Intravascular ultrasound, optical coherence tomography, and angioscopy of coronary circulation

INTRODUCTION — Invasive, radiograph coronary angiography using contrast media at the time of cardiac catheterization is the preferred diagnostic test when information regarding the extent and severity of atherosclerotic narrowing in the coronary circulation is needed. Decisions regarding the need for revascularization are based on information obtained from this procedure, as well as other clinical and noninvasive data. However, subjective interpretation of radiographic coronary angiography has observer bias and interobserver variability, which led to the development of quantitative coronary angiography.

Coronary angiography is also limited by:

●Technical limitations such as the occasional inability to optimally visualize a particular location.

●Providing information only about the contour of the vascular lumen. The components of the vascular wall are not visualized.

The intravascular imaging techniques of optical coherence tomography (OCT), intravascular ultrasound (IVUS), and coronary angioscopy (CA) provide information above and beyond that provided by radiographic coronary angiography (table 1).

This topic will discuss the technical aspects, the derived information, and the clinical applications of OCT, IVUS, and CA. An attempt will be made to provide guidance on when each procedure should be considered.

Arterial wall structure — In order to understand these imaging techniques, the interpreter must first be familiar with normal arterial wall structure:

●The intima is the inner layer that is in direct contact with the intraluminal space. The intima is normally only one to two cell layers thick, but can greatly thicken with the formation and growth of atherosclerotic plaque. It is separated from the media by the internal elastic lamina.

●The media is the middle layer of the arterial wall, and consists predominantly of homogenous layers of smooth muscle cells, which regulate vascular tone. It is separated from the adventitia by the external elastic lamina.

●The adventitia surrounds the media and is composed of fibrous connective tissue adding external support for the vessel.

INTRAVASCULAR ULTRASOUND — Intravascular ultrasound (IVUS) allows visualization of the coronary arterial wall by utilizing a miniature transducer at the end of a flexible catheter that emits ultrasound in the 10 to 40 MHz range. IVUS has become particularly useful in delineating plaque morphology and distribution, and providing a rationale to guide transcatheter coronary interventions [1].

Technology — There are two types of IVUS catheters in use today, mechanical and phased array (figure 1).

The most common IVUS catheter utilizes a single mechanical transducer mounted at the catheter tip and oriented orthogonal to the catheter major axis. The transducer quickly rotates to visualize the entire vessel in cross-section. The mechanical transducer has the advantage of a simple design with less complicated technology and a greater signal-to-power output. Thus, the image quality tends to be better with an overall resolution (with current 40 MHZ transducers) of 100 to 150 micrometers. Second generation "HD" or high density transducers use a higher frequency and/or dual transducers to achieve greater resolution. However, higher frequency transducers have poorer penetration and higher reflectivity from the blood, limiting their clinical application.

The disadvantage of mechanical transducers is the central drive shaft that decreases flexibility and prevents the concurrent use of a central guidewire. These catheters may therefore be more difficult to use in tortuous vessels.

Phased array catheters use multiple transducer elements permanently mounted along the circumference of the catheter tip. Each element sends and receives ultrasound from a sector; multiple sectors are incorporated to produce a cross-sectional image of the vessel. Phase array catheters, however, require complex programming and have suffered in image quality. Nevertheless, with the advent of higher speed computers, sophisticated software, and additional elements mounted on the catheter tip, the phased array images have made great strides within the last few years and are producing images competitive with those from mechanical transducers.

Combination IVUS – NIR (Near Infra-red spectroscopy) catheters add the NIR ability to differentiate tissue type (ie, a lipid-rich plaque) capabilities to the imaging advantages of IVUS. (See 'IVUS clinical applications' below.)

Most disposable IVUS transducers are oriented perpendicular to the length of the catheter; as a result, IVUS images are displayed as cross-sectional views of the coronary artery (figure 2A-B and image 1A-B).

The IVUS image — Ultrasound transmitted from the transducer will "bounce" back whenever it encounters an interface of different acoustic impedance. Acoustic impedance is primarily dependent upon the density of the tissue. Therefore, ultrasound emitted from the transducer will traverse the blood with minimal reflection but will be highly reflected when it meets the intima.

The reflected ultrasound from the intima is displayed as a single concentric echo (image 1A-B). All of the ultrasound, however, is not reflected by the intima; some will penetrate through to the media. Since the media is composed primarily of homogeneous smooth muscle cells, ultrasound passes through with minimal reflection and appears as a dark zone devoid of echoes.

The next layer, the adventitia, is highly reflective because it has numerous collagen fibers laid down in parallel, thereby producing multiple interfaces from which to reflect sound. The adventitia will therefore appear very bright.

