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Ann Thorac Surg 2003;76:1523-1527
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

Noninvasive evaluation of arterial grafts with newly released multidetector computed tomography

^a Department of Thoracic and Cardiovascular Surgery, The Lady Davis Carmel Medical Center, affiliated to the Rappaport Faculty of Medicine, Technion-Haifa, Israel
^b Department of Radiology, The Lady Davis Carmel Medical Center, affiliated with the Rappaport Faculty of Medicine, Technion-Haifa, Israel

Accepted for publication May 29, 2003.

^* Address reprint requests to Dr Gurevitch, Department of Thoracic and Cardiovascular Surgery, Carmel Medical Center, 7 Michal St, Haifa, Israel.
e-mail: nettag{at}barak-online.net

Abstract

BACKGROUND: High-quality postoperative imaging of bypass conduits is essential when evaluating different types of conduits, anastomoses, and surgical techniques. We investigated the potential value of the newest generation of multidetector-row computer tomographic scanners in assessing bypass grafts.

METHODS: From June to September 2002, 14 patients underwent scanning with a newly released 16-slice computed tomographic scanner (Mx8000 IDT; Philips Medical Systems) after coronary artery bypass grafting. Four patients had had minimally invasive direct coronary artery bypass grafting and 3, redo coronary artery revascularization. Contrast-enhanced computed tomographic angiography was performed using retrospective electrocardiographic gating. Scan length was 22 to 30 cm, and total scan time was 27 to 37 seconds.

RESULTS: Of the 14 patients, 8 were scanned within 1 week after operation and 6, 1 month to 12 months postoperatively. Average heart rate during the scan was 82 beats per minute (range, 60 to 97 beats per minute), and all patients were able to hold their breath for the required time. Thirty conduits were studied: 26 arterial (18 in situ left and right internal mammary artery grafts, five free right internal mammary and radial artery grafts, and three in situ right gastroepiploic artery grafts) and four vein grafts. Excellent visualization of all 30 grafts was achieved. Thirty-four of the 35 distal anastomoses were patent; one vein graft was occluded.

CONCLUSIONS: This new technology is a promising noninvasive measure to evaluate patency of bypass conduits, including the gastroepiploic artery where catheterization is usually difficult. The ability to display the vessel wall as well as its lumen might distinguish radial artery spasm from intimal hyperplasia. The superb resolution and increased scan length required to cover the entire internal mammary artery grafts—from origin to distal anastomoses—can be achieved easily in a single breath-holding owing to the increased number of slices per rotation and shortening of the gantry rotation time.

Knowledge about the patency rate of various bypass grafts is valuable in the care of patients undergoing surgical revascularization. The demonstrated improved patency rate of internal mammary artery (IMA) grafts over saphenous vein grafts [1], for example, changed the surgical approach in the late 1980s, and many surgeons currently perform multiple arterial grafting in daily practice. However, there is still uncertainty regarding both the long-term patency rate of other arterial grafts, such as the radial artery, the right gastroepiploic artery, and the free right IMA, and advanced surgical techniques such as T, Y, and sequential grafting [2].

Conventional selective angiography is the standard for assessing bypass graft patency after coronary artery bypass grafting (CABG). It is an invasive and potentially harmful procedure with a low risk of serious complications, such as conduit dissection, spasm, embolization and myocardial infarction, arrhythmia, stroke, and death.

Magnetic resonance imaging and electron-beam computed tomography have been investigated for noninvasive coronary imaging. However, both have major limitations in regard to reliable visualization of the coronary arteries [3–5]. The previous generation of multislice (two- and four-slice) computed tomographic (CT) scanners has recently been found to be a valuable and reliable diagnostic tool in assessing coronary artery disease, patient risk stratification, and bypass graft evaluation [6, 7], although, again, there are limitations, such as short scan lengths per single breath-holding and the requirement of a slow heart rate during the scan. The aim of our study was to assess the diagnostic potential of the newly released 16-slice CT scanner in evaluating bypass graft patency after operation.

