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Hellenic J Cardiol 2009; 50: 245-263

C oronary artery disease (CAD), ornarrowing of the coronary arteriesdue to atherosclerosis, remains one

of the leading causes of morbidity and mor-tality worldwide. However, a substantialnumber of patients who present with anacute coronary event due to rupture or ero-sion of an atherosclerotic plaque do not ex-perience any prior symptoms. This observa-tion emphasizes the need to improve theearly detection of atherosclerosis. Traditio-nally, imaging of the coronary arteries hasfocused on the assessment of luminal di-mensions and the presence of severe steno-sis by means of invasive coronary angiogra-phy. However, invasive coronary angiogra-phy can only assess the degree of stenosisand is less suited to evaluate the presenceof atherosclerosis, including the presenceof (potentially high-risk) plaques. As a re-sult, there is an emerging need for imagingmodalities that can identify atheroscleroticplaques with high-risk features indicatingincreased vulnerability. In this regard par-ticularly, noninvasive techniques may bevaluable, as they may identify high-risk pa-tients at a relatively early stage and mayprovide the opportunity for novel treatmentstrategies. Additionally, noninvasive imag-ing techniques may be used to monitor pro-gression and/or regression of coronary ath-erosclerosis and thus possibly to evaluatethe effectiveness of anti-atherosclerotictherapies on a larger scale.

Accordingly, the present review will

focus on invasive and noninvasive imagingmodalities for the evaluation of athero-sclerosis and detection of vulnerable le-sions in the coronary arteries.

Characteristics of the potentially “vulnerableplaque”

Due to the lack of prospective data andnatural history studies, most details con-cerning the potentially vulnerable plaquehave been derived from retrospective post-mortem studies.1-3 It has been establishedthat the majority of acute coronary events(>70%) are caused by plaque rupture fol-lowed by thrombus formation.3 The mostcommon substrate for superimposed throm-bus formation is presumed to be the thin-capped fibroatheroma; a plaque with alarge necrotic core and thin fibrous cap(<65 Ìm thick) infiltrated by macrophagesand lymphocytes (Figure 1).4 The thin fi-brous cap contains a decreased smoothmuscle content, which in certain circum-stances can rupture and cause the throm-bogenic parts of the plaque to be exposedinto the lumen. This subsequently leads tothe activation of the clotting cascade andthe formation of a thrombus that can com-promise the lumen, resulting in an acutecoronary syndrome (ACS). In the remain-ing ~30% of acute coronary events, throm-bosis may be due to other causes thanplaque rupture, including plaque erosion,intraplaque hemorrhage and calcified

Imaging of Atherosclerosis: Invasive andNoninvasive TechniquesJOELLA E. VAN VELZEN1,2, JOANNE D. SCHUIJF1, FLEUR R. DE GRAAF1, J. WOUTER JUKEMA1,2,ALBERT DE ROOS3, LUCIA J. KROFT3, MARTIN J. SCHALIJ1, JOHANNES H.C. REIBER3,ERNST E. VAN DER WALL1,2, JEROEN J. BAX1

1Department of Cardiology, Leiden University Medical Center, Leiden, 2Interuniversity Cardiology Institute,Utrecht, 3Department of Radiology, Leiden University Medical Center, Leiden, The Netherlands

Address:

Jeroen J. Bax

Leiden UniversityMedical Center,Department ofCardiology,Albinusdreef 2,P.O. Box 9600, 2300 RCLeiden,The Netherlandse-mail: j.j.bax@lumc.nl

Key words:Coronary arterydisease,atherosclerosis,imaging.

Special ArticleSpecial Article

nodules.3 The various atherosclerotic lesions andtheir association with thrombus are described inTable 1.

Additional characteristics of plaques prone torupture include large plaque volume, positive remod-eling, presence of microcalcifications and proximallocation of the lesion. It has still not been fully eluci-dated which trigger actually causes the plaque to rup-ture, although it has been postulated that inflamma-tion plays a critical role. Indeed, as shown by studiesassessing macrophage infiltration in particular, the fi-brous cap is locally deeply inflamed (Figure 1).5 In-flammation is often a result of endothelial dysfunc-tion. Initially, endothelial dysfunction results from adisturbance in blood flow (flow reversal or oscillatingshear stress) at bifurcations or tortuousness of ves-sels.6 However, it has been suggested that not onlyblood flow disturbances, but also cardiovascular riskfactors such as hypercholesterolemia, smoking and di-abetes, can induce endothelial dysfunction.7,8 Due to

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endothelial cell activation, the increased expressionof adhesion molecules (e.g. selectins, vascular cell ad-hesion molecules [VCAMs], and intercellular adhe-sion molecules [ICAMs]) promotes the infiltrationand homing of monocytes. Consequently, the mono-cytes migrate into the plaque and convert into ma-crophages, contributing to the process of atherogene-sis.7

At present, there is no widely accepted diagnostictechnique for the identification of vulnerable plaques.However, several invasive and noninvasive imagingmodalities are currently under development that mayto some extent allow the detection of plaques prone torupture.

Invasive imaging of atherosclerotic plaques

Invasive coronary angiography

Invasive coronary angiography is currently the goldstandard for the diagnosis of CAD and provides anaccurate and detailed overview of the anatomy of thecoronary artery tree, including precise quantificationof the degree of stenosis. Accordingly, the techniqueis extensively used to guide further treatment strate-gies, such as coronary angioplasty or bypass surgery.