As a result, the normal coronary artery wall produces alternating bright and dark echoes:

●A bright echo from the intima

●A dark zone from the media

●Multiple bright echoes from the adventitia

This pattern is called the normal "three-layer appearance" of a coronary artery [2,3]. The three-layer appearance is actually a simplified view since the IVUS resolution (approximately 120 micrometers) is not sufficient to detect the truly nondiseased intima (one or two cell layers thick or approximately 50 micrometers). Nonetheless, the intima is considered essentially normal if there is only a single thin concentric echo within the media. As the atherosclerotic plaque accumulates, the intimal zone will thicken on the IVUS image (image 1B and image 2). The anechoic (lack of echoes) media outlines the size a blood vessel would be if there were no intimal disease.

Ultrasonic image of an atherosclerotic plaque — Upon viewing IVUS images, plaque extent, morphology, and distribution become immediately apparent [1,2]. The tomographic cross-sectional view of the artery is ideal to discern concentric from eccentric plaque distribution. This feature makes IVUS far more accurate than angiography for assessing plaque eccentricity [1].

Another aspect of plaque distribution is the visualization of small splits in the plaque that occur spontaneously or after a coronary intervention. Plaque fissures historically were an important part of the balloon angioplasty mechanism for luminal enlargement and were associated with a favorable outcome (image 3). Small plaque fissures differ from large dissections, which extend to the media and can occur at the stent edge, and are associated with adverse outcomes (image 3).

Information about plaque composition is also available from IVUS, since denser material (calcium) will reflect more ultrasound and appear brighter on the video monitor [1]. Calcium is so dense that none of the ultrasound waves penetrate to deeper tissues, producing an acoustic shadow, which is a hallmark of calcification. Alternatively, a plaque that is less echodense than the collagen-rich adventitia is described as a "soft" plaque. This is the classic approach of using grey scale images to determine plaque composition. Plaque composition influences interventional approach; as an example, rotablator is particularly effective against calcified plaques. (See "Specialized revascularization devices in the management of coronary heart disease".)

Besides the classic grey scale interpretation of plaque characteristics, there has been active investigation attempting to get more precise plaque composition information from IVUS images. Kawasaki and colleagues have published several studies whereby they analyze the integrated backscatter (IB) signal from the radiofrequency (RF) signal of ultrasound and, based on the IB IVUS image, color code the plaque for different plaque compositions (figure 3) [4,5]. This technique has undergone pathologic validation and has shown changes in plaque composition with aggressive statin therapy.

Another approach has been to look both at the amount of returning RF signal and how that RF signal was distorted by the plaque. These changes are then put through an algorithm established from known plaque tissue types to determine the plaque composition [6]. Again, the final interpretation is color coded and superimposed onto the grey scale image. This technique has been commercialized and trademarked as "Virtual Histology." This technique is currently under investigation in different patient populations and in natural history studies.

Because of its high resolution, IVUS can precisely determine vessel and plaque size (image 2) [7]. Angiography permits measurement of luminal diameters typically in two or three orthogonal views; in contrast, IVUS provides a tomographic view (180 potential diameters) and therefore a more accurate assessment of size. Thus, the true minimal and maximal luminal diameter can be obtained with IVUS along with cross-sectional area measurements of both the lumen and vessel. IVUS has also provided information that the cross-sectional area of the plaque remains constant during vasomotion of the coronary artery [8].

Prior to a discussion of the applications of IVUS, it is important to appreciate the added information provided by this procedure when compared with conventional radiographic coronary angiography for the assessment of coronary artery disease (table 1).

Males versus females — Females have a higher mortality than males after coronary interventions; it is uncertain if this is due to qualitative differences in atherosclerosis. One study of patients with chronic angina compared IVUS findings in 169 females to those in 549 males and reported that when corrected for body surface area, females had similar external elastic membrane and luminal cross sectional areas and similar plaque burden, eccentricity, and calcium density [9]. However, these assessments were done in a variety of different vessels. When only the proximal left anterior descending or left main coronary arteries were studied, females tended to have a smaller lumen and external elastic membrane (media size) [10]. This finding has been corroborated in an IVUS study of heart transplants in which coronary arteries from a female donor became larger when placed into a male recipient [11]. There appears to be a sex-specific factor that alters coronary artery size, independent of body or heart size.

Another study found morphologic plaque differences between males and females. Males had denser, more calcified plaques than females, suggesting an earlier onset of plaque development [12].

Comparison with coronary angiography — The assessment of coronary arterial lesions is traditionally performed using radiographic coronary angiography (table 1). With radiographic coronary angiography (table 1), radiopaque contrast material is injected into the coronary artery and the lumen is imaged since the lumen appears dense to radiographs, while the arterial wall is opaque. The lumen "shadow-o-gram" is a fairly imprecise measure of luminal morphology and size (with an optimal resolution of approximately 0.3 mm) (figure 4) [13]. Because of the limitations of angiography, hazy angiographic sites could represent an irregular plaque/distorted lumen, a napkin-ring lesion, thrombus, or a dissection. IVUS is particularly useful in this situation because it immediately distinguishes between plaque and lumen irregularities, dissection, or discrete stenosis (image 3) [14].

