Noninvasive Assessment of Off-Pump Coronary Artery Bypass Surgery by 16-Channel Multidetector-Row Computed Tomography

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1 CARDIOVASCULAR Noninvasive Assessment of Off-Pump Coronary Artery Bypass Surgery by 16-Channel Multidetector-Row Computed Tomography Masato Yamamoto, MD, Fumiko Kimura, MD, Hiroshi Niinami, MD, Yuji Suda, MD, Eiko Ueno, MD, and Yasuo Takeuchi, MD Departments of Cardiovascular Surgery and Radiology, Tokyo Women s Medical University, Daini Hospital, Tokyo, Japan Background. Sixteen-channel multidetector-row computed tomography (MDCT), with higher spatial and temporal resolution, enables noninvasive visualization of images with reduced motion artifact and breathholding time. We compared images of 16-channel MDCT and selective bypass graft angiography among patients who had off-pump coronary artery bypass graft surgery. Methods. The study, conducted from April 2003 to March 2004, involved 42 patients who underwent offpump coronary artery bypass graft surgery. Samples yielded a total of 96 arterial grafts, 5 vein grafts. Sixteenchannel MDCT (LightSpeed Ultra 16; GE Healthcare, Milwaukee, Wisconsin) was performed on each patient using 500-ms or 600-ms rotation time, mm slice thickness, and mean scanning time of approximately 24 seconds. Results. If several sequential anastomoses in one graft existed, each was calculated as a separate graft. Selective bypass graft angiography yielded a patency rate of 97% (155 of 160). Multidetector-row computed tomography enabled detection of 143 of 155 patent grafts and all 5 occluded grafts visualized by selective bypass graft angiography (100% sensitivity and 93% specificity for graft occlusion after exclusion of grafts not evaluated by MDCT). In 149 graft anastomoses of 143 patent grafts viewed by MDCT, 2 significant stenoses were detected by both selective bypass graft angiography and MDCT. Twelve distal anastomoses were not evaluated by MDCT because of metallic clip artifacts. Evaluation possible graft anastomoses were 92%. Sensitivity and specificity for significant stenosis after exclusion of graft anastomoses not evaluated by MDCT were 100% and 99%, respectively. Conclusions. High-quality 16-channel MDCT images allowed detection of graft occlusion and significant stenosis of graft anastomosis after off-pump coronary artery bypass graft surgery, demonstrating an alternative tool less invasive than selective bypass graft angiography to assess grafts after surgery. (Ann Thorac Surg 2006;81:820 7) 2006 by The Society of Thoracic Surgeons Postoperative assessment of bypass conduits and anastomoses after off-pump coronary artery bypass graft surgery (OPCABG) is important to evaluate surgical techniques [1, 2]. Selective bypass graft angiography (SGA) is the current gold standard to evaluate bypass graft patency and stenosis, but SGA is invasive and carries serious risks, such as arrhythmia, graft dissection, myocardial infarction, and embolic events. These complications account for 0.14% to 0.28% mortality and 0.2% to 2.1% morbidity [3 5]. Therefore, a reliable, noninvasive method for image assessment is preferable. Both noninvasive electron-beam computed tomography and magnetic resonance imaging used for coronary assessments have been evaluated, but significant limitations remain in their production of reliable images [6 12]. Four-channel multidetector-row computed tomography (MDCT) was considered a valuable and reliable diagnostic tool in assessing bypass graft patency and stenosis [13 17], but the long breath-holding time required and Accepted for publication Aug 18, Address correspondence to Dr Yamamoto, Department of Cardiovascular Surgery, Tokyo Women s Medical University Daini Hospital, Nishiogu, Arakawa-ku, Tokyo , Japan; rhgcp476@ybb.ne.jp. poor Z-axis spatial resolution for thick slices preclude quality images of bypass graft stenosis. Sixteen-channel MDCT requires less breath-holding time with less motion artifact and has improved Z-axis spatial resolution because of increased detector number, thinner detector width, and faster rotation speed [18 23]. The modified version allows detection of coronary lesions and coronary bypass grafts with high sensitivity and specificity. We prospectively investigated the diagnostic accuracy of the 16-channel MDCT by comparing it with SGA in detecting early OPCABG graft occlusion and significant stenosis of graft anastomosis. Patients and Methods We studied 42 patients (36 men and 6 women; average age 63.6 years; range, 39 to 84) from April 2003 to March 2004, evaluating 96 arterial grafts, 5 vein grafts, and 167 graft anastomoses (including 7 proximal aortic anastomoses and 3 anastomoses of I composite grafts) using 16-channel MDCT and SGA. Samples included 41 left internal mammary arteries (LIMA), 27 right internal mammary arteries (RIMA), 23 right gastroepiploic arter by The Society of Thoracic Surgeons /06/$32.00 Published by Elsevier Inc doi: /j.athoracsur