However, the evaluation of percentage diameterstenosis has limited value in predicting future cardiacevents. Indeed, as demonstrated during the follow upof patients admitted for acute myocardial infarction,almost two thirds of plaques prone to rupture werelocated in non flow-limiting atherosclerotic lesions,and only a minority were located in severely obstruct-ed lesions.9,10 Although the likelihood of occlusion foran individual lesion is directly related to the severity ofstenosis, non-obstructive lesions are far more commonand thus may frequently cause coronary occlusiondue to their greater number (Figure 2). Accordingly,evaluation of the percentage diameter stenosis bymeans of invasive coronary angiography does not al-low differentiation between stable and unstable pla-ques.

Notably, novel promising angiographic acquisi-tion approaches have been developed recently. Oneof these acquisition methods is rotational 3-dimen-sional coronary angiography, a new imaging tech-nique in which the gantry is mechanically rotatedaround the patient, providing a multitude of X-rayprojections during a single contrast injection.11 Usingthis technique, motion information about the coro-nary arteries can be extracted, including vessel dis-

Figure 1. Histological specimen of inflamed thin-capped fi-broatheroma with trichrome stain, rendering lipid colorless, collagenblue and erythrocytes red. A: Atherosclerotic coronary artery con-taining a large lipid core and thin fibrous cap, with postmortem in-jected contrast in lumen. B: Detail of the fibrous cap, demonstratingthat the cap is heavily inflamed. The fibrous cap consists of manymacrophages and within the necrotic core extravasated erythrocytescan be seen, indicating a possible plaque rupture. Reprinted withpermission from Schaar et al.4

A

B

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Tab

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placement and pulsation.12 Furthermore, reconstruc-tion of 3-dimensional images from 2-dimensional pro-jections using specially developed dedicated softwaremay further enhance the angiographic assessment ofcoronary arteries (Figure 3). However, whether thisnovel technique will allow more accurate evaluation ofatherosclerotic plaques remains to be determined moreprecisely.13 Overall, it seems evident that invasive coro-nary angiography is an excellent modality for detectingobstructive coronary artery disease. However, detailedimaging of atherosclerosis, such as determining thepresence of vulnerable plaque characteristics, remodel-ing and inflammation, is still not feasible using thistechnique. Therefore other, more insightful modalitiesare needed for this purpose.

Intravascular ultrasound

With respect to the imaging of atherosclerosis, substan-tial progress has been achieved with the developmentof intravascular ultrasound (IVUS). IVUS is a minimal-ly invasive imaging modality that uses miniaturizedcrystals incorporated at the catheter tip to provide real-time, high-resolution, cross-sectional images of the ar-terial wall and lumen. Axial resolution is approximately150 Ìm and the lateral resolution 300 Ìm. As a result,the technique provides high-resolution images of theatherosclerotic process in the arterial wall.

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Figure 2. Bar charts representing stenosis severity and related risk of myocardial infarction (MI) as assessed by re-peated angiographic examination. As can be observed from the figure, lesions that are non-significant (<50%stenosis) on prior angiography are frequently the underlying cause of MI. Moreover, non-significant lesions out-number the more severely obstructive lesions and therefore account for the majority of MI. The bar charts wereconstructed from data published by Ambrose et al,9 Little et al,96 Nobuyoshi et al,97 and Giroud et al.98

Figure 3. A: Illustration of three-dimensional coronary modelingbased on two angiograms acquired in two different projectiongeometries. B: Modified American Heart Association (AHA)classification of coronary segments. Reprinted with permissionfrom Garcia et al.99

A

B

Importantly, the technique has been extensivelyvalidated against histological autopsy specimens ofhuman coronary arteries.14-17 Both lumen and vesseldimensions, such as plaque and vessel area, plaquedistribution, lesion length and remodeling index, canbe accurately determined in vivo. In addition, semi-quantitative tissue characterization can be achievedbased on plaque echogenicity. In conventional gray-scale ultrasound images, calcium highly reflects ultra-sound and appears as a bright and homogenous sig-nal, resulting in acoustic shadowing.15,18 In addition,the severity of calcifications can be quantified by mea-suring the angle or arc of calcium. Hypo-echoic or lowreflectance in IVUS images are usually due to lipid-laden lesions (also referred to as “soft” or “sonolucent”plaques). An example is provided in Figure 4.

Grayscale IVUS features of potentially vulnerableplaques have been evaluated prospectively by Yama-gishi et al.19 The investigators evaluated 114 coronaryplaques without luminal obstruction and assessed whichplaques were related to an acute coronary event duringa follow-up period of 21 months. Interestingly, it wasreported that large, eccentric, positive remodeled pla-ques with an echolucent zone were at increased risk ofinstability (Figure 5). In addition, several retrospectivestudies confirmed that IVUS was able to identify pla-ques at higher risk of rupture (large echolucent area,thin fibrous cap).20-22 Moreover, studies examining thedifferences between ruptured plaques and non-rup-tured plaques in the same coronary artery demonstrat-ed that the IVUS-derived lumen eccentricity index ofruptured plaques was greater.23

IVUS has also been increasingly used as the goldstandard in trials evaluating progression or regressionof plaque in the coronary arteries. Indeed, unlike an-giography, accurate quantification of plaque volumeand area is provided by IVUS. Von Birgelen and co-workers performed IVUS examination of the left maincoronary artery in 56 patients during initial angiographyand repeated imaging after 18 months.24 Adverse car-diovascular events occurred in 18 patients during followup; in patients with events, annual plaque progressionwas significantly greater than in the remaining asympto-matic patients. Hence, it seems feasible that IVUS-measured progression of coronary plaque may serve asa marker for future cardiovascular events.