IVUS has also demonstrated that apparently normal areas by angiography are often markedly abnormal (image 4). This is particularly true for smaller coronary arteries. In one study, for example, IVUS detected calcium in 73 percent of lesions in arteries with an angiographic reference lumen diameter <2 mm; angiography had a sensitivity of only 39 percent for detecting calcium in these vessels [15].

Angiography is often limited in its ability to assess the extent of atherosclerotic disease because of [16]:

●The diffuse nature of atherosclerosis

●Complex luminal shapes

●Compensatory enlargement

Diffuse lesions — "Normal" references by angiography may have as much as one-third of their cross-sectional area filled with plaque as determined by IVUS [17,18]. If the diffuse plaque is evenly distributed throughout the blood vessel without a focal encroachment on the lumen, it will not be detected by angiography (image 4 and image 5).

Thus, IVUS has changed our perception of an angiographic stenosis. We no longer assume that the nonstenotic region surrounding a discrete stenosis is normal. If the reference segment is also filled with plaque, the plaque burden at the stenosis would be more severe than angiographically apparent.

Complex luminal shapes — Experienced angiographers know that a stenosis with different severities in multiple views is an eccentric lesion. However, highly irregular lesions may not be fully appreciated in multiple views. Tomographic images may be needed to display the true luminal shape for very irregular plaques.

Compensatory enlargement — An artery is a living structure that can change and adapt. A pathologic study found that coronary arteries will enlarge to accommodate focal plaque deposition in an attempt to maintain luminal integrity [19]. Since successful compensatory enlargement will preserve the luminal contour, there will be no angiographic stenosis (since the angiogram cannot see beyond the lumen) despite the deposition of a significant plaque (figure 5). On the other hand, failure to compensate can lead to a greater degree of luminal stenosis [20]. Such lesions may also have a smaller minimum luminal and reference diameters after directional coronary atherectomy compared to those with compensatory enlargement [21].

IVUS can detect both the vessel expansion (enlargement in the media to media diameter) and the focal plaque burden. As an example, one IVUS study of 65 patients reported that three patterns accounted for 89 percent of all atherosclerotic arterial segments [22]:

●Concentric plaque with a circular lumen and a circular external elastic lamina

●Eccentric plaque with a circular lumen and an oval external elastic lamina

●Eccentric plaque with an oval lumen and a circular external elastic lamina

A circular lumen was preserved in 66 percent of all atherosclerotic arterial segments and failure to maintain a circular lumen is possibly associated with the development of a stenotic lesion at a specific site on the artery.

The presence or absence of compensatory enlargement also can determine the mechanism of response to angioplasty. Coronary arteries with compensatory enlargement show an improvement in lumen area, primarily through a reduction in plaque size without a change in vessel area; in contrast, arteries without compensatory enlargement achieve their gain in lumen area by arterial stretch without a change in plaque size [23].

IVUS clinical applications — The clinical applications of IVUS, such as optimal stent placement, clarifying the severity of intermediate coronary stenoses seen on coronary angiography, and detecting cardiac allograft vasculopathy are discussed separately in the appropriate topic reviews. (see "Clinical use of coronary artery pressure flow measurements" and "Heart transplantation: Clinical manifestations, diagnosis, and prognosis of cardiac allograft vasculopathy", section on 'Intravascular ultrasound' and "Percutaneous coronary intervention with intracoronary stents: Overview").

Near-infrared spectroscopy (NIRS) IVUS is capable of identifying lipid-rich plaques that are associated with an increased risk of subsequent rupture and the development of acute coronary syndromes (ACS). The routine use of this technology to identify nonobstructive lesions at increased risk was evaluated in a study of 1563 patients with suspected coronary artery disease who underwent radiographic coronary angiography with possible ad hoc percutaneous coronary intervention [24]. The rate of adverse events attributable to the procedure was low. The two-year incidence of non-culprit major adverse cardiovascular events was 9 percent, and NIRS IVUS identified patients and non-culprit segments at high risk for future events. It is approved by the US Food and Drug Administration and is available for clinical use. Further study is needed before we recommend routine use of this invasive technology. (See 'Technology' above and "Mechanisms of acute coronary syndromes related to atherosclerosis", section on 'Formation and progression of atherosclerotic plaques' and "Mechanisms of acute coronary syndromes related to atherosclerosis", section on 'Vulnerable plaques and future rupture'.)

OPTICAL COHERENCE TOMOGRAPHY — Optical coherence tomography (OCT) relies on the backscattering (reflection) of light (in the 1300 nm range) wavelength to obtain cross-sectional images of tissue. When applied to the coronary circulation, OCT has a 10-fold higher image resolution than intravascular ultrasound (IVUS). This allows imaging of the coronary artery in great detail and potentially providing unique insights into coronary artery pathophysiology.