2 Ann Thorac Surg YAMAMOTO ET AL 2006;81:820 7 MDCT ASSESSMENT OF OPCABG Abbreviations and Acronyms CABG coronary artery bypass graft surgery LAD left anterior descending artery LIMA left internal mammary artery MDCT multidetector-row computed tomography OPCABG off-pump coronary artery bypass graft surgery RA radial artery RGEA right gastroepiploic artery RIMA right internal mammary artery SGA selective bypass graft angiography SV saphenous vein ies (RGEA), 5 radial arteries (RA), and 5 saphenous veins (SV). The average number of distal and proximal aortic anastomoses was 3.98 per patient (range, 1 to 8 anastomoses). Sequential anastomoses were performed in 40 grafts. The anastomoses included 2 I-composite grafts of RIMA to RA and 1 I-composite graft of LIMA to RA. All 22 patients with triple-vessel disease had in-situ arterial grafts (LIMA, RIMA, and RGEA; Table 1). Off-pump CABG was performed through a median sternotomy. We prefer complete coronary revascularization with sequential grafting using primarily arterial grafts, especially in-situ arterial grafts (LIMA, RIMA, and RGEA). The LIMA, RIMA, RGEA, RA, and SV were harvested and skeletonized, and the patients were administered heparin to maintain an activated clotting time greater than 250 seconds. In all patients, the left anterior descending artery (LAD) was revascularized first. One deep pericardial suture was placed to expose the LAD territory. We used a suction-type mechanical stabilizer (Octopus 3; Medtronic, Minneapolis, Minnesota) to immobilize the target coronary artery, and a heart positioner (Starfish Heart Positioner; Medtronic) to elevate the heart in the vicinity of the circumflex and right coronary arteries. All distal anastomoses were performed with 8-0 polypropylene running sutures by side-to-side anastomosis. Proximal anastomosis to ascending aorta was performed during aortic partial clamping when RA and SV were used. Two were repeat cases, and 6 patients had chronic renal failure on hemodialysis (Table 1). The patients had neither atrial fibrillation nor any contraindication for the administration of a contrast material. The study protocol was approved by our hospital Ethics Committee, and informed consent was obtained from each patient. Selective bypass graft angiography and MDCT were performed an average of and days, respectively, after OPCABG. The mean time between SGA and MDCT was days. The mean scanning time for MDCT was seconds. Patients received 20 mg to 60 mg oral metoprolol tartrate in accordance with their weight and blood pressure 1.5 hours before the MDCT scan when their heart rates were more than 70 beats per minute, and they received 0.3 mg nitroglycerin immediately before the Table 1. Characteristics Among 42 Patients After OPCABG Surgery Characteristic 821 Patients 42 Age (years) Mean SD Range Sex Male 36 Female 6 Redo 2 CRF on hemodialysis 6 Time between OPCABG and SGA (days) Time between OPCABG and MDCT (days) Time between SGA and MDCT (days) Heart rate (beats per minute) Mean SD Range Metoprolol tartrate 30 Nitroglycerin 28 Scan time (seconds) Type of bypass graft (grafts performed sequential bypass) LIMA 41 (21) RIMA 27 (3) RGEA 23 (9) RA (including 3 of I composite graft) 5 (4) SV 5 (3) Three in-situ arterial grafts (LIMA, 22 RIMA, and RGEA) CRF chronic renal failure; LIMA left internal mammary artery; MDCT multidetector row computed tomography; OPCABG off-pump coronary bypass graft surgery; RA radial artery; R- GEA right gastroepiploic artery; RIMA right internal mammary artery; SGA selective bypass graft angiography; SV saphenous vein. MDCT scan when their systolic blood pressures were more than 100 mm Hg. Consequently, the average heart rate of patients during the MDCT scan was beats per minute (range, 46 to 90). Among the 30 patients who received metoprolol tartrate, 28 patients were given nitroglycerin (Table 1). Sixteen-channel MDCT (LightSpeed Ultra 16; GE Healthcare, Milwaukee, Wisconsin) was performed using 100 ml of nonionic contrast material (Iopamidol; 370 mg iodine/ml) in total. Scan parameters were 500-ms or 600-ms rotation time, mm slice thickness, and helical pitch (table feed per rotation divided by X-ray beam width). For best temporal resolution using multisector reconstruction, we employed a 600-ms rotation time in the 5 patients whose heart rates ranged from 75 to 85 beats per minute and 500-ms rotation time in the remaining 37 patients in our study. We performed a test scan before the actual scan using a bolus injection of 10 ml of contrast agent followed by 15 to 20 ml of saline to estimate circulation time. During the CARDIOVASCULAR

