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Original Research |
1 Clinical Department of Radiology II, Innsbruck Medical University, Anichstr.
35, A-6020 Innsbruck, Austria.
2 Clinical Department of Cardiac Surgery, Innsbruck Medical University,
Innsbruck, Austria.
3 Clinical Department of Cardiology, Innsbruck Medical University, Innsbruck,
Austria.
Received October 27, 2006;
accepted after revision April 25, 2007.
Address correspondence to G. M. Feuchtner
(Gudrun.Feuchtner{at}i-med.ac.at).
Abstract
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SUBJECTS AND METHODS. Forty-one patients (70 grafts, 46 arterial and 24 venous) underwent 64-slice CT a mean of 2.6 years after minimally invasive or conventional coronary artery bypass surgery.
RESULTS. All 70 grafts were assessable, and none of the grafts was excluded from analysis. For the detection of 50-90% graft stenosis, the sensitivity of CT was 75%, the specificity was 95%, the positive predictive value was 67%, and the negative predictive value was 97% (true disease prevalence, 8/70 grafts; 11%). Greater than 50% graft stenosis and occlusion pooled together (prevalence, 14/70; 20%) were detected with a sensitivity of 85%, specificity of 95%, positive predictive value of 80%, and negative predictive value of 96%. Vein graft disease was found in eight (42%) of 19 patent vein grafts (graft age, 15.6 ± 2.3 years). The disease was nonobstructive in three (16%) of the 19 grafts. The course of the left internal mammary artery was median retrosternal (< 1 cm deep) in 33.3% of conventionally sutured grafts.
CONCLUSION. Sixty-four-slice CT angiography can be used for accurate exclusion of greater than 50% graft stenosis, but detection of distal anastomotic stenosis is limited, and the degree of stenosis can be overestimated. The advantages of CT, however, are that it is noninvasive, vein graft disease can be diagnosed at an early stage, and complementary evaluation of extracardiac anatomic features provides useful information before coronary artery bypass grafting is redone.
Keywords: coronary artery bypass graft • 64-slice CT
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MDCT angiography is a noninvasive imaging technique that can be performed on ambulatory patients. Four- and 16-MDCT have had promising results in the assessment of bypass graft patency [2-6]. Several studies [2-6] have shown high diagnostic accuracy of 16-MDCT in the detection of complete graft occlusion. However, evaluation of the distal anastomosis and the distal target vessel has been technically limited, as by artifacts from surgical clips and by residual cardiac motion in 16-23% of grafts [2, 3]. In addition, those studies yielded limited data on the accuracy of 16-MDCT in the detection of greater than 50% bypass graft stenosis because the prevalence of stenosis was a low 1.5% [2] and 5.9% [3].
In 2005, the introduction of 64-MDCT technology offered higher temporal (83-165 milliseconds) and spatial resolution (0.4 x 0.4 x 0.4 mm3) [7] than 16-MDCT (105-250 milliseconds and 0.5 x 0.5 x 0.6 mm3) [8], which may improve visualization of grafts and distal anastomoses. Improved diagnostic accuracy in the detection of greater than 50% stenosis [9], especially in vessels larger than 1.5 mm in diameter [10], has been reported. Results of two recent studies [11, 12] suggested that 64-slice CT angiography has high diagnostic accuracy in the evaluation of both graft patency and stenosis. However, the prevalence of marked stenosis (without complete occlusion) was low in those studies (four [11] and 15 [12] vein grafts). The primary goal of this study was to compare the diagnostic performance of 64-slice CT angiography in the detection of greater than 50% distal and proximal bypass graft stenosis with the performance of invasive graft angiography. We also investigated the clinical value and performance of 64-slice CT.
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CT Examination Technique
CT examinations were performed with a 64-slice CT scanner (Sensation 64,
Siemens Medical Solutions) with 32-row detector collimation acquiring 64
x 0.6-mm slices by use of z-axis flying-focus
[7] technique, a table
translation speed of 3.8 mm/rotation, and a gantry rotation time of 0.33
seconds. Tube output was 120 kV at 700-800 mAs. ECG dose modulation was used
for patients with a stable heart rate less than 55 beats/min. Scanning
direction was craniocaudal during a single inspiratory breath-hold, and the
ECG signal was recorded simultaneously. An automated injector (Ulrich
Medizintechnik) was used for IV administration of a bolus of 120 mL nonionic
iodine contrast agent, either iodixanol (Visipaque 320, GE Amersham Health)
(n = 31) or iomeprol (Iomeron 400, Bracco) (n = 10) and a
40-mL saline flush into an antecubital vein at a flow rate of 4.5-5.5 mL/s.
