CT Angiography for Assessmentof Coronary Bypass Grafts 28

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CHAPTER 28 / CTA FOR CORONARY BYPASS GRAFTS 301

301

CT Angiography for Assessment of Coronary Bypass Grafts

MARCELLO DE SANTIS, MD

With the advent of subsecond rotation combined with prospective electrocardiogram (ECG) triggering or retrospective ECG gating, conventional computed tomography (CT) with spiral capability and superior general image quality has chal- lenged electron beam CT (EBCT) in the domain of cardiac imaging. The introduction of multislice CT scanning with the Siemens SOMATOM Volume Zoom with 4 simultaneously scanned slices, half-second rotation, and 250-ms maximum temporal resolution has recently opened new horizons for cardiac CT imaging. ECG-gated multislice spiral CT represents a leap in image quality of CT angiography (CTA) of the coronary arteries. The fast volume coverage allows scanning the heart with 1-mm slice collimation within a single breath-hold (10 cm in 25–30 s) for high-resolution imaging. Three-dimensional reconstruction with approx 1-mm slice width and submillimeter increment provide data of unique quality for visualization of the coronary arteries and of the arterial/venous grafts utilized for surgical revascularization. This chapter will be focused on the clinical background, the previous CT applications in this setting, and the recent results of multidetector-row CT (MDCT) evaluation of coronary artery bypass grafting (CABG).

CORONARY BYPASS GRAFTING—BACKGROUND Surgical revascularization for atherosclerotic heart disease is one of the milestones in medical history. Relief of angina after revascularization, improvement in exercise tolerance, and the global benefit on survival have attended this approach since the early stages of development (Fig. 1). After many surgical efforts to relieve angina pectoris, including the direct implantation of the internal mammary artery (IMA) into the myocardium (Vineberg procedure), coronary surgery moved into the modern era in the 1950s. The first direct surgical approach to the coronary circulation in a patient was likely performed by Mustard in 1953 using a carotid-to-coronary bypass, but the first clinical use of the IMA to graft a coronary vessel followed an intraoperative misadventure of William Longmire in 1958 after disintegrating a right coronary artery (1). Similarly, the first successful clinical aortocoronary saphenous vein graft

(SVG) by DeBakey and Garrett in 1964 salvaged a complicated left anterior descending (LAD) coronary endoarterectomy (2).

In the 1960s, Mason Sones showed the feasibility of selec- tive coronary arteriography and collected a large library of cineangiograms that were studied in depth by Rene Favaloro (3). Sones and Favaloro formed an innovative team that demonstrated the efficacy and safety of SVG interposition and aortocoronary SVGs for single-vessel, left main, and multivessel coronary disease, thus favoring the worldwide application of this approach. Ironically, with demonstration of the dramatic benefits obtainable by saphenous vein grafting came recognition of the ultimately palliative nature of the operation, as a result of the accelerated atherosclerosis that devel- ops within the grafted saphenous vein conduits. During the first year after bypass surgery, up to 15% of venous grafts occlude;

between 1 and 6 yr the graft attrition rate is 1% to 2% per year, and between 6 and 10 yr it is 4% per year. By 10 yr after surgery, only 60% of vein grafts are patent and only 50% of patent vein grafts are free of significant stenosis (4–6).

Although the left IMA initially fell from favor as a result of early, ill-founded concerns regarding low flow rates and technical difficulties in implantation, today it is recognized that selection of the left IMA rather than a saphenous vein as the initial conduit is the single most important factor in improved survival, freedom from cardiac events, and long-term graft patency after coronary bypass surgery (85–90% after 10 yr).

The favorable effects on mortality and morbidity are observed irrespective of age, gender, or left-ventricular function, and are particularly evident if the left IMA is implanted into a proxi- mally stenosed LAD, in view of the large area of myocardium subtended by this native vessel (7) (Fig. 2). The profound and sustained benefits afforded by the IMA grafting have given impetus to both the utilization of other arterial conduits as coronary bypass grafts (right IMA, right gastroepiploic artery, radial artery, inferior epigastric artery, and so on) and the development of minimally invasive coronary artery bypass grafting (MICABG). This innovative technique, first proposed by Benetti and colleagues in 1994, does not involve the use of cardiopulmonary bypass, or of a median sternotomy. Instead, through a small left thoracotomy, the left IMA is harvested with or without the aid of a thoracoscope, the pericardium is opened, and the arterial conduit is grafted to the LAD (8). At present, single-vessel coronary artery disease involving the LAD is the

28

From: Contemporary Cardiology: CT of the Heart:

Principles and Applications

Edited by: U. Joseph Schoepf © Humana Press, Inc., Totowa, NJ

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primary indication for MICABG. Wider application of the technique, including right IMA or gastroepiploic artery grafting to the right coronary artery, is currently under evaluation.