3 CARDIOVASCULAR 822 YAMAMOTO ET AL Ann Thorac Surg MDCT ASSESSMENT OF OPCABG 2006;81:820 7 study, 90 ml of contrast agent followed by 40 to 50 ml of saline were administered at an injection rate of 4 ml/s through a 20G needle into the antecubital vein. The scan started from 1 cm above the proximal anastomosis to the ascending aorta in cases using free grafts, or from just above the area of the right pulmonary artery in cases using in-situ arterial graft only, and ended at the area just below the diaphragm. Collected data were reconstructed using the retrospective electrocardiography-gated method and absolute antegrade reconstruction, mm slice thickness and slice interval, standard reconstruction kernel, and 20-cm displayed field of view. With our machine, the selected percentage of the reconstructed phase implied the center of the acquisition window. For example, the center of the acquisition window for a cardiac phase of 75% was located at 75% of the RR interval from the first R wave. Because we set the machine to use a multisector reconstruction algorithm as often as possible; half reconstruction was used only when multisector reconstruction was not available, namely, at approximately 60 beats per minute at a 500-ms rotation time. Multiple cardiac phases were reconstructed around end-systolic (30% to 45% of the RR interval) and enddiastolic (70% to 80%) phases. The reconstructed multiphase images were transferred to a workstation (Advantage Workstation 4.1; GE Healthcare), and the best cardiac phase was selected. When one cardiac phase was not satisfactory for all grafts and anastomoses, we selected the best cardiac phase for each graft and for each anastomosis. An independent radiologist without knowledge of the SGA results assessed the graft images taken by MDCT. The number and sites of anastomoses of the bypass grafts were known to the radiologist. Axial, three-dimensional volume-rendering, and multiplanar reformatted images were analyzed for graft occlusion and significant stenosis of graft anastomosis. Although whole courses of free grafts (RA and SV) were evaluated, in-situ arterial grafts were evaluated within limits from the area above the right pulmonary artery to the area just below the diaphragm. Because of the limited scan coverage range, the entire courses of the in-situ arterial grafts (LIMA, RIMA, RGEA) were not evaluated. 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 distal artery in sequential anastomosis. In sequential anastomosis, each of several distal anastomoses of one graft was counted as several grafts. For example, sequential grafting of LIMA to LAD to diagonal branch was counted as two grafts. Significant stenosis of graft anastomosis in a patent graft was defined as reduction of luminal diameter by 50% or more. The SGA was performed using the transfemoral or transbrachial approach in all cases. Nonionic contrast material (Iopamidol; 370 mg iodine/ml) was used in almost 100 ml to 150 ml. 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 anastomosis. In-situ arterial grafts were evaluated the entire course of proximal LIMA, RIMA and RGEA grafts. In sequential anastomosis, each of several distal anastomoses of one graft were counted as several bypass grafts. Selective bypass graft angiography was evaluated by quantitative coronary angiography with automated counter detection of vessels after catheter-based image calibration. Significant stenosis of graft anastomosis in patent graft was determined if reduction in mean diameter was 50% or more. For statistical analysis, all data were expressed as mean SD. The reliability of MDCT was evaluated by calculating sensitivity, specificity, positive and negative predictive values, and diagnostic accuracy using 2 2 contingency tables. The results of SGA served as a standard of reference. Sensitivity was calculated as true positive / (true positive false negative). Specificity was calculated as true negative / (true negative false positive). Positive predictive value resulted from true positive / (true positive false positive), and negative predictive value resulted from true negative / (true negative