Bolus-tracking technique (ascending aorta, threshold 100 H) was used as
previously described for coronary CT angiography
[13]. The scanning range was
started caudal to the aortic arch in patients with aortocoronary grafts and at
the level of the clavicles in patients with left internal mammary artery
(LIMA) grafts. A ß-blocker (5 mg metoprolol, Beloc, Schering) was
injected IV before the examination if the heart rate was greater than 85
beats/min.
CT Image Reconstruction
Transaxial slices were reconstructed at increments of 0.4 mm at an
effective slice thickness of 0.75 mm, a medium-smooth convolution kernel
(B25f+), an image matrix of 512 x 512 pixels, a field of view of 170-200
mm, and retrospective ECG gating at mid late systole (30-40%) if the heart
rate was greater than 65 beats/min and at mid late diastole (55-70%) if the
heart rate was less than 65 beats/min.
CT Image Analysis
Images were transferred to a dedicated offline computer workstation
(Leonardo, Siemens Medical Solutions) and reconstructed with multiplanar
reformation, maximum intensity projection, and volume rendering with dedicated
software (Syngo, Siemens Medical Solutions). Image quality was graded on a
3-point scale by one observer as follows: grade 1, good (no artifacts); grade
2, acceptable image quality (minor limitations, such as mild artifacts); or
grade 3, image quality insufficient because of artifacts. Bypass graft patency
and the presence of greater than 50% stenosis were evaluated by two
independent observers. Venous graft disease was classified according to the
CT-based criteria for coronary artery atherosclerosis
[14]. Plaque was
differentiated as calcifying (type 1), mixed noncalcifying and calcifying
(type 2), and noncalcifying (type 3) on the basis of CT density. The course of
the LIMA was evaluated in relation to the sternum as directly median
retrosternal (< 1 cm deep and 1 cm wide mediolaterally), left paramedian
retrosternal, or extraretrosternal.
Invasive Angiography
Invasive graft angiography was performed through 7-French femoral access in
the left or right groin by the Judkins technique with a standard fluoroscopy
unit (OEC9800, GE Healthcare, or Axiom, Siemens Medical Solutions). Two to
four projections (anteroposterior, left anterior oblique 20°, right
anterior oblique 20°, left anterior oblique 90°) were used to
visualize the grafts after injection of an iodine contrast agent (iodixanol,
Visipaque 320, GE Amersham Health).
Statistical Analysis
Statistical analysis was performed with SPSS software (version 8.0, SPSS).
The diagnostic accuracy (sensitivity, specificity, positive predictive value,
negative predictive value) of CT for the detection of greater than 50% bypass
graft stenosis was calculated. Interrater agreement was calculated with
Cohen's weighted kappa value.
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In comparison with invasive angiography, the sensitivity of 64-slice CT in the detection of 50-90% bypass graft stenosis was 75%; the specificity, 95%; the negative predictive value, 97%; and the positive predictive value, 67% (Table 3). If greater than 50% graft stenosis and occlusion (n = 6) were pooled together (Table 3), the sensitivity and specificity of 64-slice CT were 85% and 95%, respectively. The detailed diagnostic accuracy of 64-slice CT is shown in Table 3. Interobserver variability was 0.79 (kappa value).
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CT Image Quality
All 41 patients completed the CT examinations successfully, and all scans
were interpretable. Image quality was graded good in 32 (78%) and acceptable
in nine (22%) of 41 examinations. Limitations included artifacts from surgical
metal clips, mild stairstep motion artifacts, and severe calcification of the
distal coronary target vessel.
Vein Graft Disease
Vein graft disease was detected in eight (42%) of the 19 patent venous
grafts. The diseased but patent grafts had been in place a mean of 15.6
± 2.3 years (range, 13-18 years). Vein graft disease was nonobstructive
in three (16%) of 19 grafts (Fig.
5A,
5B) and obstructive (50-90%
stenosis) in five (26%) of 19 grafts (Fig.
4A,
4B). Plaque composition was
noncalcifying in three grafts and mixed noncalcifying and calcifying in five
diseased grafts (Fig. 4A,
4B). Venous bypass grafts
without signs of vein graft disease had been in place a mean of 4.5 ±
4.7 years. Five of 24 vein grafts were completely occluded.