CORONARY BYPASS GRAFTING—

PREVIOUS RESULTS OF CT EVALUATION (CONVENTIONAL SPIRAL CT AND EBCT)

Like any other vascular diagnostic field, coronary bypass grafting was subjected to CT evaluation early after its introduction in clinical practice. In comparison with conventional coro-

nary angiography (CAG), conventional CT scanning with contrast enhancement proved to have the potential to image saphenous vein grafts (9,10), but preliminary promising results on this respect were confined to simple patency assessment (occluded vs not occluded) (11,12). With spiral CT technology, it became possible to scan the entire heart during the arterial phase of contrast enhancement in a single breath-hold, thus significantly reducing both cardiac and respiratory motion artifacts; early spiral CT results, obtained in small popula- tions of CABG patients in comparison with conventional CAG, Fig. 1. Schematic drawing of conventional surgical revascularization for coronary artery disease.

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showed good sensitivity/specificity in terms of venous graft patency, but failed to define both arterial IMA graft and distal anastomosis site patencies (13).

More recent generations of spiral CT single-detector-row scanners with subsecond acquisition (0.75 s) were successfully applied to the contrast-enhanced 3D visualization of venous as well as arterial IMA grafts; in a series of 134 bypass grafts (42 IMA and 92 venous) double-blind evaluated by subsecond spiral CT and conventional CAG, Engelmann et al. found a CT-deter- mined overall sensitivity of 92% for patency with an accuracy of 88% (IMA) and 96% (venous, p = NS) (14). In addition, the newer capabilities of spiral CT imaging allowed the introduction in this setting of a new parameter of CABG patency assessment, i.e., graft flow, which can be qualitatively extracted through the graft length as obtained from multiple 3D reconstruction images at varying Hounsfield unit (HU) thresholds (faster flows correlate with longer graft lengths on most 3D reconstruction thresholds) (15). These refinements in 3D spiral CT angiography of coronary bypass grafting represented a definite step forward in CT vascular imaging and gave rise to the current competition in this respect with EBCT, whose application for CABG patency assessment has scored similar results in two recently reported prospective studies, with overall better evaluation of distal anastomosis sites due to the ECG- triggered EBCT acquisition mode (16,17).

The recent introduction of cardiosynchronization in spiral CT acquisition mode has followed the need to completely suppress cardiac motion when millimeter/submillimeter spatial resolution has to be coupled with high temporal resolution, as recom- mended in coronary vessel imaging. The crucial importance of this aspect has been recently demonstrated by using an ECG- triggered acquisition approach for spiral CT evaluation of proximal anastomoses in CABG patients (18); despite the sub- optimal temporal resolution, optimized timing of scanning as provided by the ECG-triggered mode allowed good visualization of proximal graft anastomoses in all patients, with assessment not only of overall patency (occluded vs not occluded) but also of graft disease, i.e., stenosis detection and grading (Figs. 3,4).

This innovative diagnostic capability, as offered by the cardiosynchronization of CT scanning, explains well the potential of the newer generation of MDCT scanners.

PREOPERATIVE APPLICATIONS OF MDCT

The results of several meta-analysis reports on graft failure as well as the spreading application of the minimally invasive approach for CABG surgery have ultimately led to the need for adjunctive preoperative information regarding both the native vessels and the arterial conduits. This information, which can be only partially obtained by using conventional CAG, includes:

1. precise localization of the LAD course (extra- or intramyocardial) and its relationship to adjacent structures 2. emergence, course, size, and branching of the IMAs (and

of any other arterial conduit if planned)

3. emergence, course, and size of the left subclavian artery.

Fig. 3. Source image with clear depiction of the emergence from the ascending aorta of multiple venous grafts.

Fig. 2. Schematic visualization of internal mammary artery graft implantation to LAD in the presence of coronary artery disease.

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Fig. 4. Source image with visualization of the emergence and proxi- mal portion of a venous graft with a significant stenosis at this level.

Intramyocardial LAD course is an absolute contraindication to MICABG, whereas deep, nonpalpable extramyocardial LAD is associated with a more difficult MICABG approach for correct minithoracotomy positioning; in this respect, EBCT with ECG-triggered acquisition has already been applied to visual- ize LAD anatomy before MICABG surgery with successful results in a small population of patients (19). The application of MDCT technology at higher pitches can be of value in this setting by means of fast contrast-enhanced thoracic examination (3-mm collimation), without prospective triggered or retrospective gated ECG acquisition, and with excellent anatomic depiction of LAD.

