Clinical Applications and Methods of Intravascular Imaging of Atherosclerosis

Mailing Address: Leonardo Silva Roever Borges Rua Rafael Rinaldi, 431 – Martins. Postal Code: 38400-384 – Uberlândia, MG – Brazil E-mail: leonardoroever@hotmail.com Clinical Applications and Methods of Intravascular Imaging of Atherosclerosis Leonardo Silva Roever Borges1 e Cláudio Tinoco Mesquita2 Universidade Federal de Uberlândia – Departamento de Pesquisa Clínica1 – Uberlândia, MG, Universidade Federal Fluminense – Hospital Universitário Antônio Pedro – Departamento de Medicina Clínica2 – Niterói, RJ – Brazil


Introduction
Coronary angiography has been the gold standard technique for the evaluation of coronary artery disease.[12][13] The characteristics of atherosclerotic plaques are responsible for the form of presentation of clinical events.Therefore, it is necessary to better understand its histology and morphology, as well as its evolutionary lesions, as described below: [14][15][16] • Type I lesions (initial): these are characterized by the presence of the first detectable lipid deposits in the intima and the cellular reactions associated with them.The initial histological changes at this stage are minimal, with small groups of macrophages containing lipid droplets known as foam cells; • Type II lesions include fatty streaks.Microscopically, they consist of laminated layers of foam cells attached to smooth muscle cells with fat deposits.
T lymphocytes can also be found in this type of lesion, but to a lesser extent than the macrophages;

Vascular remodeling in atherosclerosis
Vascular remodeling of the coronary artery implies geometric changes in vessel dimensions and evolves with the progression and regression of the atherosclerotic process.[19][20][21][22] Alternatively, the vascular remodeling can be classified as adaptive (compensatory, suitable for hemodynamic stimulus), or maladaptive (inadequate dysfunctional).The direction and scale of remodeling are coordinated by the production of endothelial growth factors, proteases and cell adhesion molecules in response to the changes detected in blood flow.[25]

Imaging modalities of coronary atherosclerosis
There is great advancement in imaging modalities of coronary arteries.This study provides a more accurate assessment of coronary atherosclerosis, as well as scientific advances in the pathophysiology, prevention and treatment. 26e intimal layer of the arterial wall is between the endothelium and internal elastic lamina and has, in its borders, a subendothelium formed by smooth muscle cells and fibroblasts arranged in a matrix of connective tissue.The middle layer is composed of smooth muscle cells arranged in a matrix with a small amount of elastic and collagen fibers with an average thickness of 200 μm and separated from the adventitia by the external elastic lamina.The thickness of the adventitia, the outermost layer of the arterial wall, ranges from 300-500 μm and is composed of fibrous tissue (collagen and elastin) and incorporates the vasa vasorum, nerves and lymph vessels.
This manuscript describes the main types of intravascular imaging of coronary atherosclerosis, as well as the comparison between intravascular ultrasound (IVUS) and optical coherence tomography (OCT) and their applications 27 (Chart 1).

-Intravascular Ultrasound (IVUS)
Intracoronary ultrasound is the intravascular imaging most used today.This modality requires inserting a catheter with a transducer at its tip, which emits an ultrasound signal perpendicular to its axis with a frequency of 20-70 MHz.
Detection of the intimal layer on intravascular ultrasound depends on its thickness.The minimum measurement of 160 μm is required for its definition.The thickness of the intimal layer increases with age, despite the absence of atherosclerotic lesions.Reflections of the signals emitted are received by the transducer and analyzed to generate cross-sectional images, which enables the identification of luminal borders and media and adventitia layer.It is also possible to evaluate the atherosclerotic plaque load and characterize its composition. 28,29e IVUS ability to detect the composition of the plaque surface is moderate according to studies based on the histology of atherosclerotic plaque. 30Recent studies are skeptical about the reliability of evaluating the type of plaque, especially in segments with stent and calcium deposits 31,32 as well as significant limitations of IVUS that do not allow the detection of characteristics associated with increased risk of plaque rupture (microcalcifications, neovascularization, plaque erosion, etc.). 33• accurate evaluation of the lesion length;

