Value of magnetic resonance imaging for the noninvasive detection of stenosis in coronary artery bypass grafts and recipient coronary arteries.

BACKGROUND: Magnetic resonance imaging (MRI) is a potential noninvasive diagnostic tool to detect coronary artery bypass graft stenosis, but its value in clinical practice remains to be established. We investigated the value of MRI in detecting stenotic grafts, including recipient vessels. METHODS AND RESULTS: We screened for inclusion 173 consecutive patients who were scheduled for coronary angiography because of recurrent chest pain after coronary artery bypass grafting (CABG). We studied 69 eligible patients with 166 grafts (81 single vein, 44 sequential vein, and 41 arterial grafts). MRI with baseline and stress flow mapping was performed. Both scans were successful in 80% of grafts. Grafts were divided into groups with stenosis > or =50% (n=72) and > or =70% (n=48) in the graft or recipient vessels. Marginal logistic regression was used to predict the probability for the presence of stenosis per graft type using multiple MRI variables. Receiver operator characteristics (ROC) analysis was performed to assess the diagnostic value of MRI. Sensitivity (95% confidence interval)/specificity (95% confidence interval) in detecting single vein grafts with stenosis > or =50% and > or =70% were 94% (86 to 100)/63% (48 to 79) and 96% (87 to 100)/92% (84 to 100), respectively. CONCLUSIONS: MRI with flow mapping is useful for identifying grafts and recipient vessels with flow-limiting stenosis. Flow scans could be obtained in 80% of the grafts. This proof-of-concept study suggests that noninvasive MRI detection of stenotic grafts in patients who present with recurrent chest pain after CABG may be useful in selecting those in need of an invasive procedure.

Detection of vein graft disease using high-resolution magnetic resonance angiography.

BACKGROUND: The application of previous magnetic resonance (MR) angiography techniques has enabled noninvasive differentiation between patent and occluded coronary artery bypass grafts. However, the detection of graft stenosis remains difficult. The purpose of our study was to determine the accuracy of high-resolution navigator-gated 3-dimensional (3-D) MR angiography in detecting vein graft disease. Methods and Results- MR angiography was performed in addition to coronary angiography with quantitative coronary analysis in 56 vein grafts from 38 patients (mean age 66.6+/-9.3 years), who presented with recurrent chest pain after bypass surgery. Eighteen grafts showed a luminal stenosis >/=50%, 11 grafts a stenosis >/=70%, and 6 grafts were occluded. All MR angiograms were evaluated independently by 2 blinded observers, who scored the presence of graft occlusion and graft stenosis >/=50% and >/=70% with a confidence level of 1 to 10. MR image quality was judged as insufficient in 6 grafts and these were excluded. Receiver-operator characteristic analysis revealed an area under the curve of 0.89 and 0.89 for identifying graft occlusion, 0.81 and 0.87 for stenosis >/=50%, and 0.82 and 0.79 for stenosis >/=70% for the 2 observers, respectively. Interobserver agreement in assessing graft occlusion and stenosis >/=50% and >/=70% was 94% (kappa=0.74, r=0.81), 72% (kappa=0.40, r=0.66), and 82% (kappa=0.53, r=0.72), respectively. CONCLUSIONS: High-resolution navigator-gated 3-D MR angiography allows not only good differentiation between patent and occluded vein grafts but also the assessment of vein graft disease with a fair diagnostic accuracy. This approach offers perspective as a noninvasive diagnostic tool for patients who present with recurrent chest pain after vein graft surgery.

Functional significance of stenoses in coronary artery bypass grafts. Evaluation by single-photon emission computed tomography perfusion imaging, cardiovascular magnetic resonance, and angiography.

