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Ann Thorac Surg 2005;79:585-588
© 2005 The Society of Thoracic Surgeons

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Original article: Cardiovascular

Multidetector Computed Tomography Versus Coronary Angiogram in Evaluation of Coronary Artery Bypass Grafts

^a Department of Cardiovascular Surgery, The Japanese Red Cross Nagoya First Hospital, Nagoya
^b Department of Cardiothoracic Surgery, Toyohashi Municipal Hospital, Toyohashi
^c Department of Cardiovascular Surgery, Shizuoka Saiseikai General Hospital, Shizuoka, Japan

Accepted for publication August 5, 2004.

^* Address reprint requests to Dr Song, Department of Cardiovascular Surgery, Gifu Prefectural Tajimi Hospital, 5-161 Maebata-cho, Tajimi-shi, Gifu 507-8511, Japan (E-mail: songmhmd{at}yahoo.co.jp).

	Abstract

Top Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
BACKGROUND: The new evolving multidetector computed tomography (MDCT) is thought to be promising in imaging of coronary disease. We compared the evaluating power of MDCT to that of conventional coronary artery angiogram in patients with coronary artery bypass grafting surgery.

METHODS: From April 2002 to December 2003, 50 patients underwent both MDCT and coronary artery angiogram after coronary artery bypass surgery. Overall, 170 grafts were studied. Two weeks after the surgery, 8-row MDCT was taken using contrast medium to observe the grafts patency. Three weeks after the surgery, conventional coronary angiogram was done to verify the graft status. The graft status was classified according to Fitzgibbon classification. Sensitivity, positive predictive value, and negative predictive value were calculated and compared.

RESULTS: Multidetector computed tomography succeeded in high-quality imaging of all grafts except for one (less than 1 mm gastroepiploic graft to posterior descending artery). This graft was the only graft that was interpreted differently. Overall patency rate was 97.8%. The diagnostic power of MDCT was as follows: the specificity was 99.4%, the sensitivity was 100%, the positive predictive value was 100%, and the negative predictive power was 80%.

CONCLUSIONS: In this study, 8-row MDCT showed reliable imaging power in verifying grafts status. Since MDCT is much less invasive for patients, conventional coronary artery angiogram will be substituted by evolving MDCT.

	Introduction

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The functioning status of grafts is judged primarily by patency of the grafts, and it has been verified by conventional coronary artery angiogram (CAG). Although CAG is still the standard of reference for the assessment of coronary artery grafts, it is an invasive and potentially harmful procedure with a small risk of serious events (arrhythmia, stroke, coronary artery dissection, death). Furthermore, the cost of CAG per se in Japan is about 1,900 US dollars, and the cost of multidetector computed tomography (MDCT) is 32 US dollars. In Europe and the United States, expense of CAG and MDCT is similar to that in Japan, and CAG is about ten times more expensive than MDCT. These adverse effects of CAG have led to an intensive search for noninvasive alternatives. It has recently been shown that MDCT, in combination with retrospective electrocardiographic gating, permits visualization of the coronary artery graft and its anastomotic status [1]. Initially MDCT was developed with four rows of detector; however, it has now evolved into 16 and even the flat (omni plane) panel and its application to coronary surgery has been reported [2]. Our aim in this study was to assess the diagnostic accuracy of MDCT for the verification of grafts status in coronary artery bypass grafting (CABG) and to compare the diagnostic accuracy of MDCT to that of CAG.

	Material and Methods

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Among 213 consecutive patients who underwent CABG from April 2002 to December 2003, 50 patients consented to be enrolled in this study and the rest of the 163 patients refused to participate for several reasons. The review board of the hospital approved this study and informed consent was obtained from each patient included in this study.