These advantages render OCT a recognized research tool for the assessment of atherosclerosis and useful tool for assessment of the adequacy of placement of coronary stents. OCT can be used for indications similar to the ones established for intravascular ultrasound [25] and can also be used for clinical decision making in patients with acute coronary syndrome [26].

Technology

Imaging principle — The physics of OCT (table 1) is analogous to pulse-echo ultrasound imaging, but light is used rather than sound to create the image [27]. Whereas ultrasound produces images from backscattered (reflected) sound "echoes," OCT uses infrared light waves (approximately 1300 nm wavelength) that reflect off the internal microstructure within the biological tissues.

Both the bandwidth of the infrared light used and the wave velocity are orders of magnitude higher than in medical ultrasound. The resulting resolution is one order of magnitude larger than that of IVUS: The axial resolution of OCT is about 15 μm; the lateral resolution is approximately 25 microm. However, the imaging depth of approximately 1 to 1.5 mm within the coronary artery wall is less than that of IVUS.

Analogous to ultrasound imaging, the echo time delay between emission and receipt of the light is used to generate spatial image information, the intensity of the received (reflected or backscattered) light is translated into a (false) color scale. As the speed of light is much faster than that of sound, an interferometer is required to measure the backscattered light [28]. The interferometer splits the light source into two “arms,” a reference arm and a sample arm, which is directed into the tissue. The light from both arms is recombined at a detector, which registers the so-called interferogram, the sum of reference and sample arm fields. Because of the large source bandwidth, the interferogram is non-zero only if the sample and reference arms are of equal length, within a small window equal to the coherence length of the light source.

Time domain OCT — First generation OCT used for intracoronary imaging employed “time-domain” technology. Relatively slow data acquisition and the need to clear the artery from blood during image acquisition resulted in a complex imaging procedure, limited its use.

Fourier domain OCT — Since 2008, a new generation of OCT systems (also called Fourier domain OCT systems) have been available for widespread clinical use. With these systems, the interferogram is detected as a function of wavelength, either by using a broadband source as in the time domain systems, and spectrally resolved detection, or alternatively by incorporating a novel wavelength-swept laser source [29,30]. This latter technique is also called “swept-source OCT,” or optical frequency domain imaging.

From the signal received in one wavelength sweep, the depth profile can be constructed by the Fourier transform operation. Most signals can be thought of as a summation of sine waves with different frequencies. The Fourier transform extracts those frequencies, and their relative weights, from the signal. The source wavelength in Fourier-domain OCT can be swept at a much higher rate than the position scan of the reference arm mirror in a time-domain OCT system. This development has led to faster image acquisition speeds, with greater penetration depth, without loss of vital detail or resolution, and represents a great advancement on current conventional OCT systems. Coronary arteries can be imaged with high OCT catheter pullback speeds within seconds, which allows for widespread clinical use in a broad range of patients and lesions [31].

OCT imaging technique

OCT imaging catheters and imaging procedure — OCT imaging catheters contain an OCT imaging core at their distal tip (picture 1). Similar to IVUS, the imaging core is oriented perpendicular to the length of the catheter; and is rotated during imaging. As a result, OCT images are displayed similar to IVUS as cross-sectional views of the coronary artery. Automated pullback of the OCT imaging core allows the user to scan through the coronary artery.

Intracoronary OCT [32] is performed by introducing the small (2.7 French) imaging catheter over a guide wire (0.014 inch) distally into the coronary artery using standard guide catheters (6F or larger). A motorized pullback is performed to scan the coronary artery segment. The pullback speed is typically 20 mm/sec with a frame rate of 100 frames per second or higher. Since blood scatters the OCT signal, it is temporarily cleared by an injection of radiograph contrast medium during the duration of the OCT pullback (typical flush rate 3.0 mL/s). A variety of solutions, warmed to 37° C have been used alternatively as flush medium, including Lactated Ringer’s, viscous iso-osmolar contrast media, and mixtures of lactated Ringer’s and contrast media or low molecular weight dextrose. The time needed to image a 50 mm artery segment is typically three seconds with a total volume of radiograph contrast of 10 to 12 mL, which is comparable to the amount of radiograph contrast needed for a single angiographic run.

Patient and lesion selection — All epicardial coronary arteries and venous or arterial grafts that are accessible by a guiding catheter are eligible for OCT imaging [33]. OCT should not be performed in patients with coronary artery disease and severely impaired left ventricular systolic function or those presenting with severe hemodynamic compromise, as the imaging procedure might induce brief ischemia. Further, OCT should be used with caution in patients with single remaining vessel, as any guidewire or catheter insertion carries a small risk of dissection or arterial spasm, or those with markedly impaired renal function. In these clinical circumstances, the gain in diagnostic accuracy must be balanced against potential adverse effects in individual patients.

