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Detection of Plaque Neovascularization by Optical Coherence Tomography: Ex Vivo Feasibility Study and In Vivo Observation in Patients With Angina Pectoris
Abstract: Objectives. The purposes of this study were: (1) to assess the feasibility of optical coherence tomography (OCT) for detecting neovascularization; and (2) to clarify the impact of plaque neovascularization on coronary vessel behavior over time. Background. Plaque neovascularization may be related to plaque vulnerability. Methods. In an ex vivo study, a total of 55 coronary plaques from 31 human cadavers were examined by OCT. Plaque neovascularization was diagnosed based on the presence or absence of microchannels (MCs) by OCT. In an in vivo study, we explored 83 major coronary arteries from 42 patients with angina pectoris. A total of 56 coronary plaques were selected from non-culprit (non-stented) lesions with plaque burden >40% by intravascular ultrasound (IVUS). These plaques were classified into two groups based on the presence or absence of MC by OCT. Results. In the ex vivo study, the sensitivity and specificity of OCT to detect plaque neovascularization were 52% and 68%, respectively. In the in vivo study, MC was detected in 25 plaques (44.6%). High-density lipoprotein cholesterol was significantly lower in the MC group vs the non-MC group (42.7 ± 7.8 mg/dL vs 51.6 ± 14.4 mg/dL; P=.02). Percent change in lumen volume index (VI) in the MC group was significantly greater vs the non-MC group (P=.01). Percent change in lumen VI correlated significantly with percent change in external elastic membrane VI (P=.01). Conclusions. OCT was feasible for detecting neovascularization of coronary plaques. Serial OCT examinations revealed that coronary plaques with neovascularization showed greater luminal narrowing as a result of inadequate adaptive vessel remodeling.
J INVASIVE CARDIOL 2016;28(1):17-22
Key words: interventional cardiology, optical coherence tomography
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Plaque neovascularization is known as one of the morphological characteristics of vulnerable plaque.1 Previous studies suggested that neovascularization may promote the influx of lipids and infiltrates of inflammatory cells in coronary plaques.2,3 Other studies reported that newly formed neovascularization has fragility and tendency to break and may mediate intraplaque hemorrhage, eventually resulting in luminal narrowing.2-4
Recently, a high-resolution (10 µm) optical coherence tomography (OCT) intravascular imaging device was approved and used in the clinical setting.5-7 Using this imaging device, neovascularization could be recognized as small black holes, ie, microchannels (MCs).8 It has been reported that MCs detected by OCT were associated with plaque vulnerability and predicted luminal narrowing.9,10 However, the diagnostic accuracy of OCT for detecting coronary plaque neovascularization has not been investigated. Furthermore, the impact of MCs on subsequent coronary vessel behavior is unknown. Therefore, the purposes of this study were: (1) to assess the feasibility of OCT for detecting neovascularization; and (2) to clarify the impact of plaque neovascularization on coronary vessel behavior over time.
Methods
Ex vivo study. In the ex vivo study, a total of 55 coronary plaques from 31 human cadavers were examined by OCT (19 males, 12 females; mean age, 70.8 ± 13.4 years). Segments measuring ~5 cm in length were obtained beginning at the ostial site of the three major coronary arteries harvested at autopsy. Coronary specimens were prepared for OCT imaging as we previously reported.11 The study protocol was approved by the ethics committee of Kawasaki Medical School, and written informed consent was obtained from each family. This study was in compliance with the Declaration of Helsinki with regard to investigations in humans.