Nevertheless, the main limitation of grayscaleIVUS remains its inability to accurately differentiateplaque composition. In particular, areas with low echoreflectance, such as fibrous tissue, fibro-fatty tissueand thrombus, remain hard to distinguish.14,18 More re-cently, integrated backscatter IVUS (IB-IVUS) sys-tems have been developed to overcome this problem.Using this technique, a 2-dimensional color-codedmap is constructed to reflect the tissue characteristicsof the coronary arterial wall. In a prospective study bySano et al, tissue characteristics of vulnerable plaquesin patients prior to presentation with ACS were eval-uated using IB-IVUS.25 The authors demonstratedthat the tissue characteristics of vulnerable plaquesbefore they caused an ACS were different from thoseof plaques related to stable angina. However, a lowpositive predictive value of only 42% was reported forthe identification of lipid area, indicating that further

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Figure 4. Coronary angiogram(A) of the left anterior de-scending coronary artery andcorresponding intravascular ul-trasound (IVUS) images (Band C) of a 55-year-old patientpresenting with an acute coro-nary syndrome. In panel B anIVUS frame is provided show-ing a large plaque area with anecholucent zone (arrowhead)and luminal obstruction, possi-bly suggesting the presence ofa vulnerable plaque. Panel Cshows a more distally obtainedIVUS frame with minimal pla-que burden.

A B

C

improvement is needed before application of this tech-nique is feasible.

Virtual histology intravascular ultrasound

Virtual histology intravascular ultrasound (VH IVUS)can potentially differentiate plaque composition moreaccurately than conventional grayscale IVUS. The tech-nique is based on radiofrequency analysis of intravascu-lar ultrasound backscatter signals. A combination ofspectral parameters is used to develop statistical classi-fication schemes for analysis of in vivo IVUS data in re-al time. Using these parameters, color-coded maps ofplaque composition for each cross-sectional image areprovided and are superimposed on the grayscale IVUSimages. As illustrated in Figure 6, these tissue maps candifferentiate fibrous (dark green), fibro-fatty (lightgreen), dense calcium (white) and necrotic core (red)areas. Since its introduction, the technique has beenvalidated by histology in several studies.26,27 Nair andcolleagues have shown accuracies of 90.4% for fibrous,

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92.8% for fibro-fatty, 90.9% for calcified and 89.5% fornecrotic core regions, demonstrating the potential ofthis imaging tool for analyzing plaque composition.26

The ability of VH IVUS to evaluate the presenceof vulnerable plaques was first demonstrated by Ro-driguez-Granillo et al.28 The investigators observedthat vulnerable plaques as determined on VH IVUSwere more prevalent in patients presenting with ACSthan in those with stable angina pectoris. Similar re-sults were recently reported by Pundziute and co-work-ers, who demonstrated that in culprit lesions of pa-tients with ACS the thin-capped fibroatheroma wasmore prevalent than in plaques of patients presentingwith stable symptoms (Figure 7).29 Interestingly, thepresence of positive remodeling identified by VHIVUS was found to be similarly linked to the presenceof vulnerable plaques. A retrospective study using VHIVUS demonstrated that positive remodeled plaquecontained significantly more necrotic core and featuresof high-risk plaque, whereas negative remodeled pla-ques showed a more stable phenotype.30

Figure 5. Coronary plaque in the right coronary artery (RCA) of a patient presenting with an acute coronary syndrome as evalu-ated by coronary angiography (left) and intravascular ultrasound (right). A: A mild concentric lesion in the distal part of theRCA. B: In the proximal portion, a significant eccentric lesion with an echolucent area (arrow) and high plaque burden of 67%.C: More proximally, an eccentric lesion with high echo density. Reprinted with permission from Yamagishi et al.19

B

A

C

Notably, in addition to remodeling, Valgimigli et aldemonstrated that plaque composition on VH IVUSwas influenced by the location of the plaque in thecoronary artery tree.31 As shown by VH IVUS, proxi-mal segments of coronary arteries had a larger necroticcore area when compared to distal coronary segments,whereas the other plaque components (fibrous, fibro-fatty and dense calcium) were distributed evenly alongthe coronary artery tree. Accordingly, the distancefrom the ostium was demonstrated to be inversely as-sociated with plaque vulnerability, possibly explaining

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Figure 6. Plaque characterization by virtual histology intravascular ultrasound (VH IVUS). A: Traditional grayscale intravascular ultra-sound (IVUS) frame showing coronary plaque. B: Example of VH IVUS color-coded map superimposed on grayscale IVUS frame. Thecolors correspond to different tissue types, such as fibrous (dark green), fibro-fatty (light green), dense calcium (white) and necrotic core(red). Panel B shows a plaque with predominantly necrotic core, small dense calcium deposits and a thick fibrous cap, corresponding to afibroatheroma.

the higher incidence of culprit lesions in proximalparts of the coronary artery tree.