false negative). Diagnostic accuracy was determined by (true positive true negative) / total number. The percentage and 95% confidence interval was calculated for each value. Results The SGA and MDCT were performed in all patients without complication. When electrocardiography-gated contrast-enhanced MDCT was performed using scan mode mm 16, rotation time 500-ms, pitch 0.275, tube current 350mA, and scan length 15cm, dose length product was calculated as mgy cm. Because the average ratio of effective dose to dose length product for CT of the trunk is msv (mgy cm) -1, the effective dose estimated by dose length product was calculated as 20.8 msv [24]. If several sequential anastomoses in 1 graft existed, we calculated as several grafts. Of 160 bypass grafts, including 3 I-composite graft, SGA showed 155 to be patent and 5 (2 RA distal anastomoses to native coronary artery, 2 RA distal anastomoses to next grafts in sequential anastomosis, and 1 RGEA distal anastomosis to native coronary artery) to be occluded (97% patency rate). Compared with SGA, MDCT correctly showed 143 of 155 patent (Fig 1) and 5 of 5 occluded grafts (Fig 2). Of the 12 grafts for which MDCT failed to show correct diagnosis, 2 were not evaluated because of the metallic clip artifacts of the RGEA anastomosis in 1 and motion artifacts of RGEA in the other. Therefore, evaluation possible grafts were 99% (158 of 160). Among the remaining 10 grafts, 2 (LIMA) were of competing counterflow arteries (Fig 3). Six (3 LIMA, 2 RA, and 1 RGEA) resulted from sequential anastomoses to very small native coronary arteries (diameter 1.5 mm) undetectable by the MDCT. Two (1 LIMA and 1 RGEA) poor-quality images were caused by insufficient contrast material. When 2 grafts not evalu-

4 Ann Thorac Surg YAMAMOTO ET AL 2006;81:820 7 MDCT ASSESSMENT OF OPCABG 823 CARDIOVASCULAR Fig 1. A 70-year-old man who had sequential grafting (RIMA to D1 to LAD) and LIMA to OM anastomosis. (A) Multidetector-row computed tomography (MDCT) volume-rendering image in the left anterior oblique projection showing no sequential anastomosis stenosis (white arrow), metallic clip (white arrowhead) not influencing the evaluation of anastomosis. His heart rate was 66 beats per minute at the time of MDCT examination. (B) Selective bypass graft angiography (SGA) image in 60-degree left anterior oblique projection showing no sequential anastomoses stenosis (white arrow). (D1 first diagonal branch; LAD left anterior descending artery. LIMA left internal mammary artery; OM obtuse marginal branch; RIMA right internal mammary artery.) ated by MDCT were not included in the analysis, the sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy were 100% (5 of 5), 93% (143 of 153), 33% (5 of 15), 100% (143 of 143), and 94% (148 of 158), respectively. The sensitivities of graft occlusion for LIMA, RIMA, RGEA, RA, and SV were (0 of 0), (0 of 0), 100% (1 of 1), 100% (4 of 4), and (0 of 0), respectively; and the specificities for LIMA, RIMA, RGEA, RA, and SV were 91% (64 of 70), 100% (31 of 31), 94% (31 of 33), 78% (7 of 9) and 100% (10 of 10), respectively (Table 2). Among the 143 patent grafts evaluated by MDCT, a total of 149 anastomoses including 143 distal and 6 proximal aortic anastomoses existed. Selective bypass graft angiography demonstrated significant stenosis of graft anastomosis in only 2. Multidetector-row CT correctly showed no significant stenosis of graft anastomosis in 134 of the remaining 147 and confirmed significant stenosis in the 2 demonstrated by SGA. Of 13 graft anastomoses for which MDCT failed to show correct diagnosis, 12 (7 LIMA, 3 RIMA, 1 RGEA, and 1 RA) were not evaluated because of metallic clip artifacts, and 1 Fig 2. A 70-year-old woman who had arteriosclerosis of RA graft (RIMA to RA, RA to OM to PL to PD) and LIMA to LAD anastomosis. (A) Multidetector-row computed tomography (MDCT) volume-rendering image in 148-degree left anterior oblique projection showing RIMA to I-composite graft of RA; RA to OM and RA to PL (white arrow) were patent and had no significant stenosis of graft anastomosis; RA to PD showed 100% occlusion (white dotted arrow). Her heart rate was 83 beats per minute at the time of MDCT examination. (B) Selective bypass graft angiography (SGA) image in 30-degree right anterior oblique projection showing same findings of MDCT volume-rendering image. (LAD left anterior descending artery; LIMA left internal mammary artery; OM obtuse marginal branch; PD posterior descending branch; PL posterolateral branch; RA radial artery; RIMA right internal mammary artery.)

