Evaluation of Coronary Bypass Graft Occlusion and Stenosis with 64-Detector-Row Computed Tomography Angiography

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1 Acta Radiologica ISSN: (Print) (Online) Journal homepage: Evaluation of Coronary Bypass Graft Occlusion and Stenosis with 64-Detector-Row Computed Tomography Angiography D. Oncel, G. Oncel, A. Taştan & B. Tamci To cite this article: D. Oncel, G. Oncel, A. Taştan & B. Tamci (2007) Evaluation of Coronary Bypass Graft Occlusion and Stenosis with 64-Detector-Row Computed Tomography Angiography, Acta Radiologica, 48:9, To link to this article: Published online: 04 Aug Submit your article to this journal Article views: 179 View related articles Full Terms & Conditions of access and use can be found at

2 ORIGINAL ARTICLE ACTA RADIOLOGICA Evaluation of Coronary Bypass Graft Occlusion and Stenosis with 64- Detector-Row Computed Tomography Angiography D. ONCEL, G.ONCEL, A.TAŞTAN &B.TAMCI Departments of Radiology and Cardiology, Sifa Hospital, Izmir, Turkey Oncel D, Oncel G, Taştan A, Tamci B. Evaluation of coronary bypass graft occlusion and stenosis with 64-detector-row computed tomography angiography. Acta Radiol 2007;48: Background: A noninvasive imaging modality is desirable for the evaluation of coronary bypass graft stenosis and occlusion. Purpose: To prospectively evaluate the effectiveness of 64-detector-row computed tomography (DCT) for the assessment of coronary bypass grafts. Material and Methods: Forty-two patients (35 male, seven female, mean age 66.3 years) with 103 bypass grafts (32 arterial, 71 venous) were examined with 64-DCT. The evaluations were done by two radiologists blinded to the results of quantitative coronary angiography (QCA), used as the reference standard. Results: All of the 26 occluded grafts, nine of the 10 stenosed grafts, and 66 of the 67 patent grafts were correctly diagnosed with 64-DCT angiography. The sensitivity, specificity, and positive and negative predictive values for 64-DCT in detecting graft stenosis were 90%, 99%, 90%, and 99%, respectively. For graft occlusion, all were 100%. No statistically significant difference was found between 64-DCT and QCA for the evaluation of bypass grafts. Intermodality and interobserver agreement were excellent. Conclusion: 64-DCT angiography is a reliable, noninvasive diagnostic method for the assessment of coronary bypass grafts. It can be considered as a useful tool for follow-up purposes and may function as a gatekeeper before invasive procedures. Key words: Adults; arteries; cardiac; CT angiography D. Oncel, Sifa Hospital, Fevzipasa Boulevard No: 172/2, Basmane, Izmir, Turkey (tel , fax , . dilekoncel@hotmail.com) Accepted for publication May 26, 2007 Coronary artery bypass graft surgery is an established treatment in the management of advanced coronary artery disease. The long-term clinical outcome after myocardial revascularization depends on the patency of the bypass grafts. Up to 10% of grafts become occluded during the perioperative period. Nearly 40% of patients experience recurrence of symptoms within 6 years, and 25% of bypass grafts are found to be occluded at follow-up 5 years after surgery. After 10 years, occlusion rates of 59% and 17% have been reported for venous and arterial grafts, respectively (5, 7). Conventional coronary angiography (CCA) has been considered as the reference standard for the evaluation of coronary artery bypass graft patency and stenosis. However, CCA is an invasive procedure with a significant cost and small procedure-related morbidity and mortality (3). Therefore, a noninvasive imaging modality is highly desirable. Utilization of computed tomography (CT) for the evaluation of bypass grafts has been reported since the early 1980s. Because of their large diameter, limited calcification, and relative immobility, bypass grafts and particularly saphenous vein grafts are well visualized by CT. Due to limited temporal and spatial resolution, single-detector-row spiral CT remained ineffective and was confined to simple patency assessment (22). With four-detector-row computed tomography (DCT) scanners, several studies reported good sensitivity and specificity for the detection of graft occlusion, while diagnostic performance was lower for graft stenosis and failed to define distal anastomosis site patencies (4, 12, 13, 18, 21, 23). With 16-DCT scanners, temporal and spatial resolutions were higher. Significant stenoses were better assessed and the number of unevaluable grafts was reduced. However, evaluation of distal anastomosis still remained a problem because of motion artifacts or artifacts due to surgical clip DOI / # 2007 Taylor & Francis