OCT and histological examinations. An intravascular OCT catheter (ImageWire; LightLab Imaging) was inserted into the coronary specimen and OCT images were obtained using automatic pullback devices. After OCT pullback imaging, each coronary artery was pressure fixed in 10% neutral buffered formalin. Following fixation for 48 hours, standard paraffin embedding was performed. In every 400 µm of the coronary arteries, 4 µm thick sections were cut and stained with hematoxylin and eosin. Neovascularization was defined as the presence of microvessels, forming a complex network in the coronary plaque, and supplying blood to the vessel wall.12 These neovascularizations were visualized as MCs, which were defined as no-signal tubuloluminal structures ranging from 50-300 µm in diameter in OCT images and recognized on at least 3 consecutive cross-sectional OCT images (Figure 1).10 Comparisons between OCT and histological examination were carried out strictly on corresponding sites, located according to their proximity to neighboring side branches used as anatomical landmarks. The sensitivity and specificity of OCT for detecting the neovascularization was investigated in this ex vivo study.
In vivo study. IVUS and OCT examinations of the 3 epicardial coronary arteries were performed immediately after stent implantation. In the in vivo study, a total of 83 major coronary arteries from 42 patients with angina pectoris were explored. A total of 56 coronary plaques were selected from non-culprit (non-stented) lesions with plaque burden >40% by intravascular ultrasound (IVUS). These plaques were classified into two groups based on the presence or absence of MCs by OCT. In a subset of 24 lesions from 24 patients with serial (baseline and 6-9 month follow-up) IVUS imaging, IVUS parameters including changes in vessel, plaque, and lumen were also compared between the two groups. In patients with multiple plaques at the non-culprit lesion, only a lesion that had the largest plaque burden per patient was selected for serial IVUS analysis. Patients with acute myocardial infarction, chronic total occlusion, left main disease, ostial lesion, heavily calcified lesion, or tortuous lesion were excluded.
IVUS and OCT examinations. The IVUS pullback imaging was performed in a standard fashion using automated motorized pullback (0.5 mm/s) with a commercially available 40 MHz IVUS catheter (Atlantis Pro2; Boston Scientific Corporation). After completion of IVUS imaging, the OCT imaging was also performed using the M2 OCT image wire and an occlusion balloon (LightLab Imaging, Inc) using a motorized pullback system.
Volumetric IVUS measurements of external elastic membrane (EEM), plaque plus media (P+M), and lumen in lesions with or without MC assessed by OCT were performed using echoPlaque PC-based software (Indec Systems, Inc), as previously described.13,14 Each volume was divided by the total length (10 mm) of the analyzed segment to average each parameter (volume index [VI], in mm3/mm). Percent change in each parameter during follow-up was calculated as: [%Δ VI = 100 × (VI at follow-up – VI at baseline) / (VI at baseline)]. Laboratory data were obtained 24 hours before the index procedures.
Statistical analysis. Data are presented as mean ± standard deviation for continuous variables and as frequency and percentage for categorical variables. The intraobserver and interobserver agreement for detecting the neovascularization using OCT was quantified by the κ test of concordance.15 In general, Student’s t-test was used to compare continuous variables, and the Chi2 test or Fisher’s exact test was used to compare categorical variables. The association between EEM VI and lumen VI, and P+M VI and lumen VI were investigated with linear regression analysis. Statistical analysis was performed with SPSS version 17.0 for Windows (SPSS, Inc) and P<.05 was considered statistically significant.
Results
Ex vivo study. In the ex vivo study, 6 of the 31 cadavers had a history of ischemic heart disease. A total of 55 plaques were included in the ex vivo study, and 21 plaques (38%) had neovascularization. On the other hand, OCT detected MCs in 11 plaques (20%). The sensitivity and specificity of OCT for detecting the neovascularization were 52% and 68%, respectively. Representative images of neovascularization of coronary plaque by histological examination and OCT are shown in Figure 1. The intraobserver and interobserver agreement for detecting the neovascularization using OCT was high (κ = 0.818 and κ = 0.749, respectively).
