Chapter 43 Noninvasive Coronary Plaque Imaging

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1 hapter 43 Noninvasive oronary Plaque Imaging NIRUDH KOHLI The goal of coronary imaging is to define the extent of luminal narrowing as well as composition of an atherosclerotic plaque to facilitate appropriate management. Over the last 10 years, T coronary angiograms have consistently demonstrated excellent correlation with catheter angiograms in the demonstration of the extent of luminal narrowing 1. The composition of an atherosclerotic plaque is complex, as they represent accumulations of varying quantities of lipid, collagen, necrotic debris, blood products, smooth muscle cells, macrophages and calcium. The role of plaque imaging is to differentiate the various components within the plaque to fully characterize the lesion, thus facilitating appropriate management. This especially helps in identifying individuals at risk for acute coronary syndrome, which is a frequent cause for hospitalization, sudden death, chronic disease. Often this may be the first manifestation of disease in a previously asymptomatic individual. The culprit plaque for causing acute coronary syndrome is the vulnerable plaque. These plaques have a large lipid core with a thin fibrous cap. These may rupture or develop erosions, the fibrous cap thus breached. This disruption of fibrous cap brings the lipid core in direct contact with circulating blood activating and accelerating the coagulation cascade resulting in thrombus formation at this site. onsequently, there is luminal obstruction and acute coronary syndrome, myocardial infarction or even death. In 5% 20% of cases, there may be no significant stenosis; in twothird cases, luminal stenosis may be less than 50% (see refs 2 4). Many of these vulnerable plaques may remain silent and follow a healing process to become more stable. The healing process is associated with fibrous tissue and calcification which may result in moderate to severe stenosis. These fibrocalcific plaques generally have chronic stable angina. atheter coronary angiograms and nuclear imaging do not provide any significant information regarding composition of atherosclerotic plaques; invasive techniques such as Intravascular ultrasound (IVUS) and now more recently Optical coherence tomography (OT) provide excellent visualization of the composition of atherosclerotic plaques which correlated very well with histology 5. It would be ideal if a noninvasive technique such as T coronary angiography is able to demonstrate the composition of atherosclerotic plaques. To evaluate the composition of a plaque on imaging, it is important to appreciate the gross pathological findings of plaques. The vulnerable plaque demonstrates a large lipid core with a thin fibrous cap. t the site of the plaque, the cross-sectional area of the lumen and vessel wall is larger due to atherosclerotic plaque growth. This cross-sectional area/diameter is larger than that of the normal reference vessel cross-sectional area/diameter proximal to the plaque. This is termed as positive remodelling. ratio of 1.05 is considered as positive remodelling. ratio of 1.1 is considered more specific as the specificity increases from 45% (1.05) to 78% (1.1), whereas the sensitivity reduces from 100% to 83% (see ref 6). When the diameter at site of plaque is less than that of normal vessel proximal to plaque, it is considered as negative remodelling usually due to a fibrous or calcific plaque causing fibro/calcific stenosis. The vulnerable plaque may also reveal microcalcification, often termed as spotty calcification. These are small 1 3 mm calcific deposits embedded in the soft tissue plaque often in an arc of 90 degrees. The lipid core may be surrounded by a higher density rim; this may be fibrous tissue or calcification. This has been termed as a napkin ring sign. There also may be revascularization of the plaque with presence of vasa vasorum in the plaque. 361

2 362 SETION V ardiac Imaging The advantage of T is its ability to demonstrate the density of substances visually as well as in HU values. alcium is easily demonstrated on T, as it has a very high density, seen as areas of a higher density than intraluminal contrast ( 400 HU). Intraluminal contrast is of approximately 300 HU. In view of this ability to demonstrate calcium plaques on T, plaques were classified as calcific when containing more than 50% calcific components and mixed when containing less than 50% calcium. Plaques which do not contain calcium at all were considered as soft. This classification has been considered as too simplistic to determine the composition of advanced plaques and of not great utility in management of patients as it does not identify the vulnerable plaque. The key to identifying the vulnerable plaque is the lipid core, fibrous cap, positive remodelling and spotty calcification. The lipid core is seen as a very low-density area; the HU values are less than 30 HU. Positive remodelling is well demonstrated on axial and curved Multi planar reconstruction (MPR) images. On axial images, the crosssectional area may be measured at the level of plaque as well as at a proximal level where the vessel wall is normal and devoid of atherosclerotic plaque. ratio of these cross-sectional areas is obtained to determine if there is any evidence of positive remodelling. On curved MPR, the diameter of the vessel at the level of the plaque as well as the diameter of normal vessel proximal to plaque is measured. ratio of these diameters is obtained ( Fig 43-1 to 43-8 ). Spotty calcification is seen as focal high-density areas in a low-density lipid plaque. The napkin ring sign represents a high-density ring of fibrous tissue, calcification surrounding the lipid core. The fibrous cap may be seen as a thin soft tissue band of higher density than the lipid core separating the lipid core from the contrast lumen. This requires good quality images, adequate luminal contrast enhancement, large necrotic core and high-spatial resolution. However, it is rarely identified as the spatial resolution of T is limited. The fibrous cap is usually 65 microns in thickness; the spatial resolution of T is only up to 400 microns. The major limitation of T is its spatial resolution. urrent T resolution does not allow T to match IVUS, OT and histology. Demonstrating calcific plaques and mixed plaques is not difficult for T. The differentiation of soft plaques between fibrous and lipid can be challenging. Demonstration of the fibrous cap as mentioned earlier is difficult. This may result in underestimation of vulnerable plaques as fibrous cap is not visible. ttenuation measurements may be inaccurate when plaques are of small size and irregular shapes. This is due to averaging of adjacent tissues. veraging with adjacent pixels causes overestimation of calcified plaque area due its higher density, termed as a blooming artefact. Similarly, there is also underestimation of soft plaque due to adjacent high attenuation. Microcalcifications are difficult to demonstrate as an aggregate of at least 0.4 mm surrounded by low-attenuation contrast material is required. Intraplaque haemorrhage, even if large, is difficult to detect, as it is difficult to differentiate from enhancing soft tissue, low-density calcium and fibrous tissue. The mainstays in the diagnostics of vulnerable plaque by T are the lipid core, spotty calcification and positive remodelling. Serial studies with various generations of T have compared its ability to demonstrate plaque morphology with IVUS. meta-analysis of 17 studies demonstrated an overall pooled sensitivity of 93% and specificity of 92% for detecting coronary plaques. The sensitivity and specificity for diagnosing calcified plaques were 93% and 98%, respectively, and for diagnosing non calcified, soft or fibrotic plaques were 88% and 92%, respectively 7. T coronary angiograms are an extremely useful noninvasive modality to determine the composition of atherosclerotic plaques, especially the vulnerable plaques. t present, there are limitations due to its spatial resolution; however, with technological advancements the spatial resolution will improve propelling it towards its ultimate goal of matching IVUS, OT and histology.