Material and methods

From June to September 2002, as part of our evaluation of cardiac imaging on a new 16-slice CT scanner (Mx8000 IDT; Philips Medical Systems), we randomly selected 14 patients who had undergone a CABG procedure and were either asymptomatic or had nonspecific complaints. The patients (13 men and 1 woman) underwent contrast-enhanced CT angiography of the coronary arteries and bypass grafts 3 days to 12 months after CABG. None of the patients had contraindications to receiving contrast material (such as abnormal renal function test results or known allergies). Mean age was 57 years (range, 37 to 78 years), average weight was 83 kg (range, 63 to 107 kg), and average height was 173 cm (range, 158 to 180 cm). Average number of distal anastomoses was 2.5 per patient (range, one to four anastomoses).

The new-generation multidetector-array CT scanners operate at a shorter rotation time (0.42 second) and produce up to 16 slices per rotation simultaneously. These developments allow high-speed scanning of large volumes with a high in-plane resolution as well as an improved z-axis resolution. The decrease in rotation time facilitates improvement in the temporal resolution to 210 ms. Recent modifications to the reconstruction software have further reduced the virtual temporal resolution at higher heart rates by combining the data from several heart cycles in one image, thus shortening the effective acquisition intervals to 105 ms or less. We used retrospective electrocardiographic (ECG) gating, which allows window selection and optimal gating after scan acquisition. This approach improves image quality and decreases the sensitivity to arrhythmia and ECG noise.

Patients were placed in the gantry of the CT scanner in a supine position. Leads were attached for simultaneous ECG and image recording necessary for interrelated image reconstruction. Scan volume was defined on the basis of the expected location of the coronary arteries and bypass grafts (obtained from the coronal scout view). For arterial grafts, the area to be covered was extended to the origin of the IMA and gastroepiploic artery as needed.

Fixed scanning variables included 0.42-second rotation time and tube voltage of 140 kV and 400 mA. We used a protocol of 16 slices with a collimated slice thickness of 0.75 mm. Because of the spiral motion of the detector row, the effective slice thickness was 0.8 mm with an increment of 0.4 mm. Pitch (table feed per rotation divided by the single collimation slice thickness) was set at 0.3. Depending on the volume to be covered (scan length of 22 to 30 cm), the total scan time was 27 to 37 seconds. To make it easier for the patients to hold their breath for an adequate time, they were connected to oxygen and asked to hyperventilate before the start of the scan.

To determine the exact transit time of the contrast agent from injection into an antecubital vein to appearance (as contrast enhancement) in the aortic root and coronary arteries, a 20-ml bolus of nonionic contrast agent with a high iodine content (iohexol [Omnipaque; Nycomed, Cork, Ireland], 350 mg I/mL) was injected at a rate of 4.0 mL/s. Subsequently, very good contrast between blood and surrounding tissue was achieved by injecting 100 to 120 mL of contrast material at a rate of 3.5 to 4.0 mL/s.

The acquired CT and ECG data were sent to a separate workstation, and dedicated cardiac software was used to reconstruct the images. Axial slices were reconstructed from the acquired volumetric CT data during the middle to late diastolic phase to minimize motion artifacts. The images were further processed on a separate workstation (MxView; Philips Medical Systems) and three-dimensional volume-rendering reconstruction of the heart and coronary arteries was performed. The three-dimensional images show the lumen of each vessel filled with contrast medium and might show wall calcifications and surgical clips. When this application is active, the volume can be rotated in all directions, thus allowing three-dimensional evaluation of the vessels.

Two-dimensional reconstructions (curved multiplanar reformation) of the coronary arteries and grafts were performed on several planes to assess patency of grafts and anastomoses. These two-dimensional images demonstrate the vessel wall, its lumen, and all the surrounding tissue. They are reconstructed on at least two orthogonal planes, and continuity of contrast material throughout the graft serves as an indication of patency.

Results

In all the patients, CT angiography was performed without complications. The average time needed for the procedure, including preparation and scanning, was less than 20 minutes. Reconstruction of the images and evaluation took up to 3 hours per patient. The average heart rate during the scan was 82 beats per minute (range, 60 to 97 beats per minute), and all patients were able to hold their breath for the required time.

Of the 14 patients, 8 underwent scanning within 1 week postoperatively and 6, 1 month to 12 months postoperatively. Four patients had had minimally invasive direct CABG and 3 had had redo CABG. Thirty conduits were included in this study: 26 arterial grafts (including 18 in situ left and right IMAs, five free right IMAs and radial arteries, and three in situ gastroepiploic arteries) and four saphenous vein grafts. There were 35 distal anastomoses (Table 1) and six proximal anastomoses, two tangential (K type) anastomoses between the left IMA and free right IMA and four end-to-side (T type) anastomoses between the left IMA and free right IMA or radial artery.

