Chapter 45. Utility of Computed Tomographic Coronary Angiography Post Coronary Revascularization BACKGROUND CORONARY ARTERY BYPASS GRAFTING

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1 Chapter 45 Utility of Computed Tomographic Coronary Angiography Post Coronary Revascularization SANKAR NEELAKANTAN SANJAYA VISWAMITRA SRIKANTH SOLA BACKGROUND Coronary artery disease (CAD) is the leading cause of death and disability world over. There has been an alarming increase over the past two decades in CAD prevalence and cardiovascular mortality in India and other south Asian countries. Primary treatment protocol for CAD involves myocardial revascularization procedures, namely coronary bypass surgeries (CABG) and percutaneous transluminal coronary angioplasty (PTCA) with stenting. Patients post revascularization can present with recurrent symptoms, this outcome is generally as a result of graft occlusion, in-stent restenosis (ISR) or de novo lesions 1. The long-term prognostic factors for survival after myocardial revascularization are dependent on the patency of the bypass graft/ stents and native CAD progression. Coronary angiography is the gold standard diagnostic method for assessing the status of coronary grafts/ stents. It is an invasive and expensive technique with associated complications, which are uncommon, but sometimes serious. However, the development of sophisticated multidetector computed tomography (MDCT) technology (64-slice or better) with threedimensional (3D) multiplanar applications, high spatial and temporal resolution, lower radiation and contrast dose has enabled accurate noninvasive visualization of coronary grafts/stents with submillimetre precision within a single breath-hold 2 (Figs ). CORONARY ARTERY BYPASS GRAFTING Computed tomographic coronary angiography (CTCA) is particularly effective in studying bypass grafts due to their large size, lower degree of calcifications and decreased motion when compared to native vessels. Goals of imaging in post-cabg patients: 1. To delineate the anatomy of coronary bypass grafts and native coronary arteries 2. To reassess the anatomy of subclavian arteries for left and right internal mammary artery (LIMA and RIMA, respectively) grafts 3. To guide management (PTCA vs. redo CABG vs. optimal medical therapy) in cases where there is graft failure 4. To reassess the anatomy of subclavian arteries for LIMA, RIMA grafts 5. For patients planned for redo CABG, to study the relationship of cardiovascular structures/ coronary bypass grafts to the sternum for redo sternotomy 6. To evaluate the extent and progression of atherosclerosis, particularly atherosclerotic calcification in ascending aorta 7. To assess postoperative complications like graft thrombosis and occlusion, graft malposition, graft vasospasm, graft aneurysm, pericardial and pleural effusions, sternal infection 8. Complications in the immediate postoperative period with unsuspected noncardiac findings, including pulmonary embolism, pulmonary nodules, pneumonia, mucous plugging and pneumothorax Graft evaluation: All grafts are individually assessed at the proximal anastomosis, distal anastomosis, graft body (or graft proper), distal native artery and remaining native arteries preferably using curved multiplanar reconstruction, 3D volume rendering and maximum intensity projection images. Also, look for sequential or jump graft, position of the graft in relation to the sternum and incidental findings with larger field of view. 375