3 hapter 43 Noninvasive oronary Plaque Imaging 363 F D E G H Figure () T coronary angiogram reveals proximal LD plaque with a small peripheral calcific plaque.() The HU value of plaque is 27 HU indicating lipid plaque.() There is positive remodelling at the level of the plaque as the diameter is wider at the level of the plaque. (D) atheter angiogram (E and F) IVUS and OT (G and H) demonstrate luminal narrowing by necrotic lipid plaque with intimal rupture. (ourtesy: Dr Kirti Punamiya.)

4 364 SETION V ardiac Imaging Figure ( and ) T coronary angiogram demonstrates a calcific and soft plaque in the mid-ld. The HU value of the soft component is 22 HU indicating lipid content. There is positive remodelling, as the diameter is wider at the site of the plaque. Figure ( ) T coronary angiogram reveals a soft plaque with lipid density in the proximal R. There is positive remodelling, as the diameter of the vessel is increased at the site of the plaque. There is also peripheral high density representing a napkin ring sign. Figure T coronary angiogram in 2014 demonstrates a lipid plaque in the R (). In 2017, patient presented with acute coronary syndrome, catheter angiogram revealed total occlusion of R at site of previously visualized plaque (). IVUS reveals this was due to rupture of lipid plaque. (ourtesy: Dr Kirti Punamiya.)

5 hapter 43 Noninvasive oronary Plaque Imaging 365 Figure ( ) T coronary angiogram of proximal LD demonstrates a hypodense plaque causing luminal narrowing. The density of the plaque is 62 HU indicating this is a fibrous plaque. There is a small calcific focus in the plaque representing spotty calcification. Figure T coronary angiogram demonstrates dense calcific plaques in proximal LD; there is also a soft tissue component which is very hypodense due to lipid attenuation. Figure T coronary angiogram demonstrates dense calcific plaques in proximal LD; there is also a soft tissue component which is very hypodense due to lipid attenuation.

6 366 SETION V ardiac Imaging D Figure T coronary angiogram reveals a mixed plaque which is essentially calcific causing significant LD narrowing (). atheter angiogram (). OT confirms mixed nature of plaque () with large calcific component (D) causing narrowing. (ourtesy: Dr Kirti Punamiya.) REFERENES 1. Menke, J., Unterberg-uchwald,., Staab, W., Sohns, J. M., Seif mir Hosseini,., & Schwarz,. (2013). Head-to-head comparison of prospectively triggered vs retrospectively gated coronary computed tomography angiography: Meta-analysis of diagnostic accuracy, image quality, and radiation dose. merican Heart Journal, 165(2), e3. 2. Finn,. V., Nakano, M., Narula, J., Kolodgie, F. D., & Virmani, R. (2010). oncept of vulnerable/unstable plaque. rteriosclerosis, Thrombosis, and Vascular iology, 30(7), Kolodgie, F. D., Virmani, R., urke,. P., Farb,., Weber, D. K., Kutys, R., et al. (2004). Pathologic assessment of the vulnerable human coronary plaque. Heart, 90(12), Davies, M. J. (2000). The pathophysiology of acute coronary syndromes. Heart, 83(3), aumann, S., Renker, M., Meinel, F. G., Wichmann, J. L, Fuller, S. R, ayer, R. R., et al, (2015). omputed tomography imaging of coronary artery plaque: haracterization and prognosis. Radiologic linics of North merica, 53(2), Higashi, M. (2011). Noninvasive assessment of coronary plaque using multidetector row computed tomography: Does MDT accurately estimate plaque vulnerability? (on). irculation Journal, 75, Gao, D., Ning, N., Guo, Y., Ning, W., Niu, X., & Yang, J. (2011). omputed tomography for detecting coronary artery plaques: meta-analysis. therosclerosis, 219(2),