5 CARDIOVASCULAR 824 YAMAMOTO ET AL Ann Thorac Surg MDCT ASSESSMENT OF OPCABG 2006;81:820 7 Fig 3. A 78-year-old man who had sequential grafting (LIMA to PL2 to PL1). (A) Multidetector-row computed tomography (MDCT) volumerendering image in 148-degree left anterior oblique projection showing occluded proximal LIMA before PL2 anastomosis, but no sequential anastomosis stenosis (PL2 to LIMA and LIMA to PL1) like coronary coronary bypass (white arrow). His heart rate was 67 beats per minute at the time of MDCT examination. (B) Selective bypass graft angiography (SGA) showing in 0-degree left anterior oblique projection very narrow LIMA (white dotted arrow) without demonstrating LIMA to PL2 and PL1 anastomosis because of competing counter blood flow from native left coronary artery. (C) Left coronary angiography image in 30-degree right anterior oblique projection showing retrograde enhanced LIMA (white dotted arrow) from PL2 and no sequential anastomosis stenosis from PL2 to LIMA and LIMA to PL1 (white arrow). (LIMA left internal mammary artery; PL1 first posterolateral branch; PL2 second posterolateral branch.) nonsignificant stenosis (LIMA) was misjudged as significant. Therefore, evaluation possible grafts were 92% (137 of 149). When 12 graft anastomoses not evaluated by MDCT were not included in the analysis, the sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy were 100% (2 of 2), 99% (134 of 135), 67% (2 of 3), 100% (134 of 134), and 99% (136 of 137), respectively. The sensitivities of significant stenosis of graft anastomosis for LIMA, RIMA, RGEA, RA, and SV were (0 of 0), (0 of 0), (0 of 0), 100% (1 of 1), and 100% (1 of 1), respectively; and the specificities for LIMA, RIMA, RGEA, RA, and SV were 98% (56 of 57), 100% (28 of 28), 100% (30 of 30), 100% (6 of 6), and 100% (14 of 14), respectively (Table 3). Comment Development of new devices has led to the wide practice of OPCABG, which is technically more difficult than conventional CABG. So, postoperative assessment of OPCABG conduits and anastomoses is important to evaluate the quality of surgical technique and to prevent the recurrence of angina pectoris in patients [1, 2]. Selective bypass graft angiography is one method used to confirm surgical results. However, invasive SGA after CABG has a higher risk than standard coronary angiography to produce life-threatening arrhythmia and damage of graft vessels [3 5]. In addition, cost of SGA (about $1,400 US) is almost four times more expensive than cost of MDCT (about $340 US) in Japan. At our facility, most in-situ arterial grafts are performed on patients with triple-vessel disease. Tortuosity or spasm of the arteries proximal to the grafts sometimes precludes their visualization by postoperative SGA, particularly in RGEA. Because SGA was used as a standard of reference in this study, grafts not evaluated by SGA were not included. Electron-beam computed tomography and magnetic resonance imaging are useful in noninvasively assessing graft patency, but both imaging techniques have major limitations in assessing bypass stenosis. Electron-beam Table 2. Sensitivity, Specificity, Positive Predictive Value, and Negative Predictive Value of Graft Occlusion on MDCT After Exclusion of Grafts Not Evaluated by MDCT Values for Detection of Graft Occlusion Overall LIMA RIMA RGEA RA SV Sensitivity 5/5 (100, 100) 0/0 0/0 1/1 (100, 100) 4/4 (100, 100) 0/0 Specificity 143/153 (93, 90 97) 64/70 (91, 85 98) 31/31 (100, 100) 31/33 (94, ) 7/9 (78, ) 10/10 (100, 100) Positive predictive value 5/15 (33, 9 57) 0/6 (0, 0) 0/0 1/3 (33, 20 87) 4/6 (67, ) 0/0 Negative predictive value 143/143 (100, 100) 64/64 (100, 100) 31/31 (100, 100) 31/31 (100, 100) 7/7 (100, 100) 10/10 (100, 100) Diagnostic accuracy 148/158 (94, 90 97) 64/70 (91, 85 98) 31/31 (100, 100) 32/34 (94, ) 11/13 (85, ) 10/10 (100, 100) Data in parentheses are percentage and 95% confidence interval. LIMA left internal mammary artery; MDCT multidetector row computed tomography; RA radial artery; RGEA right gastroepiploic artery; RIMA right internal mammary artery; SV saphenous vein.