Surgical Profiles
Fifty patients received 170 grafts in this study. There were 39 males and 11 females. Their ages ranged from 48 to 78 (mean 69.4). Most were indicated CABG for triple vessel disease. The average graft number was 3.4. The types of grafts used were as follows: left internal mammary artery (LIMA), 50; right internal mammary artery (RIMA), 25; radial artery (RA), 40; gastroepiploic artery (GEA), 40; and saphenous vein (SV), 15. Our choice of either off-pump or on-pump coronary artery bypass was as follows: if intraoperative epiaortic echo detected pathological calcification or atheromatous plaque in the wall of the ascending aorta, we did off-pump bypass: if there was no pathology in the aorta, we did on-pump bypass. In accordance with this strategy, 18 patients underwent off-pump beating coronary artery bypass and the rest of the 32 patients underwent on-pump arrested coronary artery bypass.

MDCT Scan Protocol
Two weeks after CABG, MDCT was taken. All patients received 50 mg atenolol orally 1 hour before the scan if the heart rate was greater than 80 bpm. All patients were in normal sinus rhythm. No nitrates were given just before the scan.

Using an 8-slice MDCT scanner (LightSpeed Plus, GE Yokogawa Medical Systems, Tokyo, Japan), a volume data set was acquired (1.25 mm collimation, gantry rotation time 0.5 second table feed 1.375 mm per rotation), covering the distance from the carina to the diaphragmal face of the heart. Tube current was modulated according to the electrocardiogram, with a maximum current of 440 mA during a time period of 0.5 seconds centered around 40% of the cardiac cycle. We continuously injected 100 mL of contrast agent at a rate of 5 mL/s. The scan time was 30 seconds and scan delay time was 22 seconds. The software BURST Plus (GE Yokogawa Medical Systems, Tokyo, Japan) was used to reconstruct the images. Axial slices were reconstructed from the acquired volumetric computed tomographic data during the middle to late diastolic phase to minimize motion artifacts. The RR interval was 40% and 92 bpm. Three-dimensional volume rendering reconstruction of the coronary arteries and grafts was performed. The volume can be rotated in all directions, thus allowing three-dimensional evaluation of the grafts.

Graft status was classified according to Fitzgibbon classification by a blind independent radiologist [3]. Fitzgibbon's definition is as follows: grade A, excellent graft with unimpaired runoff; grade B, stenosis reducing caliber of proximal or distal anastomoses or trunk to less than 50% of the grafted coronary artery; grade O, occlusion.

Coronary Angiogram
Invasive CAGs were obtained 1 week after MDCT (ie, 3 weeks after CABG) after intracoronary injection of 0.2 mg of isosorbite dinitrate. Angiograms were evaluated by a blind independent radiologist. Lesions were classified by Fitzgibbon criteria [3]. As previously stated, Fitzgibbon's definition is as follows; grade A, excellent graft with unimpaired runoff; grade B, stenosis reducing caliber of proximal or distal anastomoses or trunk to less than 50% of the grafted coronary artery; grade O, occlusion.

Diagnostic Power
The findings of MDCT and CAG were compared. We calculated sensitivity, specificity, positive predictive value, and negative predictive value.

	Results

Top Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
Table 1 shows the grafts patency read on CAG and MDCT. The LIMA and RA were bypassed mainly to left anterior and left circumflex systems and they showed 100% patency. The RIMA was bypassed mainly to the circumflex system and it showed 96.0% (1 occluded out of 24) patency rate. The GEA was bypassed mainly to the right posterior descending system and it showed 97.5% (1 occluded out of 39) patency rate. The SV was bypassed mainly to the right or left circumflex systems and it showed 86.7% (2 occluded out of 13) patency rate. One occluded RIMA graft was also read as occluded on MDCT. Two occluded SV were also read as occluded on MDCT. The MDCT said there were two occluded GEA grafts; one was the same graft as shown by CAG, the other was read patent but MDCT finding was interpreted to be occluded. This graft had 1 mm diameter and was anastomosed to less than 1 mm right posterior descending artery.

View larger version (149K): [in this window] [in a new window]   Fig 1. Volume-rendered three-dimensional computed tomographic imaging from a 70-year-old patient two weeks after coronary artery bypass grafting. The left internal mammary artery was anastomosed to the left anterior descending artery, which was visualized vividly even in the presence of clips. The radial artery was anastomosed to the obtuse marginal artery, which was wide open. The posterior descending artery was bypassed by the right gastroepiploic artery, which was also wide open.