A technical draw-back is that plaques located at the very ostium of the left or right coronaries cannot be accurately addressed by OCT, as it is difficult to clear the artery from blood during a nonselective guide catheter position, required for the visualization of the ostium.

Safety — The applied energies in contemporary, intravascular OCT are relatively low (output power in the range of 5.0 to 8.0 mW) and are not thought to cause functional or structural damage to the tissue.

The principle safety considerations relate to the possible induction of ischemia due to the need of blood displacement for image acquisition. Current OCT systems allow for very fast data within a few seconds and therefore are unlikely to lead to significant ischemia [34]. In a report of 114 OCT acquisitions in 90 patients, the procedure was successful in 89. No patients suffered contrast induced nephropathy and no major complications were recorded [35]. One patient had a transient vessel spasm that was resolved with intracoronary administration of nitrates. During frequency domain-OCT images acquisition no ischemic electrocardiographic changes occurred. Ventricular ectopic beats were found only in three patients while other major arrhythmias (ventricular tachycardia or fibrillation) were not observed.

Concomitant therapy — Similar to other diagnostic coronary instrumentation such as IVUS, patients should be anticoagulated (typically with heparin) during the OCT procedure. The OCT catheter should be introduced distally into the coronary artery after the administration of intracoronary nitroglycerin to minimize the potential for catheter-induced vasospasm.

Some clinicians prefer to prescribe dual antiplatelet therapy for 14 days or up to one month after OCT imaging, as invasive imaging has been shown to cause limited endothelial damage in animal experimental studies.

The OCT image

Normal coronary morphology — OCT creates cross-sectional images of the coronary artery wall, based on the optical interaction of the emitted light with the vessel wall. Light is typically reflected and backscattered in biological structures with different optical indices. In normal coronary arteries, light is highly reflected by the internal external elastic membranes. As a result, the normal coronary artery wall appears as a circular structure with three concentric layers at OCT images (picture 2). The innermost signal-rich layer reflects the internal elastic membrane, the middle dark layer represents the media, and the outer, signal rich layer represents the external elastic lamina [36,37]. (See 'Arterial wall structure' above.)

A normal three-layer appearance by OCT is not synonymous to three-layer appearance by IVUS. While OCT (resolution approximately 15 microm) is able to visualize a normal, nondiseased coronary artery, the resolution of IVUS (approximately 120 microm) is not able to visualize truly nondiseased vessels (figure 6). Thus, OCT can confirm the absence of significant atherosclerosis or indicate the degree of subclinical atherosclerotic lesion formation. Serial measurements can be performed to monitor the structural changes that occur in the vessel wall over time.

Atherosclerotic plaques — OCT has the ability to characterize the structure and extent of coronary artery disease in greater detail than IVUS or angioscopy (figure 7). The various components of atherosclerotic plaques have different optical properties. Typically, OCT images are interpreted by visual assessment of the signal intensity and geometry. Good inter- and intraobserver agreement for visual plaque characterization using criteria as described below have been reported [38,39]. Compared to IVUS, OCT has a higher accuracy to detect early atherosclerosis, necrotic core or lipid-rich tissues, a higher accuracy to detect thrombi and allows for visualization of calcifications without blooming artefact, which typically causes overestimation of the calcium extent by IVUS.

When atherosclerotic lesions are present, OCT can provide details on the tissue composition. The following classifications have been used and validated using in-vitro studies:

●Fibrous plaques are typically rich in collagen or muscle cells and have a homogeneous OCT signal. Calcifications within plaques are identified by the presence of well-delineated, low back scattering, signal-poor heterogeneous regions.

●Necrotic cores or lipid-rich tissues are less well-delineated than calcifications, appearing as diffusely-bordered, signal-poor regions with overlying signal-rich bands, corresponding to fibrous caps. The superiority of OCT for lipid-rich plaque detection has been confirmed in other studies comparing OCT, IVUS, and IVUS-derived techniques for plaque composition analysis [36,40].

●A specific type of plaque is called the thin-cap fibroatheroma. Thin fibrous cap atheroma are considered the most important morphologic substrate for a plaque at high risk of rupture and causing acute coronary syndrome. (See "Mechanisms of acute coronary syndromes related to atherosclerosis".)

OCT allows the diagnosis of thin fibrous cap atheroma with a sensitivity of 90 percent and a specificity of 79 percent when compared to histopathology [41] and for accurate measurement of the fibrous cap thickness with low variability [42,43]. Ongoing research suggests that the ability of OCT to measure changes in the fibrous cap thickness could be used to monitor the effect of therapeutic agents aiming at plaque stabilization (image 6). Exploratory registries evaluating the fibrous cap thickness in patients on statin therapy suggest a trend towards an increased cap thickness [44] and lower incidence of plaque rupture under statin therapy [45]. Such data, however, need to be confirmed by adequately powered, prospective studies.