3 Evaluating Coronary Bypass Grafts with 64-DCT Angiography 989 material or calcification (1, 6, 19, 20, 21). With 64- DCT technology, the superior temporal and spatial resolution further improved image quality (10, 11, 14 17). Therefore, the aim of our study was to evaluate the diagnostic performance of 64-DCT for the assessment of coronary artery bypass grafts. Material and Methods Patient population The study was approved by the institutional review board, and all patients gave informed consent to participate in the study. In this prospective study, 42 patients with previous bypass grafts (35 male, seven female, mean age 66.3 years, range years) were examined between January 2005 and September A total of 103 bypass grafts were examined. Thirty-two of them were arterial grafts (28 left internal mammarian artery [LIMA] and four right internal mammarian artery [RIMA]) and 71 of them were saphenous vein grafts. All were single grafts. The types and localizations of bypass grafts are summarized in Table 1. The localizations of bypass grafts were specified as outlined by the American Heart Association, which defines coronary arteries in terms of 16 segments (2). The mean interval after bypass graft operation was 50.9 months (range months). In all patients, there was suspected bypass graft stenosis or occlusion depending on patient complaints (angina-like symptoms) and/or laboratory findings (positive or suspected stress ECG tests and/or myocardial perfusion scintigraphies). All patients were scheduled for CCA, performed 1 2 days after the CT examination. Patient characteristics are summarized in Table 2. The exclusion criteria for CT were as follows: unstable clinical condition, previous allergic reaction to iodinated contrast agents, irregular heart rate, atrial fibrillation, renal dysfunction (elevated serum creatinine levels w1.5 mg/dl), and failure to follow breath-hold commands. The study protocol included intravenous metoprolol administration in patients with heart rates w70 bpm. Therefore, patients with any contraindications concerning Table 2. Clinical characteristics (42 patients) Age (mean SD), years Male, % 83 Cardiac risk factors Hypertension*, % 43 Hypercholesterolemia{, % 57 Diabetes mellitus, % 45 Current smoking, % 31 Family history of coronary artery disease, % 45 Body-mass index (mean SD), kg/m Serum creatinine (mean SD), mg/dl Previous use of heart-rate-lowering medication, % 74 Reasons for referral to invasive coronary angiography Angina-like symptoms, % 57 Positive or suspected stress ECG test, % 29 Positive or suspected myocardial perfusion 14 scintigraphy, % * Blood pressure >160/95 mmhg, or treatment for hypertension. {Total cholesterol w200 mg/dl, or treatment for hypercholesterolemia. beta-blocker administration (asthma, a v block, congestive heart failure, etc.) were also excluded. During the study period, 27 patients were excluded from the study group either due to the abovementioned contraindications (n515) or due to refusal to participate in the study (n512). CT coronary angiography: scan protocol and reconstruction All CT examinations were performed using a 64- DCT scanner (Sensation Cardiac 64; Siemens, Forcheim, Germany). To achieve optimal image quality, in patients with heart rates exceeding 70 bpm, 5 20 mg intravenous metoprolol (Beloc 5 mg/5 ml; AstraZeneca, Istanbul, Turkey) was administered immediately prior to scanning. In our study group, 35 patients had prescan heart rates (70 bpm (31 of them were already on beta-blocker treatment). Intravenous metoprolol was administered to the remaining seven patients, and in all of them heart rates below 70 bpm were maintained. Also, we administered sublingual nitrate (Isordil 5 mg, isosorbide dinitrate; Fako, Istanbul, Turkey) before scanning to standardize vasomotor tone in the coronary arteries beyond the graft anastomosis. Scan parameters were as follows: slice collimation mm62, rotation time 0.33 ms (effective Table 1. Type and localization of bypass grafts Localizaton of the bypass graft implantation (segment number according to AHA classification) Type of bypass graft n Saphenous vein LIMA RIMA Total LIMA: left internal mammarian artery; RIMA: right internal mammarian artery; AHA: American Heart Association.