6 Ann Thorac Surg YAMAMOTO ET AL 2006;81:820 7 MDCT ASSESSMENT OF OPCABG Table 3. Sensitivity, Specificity, Positive Predictive Value, and Negative Predictive Value of Significant Stenosis of Graft Anastomosis on MDCT After Exlusion of Graft Anastomoses Not Evaluated by MDCT Values for Detection of Significant Stenosis of Graft Anastomosis Overall LIMA RIMA RGEA RA SV 825 CARDIOVASCULAR Sensitivity 2/2 (100, 100) 0/0 0/0 0/0 1/1 (100, 100) 1/1 (100, 100) Specificity 134/135 (99, ) 56/57 (98, ) 28/28 (100, 100) 30/30 (100, 100) 6/6 (100, 100) 14/14 (100, 100) Positive predictive value 2/3 (67, ) 0/1 (0, 0) 0/0 0/0 1/1 (100, 100) 1/1 (100, 100) Negative predictive value 134/134 (100, 100) 56/56 (100, 100) 28/28 (100, 100) 30/30 (100, 100) 6/6 (100, 100) 14/14 (100, 100) Diagnostic accuracy 136/137 (99, ) 56/57 (98, ) 28/28 (100, 100) 30/30 (100, 100) 7/7 (100, 100) 15/15 (100, 100) Data in parentheses are percentage and 95% confidence interval. LIMA left internal mammary artery; MDCT multidetector row computed tomography; RA radial artery; RGEA right gastroepiploic artery; RIMA right internal mammary artery; SV saphenous vein. computed tomography yields poor image quality for three-dimensional visualization of the coronary arteries as a result of limited spatial resolution because only a 3-mm slice can be obtained with sequential electrocardiography triggering [7, 8]. Magnetic resonance imaging provides information about graft morphology and function, but morphologic delineation of bypass grafts could be improved by contrast-enhanced three-dimensional magnetic resonance angiography. However, contrastenhanced magnetic resonance angiography is strongly affected by metallic clip artifacts and requires improved spatial resolution to enable depiction of distal anastomosis [9 12]. A recent study of four-channel MDCT angiography reported sensitivity of 97% and specificity of 98% for graft occlusion. However, requirements such as minimum heart rate and long breath-holding time to cover scan length and limited spatial resolution caused by thick collimation reduced ability to evaluate stenosis of patent grafts (evaluation-possible grafts: 62%) [13]. Sixteen-channel MDCT, which obtains 16 thin slices of 0.5 to 0.75 mm thickness per 375-ms to 500-ms gantry rotation, enables scanning of a large volume during a single breath hold with improved spatial and temporal resolutions. Therefore, recent reports on 16-channel MDCT have shown high-quality images and satisfactory accuracy in evaluating coronary artery stenosis and bypass graft patency [18 23]. We analyzed several kinds of grafts (mainly in-situ arterial grafts), sequential grafting, and composite arterial grafts performed by OPCABG and achieved satisfactory MDCT images for diagnosis. Specificity in graft occlusion was 100% in both RIMA and SV grafts because most RIMA grafts were anastomosed to the LAD, which is easily revealed with MDCT because of the straight course and relatively large diameter of this artery and because SV grafts were large in diameter. Six sequential grafts (3 LIMA, 2RA, and 1 RGEA) undetectable by MDCT were connected to small native coronary arteries (3 branches of circumflex artery, 2 diagonal branches, and 1 posterior descending branch). In addition to their small diameter, the branches of circumflex artery and diagonal branch arteries are easily affected by cardiac motion artifact because they course along the atrioventricular groove or the left ventricle. Multidetector-row CT demonstrated 2 LIMA grafts to be occluded, whereas SGA showed their patency with a competing counter blood flow from the native coronary artery (Fig 3). This false positive was caused by delayed LIMA blood flow resulting from a competing counter blood flow between the moderate stenosis of a native coronary artery and the smaller diameter of the LIMA graft [15]. In this study, 16-channel MDCT scanning was not extended to cover the entire course of proximal LIMA, RIMA and RGEA grafts, where obstructions are not frequently found by SGA. To view the entire course of grafts, beginning from their proximal sites (subclavian artery, celiac artery) to distal anastomoses, a long distance must be covered in the cranio-caudal direction while keeping good spatial resolution (namely, thin slice collimation and small pitch), which requires longer breath-holding time and increased radiation dose. We considered essential evaluation