Interestingly, in addition to evaluating progres-sion or regression in plaque burden, VH IVUS mayalso have the ability to monitor changes in plaquecomposition (and possibly even plaque vulnerability)after treatment with anti-atherosclerotic therapy. Ser-ruys et al assessed the effect of the direct lipoprotein-associated phospholipase A2 (Lp-PLA2) inhibitordarapladib on plaque composition by VH IVUS.32

The investigators showed that necrotic core size in-creased in patients receiving placebo. In contrast, Lp-PLA2 inhibition prevented further progression ofnecrotic core, suggesting stabilization of atherosclero-sis.

Although VH IVUS is a promising imaging mo-dality for plaque characterization, some limitationsremain. Importantly, detection of the thin fibrouscap (<65 Ìm) is not yet feasible as VH IVUS has alimited radial resolution of only 100 Ìm. However,with the introduction of 40 MHz catheters imagingof the thin fibrous cap may eventually become possi-ble.

Optical coherence tomography

Optical coherence tomography (OCT) is a uniquehigh-resolution imaging technique that uses low co-herence, near infrared light for intravascular imagingof the coronary artery wall. It has an excellent spatialresolution of 10-20 Ìm, which is ten times higher thanthe resolution of IVUS. Furthermore, using histologi-cal controls, it has been demonstrated that OCT is su-

Figure 7. The prevalence of thin-capped fibroatheroma (TCFA)in patients presenting with stable symptoms versus patients pre-senting with an acute coronary syndrome (ACS) evaluated by vir-tual histology intravascular ultrasound. TCFAs were more fre-quently observed in plaques of patients with ACS (32%) as com-pared to patients with stable symptoms (3%). The bar chart wasconstructed with data from Pundziute et al.29

A B

perior to IVUS in detecting important features of vul-nerable plaque components, including thickness of fi-brous cap, thrombus and density of macrophages.33-35

One of the first investigations to demonstrate thefeasibility of plaque characterization with OCT in vi-vo was performed by Jang et al.36 Using this techni-que the authors reported a higher frequency of thin-capped fibroatheroma in patients with ACS as com-pared to patients with stable angina pectoris. Kubo etal compared the assessment of culprit plaque mor-phology by OCT to grayscale IVUS and coronary an-gioscopy.37 The authors concluded that OCT was su-perior in identifying the thin-capped fibroatheromaand thrombus, and that OCT was the only modalitythat could distinguish the thickness of the fibrous cap(Figure 8).

Another interesting feature of OCT is that it en-ables quantification of macrophages within fibrouscaps. Tearney and colleagues showed in vitro, by com-paring OCT images to histological specimens, that ahigh positive correlation exists between OCT measure-ments and fibrous cap macrophage density (r=0.84).38

In vivo, Raffel and colleagues demonstrated a signifi-cant relationship between systemic inflammation (whitecell blood count) and macrophage density in fibrouscaps identified by OCT.39

At present, it is important to realize that there aresome important limitations in the use of OCT. Bloodleads to significant attenuation of the emitted in-frared light, therefore regular saline flushes or bal-loon occlusion of the artery is necessary for adequateimaging. Consequently, data acquisition is time-con-suming and is therefore limited to focal lesion explo-ration. Furthermore, the penetration depth of near

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infrared light is only 1-2 mm. As a result, OCT is notable to visualize the complete plaque and vessel wall,and quantitative measurements of plaque and/or lipidvolume are currently not possible. However, a sec-ond-generation OCT technology, namely optical fre-quency domain imaging (OFDI), has recently beendeveloped to enable imaging of the coronary arterieswith a short, non-occlusive saline flush and rapid spi-ral pullback.40

Other intracoronary techniques

Intravascular ultrasound palpography

Intravascular palpography is a technique based on in-travascular ultrasound. This imaging modality allowsthe assessment of local mechanical tissue propertiesby assessing tissue deformation or strain. At a givenpressure limit, fatty tissue components will showmore deformation than fibrous components. Accord-ingly, palpography uses these differences in tissue de-formation to differentiate between various plaquecomponents. Indeed, differences in strain between fi-brous, fibro-fatty and fatty components of the plaqueof coronary and femoral arteries have been reportedin vitro.41 In addition, a distinctive strain pattern wasfound with a high sensitivity and specificity (89%) forthe detection of thin-capped fibroatheroma in post-mortem coronary arteries. Schaar et al performed thefirst clinical study using palpography in patients to as-sess the incidence of vulnerable plaque.42 In 55 pa-tients presenting with stable symptoms, unstable sym-ptoms and acute myocardial infarction, palpographywas performed and the number of deformable pla-

Figure 8. Intraluminal thrombi in corresponding images of optical coherence tomography (A), coronary angioscopy (B), and in-travascular ultrasound (C). A: Thrombus with optical coherence tomography signal attenuation (T). B: Large white thrombus(WT) and small red thrombus (RT) adhering to a rough surface of yellow plaque. C: Thrombus (arrows) identified as a mass pro-truding into the vessel lumen from the surface of the vessel wall. Reprinted with permission from Kubo et al.37

A B C

ques was assessed. The investigators reported that pa-tients with stable angina pectoris had significantly few-er deformable plaques (high strain spots) per vessel ascompared to patients presenting with unstable anginapectoris or acute myocardial infarction (Figure 9).Thus, although additional validation is required, in-travascular ultrasound palpography appears to havepotential for the identification of vulnerable plaquecharacteristics.