4 990 D. Oncel et al. temporal resolution 165 ms), tube voltage 120 kv, tube current (effective mas) 900 mas, pitch 0.2. With "flying spot" technology, the focal spot is continuously modified between two points aligned on the z-axis, and the number of simultaneously acquired slices is doubled (double-z sampling). The scans were performed in the craniocaudal direction with subjects in the supine position, and the scan time was approximately s (depending on the volume of coverage) in a single breath-hold. The scanning range was planned individually for each patient to include the extent of the coronary artery bypass grafts, ranging from the subclavian artery (including the proximal segment of the IMA grafts) to the apex of the heart. The mean craniocaudal distance of the volume data set was 17 cm (range cm). CT angiography was triggered automatically by the arrival of the main contrast bolus (automatic bolus tracking). A prescan was taken at the level of the aortic root, and a region of interest (ROI) was identified on the ascending aorta. We injected 100 ml nonionic contrast medium (Iomeron 350/ ml, iomeprol; Bracco, Milan, Italy) into an antecubital vein through an 18-gauge catheter at a flow rate of 5.5 ml/s. As soon as the signal density level in the ascending aorta reached a predefined threshold of 150 Hounsfield units (HU), the scan started. The injection of contrast medium was followed by a 40-ml saline chaser bolus at a flow rate of 4 ml/s to wash out contrast from the right ventricle. During the scan, ECG was recorded simultaneously. Continuous data acquisition allowed slice reconstruction at different time positions within the cardiac cycle. In each patient, images were first reconstructed at 65%, 70%, and 75% of the R R interval. If motion artifacts were present, additional reconstruction windows were applied. The reconstruction interval with the least motion artifacts was used for further analysis. When necessary, R-wave indications were manually repositioned to improve the quality of synchronization. Prospective ECG tube-current modulation (ECG pulsing) for the reduction of radiation dose was applied in all patients. Axial images were reconstructed with a slice thickness of 0.75 mm and slice increment of 0.5 mm. For the reconstruction of images, standard medium-soft kernels (B20f) were used. CT coronary angiography was performed successfully in all patients without any complications. CT coronary angiography analysis Evaluation of the images was done by two radiologists (D.O., G.O.), with 5 years of experience regarding cardiac CT, independently and blinded to the results of CCA. The number and course of the coronary artery bypass grafts as obtained from surgical reports were known to both radiologists for each patient. In case of disagreement, a final decision was obtained by consensus. For the evaluation of bypass grafts, axial source images, curved multiplanar reformations (MPR), maximum intensity projections (MIP), and volumerendered images (VR) were used. For post-processing, a Leonardo workstation (Siemens, Forcheim, Germany) was used. Significant stenosis was defined as narrowing of the vessel diameter exceeding 50%. Vessel diameters were measured on reconstructions perpendicularly oriented to the vessel course. Each bypass graft was evaluated to be patent, stenosed, or occluded. Along with the bypass grafts, proximal and distal anastomoses were also evaluated. Image quality was determined on the basis of the presence of artifacts degrading the image, such as motion artifacts, calcification, or metallic surgical materials (sutures, clips, markers, and sternal wires) (1, 4). A three-point grading scale was determined accordingly: 1, non-diagnostic/poor (severe artifacts making evaluation impossible, non-adequate quality for diagnosis); 2, moderate (mild/moderate artifacts but acceptable for routine clinical diagnosis); 3, excellent (no artifacts, with clear delineation of the entire graft). The bypass grafts with score 1 (poor image quality/non-diagnostic) were not considered for further evaluation. The analysis was done on a lesion-by-lesion basis for both false-positive and false-negative results, and the possible reason for each misdiagnosis was evaluated retrospectively. In addition to the overall diagnostic performance of 64-DCT coronary angiography in detecting significant stenoses and occlusion, diagnostic performance was evaluated separately with respect to image quality scores and graft type as well as on a patient-by-patient basis. Conventional coronary angiography: reference standard CCA was performed in all patients by using a biplanar angiography system (Siemens, Erlangen, Germany) with standard techniques, carried out by two cardiologists (A.T. and B.T., with 10 and 5 years of angiography experience, respectively) within 1 2 days after the CT examination. The angiograms were evaluated by one cardiologist