2 376 SECTION V Cardiac Imaging Imaging considerations for bypass grafts 3,4 : Saphenous vein graft (SVG): SVGs [also referred to as reversed SVGs (RSVGs)] are typically attached proximally on the anterior wall of the ascending aorta and distally below the stenosis or obstruction. For grafts to the left anterior descending artery (LAD) or left circumflex artery (LCX) territories, a proximal anastomosis is made on the left side of the aorta and stabilized on the main pulmonary artery. In the case of grafts to the right coronary artery territory, the saphenous vein is anastomosed to the right side of the aorta, permitting it to course towards the right atrioventricular groove. Chance for occlusion is around 50% in the first 5 years. Graft body occlusion is relatively common. Internal mammary artery (IMA): The IMA has become the conduit of choice for revascularization of obstructed coronary arteries. The graft is conventionally used in situ from its proximal origin at the left subclavian with distal anastomosis below the occluded LAD or diagonal branches. These grafts are preferred over venous grafts as they are larger in calibre, less prone to occlusion and more pressure resistant. Graft stenosis is mostly noted at the level of the distal anastomosis. There is a possibility of clip artefacts at this region; however, if the graft body is patent, then the clip artefact is ignored and the graft is assessed as patent. RIMA graft is the only exception to this. Radial artery (RA): It is harvested from the nondominant arm and used in combination with other arterial grafts or as an independent graft. The RA is generally used as an alternative to a venous graft or when a third graft is required. It is most commonly used as an independent conduit to perfuse the left cardiac territory. It can be used, however, as part of a Y configuration or a conduit to perfuse the distal right coronary artery (RCA) or posterior descending artery (PDA). RA grafts are prone for diffuse spasm and can appear as a diffuse long segment narrowing. The most common site for focal stenosis is the distal anastomosis. Other arteries like the right gastroepiploic, ulnar, left gastric, splenic, thoracodorsal and lateral femoral circumflex arteries can be used in cases where no alternative arterial conduits are present. Result interpretation: 1. If all grafts are occluded, evaluate the native coronary arteries and consider redo CABG or limited PTCA, as flow could be reestablished postintervention. 2. If the graft is occluded and the distal native vessel is also occluded, no invasive management is warranted; the patient should be put on medical management. 3. If all native coronary arteries are occluded and one CABG graft is patent, this vessel is called last remaining patent artery (LRPA) and is a high-risk situation; consider percutaneous coronary intervention (PCI) in case of stenosis. CORONARY STENTING PCI involving the placement of stents is the main nonsurgical procedure for myocardial revascularization. Newer generation MDCT scanners enable direct visualization and assessment of stent lumen and assessment of in-stent patency. Goals of imaging in patients with coronary stents: 1. To visualize and accurately assess stent patency, ISR or neointimal hyperplasia. 2. To look for stent fracture. 3. To guide repeat interventions in case of stent failure. Challenges with MDCT evaluation of coronary stent patency 1,5 : The degree of artefacts varies with the material composition of the stent used. Most stents are made of stainless steel, but newer generation of stents are made of tantalum, cobalt alloys, platinum, nitinol and titanium. Stents made of tantalum create the most intense beam hardening artefacts, whereas titanium and nitinol stents cause the least artefacts. Stent design and strut diameter can affect the magnitude of artefacts and limit accuracy in evaluation of coronary stent patency. Stent diameter is an important factor in-stent lumen assessment. In general, stents with a diameter 3.5 mm are better visualized. Image analysis for post-pci and stented patients: Using sharp kernel and wider window width (width of 1200), in case of dual energy CT technology, subtraction can be done for specific stent compositions, which helps in-stent delineation. Coronary stents are considered occluded if there is complete absence of contrast inside the stent lumen with decreased or absent distal runoff. Visualization of contrast in the vessel distal to the stent does not necessarily indicate patency because it may be due to retrograde filling. Therefore, reduced contrast enhancement distally implies occlusion or retrograde perfusion 1.

3 Chapter 45 Utility of Computed Tomographic Coronary Angiography Post Coronary Revascularization 377 Restenosis is a critical factor for the long-term success of PTCA and is defined as a reduction in 50% of the stent lumen diameter. ISR is less frequent in drug-eluting stents (,10%) compared to uncoated metallic stents (almost 40%). Acute stent thrombosis can occur during the 24 h after intervention, whereas subacute thrombosis typically occurs between 1 and 30 days after implantation of the coronary stent. Drug-eluting stents are associated with delayed in-stent thrombosis, usually after 30 days of stent placement. Balloon angioplasty is the most common procedure for the treatment of ISR 1,5. Also, rule out stent fracture, which is identified by fragmentation and/or migration of the stent. Figs 45-1 to 45-4 demonstrate illustrative examples of the utility of CT coronary angiography in patients with previous coronary revascularization. A: Coronal reformatted image B, C: Volume rendered 3D images (VRT) Occlusion of SVG to PLB graft just distal to its orgin Aorta Angle: cm Ex: Apr 04 RSVG GRAFT SITE OF ANASTOMOSIS RCA PDA PLVB Figure A 62-year-old man (status post CABG 2010) presents with angina. In this patient with prior CABG, CTCA showed patent LIMA to LAD, SVG to first diagonal and SVG to PDA. There was evidence of occlusion in the proximal aspect of SVG to posterior left ventricular branch (PLB or PLVB) as shown in the VRT image (B); however, the native PLVB distal to anastomosis was patent. Hence, redo CABG was advised. DISTAL SVG SEVERE STENOSIS ATRETIC LIMA P SVG TO PDA SEVERE STENOSIS IN DISTAL GRAFT NATIVE LAD NATIVE LAD SHOWS DIFFUSE SEVERE DISEASE A B C I I Figure A 57-year-old man (known case of CAD, status post CABG in 2005) presented with exertional dyspnea class II since 3 months. Reformatted and VRT images showing distal severe stenosis in SVG to PDA; LIMA to LAD atretic with diffuse severe disease in native LAD. RA-obtuse marginal 1 (OM1) also showed subtotal occlusion. In this patient with prior CABG, since native PDA was the LRPA with focal stenosis of body of SVG graft, redo CABG was deferred. Hence, invasive coronary angiography and PTCA was advised.