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CT Examination
The mean heart rate was 67.8 beats/min (range, 40-91 beats/min). The time
point of the image reconstruction window was mid late systole (30-40% of
RR-interval) in 20 (49%) and mid late diastole (55-70% of RR-interval) in 21
(51%) of the 41 patients. In the latter group, a second image data set at 35%
of the RR-interval was required for two patients in order to display the right
coronary artery without motion artifacts. All patients were in sinus rhythm
and had premature beats; one patient had a right ventricular pacemaker. ECG
editing [15] was performed in
10 (24%) of the cases because of supraventricular or ventricular extramature
beats or for smoothing of heart-rate irregularities to avoid stairstep
artifacts. A ß-blocker was given to three (7%) of the 41 patients.
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Our findings are in accordance with those of two recent studies [11, 12] that involved 31 and 52 patients. In the first study [11], good visualization of the distal anastomosis was found in 94% of grafts. Diagnostic confidence for the evaluation of distal runoff disease was good in only 75% of grafts, and overestimation of distal stenosis was frequent, particularly when the distal coronary vessel was small, resulting in a low positive predictive value of 50%. The respective reported sensitivities for the detection of greater than 50% graft stenosis and occlusion were high at 98% [11] and 100% [12] with respective specificities of 89% and 89%, positive predictive values of 90% and 89%, and negative predictive values of 97% and 100%.
Unlike invasive angiography, 64-slice CT can be used for evaluation of the vessel wall. Vein graft disease can be identified in early stages, before graft obstruction develops (Fig. 5A, 5B). According to the CT-based criteria for evaluation of coronary atherosclerosis, noncalcifying plaques (Fig. 4A, 4B) can be differentiated from calcifying plaques (Fig. 5A, 5B) on the basis of CT density [14]. With respect to the pathogenesis of vein graft disease [1] initially manifesting as neointimal hyperplasia [16, 17] and followed by an atherosclerosis-like process, noncalcifying plaques can be the early stages of vein graft disease. In our study, 16% of patent vein grafts had signs of vein graft disease without obstruction, and all diseased grafts had a noncalcifying plaque component. These patients may benefit from medical treatment [1, 16] to prevent progression of vein graft disease.
Sixty-four-slice CT allows comprehensive evaluation of extracardiac thoracic anatomic features, including the course of the LIMA, which can be classified in relation to the sternum. For example, awareness of a median retrosternal proximal course of the LIMA, found in 33% of LIMA conduits, can help cardiac surgeons planning repeat CABG avoid iatrogenic injury to the LIMA during sternotomy. Aortocoronary vein grafts to the right coronary artery also can take a directly median retrosternal course.
Native coronary arteries should be examined and evaluated with regard to the presence of de novo greater than 50% stenosis, which can explain the recurrence of angina pectoris if grafts are patent [12]. The first studies of 64-slice CT coronary angiography have shown high diagnostic accuracy in the detection of greater than 50% coronary stenosis in unselected patients [9, 10]. One study [11] showed good accuracy (sensitivity, 97%; specificity, 86%) of 64-slice CT in the detection of greater than 50% coronary stenosis in patients who had undergone CABG in the absence of severe coronary calcification.
Limitations of CT
Absolute cardiac arrhythmia (e.g., persistent atrial fibrillation) causes
image artifacts. Therefore, patients with atrial fibrillation can be examined
successfully only if their medication allows sufficient heart rate control.
Radiation exposure during coronary 64-slice CT ranges between 9.4 and 14.8 mSv
(mean, 11 mSv) [18], causing
concern because the radiation dose is slightly greater than these values. The
effective radiation dose of interventional coronary angiography (3-10 mSv)
[19] is clearly lower, but the
dose from myocardial SPECT is higher (20 mSv)
[20]. However, the mean age of
patients who have undergone CABG is usually high (61.8 years in our study
population). The effect of radiation dose on the excess lifetime risk of
cancer mortality declines significantly with age and is very small among
patients older than 65 years
[21]. Iodine contrast agents
cannot be administered to patients with renal dysfunction, known allergy, or
hyperthyreosis.
In conclusion, 64-slice CT angiography is an accurate imaging technique for the exclusion of greater than 50% graft stenosis and occlusion. It can be used in clinical practice as a noninvasive alternative imaging technique if bypass dysfunction is clinically suspected but cardiac catheterization is not primarily indicated (i.e., in case of equivocal or controversial clinical symptoms and results of pretest) or if the patient would be at high-risk during intervention because of comorbid conditions. Although detection of distal anastomotic stenosis remains difficult, 64-slice CT has the advantages of noninvasiveness, identification of vein graft disease in early stages, and comprehensive information about extracardiac anatomic features, such as the course of the LIMA, which is useful to know before CABG is repeated.
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