The proximal origin of the internal mammary artery, either right or left, is on the concavity of the subclavian artery, just opposite to the thyrocervical trunk, which is the second branch on the convexity of the subclavian artery (the first branch is the vertebral artery). After crossing the subclavian veins, the IMAs line the sternum on both sides for a distance of approx 1–2 cm from the sternal border, and they are accompanied in general by one or two internal mammary veins. After a proximal, medial thymic branch, the IMA anastomoses with the intercostal arteries beyond each rib until it reaches the sixth intercostal space, where it divides into two major branches: the craniocaudal branch enters the sheath of the musculus rectus abdominis and anastomoses with the superior epigastric artery, whereas the major lateral branch follows the cartilaginous arch of the ribs.

In a series of 262 consecutive patients undergoing cardiac cath- eterization prior to CABG, Bauer et al. found surgically signifi-

cant anomalies, eventually correlated to graft failure, in 79/262 patients (30%) including common origin of another large artery, large side branches, tortuosity, atypical origin/course, hypo- plasia, and atherosclerotic lesions, resulting in more difficult IMA preparation in 68/262 patients (26%) and complete modi- fication of surgical strategy in 11/262 patients (4%) (20). These findings, coupled with the above-mentioned LAD anatomic assessment and added to the left subclavian artery evaluation for suspected extensive brachiocephalic atherosclerosis, strongly recommend pre-MICABG fast contrast-enhanced MDCT ungated thoracic examination for optimal surgical planning, as already suggested by our experience in this setting (21) (Fig. 5).

Another fundamental preoperative application of MDCT is represented by redo CABG surgery. From an epidemiologic point of view, further revascularization, either reoperative bypass surgery or percutaneous intervention, is required in 4%

of patients by 5 yr, 19% of patients by 10 yr, and 31% of patients by 12 yr after initial bypass surgery (22). Despite the increasing numbers of patients undergoing second and third reoperations, repeat revascularization has considerable limitations, taking into account the perioperative morbidity and mortality, which escalate further as the clinical benefits diminish. As compared with initial surgery, reoperation carries a higher mortality rate (3 to 7%), with a high rate of perioperative myocardial infarc- tion (4 to 11.5%). Redo surgery is also associated with less complete relief of angina and with reduction in saphenous vein graft patency as compared with initial bypass surgery (23,24).

Extremely meticulous planning of surgical approach is therefore mandatory in this subgroup of redo CABG patients and must be defined taking into account several parameters, both cardiovascular (native CAD, venous-IMA graft patencies, and courses) and thoracic (sternotomy, rib cage, mediastinum, lung Fig. 5. Sagittal thin maximum intensity projection reconstruction of the entire course of left internal mammary artery.

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Fig. 6. Volume-rendering 3D reconstruction of multiple venous bypass grafts.

parenchyma). In this respect, ungated MDCT has already been successfully applied as a preoperative tool in the case of redo CABG surgery (25); 3D visualization of cardiovascular as well as thoracic structures, as offered by the high-resolution MDCT technology, allows user-friendly appreciation of complex post- surgical anatomy and therefore a more confident approach for the surgeon.

POSTOPERATIVE APPLICATIONS OF MDCT To date, few studies have been prospectively addressed to the noninvasive evaluation of coronary bypass grafts by means of ECG-gated MDCT acquisition (Fig. 6). Previous MDCT reports on this respect (25) were both restricted to small popu- lations of patients and, much more important, ungated, thus without visualization of distal anastomosis sites. Cardio- synchronization for MDCT evaluation of CABG was first proposed by von Smekal et al. by means of an ECG-triggered approach providing sufficient volume coverage (120- to 140-mm scan range) within a single breath-hold with 0.5-s rotation and 4 × 2.5-mm collimation. However, due to the lack of spatial resolution with 2.5-mm slice width, the distal anastomoses and distal patency of native coronary vessels cannot be evaluated with this approach (26).