4 378 SECTION V Cardiac Imaging RIMA GRAFT RSVG GRAFT RCA LAD A B C Figure A 55-year-old man (known case of CAD) presented with atypical chest pain since 1 month. 3D volume rendered images (A, B) showing origin of RIMA (red arrow) and RSVG (yellow arrow) in a patient with dextrocardia and sided aortic arch. Coronal oblique reformatted image (C) showing occlusion of RSVG to OM graft (red arrow). LIMA GRAFT NATIVE LAD OCCLUDED SVG TO D1 SVG TO RI PROXIMAL LCX OCCLUSION LCX STENT Figure A 57-year-old man presented with angina on exertion (status post CABG in 2007). In this case, the LCX stent showed proximal occlusion; SVG to D1 and SVG to ramus intermedius (RI) were completely occluded. Hence, LIMA to LAD was the LRPA; however, LAD showed proximal occlusion. Redo CABG/PCI options are not available in this case. So this patient was advised optimal medical management. FUTURE CONSIDERATIONS Recent advancements in imaging technology have dramatically expanded the capabilities of CT for noninvasive coronary imaging. The newer 256- and 320-slice scanners have an increased longitudinal coverage with improved temporal resolution. These improvements aid in reducing artefacts during image acquisition. Dual-source CT has emerged in evaluation of coronary arteries in patients with elevated heart rates and arrhythmias while simultaneously assessing global ventricular function, regional wall motion and cardiac valves. The use of prospective gating has led to a reduction in radiation dose in patients compared with retrospective ECG gating without compromising the diagnostic accuracy of coronary CTCA 6. A recent development is CT fractional flow reserve (FFR CT ) that is used to determine the functional significance of stenotic/occlusive lesions using CT data. Potentially, the addition of this technology could make CTCA a comprehensive noninvasive method in coronary assessment, comparable to the more invasive coronary angiography 7. Role of CT perfusion and metabolic imaging will provide combined anatomical and functional information while evaluating revascularized hearts, which will aid in decision making.

5 Chapter 45 Utility of Computed Tomographic Coronary Angiography Post Coronary Revascularization 379 CONCLUSION MDCT coronary angiography is an accurate and essentially noninvasive tool for the assessment of coronary artery bypass grafts and stents. Recent and future technological advances can further improve image quality and provide critical prognostic information. References 1. Lu, M., Jen-Sho Chen, J., Awan, O., & White, C. S. (2010). Evaluation of bypass grafts and stents. Radiologic Clinics of North America, 48(4), Baumüller, S., Leschka, S., Desbiolles, L., Stolzmann, P., Scheffel, H., Seifert, B., et al. (2009). Dual-source versus 64-section CT coronary angiography at lower heart rates: comparison of accuracy and radiation dose. Radiology, 253(1), Sun, Z., & Sabarudin, A. (2013). Coronary CT angiography: State of the art. World Journal of Cardiology, 5(12), Mahnken, A.H. (2012). CT imaging of coronary stents: Past, present, and future. ISRN Cardiology, 2012 [Article ID , 12 pp.]. 5. Han, R., Sun, K., Lu, B., Zhao, R., Li, K., & Yang, X. (2017). Diagnostic accuracy of coronary CT angiography combined with dual-energy myocardial perfusion imaging for detection of myocardial infarction. Experimental and Therapeutic Medicine, 14(1), Nørgaard, B. L., Jensen, J. M., & Leipsic, J. (2015). Fractional flow reserve derived from coronary CT angiography in stable coronary disease: A new standard in noninvasive testing? European Radiology, 25(8), Meinel, F. G., Wichmann, J. L., Schoepf, U. J., Pugliese, F., Ebersberger, U., Lo, G. G. et al. (2017). Global quantification of left ventricular myocardial perfusion at dynamic CT imaging: Prognostic value. Journal of Cardiovascular Computed Tomography, 11(1),

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