View larger version (111K): [in this window] [in a new window]   Fig 1. Two-dimensional computed tomographic angiographic scan from a 62-year-old patient who had had minimally invasive direct coronary artery bypass grafting 5 months earlier. The entire left internal mammary artery (LIMA) is visible from its left subclavian origin to its distal anastomosis to the left anterior descending coronary artery (Dist. LAD). (Inset) A focused-angle view of the silky attachment to the coronary artery.

View larger version (119K): [in this window] [in a new window]   Fig 2. Three-dimensional computed tomographic angiographic image of heart of a 67-year-old patient who had undergone myocardial revascularization 4 days prior to scanning. The diseased right coronary artery (RCA) is clearly visible through this view portraying several major stenoses of the proximal and middle RCA and a calcification in its proximal section. The right gastroepiploic artery (Rt GEA) is attached to the posterior descending artery (PDA). The in situ right internal mammary artery (RIMA) can be traced from its right subclavian artery origin down toward the left anterior descending coronary artery (LAD) and parts of the left internal mammary artery (LIMA) that supply the posterior marginal branches (MARG) of the circumflex coronary artery can be seen in this projection.

View larger version (85K): [in this window] [in a new window]   Fig 3. Volume-rendered three-dimensional and (inset) two-dimensional computed tomographic angiographic images from a 42-year-old patient 4 days after coronary artery bypass grafting. The left internal mammary artery (LIMA) can be seen from its left subclavian origin going toward the left anterior descending coronary artery (LAD). The right internal mammary artery (RIMA) is proximally attached in a T-shaped anastomosis to the LIMA and sequentially supplies the second marginal circumflex (just before a bifurcation) and posterior descending coronary arteries.

Postoperative imaging of bypass conduits and anastomoses after surgical myocardial revascularization is necessary to evaluate the quality of the surgical technique, anastomoses, and bypass grafts. Improved patency of IMA grafts over vein grafts [1], renewed role for the radial [8] and right gastroepiploic arteries as important arterial conduits for myocardial revascularization, and the feasibility and the advantage of several surgical techniques such as T or Y grafting and sequential anastomoses [2] have all been demonstrated by postoperative imaging and thus changed the surgical approach for patients undergoing CABG during the 1990s.

Stress testing [9], radionuclide ventriculography [10], and thallium 201 myocardial perfusion scintigraphy [11] are commonly used to indirectly determine the status of bypass grafts by noting ECG changes, regional wall abnormalities, or changes in regional myocardial perfusion. However, there is no substitute for images of the graft itself, and to date, the gold standard for determining patency of bypass grafts is selective coronary angiography. This is an invasive procedure that is costly, uncomfortable for the patient, and not without complications. Most patients who are asymptomatic or who have minimal or nonspecific symptoms after operation are reluctant to undergo this procedure. Furthermore, direct catheterization of arterial grafts takes longer and is more painstaking than catheterization of native coronary arteries. It involves higher radiation exposures and larger volumes of contrast material and carries higher complication rates. Postoperative selective catheterization of the right gastroepiploic artery is not an easy task, even for the experienced invasive cardiologist.

Magnetic resonance imaging has been shown to be a useful noninvasive method to determine bypass graft patency [12]. However, this technique is cumbersome and time-consuming, and image resolution is currently poor [5].

Electron-beam computed tomography is a cross-sectional imaging technique with high spatial and temporal resolution, and image acquisition can be triggered by the patient's electrocardiogram. This technique is suitable for cardiac imaging with prospective ECG gating. In a 1998 study, the results of electron-beam computed tomography were compared with those of invasive coronary angiography. Electron-beam computed tomography had a sensitivity of 92% and a specificity of 94%. However, that study later was criticized for excluding one quarter of the arteries studied because of technical limitations, thus dropping its positive predictive value to only 37% [13].

Multidetector-row CT (MDCT) scanners have recently been used for noninvasive imaging of coronary artery disease and determining the patency of bypass grafts [6, 7]. In addition to providing very good full-length visualization of conduits from origin to distal anastomosis (see Figs 1–3), the technique has the added benefit of demonstrating the precise anatomy of the grafts and surrounding structures, the conduit wall, and its lumen. This allows differentiation between vessel spasm and various intraluminal pathological conditions.