Retrospective ECG gating applied to contrast-enhanced MDCT scanning, as already showed in the technical part of this book, is undoubtedly the method of choice for an optimal cardiosynchronization of CTA raw data collection; this approach, coupled with extensive volume coverage and submillimeter spatial resolution, provides unique visualization of arterial-venous grafts almost completely free of cardio-respiratory motion along their entire course at the expense of increased radiation exposure (Fig. 7 A,B). Using 0.5-s rotation, 4 × 1 mm

collimation for 1.25-mm slice width, 120 kV, 300 mA, and a spiral pitch between 1.5 and 2.0, a scan range of 120 mm can be covered within a 30- to 35-s breath-hold. Owing to the spiral pitch higher than 1.5, reconstruction is restricted to the single- segment multislice cardiac volume (MSCV) reconstruction algorithm (27) with a fixed temporal resolution of 250 ms for heart rates up to approx 74 bpm; this temporal resolution is usually sufficient for motion-free MDCT imaging of coronary bypass grafts and their distal anastomoses, whose motion amplitude is significantly lower than that of native vessels (Figs. 8 and 9). Patient preparation and image postprocessing with 3D reconstruction algorithms are obviously similar to those usually employed for coronary MDCT imaging protocols.

Fig. 7 (A,B). Volume-rendering 3D reconstruction views of a single venous graft to the posterior descending artery in a patient with significant right coronary artery disease.

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Fig. 8. Source image showing the distal anastomosis site of a left internal mammary artery implantation to left anterior descending artery.

Regarding the preliminary results, Nieman et al. first reported 88% (15/17) of evaluable bypass grafts in a restricted population of four patients, with overall detection of 4/5 graft lesions in comparison to conventional CAG (28). Moreover, a wider population of 65 patients with a total of 182 bypass grafts was prospectively evaluated by Ropers et al., comparing MDCT and conventional CAG results in order to assess overall accu- racy in detecting graft occlusion and stenosis (29); higher sen- sitivity (98%) and specificity (99%) than ever before were obtained by MDCT in terms of bypass graft patency assessment, whereas a satisfactory diagnostic accuracy for the detection of high-grade bypass stenoses (sensitivity 75%, specificity 92%) was achieved when grafts were imaged with sufficient quality (Figs. 10 and 11).

Distal anastomoses visualization represents one of the crucial aspects of bypass grafting MDCT imaging, and its full appreciation depends on the rigorous application of the acquisition technique as well as on the correct utilization of 3D reconstruction algorithms usually available, in order to obtain evidence-based images (Figs. 12–14). In addition, MDCT technology is generally well suited for stenting evaluation and therefore may be successfully applied even in those infrequent cases of graft disease percutaneous treatment (Fig. 15).

The major drawback of such noninvasive approach seems at the moment to be the significant number of unevaluable grafts;

as stressed in the study of Ropers et al. (29), only 62% of the patent bypass grafts could be evaluated for the presence or absence of high-grade stenoses. In general, metal and motion

artifacts represent the major causes for impaired image quality in this setting, and further improvements of MDCT technology are expected to reduce in the next future the percentage of unevaluable grafts and to optimize visualization and evaluation of distal anastomosis sites.

REFERENCES

1. Shumacker HB. The Evolution of Cardiac Surgery. Indiana Univer- sity Press, Bloomington, IN: 1992.

2. Garrett HE, Dennis EW, DeBakey ME. Aortocoronary bypass with saphenous vein graft: seven-year follow-up. JAMA 1973;223:

792–794.

3. Favaloro RG. Critical analysis of coronary artery bypass graft surgery: a 30-year journey. J Am Coll Cardiol 1998;31:1B–63B.

4. Campeau L, Enjalbert M, Lesperance J, et al. The relation of risk factors to the development of atherosclerosis in saphenous vein bypass grafts and the progression of disease in the native circulation: a study 10 years after aortocoronary bypass surgery. N Engl J Med 1984;311:1329–1332.

5. Bourassa MG. Fate of venous grafts: the past, the present and the future. J Am Coll Cardiol 1991;5:1081–1083.

6. Fitzgibbon GM, Kafka HP, Leach AJ, et al. Coronary bypass graft fate and patient outcome: angiographic follow-up of 5065 grafts related to survival and reoperation in 1388 patients during 25 years.

J Am Coll Cardiol 1996;28:616–626.

7. Loop FD. Internal-thoracic-artery grafts: biologically better coronary arteries. N Engl J Med 1996;334:263–265.

8. Calafiore AM, Angelini GD, Bergsland J, et al. Minimally invasive coronary artery bypass grafting. Ann Thorac Surg 1996;62:

1545–1548.

9. Brundage BH, Lipton MJ, Herfkens RJ, et al. Detection of patent bypass grafts by computed tomography: a preliminary report. Circu- lation 1980;61:826–831.

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Fig. 9. Source image with evidence of venous graft anastomosis to the second diagonal branch.

10. Ullyot DJ, Turley K, McKay CR, et al. Assessment of saphenous vein graft patency by contrast-enhanced computed tomography.

J Thorac Cardiovasc Surg 1982;83:512–518.