of graft patency and stenosis of graft anastomosis including distal and proximal aortic anastomosis and did not scan the entire course of the in-situ arterial grafts. However, the long distance of the entire in-situ grafts could be easily covered within a fairly short breath hold with newly developed 64- channel MDCT systems. On MDCT, artifacts from metallic vascular clips, which may obscure the body and the anastomosis of grafts, prevent visualization. Because we performed side-to-side anastomosis even for distal anastomosis, metallic clips were always necessary to block blood flow from the distal grafts. To avoid metallic artifact on MDCT, for distal anastomosis in 18 grafts, we used Absolok Extra Ligating Clip (Ethicon Endo-Surgery, Cincinnati, Ohio), which is made of violet polydioxanone. However, these comparatively large clips in the anastomosis cause technical difficulty when suturing the edge near the clip. Therefore, we returned to using blocking metallic clips on distal grafts, placing them slightly away from the distal anastomosis to avoid metallic artifacts as much as possible. Although this method allows detection of anastomosis on MDCT, the metallic clips often obscured the native coronary arteries just distal to the graft anastomoses (Fig 4). A recent report indicated that heart rate is not a crucial determinant for the quality of diagnostic im-

7 CARDIOVASCULAR 826 YAMAMOTO ET AL Ann Thorac Surg MDCT ASSESSMENT OF OPCABG 2006;81:820 7 Fig 4. A 66-year-old man who had sequential grafting (RGEA to PD1 to PD2). (A) Multidetector-row computed tomography (MDCT) volumerendering image showing no sequential anastomosis stenosis (RGEA to PD1 and RGEA to PD2 [white arrow]). Metallic clip artifact (white arrowhead) obscures distal native coronary artery after distal RGEA graft anastomosis. His heart rate was 80 beats per minute at the time of MDCT examination. (B) Multidetector-row computed tomography multiplanar reformatted image also showing no sequential anastomosis stenosis (RGEA to PD1 and RGEA to PD2 [white arrow]). Metallic clip artifact (white arrowhead) obscures distal native coronary artery as shown on the volume rendering image. (C) Selective bypass graft angiography (SGA) image in 30-degree right anterior oblique projection showing no sequential anastomosis stenosis (RGEA to PD1 and RGEA to PD2 [white arrow]). (PD1 first posterior descending branch; PD2 second posterior descending branch; RGEA right gastroepiploic artery.) ages by multisector reconstruction [20]. 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. In addition, with our machine, in patients with heart rates of 75 to 85 beats per minute, this technique requires a 600-ms rotation time, which lengthens breath-holding time. Therefore, we gave metoprolol tartrate orally to obtain sufficient image quality with 500-ms rotation time in patients with heart rate greater than 70 beats per minute. Although we examined a large number of samples, surgery resulted in only 5 graft occlusions and 2 significant stenoses of graft anastomosis, which led to a low positive predictive value. A larger occlusion and stenosis sample is needed to confirm our results. Despite a few unavoidable limitations, our study results showed that MDCT allows accurate assessment of graft occlusion and significant stenosis of graft anastomosis after OPCABG. The sensitivity and negative predictive value for graft occlusion and significant stenosis of graft anastomosis were all 100%. Because no false negative result was shown in graft occlusion or in significant stenosis of graft anastomosis, we consider MDCT the first choice for post-opcabg graft assessment and the more physically invasive and expensive SGA as the next choice when MDCT shows graft occlusion or significant stenosis of graft anastomosis or cannot permit evaluation of the state of graft. In conclusion, 16-channel MDCT is an effective noninvasive technique to assess post-opcabg grafts. References 1. Diegeler A, Matin M, Kayser S, et al. Angiographic results after minimally invasive coronary bypass grafting using the minimally invasive direct coronary bypass grafting (MID- CAB) approach. Eur J Cardiothorac Surg 1999;15: Wiklund L, Johansson M, Brandrup-Wognsen G, Bugge M, Radberg G, Berglin E. Difficulties in the interpretation of coronary angiogram early after coronary artery bypass sur-

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