A recent study [14] involving coronary CT angiography using four-slice MDCT scanners with retrospective ECG gating and 1-mm slice thickness reported a sensitivity of up to 93% and a specificity of 98% with a negative predictive value of 98% for major stenoses. Other studies have been less enthusiastic. Vogl and colleagues [15] obtained a sensitivity of 74.7% using MDCT angiography, and Giesler and coauthors [16] concluded that the ability of four-slice MDCT scanners to accurately evaluate coronary artery stenosis decreases from 62% to 33% as the patient's heart rate increases to more than 70 beats per minute. Although several centers have found four-slice MDCT angiography valuable for noninvasive imaging of coronary artery and bypass grafts, it remains limited by the patient's heart rate and the scan length to be covered within a single breath-holding.

The new generation of MDCT scanners allows simultaneous acquisition of 16 slices per rotation and a gantry rotation time of 0.42 second. These developments permit high-speed scanning of larger volumes during a single breath-holding. They also provide images with submillimeter isotropic resolution and improved temporal resolution and have recently been shown to be highly accurate (93% positive and negative predictive values) in the detection of coronary artery stenoses [17].

In our study, the 16-slice scanner demonstrated excellent image quality. The primary goal of the study was to present the feasibility of this new technique for bypass graft imaging. Interpretation of the anatomical findings is quite trivial using this high-resolution volume-rendered three-dimensional scanning, and the images can easily obscure major disease in the coronary arteries (see Fig 2) and the occluded vein graft (Fig 4). The ability to demonstrate the vessel wall in addition to its lumen is an advantage of CT angiography over conventional coronary angiography, which permits only intraluminal investigation. This property, theoretically, may distinguish between radial artery spasm and intimal hyperplasia. Another advantage is the clear demonstration of ostial lesions, such as ostial left main coronary artery disease [17], lesions that might be missed with conventional coronary angiography because of positioning of the catheter tip beyond the arterial ostium.

View larger version (124K): [in this window] [in a new window]   Fig 4. Three-dimensional image from a 54-year-old patient with atypical chest pain made 10 months after coronary artery bypass grafting. The saphenous vein graft (SVG) to the first marginal (Marg) branch was occluded from its origin in the aorta, and only a very short stump is shown. Saphenous vein grafts to the first diagonal (D1) branch and posterior descending artery (PDA) were patent. The native right coronary artery (RCA) showed a short, tight stenosis in its distal part, which correlated with the preoperative catheterization findings. A patent left internal mammary artery graft to the distal left anterior descending coronary artery was also found (not shown).

Computed tomographic angiography is currently a well-established noninvasive substitute for invasive angiography to evaluate blood vessels of the entire circulation, such as the aorta and the carotid, intracranial, renal, and iliac arteries. The problem in evaluating the coronary circulation is to overcome motion artifacts created by the beating heart, and this was addressed by slowing the heart rate, gating, multislicing, short gantry rotation time and higher resolution, and high-speed scanning. Angiographic correlation has been used previously for the coronary circulation and was found to be promising [3–7, 13–17]. Although not proven, we assume that given the improved scanning technology and the fact that these vessels are less mobile than the coronary arteries, bypass graft imaging should currently be even better. Nevertheless, a prospective randomized study comparing this new technique with invasive angiography is necessary.

The 16-slice CT scanner allows high-resolution imaging of the entire length of bypass grafts, from their origin (ie, the left or right subclavian artery for the IMA and the gastroduodenal artery for the right gastroepiploic artery) to the distal anastomoses, during a single breath-holding and without the need to slow the patient's heart rate. It is an outpatient, noninvasive procedure that is less expensive and easier for the patient and that has a lower complication rate than invasive coronary angiography. We believe CT angiography using the new-generation MDCT scanners is an important tool for quality control, providing valuable information in the care and follow-up of patients undergoing CABG. Larger prospective studies are necessary to determine its potential to replace the current more invasive selective coronary angiography.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Gurevitch, J. Articles by Aravot, D. J. Search for Related Content PubMed PubMed Citation Articles by Gurevitch, J. Articles by Aravot, D. J. Related Collections Coronary disease