In vivo study. In the in vivo study, coronary plaques were classified into two groups based on the presence or absence of MC in OCT images (Figure 2). The clinical characteristics and medications are shown in Tables 1 and 2. There were no significant differences in clinical characteristics and medications between the two groups. Statins were similarly prescribed in both groups. Laboratory data are shown in Table 3. Although serum low-density lipoprotein (LDL) cholesterol level was similar, high-density lipoprotein (HDL) cholesterol was significantly lower in the MC group vs the non-MC group. Quantitative coronary angiography findings were shown in Table 4. There were no significant differences in quantitative coronary angiography findings between the two groups.
In a subset of patients with serial IVUS imaging, there were again no significant differences in clinical characteristics and medications between the two groups. Serial IVUS assessments of the MC and non-MC groups are shown in Table 5. There were no significant differences in EEM VI, and P+M VI as well as lumen VI at baseline between the MC and non-MC groups. EEM VI and P+M VI did not change during follow-up in either group (MC group P=.32 and P=.40; non-MC group P=.93 and P=.98, respectively). Although lumen VI in the non-MC group did not change during follow-up, lumen VI in the MC group decreased significantly (MC group P=.03; non-MC group P=.94, respectively). As a result, percent change in lumen VI was significantly greater in the MC group vs the non-MC group (P=.01). As shown in Figure 3, serial change in lumen VI did not correlate with serial change in P+M VI, whereas serial change in VI positively and significantly correlated with serial change in EEM VI, suggesting that a lack of adaptive vessel remodeling, rather than plaque accumulation, contributed to luminal narrowing. Serial IVUS and OCT images of a case with coronary plaque with MC are shown in Figure 4.
Discussion
The main findings of this study were: (1) OCT reasonably detected plaque neovascularization as MCs, with moderate sensitivity and specificity; and (2) MCs were observed in about one-half of non-culprit coronary lesions in angina patients. Those with MCs had lower HDL cholesterol levels vs those without MCs; and (3) the lumen VI assessed by IVUS in the MC group was significantly decreased during follow-up period vs the non-MC group. The change of the lumen VI was significantly correlated with the change of the EEM VI.
The precursor of plaque rupture resulting in acute coronary syndrome is known to be vulnerable plaque, which is characterized by neovascularization, thin-cap fibroatheroma (fibrous cap thickness <65 µm), large lipid core, and macrophage.1,4,16 Neovascularization of coronary plaque (one of the characteristics of vulnerable plaque) could not be visualized in vivo due to lack of high-resolution imaging modality. Recently, a high-resolution (10 µm) OCT intravascular imaging device was approved and used in the clinical setting.5 Using OCT, neovascularization could be visualized in the previous case report.8 However, to the best of our knowledge, the accuracy of detection of neovascularization with OCT has not yet been validated. Our study was the first to evaluate the feasibility of OCT for detecting neovascularization of coronary plaques.
A previous study revealed that culprit plaques with neovascularization had vulnerable features, such as thinner fibrous cap, greater lipid arc, longer lipid core length, and more frequent thin-cap fibroatheroma vs those without neovascularization in patients with unstable angina pectoris.9 In the present in vivo study, HDL cholesterol was significantly lower in the MC group vs the non-MC group, although statin use was similar in both groups. These findings from the previous report, as well as our data, suggested that neovascularization was associated with not only vulnerable plaque, but also vulnerable patients who had dyslipidemia. However, less is known about the natural history of coronary plaque with neovascularization in vivo. Vulnerability of the coronary plaque with neovascularization is thought to be due to intraplaque hemorrhage from immature and leaky neovascularization, eventually resulting in luminal narrowing or occlusion.4,17,18 In fact, Uemura et al reported that neovascularization was one of the predictors of subsequent luminal narrowing in patients with coronary artery disease (odds ratio, 20.0; P<.01).10 These findings of an angiographic study were compatible with our in vivo serial study data. Luminal narrowing might occur together with increasing P+M VI because of intraplaque hemorrhage. However, our data demonstrated that P+M VI in the MC group was not increased significantly vs the non-MC group. A previous study reported that vulnerable plaque, such as thin-cap fibroatheroma detected and defined by radiofrequency IVUS, caused a major adverse cardiovascular event in 4.9% at a median follow-up of 3.4 years.19 In our in vivo serial study, neovascularization was observed in 48% of the plaques. Considering that this discrepancy between low major adverse cardiovascular event rates arises from vulnerable plaque and a high incidence of coronary plaque with neovascularization, acute coronary events with massive intraplaque hemorrhage might not have occurred during the limited follow-up period in our study, and in fact, no cardiac event related to the study lesion was observed. A possible mechanism for the decreasing lumen VI without increasing P+M VI during our follow-up period was a lack of adaptive vessel remodeling. In the present study, the change of the lumen VI was positively correlated with the change of EEM VI. According to a study by Glagov et al,20 luminal narrowing might be delayed until the lesion has >40% plaque burden due to adaptive vessel remodeling, which was recognized as the compensatory enlargement of vessel area preserving lumen area. We enrolled and analyzed lesions with >40% plaque burden at baseline. Therefore, adaptive vessel remodeling might be diminished, and coronary plaques with neovascularization showed greater luminal narrowing as a result of inadequate vessel remodeling. Further studies are needed to clarify the long-term impact of plaque neovascularization on coronary vessel behavior.