OBJECTIVES: This study was designed to perform a head-to-head comparison between single-photon emission computed tomography (SPECT) and cardiovascular magnetic resonance (CMR) to evaluate hemodynamic significance of angiographic findings in bypass grafts. BACKGROUND: The hemodynamic significance of a bypass graft stenosis may not always accurately be determined from the coronary angiogram. A variety of diagnostic tests (invasive or noninvasive) can further characterize the hemodynamic consequence of a lesion. METHODS: Fifty-seven arterial and vein grafts in 25 patients were evaluated by angiography, SPECT perfusion imaging, and coronary flow velocity reserve determination by CMR. Based on angiography and SPECT, four different groups could be identified: 1) no significant stenosis (<50%), normal perfusion; 2) significant stenosis (>/=50%), abnormal perfusion; 3) significant stenosis, normal perfusion (no hemodynamic significance); and 4) no significant stenosis, abnormal perfusion (suggesting microvascular disease). RESULTS: A complete evaluation was obtained in 46 grafts. Single-photon emission computed tomography and CMR provided similar information in 37 of 46 grafts (80%), illustrating good agreement (kappa = 0.61, p < 0.001). Eight grafts perfused a territory with scar tissue. When agreement between SPECT and CMR was restricted to grafts without scar tissue, it improved to 84% (kappa = 0.68). Integration of angiography with SPECT categorized 14 lesions in group 1, 23 in group 2, 6 in group 3, and 3 in group 4. Single-photon emission computed tomography and CMR agreement per group was 86%, 78%, 100%, and 33%, respectively. CONCLUSIONS: Head-to-head comparison showed good agreement between SPECT and CMR for functional evaluation of bypass grafts. Cardiovascular magnetic resonance may offer an alternative method to SPECT for functional characterization of angiographic lesions.

Evaluation of saphenous vein coronary artery bypass graft flow by cardiovascular magnetic resonance.

Cardiovascular magnetic resonance (CMR) with flow velocity mapping has emerged as a noninvasive method to measure flow in saphenous vein coronary artery bypass grafts. The aim of the current study was to retrospectively test two previously described analysis methods on a large CMR data set and to compare their diagnostic accuracy in detecting diseased vein grafts. In 125 vein grafts of 68 patients, volume flow parameters (volume flow, systolic and diastolic peak flow, diastolic-to-systolic flow ratio at rest and during adenosine stress, and flow reserve) were derived from the velocity maps. Method 1 implemented basal flow < 20 ml/min or flow reserve < 2, yielding a sensitivity and specificity of 70% and 38% in the detection of a diseased graft or recipient vessel. Method 2 used receiver operating characteristic (ROC) curve analysis and implemented all significant volume flow parameters in a logistic regression model, yielding a sensitivity of 74% with a specificity of 68% in the detection of a diseased graft or recipient vessel. Evaluating single and sequential grafts separately, this method yielded a sensitivity and specificity of 79% and 87% for single grafts, and 62% and 94% for sequential grafts in the detection of > or = 50% stenosis in grafts or recipient vessels. Cut-off values were formulated for the respective volume flow parameters, which maximally separate grafts with and without > or = 50% stenosis. Using ROC curve analysis with logistic regression the specificity of the analysis method improved considerably. For the current data set the best results were acquired when single and sequential grafts were separately analyzed.

Hemodynamic evaluation of saphenous vein coronary artery bypass grafts: relative merits of Doppler flow velocity and SPECT perfusion imaging.

BACKGROUND: Coronary angiography is considered the gold standard in evaluating vein graft disease; however, angiography does not allow assessment of hemodynamic consequences of lesions. In this study hemodynamic consequences of significant stenoses in vein grafts were evaluated by Doppler velocity assessment, and results were compared with single photon emission computed tomography (SPECT) perfusion imaging. METHODS AND RESULTS: Angiography was performed in 58 patients after coronary artery bypass grafting because of recurrent chest pain. During the procedure, Doppler velocity measurements were acquired before and after administration of adenosine. Of 58 patients (with 78 vein grafts), 20 patients (with 24 vein grafts) underwent SPECT perfusion imaging. Grafts were divided into those with nonsignificant percent diameter stenosis (< 50%) (n = 49) and those with significant percent diameter stenosis (> or =50%) (n = 29). When a cutoff value for coronary flow velocity reserve (CFVR) of 1.8 was applied, modest agreement (69%, kappa = 0.25, P < .05) between CFVR and angiography was shown. Agreement between SPECT and angiography was also modest (63%, kappa = 0.28, P = not significant). SPECT and CFVR provided comparable information in 20 of 24 grafts with available SPECT, illustrating good agreement (83%, kappa = 0.61, P = .001). CONCLUSIONS: Significant stenoses in vein grafts require further exploration to assess their hemodynamic significance. The Doppler velocity results agreed better with SPECT perfusion imaging than with percent diameter stenosis in the evaluation of vein graft function.

Vein graft function improvement after percutaneous intervention: evaluation with MR flow mapping.

