroepiploic artery (RGEA) at a single breath hold in one photograph.
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Several types of graft were analyzed, as well as sequential and composite arterial grafts; 64-MDCT images satisfactory for diagnosis were obtained. The specificity of graft occlusion was higher in LITA or RITA grafts because most were anastomosed to the LAD which is easily revealed by MDCT because of its straight course and relatively large diameter, and in SVGs because they were large in diameter. The 45 sequential grafts undetected by 64-MDCT were connected to small native coronary arteries (38 branches of the circumflex artery, 5 diagonal branches, and 2 posterior descending branches). In addition to their small diameter, branches of the circumflex artery and diagonal branch arteries are easily affected by cardiac motion artifacts because they course along the atrioventricular groove or the left ventricle.
The best quality images are always obtained in patients with a low heart rate, but a recent report indicated that heart rate is not a crucial determinant of the quality of diagnostic accuracy by multisector reconstruction.13–15 In patients with rapid heart rates, multisector reconstruction was superior to half reconstruction in obtaining images with fewer motion artifacts. However, multisector reconstruction is not always appropriate because temporal resolution with this technique varies with the patient’s heart rate. Therefore, we gave metoprolol tartrate if the heart rate exceeded 80 beats·min–1.
In this study, there were only 2 graft occlusions and 2 significant stenoses of graft anastomoses, which led to a low positive predictive value. A larger sample is needed to confirm our results. Other study limitations include the fact that we do not have specific data on radiation exposure or the amount of contrast medium required for 64-MDCT scanning. The Aquilion 64-MDCT needs a higher radiation exposure (15–20 mSv) than CAG (5–10 mSv). The volume of contrast medium is higher for CAG (100–150 mL) than 64-MDCT (80–100 mL). These issues were not considered in the conclusions of the study. Despite this, our results show that 64-MDCT allowed accurate assessment of graft occlusion and significant stenosis of graft anastomosis after CABG. Because there were no false negative results, we consider 64-MDCT to be the first choice for post-CABG graft assessment, and the more invasive CAG as the 2nd choice when 64-MDCT shows graft occlusion or significant graft anastomosis stenosis, or if it cannot evaluate the state of the graft. Although our results indicate the usefulness of MDCT as a screening modality for post-CABG evaluation, it should be accepted for clinical application only when the safety of MDCT technology has been demonstrated.
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REFERENCES
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ABSTRACT
INTRODUCTION