●Thrombi are identified as masses protruding into the vessel lumen discontinuous from the surface of the vessel wall. Red thrombi consist mainly of red blood cells; relevant OCT images are characterized as high-backscattering protrusions with signal-free shadowing. White thrombi consist mainly of platelets and white blood cells and are characterized by a signal-rich, low-backscattering billowing projections protruding into the lumen [42]. OCT is highly sensitive in diagnosing intracoronary thrombi, as the high contrast between the lumen and the surrounding structures facilitate the diagnosis. This is in contrast to IVUS where it is often difficult to differentiate thrombi from the blood-filled lumen.

●Less well-validated entities are local macrophage accumulations and neovascularization. Macrophages can be seen by OCT as signal-rich, distinct, or confluent punctate dots that exceed the intensity of background speckle noise [46]. They may be seen at the boundary between the bottom of the cap and the top of a necrotic core.

●Due to its high spatial resolution, OCT is the only technique able to detect eroded plaques [47].

Likewise, experts believe that the visualization of vasa vasorum and neovascularization is possible; however, no substantial validation studies have been published. Neovascularization within the intima appears as signal poor voids that are sharply delineated and usually contiguous and seen on multiple frames [48].

Assessment of acute stent placement — IVUS has been used to assess the acute result following stenting (picture 3), giving valuable information on extent of stent expansion and apposition against the vessel wall. OCT, with its high contrast between the lumen and the vessel wall as well as the high resolution, allows one to visualize stented vessels in greater detail than IVUS [49]. OCT can also demonstrate periprocedural vessel trauma including coronary artery dissection (image 7).

From a clinical view point, a stent should be expanded with all struts apposed to the vessel wall in order to allow for optimal blood flow. However, IVUS and OCT studies have shown that stent struts are not always apposed against the coronary wall, despite a good angiographic result. The risk for stent strut mal-apposition might be increased in certain stent designs (thick struts, closed-cell design), in conditions where two stents have been implanted in an overlapping way in order to completely cover a long coronary lesion, and in complex lesions. When inadequate stent expansion or strut mal-apposition is recognized during the percutaneous coronary intervention procedure, it can be correct by the use of adequate balloons for further dilatation. While these findings are helpful for the improvement of future stent designs, the clinical relevance and potentially long-term sequelae of mal-apposed struts as detected by OCT are unknown.

Assessment of long-term stent outcome — Long-term stent strut/vessel wall interaction is of interest to both researchers and clinicians. The stability of the acute result, the identification of complex anatomy that is not accessible by angiography or IVUS, and the understanding of reasons for stent failures, when they do occur, are examples of information that is of interest. In contrast to IVUS, OCT can reliably detect and quantify early and very thin layers of tissue coverage on stent struts, even in drug-eluting stents with very thin layers of neointima, often below 80 μm in thickness, with high reliability (picture 4) [50].

OCT has been employed in prospective clinical trials to compare the long-term outcome of various stents:

●The LEADERS randomized trial was a comparison of a biolimus-eluting stent (BES) with biodegradable polymer with a sirolimus-eluting stent (SES) using a durable polymer. (See "Bioresorbable scaffold coronary artery stents".)

Fifty-six consecutive patients underwent OCT during angiographic follow-up at nine months. Strut coverage at an average follow-up of nine months was significantly more complete in patients allocated to BES when compared to those with SES (weighted difference -1.4%, 95% CI, -3.7 to 0.0) [51].

In the Harmonizing Outcomes With Revascularization and Stents in Acute Myocardial Infarction (HORIZONS-AMI) trial, patients with ST elevation myocardial infarction were randomized to paclitaxel-eluting stent (PES) or bare-metal stent (BMS) implantation [52]. (See "Primary percutaneous coronary intervention in acute ST-elevation myocardial infarction: Periprocedural management", section on 'Selection of stent type'.)

The OCT substudy revealed that implantation of PES as compared with BMS significantly reduced neointimal hyperplasia but resulted in higher rates of uncovered and mal-apposed stent struts at 13-month follow-up (1.1±2.5 percent in BMS lesions versus 5.7±7.0 percent in PES lesions). Likewise OCT was employed to study tissue coverage at follow-up in bioresorbable scaffolds [53]. OCT was able to visualize the particular structure of the scaffold struts, the tissue coverage over time, as well as the changes in the optical properties of the vascular tissue during the bioresorption process.

While these observations are important to understand differences in-stent design, further studies are required to determine the clinical significance of these findings. Today, no threshold for coverage is established. However, OCT is the only imaging modality to date that offers, within the discussed limits, the possibility to understand tissue coverage and neointima formation in drug-eluting stents over time. Large scale, prospective studies, might be useful to address vexing clinical questions such as the relationship of drug-eluting stent deployment, vascular healing, the true time course of endothelial stent coverage, and late-stent thrombosis. This may also better guide the optimal duration of dual antiplatelet therapy that currently remains unclear and rather empiric.