	Comment

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We have been working on the usefulness of MDCT and published decent MDCT images in CABG grafts [8, 9]. In this study we found three observations. First, MDCT was as accurate as CAG in discriminating graft status. Second, MDCT was much less invasive than CAG for patients, both physically and economically. Third, still, a very small vessel less than 1.0 mm diameter was a hazard to see on the 8-row MDCT.

Accuracy of MDCT in CABG patients was first reported in 2003 by Gurevitch and colleagues [2] and Herzog and colleagues [10]. Gurevitch and colleagues found that excellent visualization of all 30 grafts was achieved. They added the superiority of MDCT to CAG; that the ability to display the vessel wall as well as its lumen might distinguish radial artery spasm from intimal hyperplasia, which Herzog also observed. Another advantage inferred by Gurevitch and colleagues was the clear demonstration of ostial lesions, such as ostial left main coronary artery disease, lesions that might be missed with conventional CAG because of positioning of the catheter tip beyond the arterial ostium. Herzog addressed that MDCT should be regarded as an ideal additive planning tool for complex minimally invasive procedures such as totally endoscopic CABG or minimally invasive direct CABG. Our results were in accordance with their results. A 100% positive predictive value means MDCT is as accurate as CAG, besides a 100% sensitivity and a 99.4% specificity. These good numbers were in contrast with less encouraging numbers from Kopp and colleagues [5] and Vogl and colleagues [11] because they used 4-row MDCT not 8-row MDCT. By applying 16-row MDCT, Gurevitch and colleagues [2] attained 100% power of imaging grafts.

In Japan, greater than 20,000 coronary operations are being done annually and its medical insurance budget has been debated because of its explosion. Still, CABG patients are discharged only after having them catheterized and CAG is the golden-standard routine practice after CABG. Coronary angiogram costs ten times as much as MDCT in Japan, and this makes physicians and surgeons have hope that MDCT will be a substitute for CAG. We observed in this study that it is not only feasible but practical. As arterial grafts are known to have better patency rates and better long-term prognosis, we have been trying to revascularize patients by using arterial grafts. There is no doubt about the excellence of the LIMA and RIMA, and RA is a very good conduit in terms of its good patency and its easy-to-handle characteristics. The GEA has been frequently used since the report by Suma and colleagues [12] showing that long-term result was good, and it is believed to be a very convenient graft to the right coronary artery system in Japan. In off-pump cases, in situ arterial grafts are mandatory and we are taking special care in harvesting those arterial grafts, in order to avoid any injury, by using an ultrasonic scalpel.

Even by 8-row MDCT, a less than 1 mm graft and artery are still too small to attain complete visualization on MDCT. This will be overcome by 16-row or flat panel MDCT because resolution power will become much higher. Increased detector number leads to increased temporal resolution, total workflow, breath hold time, and high resolution. Especially, temporal resolution improves with high gantry rotation time and heart rate does not affect the optimal temporal resolution. The new anastomosing device such as the U-clip and magnet have a possibility in that they produce artifact effects in the volume rendered image. But, so far, we have found no reports regarding the MDCT image of coronary artery bypass using the U-clip and magnet. Possibly, because of the difference of the CT number between the U-clip and magnet and coronary artery and grafts, we can expect a fine discriminating image on MDCT.

Study limitations include the following. Clearly, 50 patients were too small to infer the definite effectiveness of MDCT. A prospective randomized control study on a larger volume will be necessary to conclude the superiority of MDCT over CAG.

In summary, it is shown in this 50 patient study that MDCT is a diagnostic modality of high sensitivity and specificity. It will replace CAG after a larger volume study confirms its superiority and as MDCT keeps evolving.

	Acknowledgments

Top Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
The authors thank Liya Jeon for her help in manuscript preparation.

	References

Top Abstract Introduction Material and Methods Results Comment Acknowledgments References

 

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3. Fitzgibbon GM, Kafka HP, Leach AJ, Keon WJ, Hooper GD, Burton JR. Coronary bypass graft fate and patient outcome: angiographic follow-up of 5065 grafts related to survival and reoperation in 1388 patients during 25 years J Am Coll Cardiol 1996;28:616-626.[Abstract]
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