Intracoronary angioscopy

Intracoronary angioscopy is an imaging technique thatuses optical fibers to allow direct visualization of theplaque surface, the presence of thrombus and the colorof the luminal surface. A normal artery appears as glis-tening white, whereas a plaque can be categorized basedon its angioscopic color, such as yellow or white. Addi-tionally, thrombus can be identified as white (plateletrich) or red (platelets and erythrocytes) (Figure 8B).Uchida and co-workers performed intracoronary an-gioscopy in 157 patients presenting with stable angina.43

In a 12-month follow-up period, ACS occurred morefrequently in patients with glistening yellow plaques(69%) than in those with white plaques (3%).

Intracoronary angioscopy can also be applied as atool for monitoring changes in plaque morphology

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Figure 9. Bar chart representing the relation between the numberof high strain spots as assessed by palpography and clinical presen-tation in 55 patients. High strain spots correspond to the more vul-nerable plaques. More high strain spots were demonstrated by pal-pography in patients presenting with unstable angina pectoris andacute myocardial infarction as compared to patients presentingwith stable angina pectoris. Bar chart constructed with data fromSchaar et al.42

following pharmaceutical therapy. Using this tech-nique, Takano and colleagues were able to demon-strate an effect of preventive treatment with atorvas-tatin.44 Interestingly, lipid-lowering therapy with ator-vastatin changed plaque color and morphology as de-termined by angioscopy, thereby suggesting plaquestabilization.

A major limitation of angioscopy remains that, aswith OCT, the technique requires a blood-free field,while investigation is restricted to a limited part of thevessel.

Noninvasive imaging of atherosclerotic plaques

Calcium score

It has been well established that the presence of coro-nary artery calcifications (CAC) confirms the presenceof atherosclerosis. In fact, an association between visi-ble CAC on invasive coronary angiography and the riskof cardiovascular events was demonstrated in the early1980s.45 The introduction of electron beam computedtomography (EBCT) allowed the noninvasive evalua-tion of CAC and resulted in the development of thewidely established quantification method by Agat-ston.46 More recently, assessment of CAC is performedby means of multislice computed tomography (MSCT)(Figure 10).

The relation between the presence and extent ofCAC and the presence of coronary artery stenosis hasbeen assessed in several studies.47-49 As expected, ahigh sensitivity of CAC for the presence of obstructiveCAD has been reported. However, extensive calcifica-tions can be present in the absence of luminal narrow-ing. As a result, the specificity for obstructive CAD islow. Accordingly, the technique may be more suited toprovide an estimate of total plaque burden rather thanstenosis severity.

Importantly, data concerning the calcified plaqueburden have been shown to translate into prognosticinformation. Indeed, the value of CAC scoring for riskstratification has been extensively studied. A large clini-cal trial by Greenland and colleagues showed the dis-tinct incremental value of CAC scoring over the Fram-ingham risk score in asymptomatic patients.50 In addi-tion, Detrano et al demonstrated that CAC performedequally well among the four major racial and ethnicgroups.51 In a even larger cohort of 25,253 asymptoma-tic individuals, Budoff and colleagues confirmed thatCAC was an independent predictor of mortality andthat risk scores increased proportionally with higher

CAC scores (Figure 11).52 Particularly in patients ini-tially classified as being at intermediate risk, knowledgeof the extent of CAC may be valuable for refining riskstratification and determining further management.

In addition to risk stratification, it has been suggest-ed that CAC scoring may allow noninvasive monitoringof changes in atherosclerotic plaque burden. Several in-vestigations have demonstrated a halt in progression oreven regression of coronary calcifications as a result ofreductions in serum low-density lipoprotein (LDL) cho-lesterol concentrations.53 However, other investiga-

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tions failed to show such an effect, despite effectivereductions in systemic inflammation or LDL choles-terol concentrations. It is possible that changes in calci-fied plaque burden may not adequately reflect changesin total atherosclerotic plaque burden. Moreover, it hasbeen suggested that plaque stabilization may even beassociated with a relative increase of coronary calcifica-tions rather than a decrease. Indeed, it is important torealize that the presence or absence of calcium itself isnot a direct marker for vulnerability. Since no informa-tion is obtained about the presence of non-calcified

Figure 10. Example of coronary calcium on noncontrast-enhanced multislice computed tomogra-phy (MSCT) axial images. Calcifications appearas bright white dense structures on MSCT. PanelA shows a 57-year-old patient without evidenceof coronary calcifications in the left anterior de-scending coronary artery (LAD). Panel B shows a53-year-old patient with calcifications in the LAD.AO – aorta.

Figure 11. Cumulative survival by coronary artery calcification score, adjusted for risk factors such as age,hypercholesterolemia, diabetes, smoking, hypertension, and a family history of premature coronary arterydisease. Increasing calcium scores were associated with worse survival and each increment of calcium scorewas associated with significant increased risk of all-cause mortality. Reprinted with permission from Budoffet al.52

A B

plaques, CAC scoring does not allow for a reliable dis-tinction between potentially unstable versus stable pla-ques.54

Multislice computed tomography angiography

MSCT is a rapidly evolving imaging tool that allowsthe noninvasive visualization of coronary atheroscle-rosis. Since the introduction of 4-slice scanners, thetechnique has developed rapidly and 64-slice and even320-slice systems are currently available. The temporaland spatial resolution have improved accordingly, re-sulting in superior image quality and diagnostic accu-racy for the detection of CAD. Although the resolutionof MSCT remains inferior to that of invasive coronaryangiography, high diagnostic accuracies have beendemonstrated for the detection of significant CAD.55

Additionally, the technique may be of use in theworkup of patients presenting to the emergency de-partment with suspected ACS. Promising results werereported by Hoffmann and co-workers, who demon-strated that the absence of significant coronary arterystenosis (73 of 103 patients) and non-significant coro-nary atherosclerotic plaque (41 of 103 patients) onMSCT accurately ruled out ACS.56 Thus, a high nega-tive predictive value was observed, indicating thatMSCT angiography may be a valuable gatekeeper forinvasive coronary angiography.