5 Evaluating Coronary Bypass Grafts with 64-DCT Angiography 991 (A.T.). The cardiologist was aware of the number and locations of grafts, as obtained from surgical reports, but blinded to the results of CT angiography. Each bypass graft was evaluated to be patent, significantly stenosed (defined as a diameter reduction of w50%), or occluded by using the quantitative coronary analysis method. Distal and proximal anastomosis sites were also included in the evaluation. No intracoronary isosorbide dinitrate was employed. Statistical analysis Statistical analysis was performed using commercially available statistical software (SPSS 11.5 for Windows; SPPS Inc., Chicago, Ill., USA). The diagnostic performance of 64-DCT coronary angiography for the evaluation of bypass graft stenosis and occlusion was presented as sensitivity, specificity, and positive (PPV) and negative predictive values (NPV). These diagnostic parameters were expressed with a 95% confidence interval (CI). CCA was regarded as the standard of reference. The McNemar test was used to discover whether there was a statistically significant difference between 64-DCT angiography and CCA in detecting bypass graft stenosis and occlusion. A P value v0.05 was considered to be statistically significant. To avoid bias caused by multiple bypass grafts per patient, the P value was adjusted by using the correction factor C, as defined by GONEN et al. (8). Intermodality and interobserver agreements between 64-DCT angiography and CCA in detecting bypass graft stenosis and occlusion were determined by kappa statistics. According to LANDIS and KOCH (9), a kappa value of 0 indicates poor agreement, slight agreement, poor agreement, moderate agreement, good agreement, and excellent agreement. A power analysis to determine the number of patients to be included in the study was not performed. Results Conventional coronary angiography In conventional angiography, 26 of 103 bypass grafts showed total occlusion (25%). Twenty-two of the occluded grafts were saphenous vein grafts (31%, 22/71), and four of them were LIMA grafts (14%, 4/28). In 10 bypass grafts, significant (>50% narrowing of lumen diameter) stenosis was found (10%, 10/103). Six of the stenoses were in LIMA grafts (21%, 6/28), one was in a RIMA graft (25%, 1/4), and three were in saphenous vein grafts (4%, 3/ 71). The other 67 bypass grafts were shown to be patent with no stenosis (65%, 67/103). In the patient-based analysis, bypass graft stenosis and/or occlusion were found in 29 (69%, 29/42) patients. The remaining 13 (31%, 13/42) patients had patent bypass grafts with no stenosis or occlusion. CT coronary angiography The mean heart rate during the scan was 59 bpm (SD 4.6). In CT angiography, all 26 occluded bypass grafts were correctly diagnosed (Figs. 1 and 2). Image quality assessment was done in the remaining 77 bypass grafts. The image quality was excellent (score 3) in 66 (86%) and moderate (score 2) in 11 bypass grafts (14%). No bypass graft was evaluated to be of poor image quality (score 1). Therefore, all bypass grafts were included in the analysis. The reasons for decreased image quality are summarized in Table 3. Among 10 bypass graft stenoses, nine were correctly demonstrated (Fig. 3A C). One stenosed bypass graft was misdiagnosed as normal. Three of the stenoses were located in saphenous vein grafts. One of these stenoses was located at the distal anastomosis site, and two were at the body of the grafts. All of them were correctly diagnosed, and image quality was excellent in all cases (score 3). One of the stenoses was located at the distal anastomosis site of a RIMA graft. Image quality was excellent (score 3), and it was correctly diagnosed. The remaining six stenoses were located at LIMA grafts. Three of the stenoses were located at the body of the graft. The image quality was excellent (score 3), and all were correctly demonstrated. Three stenoses