OCT clinical applications — Today, no clinical indications for OCT imaging are established. There are no randomized data supporting a prognostic role for OCT in catheter-based intervention. However, there is broad expert agreement that the detailed, easily accessible, and interpretable information of OCT on the presence of atherosclerosis, its extent, lumen narrowing, as well as on the result of any interventional measure can be of clinical value, at least in individual patients and in specific clinical scenarios [54,55]. Preliminary data on OCT indicate that it can change the operator’s intention-to-treat and modify the overall revascularization strategy, potentially avoiding unnecessary interventional procedures. OCT might be efficient in complex interventions including treatment of left main stem, bifurcations, as well as in all cases of angiographically ambiguous lesions, and in-stent failures.

Two other potential uses of OCT are identification of an angiographically unclear lesion and assessment of stent failure; both are discussed directly below.

Compared with IVUS and CA — As mentioned directly above, no clinical indications for OCT imaging are established. An attempt was made to understand the role of OCT in patients undergoing PCI in the ILUMIEN III: OPTIMIZE PCI trial, which randomly assigned patients with native coronary artery lesions to OCT guidance, intravascular ultrasound (IVUS) guidance, or coronary angiography-guided stent implantations [56]. The primary efficacy end point was post-PCI minimum stent area (measured by OCT at a masked independent core laboratory at the completion of enrolment) in all patients and the primary safety end point was procedural major adverse procedural complications. The study demonstrated that post-PCI minimum stent area by OCT guidance was noninferior to IVUS. Neither IVUS nor OCT were superior to angiography-guided PCI. Major complications were 3 percent or less in the three groups.

Patients with ACS and angiographically unclear culprit — The optimal management of acute coronary syndromes usually relies on rapid treatment of the culprit vessel. The culprit vessel is sometimes indicated by the localization of the pathognomonic electrocardiographic changes, or by the finding of a thrombotic, hazy lesion by radiographic coronary angiography. However, the identification of the culprit lesion can be challenging in some individuals, especially when multi-vessel disease is present. Similarly, in 15 percent of the patients undergoing primary percutaneous coronary artery intervention for ST elevation myocardial infarction, angiography shows a patent infarct-related vessel with TIMI 3 flow [57]. OCT can provide accurate information on the superficial composition of the plaque, can identify ruptured plaques, and most importantly can reveal thrombosed lesions, and thus identify the culprit lesion (image 8) [58].

Assessment of stent failure — Reasons for drug-eluting stent failure, including restenosis and stent thrombosis, are poorly understood. In the catheterization laboratory, OCT can be very useful in the evaluation of the causes for stent failure in any given patient and guide treatment decisions (picture 5) [59]. OCT is able to differentiate mechanical stent failure (such as incomplete stent expansion or stent fracture) from impaired healing (such as absence of strut coverage, absence of homogenous coverage, or [late] strut mal-apposition).

PCI of complex lesions — The OCTOBER trial of 1201 patients [60] and ILUMIEN IV trial of 2487 patients [61] studied whether OCT-guided PCI versus angiographic-guided PCI of complex lesions prevented major adverse coronary events and target vessel failure, respectively. Both studies followed participants for outcomes over a two-year period. Results were mixed as follows:

●In the OCTOBER trial, there were lower rates of major adverse cardiac events in the OCT-guided versus angiography-guided PCI groups (10.1 versus 14.1 percent; hazard ratio [HR] 0.70; 95% CI 0.50-0.98) [60]. Procedure-related complications occurred in 6.8 versus 5.7 percent in the OCT-guided versus angiography-guided PCI group.

●In the ILUMIEN IV trial, there were similar rates of target-vessel failure within two years in the OCT and angiography groups (Kaplan-Meier estimates 7.4 and 8.2 percent; HR 0.90; 95% CI 0.67-1.19) [61]. OCT-related adverse events occurred in one patient in the OCT group and in two patients in the angiography group.

CORONARY ANGIOSCOPY — Coronary angioscopy (CA) is an intravascular technique that allows direct visualization (using high intensity cold light) of the internal surface of the coronary circulation. This technique provides information concerning the pathology of coronary lesions and insight into the pathophysiology of acute coronary syndromes, thereby aiding diagnosis and treatment. Due to technical limitations, CA has not become broadly incorporated into routine clinical practice.

Imaging technique — The angioscope is a disposable coronary polyethylene catheter with a 4.5 French (1.5 mm) outer diameter that can be inserted through an 8 French percutaneous coronary intervention (PCI) guiding catheter.

The catheter consists of two coaxial catheters. The inner catheter contains the optical fibers, and a small secondary channel permits inflation of a low-pressure occlusion balloon, or cuff, located at the tip of the outer catheter. The main lumen of the outer catheter is used for irrigation. The fluid exits the catheter just distal to the occlusion cuff.