Chart 1 IVUS and OCT and their applications
• accurate evaluation of the vessel size; • better evaluation for direct stent implant. -Post-intervention: • accurate evaluation of the apposition of stent struts; • avoid gaps (implanting > 2 stents).
-Allows better understanding the various stages of atherosclerotic disease and vascular response to treatment.
-Detects arterial structures and helps determining different histological constituents.
- -Limits the study of the microstructure, resulting in a sensitivity of only 37% for the detection of plaque rupture.
-Its low axial penetration (1.5-2.0 mm) does not provide optimal view of the arterial wall, especially in large vessels, in which the outer layers of the artery cannot be identified.

Review Article
The main indications for the use of intravascular ultrasound are: a) evaluation of moderate coronary lesions; b) evaluation of ambiguous lesions in the left main coronary artery; c) detection of unstable plaques; d) the guide method in the implant of coronary stents (bare-metal and/or drug-eluting stents).Figure 1 shows the image of a normal coronary artery by the IVUS. 34Chart 2 shows the differences in the IVUS modes of operation.

IVUS with virtual histology
[37] Based on the percentage composition of the plaque and the location of necrotic and calcium contents in relation to the vascular lumen, the following classification of lesions is obtained by virtual histology: [38][39] • fibrotic plaque: predominant fibrotic tissue without confluent areas of necrotic or calcium tissue; IVUS image showing the trilaminate appearance of the normal coronary artery wall.Source: adapted from Dash et al. 34 IVUS: intravascular ultrasound.
• calcified fibroatheroma: presence of calcium confluent areas (> 10% of the plaque area percentage) in three or more consecutive sections; • fibroatheroma: presence of confluent areas of necrotic tissue (> 10% of the percentage of plaque area) in three or more consecutive sections; • thin cap fibroatheroma (TCFA): presence of confluent areas of necrotic tissue (> 10% of the percentage of plaque area) in three or more consecutive sections and in direct contact with the lumen.
Figure 2 represents the morphology of the atheroma plaque. 40Figure 3 shows the types of images that can be viewed from the atherosclerotic plaque 41 and Figure 4 shows the types of plaques. 42

-Optical coherence tomography (OCT)
5][46] Its imaging limitations are: incapacity to penetrate the lipid-rich cores and tissue penetration (interval: 2-3 mm), which repeatedly inhibits the image of the entire atheroma plaque; limitations in portraying the longitudinal 3D morphology of the plaque on the artery.Figure 4 shows coronary OCT image.Chart 3 describes the particularities of the OCT in different types of plaque.

-Magnetic resonance intravascular spectroscopy
Magnetic resonance intravascular spectroscopy involves advancing the catheter with a magnetic resonance probe at its tip, which creates a field of vision with a 60 o radial sector, which allows the identification of the lipid component into two zones: superficial (0-100 μm) and deep (100-250 μm).[46][47][48]

-Intravascular magnetic resonance imaging
Intravascular magnetic resonance imaging shows the plaque behind the calcified tissue.Its limitations include the noise introduced by the movements, the heat generated during the imaging and increased time required to acquire the images, which would limit its application in humans.Recent advances in intravascular magnetic resonance imaging enabled high-resolution (80 μm) real-time study of the in vivo image. 49

-Intravascular photoacoustics (IP)
Intravascular photoacoustic imaging allows identifying many plaque components and is able to differentiate plaques rich in fibrous tissue lipids and detect the presence of neoangiogenesis. 50,51In addition, -Slightly easier to set up.
-Simultaneous display of blood flow in color, which facilitates the distinction between lumen and wall limits.-Artifacts and image can be problematic, immediately adjacent to the transducer.
-It is necessary to use the closer field ring down the subtraction.
markers can be used to detect cells or molecules that are involved in atherosclerosis and inflammation process (macrophages, metalloproteinases, selectins). 52,53e significant advantages of IP imaging are the ability to view the stent morphology and its high penetration and lateral and axial resolution which allow a more detailed and thorough assessment of the vessel wall. 54igure 5 shows a coronary IP image. 55