were located at distal anastomosis sites. Two of them were correctly diagnosed. Image quality was moderate (score 2) for one of them and excellent for the other (score 3). Among the 67 patent bypass grafts, 66 were correctly diagnosed (Fig. 3E H). One patent bypass graft was misdiagnosed as stenosis. Regarding the false-negative result, the stenosis of the graft, which was localized at the distal anastomosis site of a LIMA graft implanted to the proximal segment of the LAD, was considered as a motion artifact resulting from breath-hold insufficiency. In the case of the one false-positive result, the motion artifact secondary to arrhythmia during the scan mimicked a significant stenosis at the distal anastomosis site of a LIMA bypass graft implanted to the proximal segment of the first diagonal branch of the LAD. In both cases, image quality was

6 992 D. Oncel et al. A B Fig. 1. A 64-year-old male patient with saphenous vein LAD, LIMA D1, and saphenous vein RCA bypass grafts. A. A curved MPR image of the CT angiography image demonstrates the total occlusion of the saphenous vein RCA bypass graft. In the lumen of the graft, thrombosis can be visualized. There is also wall calcification. There are metallic artifacts due to sternal wires, but they do not obscure the visualization of the graft. B. A volume-rendered CT angiography image shows only the stump of the saphenous vein RCA graft on the aorta, corresponding to saphenous vein bypass graft occlusion. The saphenous vein LCX bypass graft could not be visualized on conventional angiography. considered to be moderate (score 2) due to motion artifacts that contributed to the misdiagnosis. According to these results, the sensitivity, specificity, PPV, and NPV for 64-DCT in detecting bypass graft occlusion was 100% for all. For bypass graft stenosis, the sensitivity, specificity, PPV, and NPV was calculated as 90% (9/10, 95% CI 84 96), 99% (66/67, 95% CI ), 90% (9/10, 95% CI A B Fig. 2. A 67-year-old male patient with LIMA LAD and saphenous vein D1 bypass grafts. A. A curved MPR image of the CT angiography image demonstrates total occlusion of the LIMA graft. No flow density is observed within the lumen, and only metallic side-branch clips are visible along the course of the graft. However, the distal LAD demonstrates normal flow, which is supplied by the patent saphenous vein D1 bypass graft. B. A volume-rendered CT angiography image demonstrates the occluded LIMA graft and patent saphenous vein graft. Note the normal flow in the distal LAD. The LIMA LAD graft could not be visualized with conventional angiography, confirming occlusion of the graft.

7 Evaluating Coronary Bypass Grafts with 64-DCT Angiography 993 Table 3. Reasons for decreased (moderate) image quality in 11 bypass grafts Reason for moderate image quality Saphenous vein LIMA RIMA Motion artifacts Arrhytmia during scan Breath-hold insufficiency Patient movement Metallic artifacts Sternal wire Side-branch clip Total LIMA: left internal mammarian artery; RIMA: right internal mammarian artery ), and 99% (66/67, 95% CI ), respectively. The overall sensitivity, specificity, PPV, and NPV for 64-DCT in detecting bypass graft occlusion and stenosis was 97% (35/36, 95% CI 94 99), 99% (66/67, 95% CI ), 97% (35/36, 95% CI 94 99), and 99% (66/67, 95% CI ), respectively. The diagnostic performance of CT angiography in the evaluation of bypass graft stenosis with respect to graft type and image quality is summarized in Table 4. In the patient-based analysis, all patients with bypass graft stenosis and/or occlusion were correctly diagnosed (n529). Also, all patients with patent bypass grafts with no stenosis or occlusion were correctly classified (n513). In patients with false-positive and false-negative results, there were additional bypass grafts that were correctly diagnosed as occluded or stenosed. Therefore, sensitivity, specificity, PPV, and NPV for the classification of patients with or without bypass graft occlusion and/or stenosis was 100% for all. According to kappa statistics, agreement between 64-DCT and CCA was excellent (k50.885) for the detection of coronary bypass graft stenosis (Table 4). Interobserver agreement was excellent concerning coronary bypass graft stenosis (k50.95) and occlusion (k51). With the McNemar test, the corrected P value was calculated as This demonstrated that there was no statistically significant difference between 