MSCT is not only able to identify coronary arterystenosis, but also has the potential to provide informa-tion about lesion morphology and plaque composition.As illustrated in Figure 12, the technique can distin-guish non-calcified, mixed and calcified plaques. Due tothe substantially higher density values, identification ofcalcified plaque is relatively simple on MSCT. Howev-er, identification of non-calcified plaque is more de-manding because of the more subtle difference in at-tenuation and relatively larger influence of body-massindex, cardiac output and amount of contrast injected.Interestingly, a comparison between density measure-ments of non-calcified plaques on MSCT and invasiveIVUS showed that the attenuation within hyper-echoic(fibrous) plaques was higher than within hypo-echoic(lipid-rich) plaques (mean attenuation values of 121 ±34 HU versus 58 ± 43 HU).57 However, for individuallesions a substantial overlap between hyper-echoic andhypo-echoic attenuation values was observed, indicat-ing that at this stage further characterization of non-cal-cified plaque is not feasible.

Plaque composition as evaluated by MSCT hasbeen linked to clinical presentation. Motoyama and col-

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Figure 12. Example of plaque imaging performed on 320-slicemultislice computed tomography coronary angiography. A:Curved multiplanar reconstruction of the left anterior descendingartery (LAD) with non-calcified plaque (arrow). B: Curved multi-planar reconstruction of the LAD demonstrating mixed plaque(arrow). C: Curved multiplanar reconstruction of the right coro-nary artery demonstrating calcified plaque (arrow).

leagues compared plaque morphology on MSCT in 38patients with ACS versus 33 patients with stable anginapectoris and demonstrated that plaques associated withACS showed lower density values, positive remodelingand spotty calcification.58 Pundziute and colleaguescompared plaque characteristics on 64-slice MSCTand VH IVUS in patients with ACS and stable anginapectoris and demonstrated that non-calcified (32%)and mixed plaques (59%) were more frequently pre-sent in ACS.29 In line with these findings, using 64-sliceMSCT, Henneman et al demonstrated in 40 patientssuspected of ACS that CAC was absent in a large pro-portion of patients (33%). However, as illustrated inFigure 13, in these patients non-calcified plaques werehighly prevalent (39%).59 As a result, atherosclerosisand even obstructive CAD were frequently observed,even in the absence of detectable calcium. Thus, the in-vestigators suggested that in patients presenting with

A

B

C

ACS, the absence of CAC does not reliably excludeCAD.

Preliminary studies have suggested that informa-tion about atherosclerosis derived from MSCT angiog-raphy may also provide prognostic information.60,61 In-terestingly, van Werkhoven et al demonstrated that thepresence of a substantial non-calcified plaque burdenwas an independent predictor of events (all-cause mor-tality, non-fatal myocardial infarction, and unstableangina requiring revascularization).62 However, furtherinvestigations are required in larger patient populationsto confirm these observations.

In addition, MSCT may potentially be applied tomonitor the progression and/or regression of coronaryplaque burden. Preliminary results from an experimen-tal animal model indicated that MSCT could accuratelydocument serial changes in aortic plaque burden thatcorrelated well with measurements derived from mag-netic resonance imaging (MRI).63 In humans, Burg-stahler and co-workers studied the effect of lipid-lower-ing therapy on coronary plaque burden with MSCT af-ter one year.64 Although no differences were found intotal plaque burden and CAC, a significantly lowernon-calcified plaque burden was demonstrated afterlipid-lowering therapy.

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Figure 13. Prevalence of different plaque types in patients pre-senting with suspected acute coronary syndrome (ACS). A highprevalence of non-calcified and mixed plaques was observed in pa-tients presenting with suspected ACS. Pie chart constructed withdata from Henneman et al.59

While MSCT angiography may have potential forthe noninvasive evaluation of plaque composition andsubsequent identification of patients at higher risk ofevents, several important limitations remain. Firstly, thetechnique is associated with radiation exposure, al-though significant dose reductions have been achievedwith recent advances in scanner hardware and acquisi-tion protocols.65-67 In addition, the resolution remainsinferior compared to invasive atherosclerosis imagingtechniques and no validated algorithms are currentlyavailable for the quantification of observations. Furtherimprovement in plaque characterization, however, isexpected with the development of dual-energy MSCTor dedicated contrast agents.