-Infrared fluorescence (IRF)
Infrared fluorescence imaging is a modality that is rapidly evolving in the study of atherosclerosis.This method is based on the injection of agents that have the ability to attach molecules and cause fluorescence when viewed with infrared light.advances in molecular biology have allowed the development of several markers, such as thrombin, metalloproteinases 2 and 9, cathepsin K, D and S. [57][58][59][60][61][62] A number of in vitro and in vivo studies were tested for the feasibility and accuracy of IRF, providing evidence that it can be used to study the plaque biology, inflammation and neovascularization. 63,64luorescence spectroscopy (FS) Fluorescence spectroscopy is based on the evaluation of the time needed to resolve the fluorescence emitted after the molecules have been excited by light.Several experimental studies have shown that FS is able to discriminate between different degrees of atherosclerotic lesions and detect the presence of macrophages.

-Infrared spectroscopy (IS)
Infrared spectroscopy allows evaluating the chemical composition of the plaque and the lipid component.The IS ability to detect lipid-rich plaques was evaluated and compared to histology.The result was reliable in relation to the lipid component in 83% of the vessels studied. 69,70e IS allows identifying the superficial plaque (cap thickness < 450 μm) and the width of the lipid cores (circumferential measurement > 60 o , plaque thickness > 200 μm) and detecting lipid-rich plaques located behind calcified deposits. 71,72Its limitations include the inability to view the lumen and the outer wall of the vessel to quantify the atheroma load and retract plaque characteristics associated with increased vulnerability, such as fibrous cap thickness, integrity, the presence of thrombi and neovascularization.

-Raman spectroscopy (RS)
Raman spectroscopy involves the scattering of light through the molecules. 43[75][76] RS allows quantifying the effect of treatment on the composition of the plaque surface and identifying vulnerable plaques with high sensitivity and specificity (79% and 85%, respectively). 77Its limitations are the inability to show the morphology of the vessel wall lumen and the plaque and to measure their dimensions.
In summary, advances in intravascular imaging modalities greatly contributes to the evaluation of atherosclerosis, enabling high-resolution evaluation of plaque characteristics with better characterization and quantification of atherosclerosis.Randomized controlled trials with large numbers of patients should be conducted to better understand the real impact of their use in terms of prevention and treatment.
of stenosis severity -Evaluation of atherosclerotic plaque -Evaluation of stent coverage and position -PCI guide Advantages -Pre-intervention: Distinguishes different degrees of atherosclerotic changes and various types of plaques.-Allows extraordinary evaluation of the characteristics and thickness of the fibrous cap.-Evaluates the neointimal coverage, tissue patterns for stent strut and apposition.Disadvantages -Invasive test.
quality and easier to interpret.

Traps-
Requires saline washing to provide a pathway for the ultrasound beam fluid (air bubbles may degrade image quality).

Figure 3
Figure 3 Intravascular imaging of atherosclerosis From the radio frequency backscattering data, different types of information can be seen: (1) Virtual histology, (2) retinography, (3) integrated backscattered image with intravascular ultrasound (4) iMAP.Virtual histology can detect four types of tissues: necrotic core, fibrous tissue, fibrofatty tissue and dense calcium tissue.Plaque strain on retinography is reported in strain values, which are further classified into four categories according to the Rotterdam classification.The tissues characterized by integrated backscattered image with intravascular ultrasound are the lipid, fibrous and calcified tissues; iMAP detects the fibrous, lipid, necrotic and calcified tissues.Source: adapted from García-García et al.41

Figure 4 Figure 5
Figure 4 Measurement of the fibrous cap.Sample containing atherosclerotic plaque with different fibrous cap thicknesses.A and B: thin fibrous cap, measuring 40 μm.C and D: thick fibrous cap, measuring 250 μm.Source: adapted from Bezerra et al.42