PURPOSE: To provide functional reference values in single and sequential vein grafts by using magnetic resonance (MR) flow mapping and to examine the effect of percutaneous intervention (PCI) on coronary artery bypass graft function. MATERIALS AND METHODS: Fast MR flow mapping at baseline and during adenosine-induced stress was performed in 39 nonstenotic single vein grafts and 20 nonstenotic sequential vein grafts, as well as in 15 stenotic vein grafts before and 7.3 weeks +/- 1.5 after successful PCI. We evaluated the following parameters (in terms of mean values +/- SDs): average peak velocity (APV) at baseline, stress APV, and velocity reserve. Parameters in nonstenotic single and sequential vein grafts were compared by means of unpaired two-tailed Student t testing. To evaluate changes in velocities before and after PCI, a paired two-tailed Student t test was used. P <.05 was considered to indicate a statistically significant difference. RESULTS: Reference values in single vein grafts for baseline APV, stress APV, and velocity reserve were 8.6 cm/sec +/- 3.4, 20.2 cm/sec +/- 9.5, and 2.4 +/- 0.8, respectively. In sequential vein grafts, significantly higher values for baseline APV (12.2 cm/sec +/- 5.0) and stress APV (27.2 cm/sec +/- 10.6) but a similar velocity reserve (2.3 +/- 0.7) were found. Significant improvements were observed after PCI in baseline APV (before PCI: 9.2 cm/sec +/- 6.6; after PCI: 12.9 cm/sec +/- 7.9; P =.008) and stress APV (before PCI: 12.9 cm/sec +/- 6.3; after PCI: 27.1 cm/sec +/- 13.9; P <.001). No improvement in velocity reserve was observed. CONCLUSION: Significantly higher absolute velocity and flow values were observed in sequential versus single vein grafts, underscoring the need for separate functional reference values for different graft types. Graft function showed significant improvement after PCI to the point that it was restored or nearly restored to reference values.

Blood flow in coronary artery bypass vein grafts: volume versus velocity at cardiovascular MR imaging.

Forty-nine patients with previous bypass surgery underwent coronary angiography and cardiovascular magnetic resonance (MR) imaging of single-vein bypass grafts. Volume flow and velocity analyses were performed and compared on MR velocity maps. Bland-Altman analysis showed close agreement between the two types of analysis. Comparison of areas under the receiver operating characteristic curve revealed no significant differences between the analyses for detection of stenoses of 70% or greater. Diagnostic accuracy for volume flow and velocity parameters was 92% and 93%, respectively. Velocity analysis appears to be the preferred method, because it is less time-consuming and has a similar diagnostic accuracy to volume flow analysis.

MR flow mapping in coronary artery bypass grafts: a validation study with Doppler flow measurements.

PURPOSE: To validate fast magnetic resonance (MR) flow mapping with intravascular Doppler flow measurements in vitro and in patients with nonstenotic and stenotic coronary artery bypass grafts. MATERIALS AND METHODS: MR and Doppler flow measurements were performed in a small-diameter flow phantom with physiologic flow conditions and at baseline and during adenosine stress in 27 grafts in 23 patients, who were scheduled for cardiac catheterization. At invasive analysis, the grafts were divided into those with stenosis of less than 50% (nonstenotic) and those with stenosis greater than or equal to 50% (stenotic). In vitro velocity values and velocity values in nonstenotic and stenotic grafts were compared with linear regression analysis, and the in vitro interstudy variability was determined. RESULTS: Excellent correlations in average peak velocity (r = 0.99, P <.001) and diastolic peak velocity (r = 0.99, P <.001) were demonstrated in vitro between MR and Doppler flow measurements, with less than 5% interstudy variability. MR and Doppler flow measurements revealed good correlations in peak velocity and velocity reserve both in nonstenotic (n = 20) (average peak velocity: r = 0.81, P <.001; diastolic peak velocity: r = 0.83, P <.001; velocity reserve: r = 0.56, P =.010) and stenotic (n = 7) (average peak velocity: r = 0.83, P <.001; diastolic peak velocity: r = 0.78, P =.001; velocity reserve: r = 0.70, P =.078) grafts. CONCLUSION: Fast MR flow mapping provides noninvasive measures of peak velocity and velocity reserve, which closely correlate with Doppler values both in vitro and in nonstenotic and stenotic grafts.