64- DCT and CCA for the detection of coronary bypass graft stenosis and occlusion. Discussion CCA has been considered the imaging modality of choice for the assessment of coronary artery bypass grafts in patients with recurrent symptoms after bypass graft placement (7). In parallel with the advances in CT technology, multidetector-row CT has emerged as an alternative, noninvasive imaging method in the evaluation of coronary artery bypass grafts. In previous studies with 4-DCT scanners, the main problems were the significant number of unevaluable grafts, failure to define distal anastomosis site patencies, and long scan times (4, 12, 13, 18, 23). With 16-DCT scanners, the number of unevaluable grafts was reduced and scan times were more favorable. Also, the diagnostic performance regarding bypass graft patency and stenosis was high. However, a considerable number of patent grafts were still not eligible for the analysis of highgrade stenosis, especially at distal anastomosis sites (1, 6, 19, 20). Sixty-four-DCT scanners provide superior spatial and temporal resolution. With increased spatial resolution, the blooming effect and beamhardening artifacts related to metallic materials are decreased substantially, while an effective temporal resolution of 165 ms helps to minimize motion artifacts (10, 14, 16). Scan times are also shorter. These improvements are expected to increase the diagnostic performance accordingly. Previously published studies with 64-DCT reveal favorable results in the evaluation of bypass grafts and anastomosis sites, as well as native coronary arteries (11, 15, 17). Our study, in parallel with the literature, demonstrated that 64-DCT performed well in the determination of bypass graft occlusion and stenosis. In our study, all bypass grafts were interpretable. We achieved excellent image quality in 86% (66/77) of visible bypass grafts, and no bypass graft had to be excluded from the study due to poor image quality. This is very important, because evaluation of all bypass grafts is a prerequisite for CT angiography to become a clinically relevant tool for the assessment of patients with suspected bypass graft stenosis or occlusion. In accordance with previously published articles and our own experience, the image quality of different parts of the bypass graft was usually not uniform (1, 13, 18, 23). The proximal segments were rarely affected by motion artifacts, while the body and in particular the distal anastomoses of the bypass grafts were more susceptible to these artifacts. Also, the effect of motion artifacts in image quality was more pronounced in arterial grafts due to thinner luminal diameters. In our study, both of the two misdiagnoses (one falsenegative and one false-positive result) were at the

8 994 D. Oncel et al. A B C D E F G H Fig. 3. A 57-year-old female patient with LIMA LAD, saphenous vein D1, and saphenous vein LCX bypass grafts. A. A curved MPR image of CT angiography image shows significant stenosis in the proximal part of the saphenous vein LCX graft (arrow). B. MIP image of the CT angiography demonstrates stenosis of the saphenous vein LCX bypass graft (arrow). C. Volume-rendered CT angiography image demonstrates the stenosis (arrow). Also, the LIMA LAD and saphenous vein D1 bypass grafts are seen to be patent. D. Conventional angiography image (left anterior oblique view, cranial angulation) depicts stenosis of the saphenous vein LCX bypass graft. E. MIP image of CT angiography demonstrates the LIMA LAD and saphenous vein D1 bypass grafts with no significant stenoses. The distal anastomosis sites are well shown as normal. F. Volume-rendered CT angiography image shows normal LIMA LAD and saphenous vein D1 bypass grafts and distal anastomosis sites. G. Conventional angiography image (right anterior oblique view) depicts the normal LIMA LAD bypass graft. H. Conventional angiography image (right anterior oblique view) depicts the normal saphenous vein D1 graft. distal anastomosis sites of LIMA grafts and resulted from motion artifacts (secondary to breath-hold insufficiency and unexpected arrhythmia during the scan, respectively). Therefore, motion artifacts, although minimized substantially with 64-DCT scanners, still remain a major cause of image degradation. We used beta-blockers to decrease heart rates below 70 bpm, and this helped to eliminate residual cardiac motion artifacts and improve image quality.