11. Godwin JD, Califf RM, Korobkin M, et al. Clinical value of coronary bypass graft evaluation with CT. AJR Am J Roentgenol 1983;140:

649–655.

12. Daniel WG, Dohring W, Stender HS, et al. Value and limitations of computed tomography in assessing aortocoronary bypass graft patency. Circulation 1983;67:983–987.

13. Tello R, Costello P, Ecker CP, et al. Spiral CT evaluation of coronary artery bypass graft patency. J Comput Assist Tomogr 1993;17:

253–259.

14. Engelmann MG, von Smekal A, Knez A, et al. Accuracy of spiral computed tomography for identifying arterial and venous coronary graft patency. Am J Cardiol 1997;80:569–574.

15. Tello R, Hartnell GG, Costello P, et al. Coronary artery bypass graft flow: qualitative evaluation with cine single-detector row CT and comparison with findings at angiography. Radiology 2002;224:

913–918.

16. Ha JW, Cho SY, Shim WH, et al. Noninvasive evaluation of coronary artery bypass graft patencyusing three-dimensional angiography obtained with contrast-enhanced electron beam CT. AJR Am J Roentgenol 1999;172:1055–1059.

17. Lu B, Dai RP, Jing BL, et al. Evaluation of coronary artery bypass graft patency using three-dimensional reconstruction and flow study on electron beam tomography. J Comput Assist Tomogr 2000;24:663–670.

18. von Smekal A, Lachat M, Wildermuth S, et al. Proximal anastomoses of aortocoronary bypasses. Evaluation with ECG-triggered single- slice computerized tomography. Radiologe 2000;40:130–135.

19. Ohtsuka T, Takamoto S, Endoh M, et al. Ultrafast computed tomography for minimally invasive coronary artery bypass grafting.

J Thorac Cardiovasc Surg 1998;116:173–174.

20. Bauer EP, Bino MC, von Segesser LK, et al. Internal mammary artery anomalies. Thorac Cardiovasc Surg 1990;38:312–315.

21. De Santis M, Quagliarini F, Leonetti C, et al. MDSCT pre-operative evaluation of internal mammary arteries (IMAs) in patients candi- date to minimally invasive coronary artery bypass grafting (MICABG) (abs). ECR, Vienna, Austria: 2003.

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Fig. 12. Volume-rendering 3D reconstruction of a patent distal anas- tomosis between internal mammary artery graft and left anterior descending artery.

Fig. 13. Volume-rendering 3D reconstruction of a patent internal mammary artery graft implanted to left anterior descending artery.

Fig. 11. Axial image showing a completely occluded graft closer to a small patent right coronary artery and an occluded circumflex artery in the presence of patency of the corresponding graft.

Fig. 10. Axial visualization of a diseased venous graft on the left side of the pulmonary trunk in a patient with internal mammary artery implantation.

22. Weintraub WS, Jones EL, Craver JM, et al. Frequency of repeat coronary bypass or coronary angioplasty after coronary artery bypass surgery using saphenous venous grafts. Am J Cardiol 1994;73:103–112.

23. Loop FD, Lytle BW, Cosgrove DM, et al. Reoperation for coronary atherosclerosis: changing practice in 2509 consecutive patients. Ann Surg 1990;212:378–386.

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24. Cameron A, Kemp HG Jr, Green GE. Reoperation for coronary artery disease: 10 years of clinical follow-up. Circulation 1988;78 (Suppl I):

I/158–I/162.

25. Yamaguchi A, Adachi H, Ino T, et al. Three-dimensional computed tomographic angiography as pre-operative evaluation of a patent internal thoracic artery graft. J Thorac Cardiovasc Surg 2000;120:

811–812.

26. von Smekal A. The potential of cardio-computed tomography.

Multislice CT: a practical guide. Proceedings of the 5th International Somatom CT User Conference. Zurich, June 2000.

Fig. 15. Multiplanar reformation 3D reconstruction of a patent stent for venous graft disease.

Fig. 14. Volume-rendering 3D reconstruction of a patent distal anas- tomosis between venous graft and obtuse marginal.

27. Ohnesorge B, Flohr T, Becker CR, et al. Cardiac imaging by means of electrocardiographically gated multisection spiral CT: initial experience. Radiology 2000;217:564–571.

28. Nieman K, Oudkerk M, Rensing BJ, et al. Coronary angiography with multi-slice computed tomography. Lancet 2001;357 (9256):

599–603.

29. Ropers D, Ulzheimer S, Wenkel E, et al. Investigation of aortocoronary artery bypass grafts by multislice spiral computed tomography with electrocardiographic-gated image reconstruction. Am J Cardiol 2001;88:792–795.