There were two different origins of neovascularization: (1) internal neovascularization (originating directly from the main lumen); and (2) external neovascularization (originating from a major coronary branch).21 The majority of neovascularizations arise from the adventitia as external neovascularization and penetrate only the outer side of the coronary plaque.22,23 As coronary plaque volume increases in the setting of coronary artery disease, proliferation of neovascularization at the inner side of the coronary plaque is observed. OCT could visualize only the inner side of the coronary plaque due to the limited depth of OCT penetration; thus, information on the neovascularization at the outer side of the coronary plaque might be missed. Indeed, in our ex vivo study, OCT examination of neovascularization in coronary plaque was feasible, but had low sensitivity for the detection of neovascularization. A time-domain OCT system was used for OCT imaging in both present studies. The penetration of frequency-domain OCT (the so-called second-generation OCT system) has improved, and thus, the detection of neovascularization using a frequency-domain OCT system might be more accurate compared with a time-domain OCT system.24
In the present study, we assessed the clinical impact of neovascularization of coronary plaque, which is one of the characteristics of vulnerable plaque. OCT can offer unique insights into vulnerable plaque characterized as not only neovascularized, but also thin-cap fibroatheroma (fibrous cap thickness <65 µm), large lipid core, and macrophage.16,25,26 If a large number of lesions were investigated in vivo, OCT could provide a comprehensive assessment of plaque vulnerability. In other words, complex characteristics of vulnerable plaque assessed by OCT in vivo might be useful to detect truly “vulnerable” plaque that cause thrombotic events in the future with a high probability. In addition, MCs evaluated by OCT might be a useful surrogate marker of plaque vulnerability, and thus the effect of a medically therapeutic agent (such as the effect of a statin on plaque stability) could be assessed in vivo.
Study limitations. This was a retrospective study with a small sample size. Because of the limited number of patients in the in vivo study, a comparison of OCT findings between patients with stable vs unstable angina pectoris was not performed.
Conclusion
OCT was feasible for detecting neovascularization in coronary plaques. Serial OCT examinations revealed that coronary plaques with neovascularization showed greater luminal narrowing as a result of inadequate adaptive vessel remodeling.
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From the 1Department of Cardiology, Kawasaki Medical School, Kurashiki, Japan; and 2First Department of Internal Medicine, Nara Medical University, Kashihara, Japan.
Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. The authors report no conflicts of interest regarding the content herein.
Manuscript submitted May 11, 2015, provisional acceptance given June 10, 2015, final version accepted June 24, 2015.
Address for correspondence: Shiro Uemura, MD, PhD, Department of Cardiology, Kawasaki Medical School, 577 Matsushima, Kurashiki, 701-0192, Japan. Email: suemura@med.kawasaki-m.ac.jp