9 Evaluating Coronary Bypass Grafts with 64-DCT Angiography 995 Table 4. Diagnostic performance of 64-DCT angiography in the evaluation of bypass graft stenosis with respect to graft type and image quality Graft type and image Sensitivity, % Specificity, % PPV, % NPV, % quality Number TP TN FP FN (95% CI) (95% CI) (95% CI) (95% CI) Kappa index, k LIMA (5/6) 94 (17/18) 83 (5/6) 94 (17/18) (68 99) (85 100) (68 99) (85 100) Moderate (score 2) (1/2) 80 (4/5) 50 (1/2) 80 (4/5) 0.3 (13 87) (51 100) (13 87) (51 100) Excellent (score 3) (4/4) 100 (13/13) 100 (4/4) 100 (13/13) 1 RIMA (1/1) 100 (3/3) 100 (1/1) 100 (3/3) 1 Moderate (score 2) NA 100 (1/1) NA 100 (1/1) NA Excellent (score 3) (1/1) 100 (2/2) 100 (1/1) 100 (2/2) 1 Saphenous vein (3/3) 100 (46/46) 100 (3/3) 100 (46/46) 1 Moderate (score 2) NA 100 (3/3) NA 100 (3/3) NA Excellent (score 3) (3/3) 100 (43/43) 100 (3/3) 100 (43/43) 1 Total (9/10) 99 (66/67) 90 (9/10) 99 (66/67) (84 96) (97 100) (84 96) (97 100) Moderate (score 2) (1/2) 89 (8/9) 50 (1/2) 89 (8/9) (20 80) (71 100) (20 80) (71 100) Excellent (score 3) (8/8) 100 (78/78) 100 (8/8) 100 (78/78) 1 TP: true positive; TN: true negative; FP: false positive; FN: false negative; PPV: positive predictive value; NPV: negative predictive value; LIMA: left internal mammarian artery; RIMA: right internal mammarian artery; CI: confidence interval; NA: not applicable. In our study, metallic artifacts caused reduced image quality in only three bypass grafts (all arterial). However, they did not lead to misdiagnosis, and the images were eligible for correct evaluation. This low number can be explained by the improved spatial resolution of 64-DCT, resulting in decreased metallic artifacts. Concerning image quality and the evaluation of stenosis, the better performance of CT angiography in venous grafts compared to arterial grafts was probably a result of their larger diameters and fewer metallic artifacts. In our study, the overall diagnostic accuracy of 64-MDCT in detecting bypass graft occlusion and stenosis was very high, and in the patient-based analysis all patients with or without bypass graft occlusion and/or stenosis were correctly classified. Therefore, when referral to catheterization is questionable, CT coronary angiography may identify patients without bypass graft occlusion and/or stenosis, and decrease the number of unnecessary invasive procedures. We acknowledge certain limitations in our study. First, our patients comprised a high pre-test probability population referred for invasive angiography with clinical suspicion of bypass graft occlusion or stenosis, which may have resulted in an overestimation of the ability of 64-DCT to detect and to rule out bypass graft stenosis and occlusion. Wider applicability to an asymptomatic population with previous bypass grafts may yield different results. Another limitation may be the assessment of the stenosis of bypass grafts only. Native coronary arteries were not included in the analysis. Also, while evaluating the image quality of bypass grafts, we did not divide the grafts into segments. However, this did not complicate the evaluation, because image quality was excellent in a substantially high number of grafts (86%, 66/77). The radiation exposure inherent with CT was another important limitation of the study. CT coronary angiography was associated with substantial irradiation to the patients, even though we applied ECG pulsing for dose-saving purposes in all of our patients. In conclusion, our results with 64-DCT demonstrated that CT coronary angiography is a reliable, noninvasive diagnostic method for the evaluation of coronary artery bypass graft occlusion and stenosis. It provides important diagnostic information in a faster, less expensive, more patient-friendly, and safer manner than CCA. It can thus be considered as a useful tool, especially for follow-up purposes, and may function as a gatekeeper before invasive coronary procedures. Future improvements in CT

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