Magnetic resonance imaging

MRI is a versatile imaging technique with a high poten-tial to visualize vessel anatomy. The technique is able todifferentiate atherosclerotic tissue without exposure toradiation, using features such as chemical composition,water content, molecular motion, or diffusion. Due torecent improvements in MR techniques, such as high-resolution and multi-contrast MR—T1- and T2-weight-ed, proton density (PD) weighted and time-of-flight(TOF) imaging—plaque characterization has becomepossible, as demonstrated in experimental models, his-tological specimens, human carotid arteries and theaortic wall in vivo (Table 2).68-71 Fayad and colleaguesassessed aortic wall plaque composition with MR im-ages matched to transesophageal echocardiograms,demonstrating a strong correlation for plaque compo-sition, thickness and extent.68 In several studies, thepotential of MRI to characterize different plaque char-acteristics, including the fibrous cap, lipid core and eventhe presence of hemorrhage in human carotid athero-sclerotic plaques (Figure 14), has been demonstrat-ed.72,73 In addition, a good correlation has been iden-tified between fibrous cap integrity on MRI and histo-pathological specimens.69

Table 2. Multicontrast weightings and corresponding plaque characterization on magnetic resonance imaging. Intensities are relative tothat of the sternomastoid muscle. Table modified from Yuan et al.94

Component TOF T1-weighted T2-weighted PD-weighted

Hemorrhage High High - moderate Variable Variable Lipid-rich necrotic core Moderate High Variable High Calcification Low Low Low Low Fibrous tissue Moderate - low Moderate Variable High

TOF - time of flight; PD - proton density.

Plaque imaging of the coronary arteries with MRIremains challenging, since deep location, motion andrespiratory artifacts, and small caliber vessels remainobstacles for accurate coronary visualization and plaquedifferentiation. Nevertheless, several novel approachesfor coronary plaque imaging are currently under devel-opment and may potentially allow accurate evaluationof atherosclerotic plaque in the coronary arteries.74 Inparticular “black-blood” techniques (an imaging ap-proach in which the blood appears black and the arteri-al wall can be seen) are promising for accurately por-traying plaque presence, size and morphology with sub-millimeter resolution and high reproducibility.75 Kim etal recently applied a novel 3-dimensional free breathingblack-blood fast gradient technique with real-time mo-tion correction developed by Botnar et al to evaluatepatients with non-significant CAD and compared thesepatients to a control group without CAD.76,77 The in-vestigators demonstrated that MRI could identify sig-nificantly increased vessel wall thickness with preservedlumen size in patients with non-significant CAD.

High-resolution MRI, in combination with mole-cular contrast agents targeted to specific cells or mol-ecules, offers an interesting alternative approach formore detailed plaque characterization.78-80 In particu-lar, contrast agents dedicated to the identification ofvulnerable plaque components are of considerable in-terest. Paramagnetic contrast agents such as gado-

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Figure 14. Example of magnetic resonance image (T2-weighted) ofthe carotid arteries. A stenotic lesion of the right internal carotidartery can be observed just distal to the bifurcation (arrow). The ar-rowhead indicates a high signal artifact from the closely placed su-perficial phase-array coil. Finally, in the enlargement, a hypo-intensesignal within the plaque, corresponding to lipid accumulation, can beobserved. Reprinted with permission from Corti et al.100

linium (T1-shortening contrast with a high affinity forlipid-rich lesions) are able to assess the more subtle dif-ferences in plaque composition.79 Furthermore, T2-shortening contrast agents such as ultra small super-paramagnetic particles of iron oxide (USPIOs) havebeen studied both in vitro and in vivo. Interestingly,these particles were found to accumulate in plaqueswith a high macrophage content and caused a signaldecrease in MR images.81 Additionally, promising re-sults have been achieved with fibrin-targeted contrastagents, which have the potential to allow noninvasivemolecular imaging of thrombus. Spuentrup and col-leagues demonstrated in an experimental animal mod-el that, using these agents, acute pulmonary, cardiacand coronary thrombosis could be accurately visualizedby MR imaging.82 Furthermore, continued advances inradiofrequency hardware have resulted in an increasein the operating field strength from 1.5 T (tesla) to 3 Tand even 7 T. At 3 T an approximately twofold increasein signal-to-noise ratio can be obtained, resulting in afourfold reduction in scanning time and a significantincrease in temporal resolution.

Recent studies also support MRI as an effectivetool to evaluate plaque regression following lipid-low-ering therapy. Corti et al demonstrated in 18 hypercho-lesterolemic patients that MRI could document amarked reduction in atherosclerotic lesion size inducedby statin therapy in humans.83 Accordingly, MRI maybecome a particular attractive modality for the nonin-vasive monitoring of the effect of anti-atheroscleroticinterventions in vivo.

Nevertheless, detailed characterization of plaque,including the identification of high-risk features, re-mains difficult at present. Although much is expectedfrom current developments, evidently more data areneeded before plaque characterization with MRI maybe used clinically for the identification and manage-ment of patients at risk.

Molecular imaging with nuclear techniques

Using dedicated tracers, nuclear imaging techniquessuch as single photon emission tomography (SPECT)and positron emission tomography (PET) can targetdistinct mediators and regulators involved in the cas-cade of atherosclerosis. As a result of increasing know-ledge regarding the pathophysiology of atherosclerosis,several radionuclide-labeled tracers that serve as mark-ers of inflammation, angiogenesis, apoptosis and lipidmetabolism have been developed for plaque imaging(Table 3).

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Tab

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or.

Matrix metalloproteinases (MMP) are releasedby activated macrophages and are therefore used toidentify proteolytic activity in atherosclerotic lesions.MMPs modulate the degrading of the extracellularmatrix and the thin fibrous cap of an atheroscleroticlesion, contributing to the vulnerability of the plaque.The feasibility of in vivo imaging of MMP activity us-ing radionuclide-labeled MMP inhibitors has beenshown in several animal models.84-86

Additionally, it has been proposed that apoptosis isone of the features of an atherosclerotic unstable lesionand that apoptosis consequently leads to growth of thenecrotic core and influences plaque stability. AnnexinA5 has a high affinity for phosphatidylserine (exposedon the plasma membrane of apoptotic cells) and there-fore radionuclide-labeled Annexin A5 can be used as amarker of apoptotic cells in atherosclerotic lesions. Inexperimental models, a direct correlation was demon-strated between Annexin A5 uptake, macrophage bur-den and histologically demonstrated apoptosis.87 In asmall patient cohort with a history of transient ischemicattack, Annexin A5 imaging of carotid atherosclerosiswas performed by Kietselaer et al.88 Imaging was per-formed before carotid surgery and correlated withhistopathology findings. The investigators reportedthat Annexin A5 uptake in carotid lesions correlatedhighly with plaque instability. However, only prelimi-nary data are available and further research in hu-mans is necessary.

Finally, PET imaging with F18-fluorodeoxyglu-cose (FDG) is currently considered to be one of themost promising imaging modalities for the identifica-tion of vulnerable lesions. FDG is a radionuclide trac-

er that competes with glucose for uptake into metaboli-cally active cells, especially macrophages, and enablesquantification via PET. Within carotid artery athero-sclerotic plaques, Rudd et al demonstrated with FDGPET that FDG was taken up by resident macrophagesin atherosclerotic plaque but not by surrounding cellu-lar plaque components.89 The authors suggested thatFDG may be capable of imaging and possibly evenquantification of plaque inflammation. In addition,FDG PET could potentially be used to serially monitorchanges in atherosclerotic plaque macrophage content.In an experimental rabbit model, Worthley and co-workers demonstrated that assessment of progressionand/or regression of macrophage content in athero-sclerotic plaques was feasible using this noninvasivetechnique.90 In addition, Tahara et al showed in 43 pa-tients that FDG PET, co-registered with computed to-mography data, was able to visualize significantly re-duced plaque inflammation following 3 months’ treat-ment with simvastatin.91

However, thus far FDG imaging of the coronaryarteries has been challenging because of cardiac mo-tion, FDG uptake in the myocardium, and the limitedresolution of PET. Co-registration of the functionalimages with high-resolution anatomical data obtainedwith MSCT in combination with dedicated protocolsto suppress myocardial uptake could possibly over-come this limitation (Figure 15).92,93

Summary and conclusions

Plaque rupture followed by coronary occlusion due tothrombosis is responsible for a large number of acute

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Figure 15. Example of the co-registra-tion of functional imaging with F-18fluorodeoxyglucose (FDG) positronemission tomography (PET) andanatomical imaging with multislicecomputed tomography (MSCT). A:On MSCT axial images a non-calcifiedplaque in the left main coronary artery(arrow) was identified. B: Correspond-ing image after fusion with F-18 FDGPET, localizing the inflammatory PETsignal with a maximal standard uptakevalue of 2.1 to the non-calcified plaqueseen in the left coronary artery (ar-row). Reprinted with permission fromAlexanderson et al.92

A B

coronary events. Identification of lesions before theyrupture would allow the initiation of aggressive sys-temic or even local therapy and could potentially im-prove outcome. In the absence of natural history da-ta, the precursor of vulnerable lesions remains largelyunknown and most details have been derived fromretrospective postmortem studies. On the basis ofthese investigations, it has been suggested that themost common substrate for superimposed thrombusformation is the thin-capped fibroatheroma, a plaquewith a large necrotic core and an inflamed thin fi-brous cap (<65 Ìm thick) infiltrated by macrophagesand lymphocytes.

As discussed in the current review, extensive ef-fort has been invested in the development of imagingtools to characterize coronary atherosclerosis, withthe ultimate goal of detecting vulnerable lesions. Tothis end, several techniques are currently under inves-tigation, with each technique having specific advan-tages as well as limitations. Importantly, the clinicalrelevance in terms of predicting outcome and chang-ing management remains to be established for all cur-rently available techniques.

At present, invasive techniques, such as OCT andVH IVUS, provide the most detailed information andare currently employed in prospective natural historystudies. Given their invasive nature, the application ofthese techniques will remain largely restricted tosymptomatic high-risk patients, whereas a noninva-sive technique would allow application on a widerscale. At present, noninvasive approaches cannot pro-vide detailed characterization of the individual vul-nerable coronary plaque. However, direct in vivocomparisons with invasive modalities may substantial-ly improve our understanding and interpretation ofnoninvasive observations. Consequently, this infor-mation may be translated into enhanced strategies forrisk stratification. In addition, the measurements ofplaque vulnerability obtained with either invasive ornoninvasive imaging techniques may be used as a sur-rogate endpoint for prospective anti-atherosclerotictherapy trials.32 Possibly, the combination of imagingtechniques targeting both morphological and func-tional characteristics may be of particular value.

Evidently, large prospective studies are needed tofurther define the potential role of each imaging tech-nique in the identification of vulnerable plaques.Moreover, much uncertainty remains concerning howthese vulnerable lesions should be treated. In addi-tion to an increased intensity of systemic therapy,such as aspirin and statin therapy, local or regional

therapeutic approaches (such as plaque sealing) havealso been suggested. However, no robust data are cur-rently available to support their effectiveness. Poten-tially, imaging techniques may prove of great value inthe development of such individually targeted treat-ment strategies.

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