An optical coupler connects the proximal image bundle aperture to the camera and to the light source. The combination of several features, such as field of view, scope flexibility, and the moving optical bundle during imaging, provides excellent image resolution. Lack of a mechanism to steer the lens tip remains an important limitation, particularly in large and/or tortuous vessels.

A 0.014 inch PCI guidewire is first inserted through an 8 French guiding catheter and positioned distally in the target artery. To permit optimal guidance of the lens tip and avoid wire-trapping in tortuous vessels, we prefer the use of guidewires that offer more support than "floppy" wires.

The angioscope is carefully advanced over the guidewire and irrigation is begun to eliminate air bubbles before advancing into the coronary artery. Under fluoroscopy, the angioscope is then advanced into the coronary artery and the occlusion cuff is positioned at the selected site. While keeping the occlusion cuff inflated, this catheter can be advanced or withdrawn 5 cm over the guidewire inside the outer catheter

Irrigation is then started and the occlusion cuff is inflated after two to five seconds. As soon as a satisfactory image is obtained, the inflation is stopped, recording on videotape is started, and the probing fiber is slowly advanced over the guidewire under continuous irrigation.

Since the combination of proximal occlusion and irrigation produces myocardial ischemia and chest pain, it is advisable to limit each inflation to 30 seconds. The procedure can be repeated after a brief interval of balloon deflation and coronary reperfusion with blood.

Lesion selection — At present, the primary clinical indication for performing CA is to evaluate, in detail, specific characteristics of a coronary lesion of interest. This is usually determined by evaluating the coronary angiograms performed prior to angioscopy. Often, lesions potentially associated with an ongoing acute coronary syndrome would be ideal candidates. There are no guideline indications for angioscopy.

The proximal 2 cm of all three major coronary arteries cannot be imaged because of the relatively large distance between the occlusion cuff and the lens tip. As a result, the angioscope is ideal to visualize the midsegments of the main divisions and also large side branches. If the internal luminal diameter is between 2.5 and 3.5 mm, imaging is usually excellent for the entire vessel. As the vessel diameter increases beyond 3.5 mm, the quality of the imaging decreases. In addition, if the left circumflex artery is excessively tortuous and has a sharp takeoff, it is better to avoid visualization of the vessel because the main catheter body is quite stiff. During the procedure, it is recommended to keep the occlusion cuff straight to avoid an eccentric and wall directed field of view.

Correlation with histopathology — A close correlation is observed between the findings from histopathology and those obtained with CA. One study evaluated 70 postmortem coronary artery segments by angioscopy, intracoronary ultrasound, and histologic examination [62]. Angioscopic findings were classified as normal artery, stable atheroma, disrupted atheroma, and thrombus. Using histologic findings as reference, angioscopy had a sensitivity and specificity of 95 percent for all pathologic findings except for disrupted atheroma, a lesion occasionally missed by this technique. However, the sensitivity of angioscopy for thrombus was excellent (100 percent) and vastly superior to that of ultrasound (57 percent).

Since there was a lack of consensus in reporting the qualitative type of imaging because of subjectivity in interpretation, it became necessary to establish a classification of angioscopic findings. In October 1992, European angioscopy investigators established a simplified "Ermenonville" classification based upon an unbiased review of videotapes by several angioscopy investigators [63]. The classification system is often used to assist in the clinical documentation of angioscopy results. The following parameters are described:

●Image quality

●Obtained image of the target vessel

●Normal vessel

●Narrowing

●Shape of narrowing

●Thrombus

●Atheroma

●Post-angioplasty dissection

●Wall haemorrhage

The CA image — Angioscopy visualizes the inner lumen of the coronary artery wall and intraluminal changes.

Normal coronary artery — The normal coronary artery surface is smooth in contour, without any protruding structure, and has a uniform yellow-white or grayish-white color.

Abnormal coronary findings — Abnormal findings in the coronary artery include the presence of atheroma, thrombus, dissection, subintimal hemorrhage, luminal narrowing, and restenosis. Coronary stents, and the degree which they have been endothelialized, can also be analyzed.

Atheroma — Atheroma are usually yellow, and may be smooth and lining the lumen, or protruding. A fibrous plaque is a white, nonmobile elevation and/or protrusion that is focal or diffuse. If the atheroma is complicated, it may or may not be mobile, patchy colored, and with or without visible cracks or fissures on the luminal surface. (See "Mechanisms of acute coronary syndromes related to atherosclerosis".)

Thrombus — Thrombi are defined as masses that are red, white, or both, and adhere to the intima and/or protrude into the lumen. Thrombi are, by definition, not located within the wall but on its surface. Intracoronary thrombi may vary in size from small patches on the surface to large, protruding, and obstructing masses. This mass is not dislodged by Ringer lactate infusion. Pathologic studies of coronary thrombi have shown that white thrombi are platelet-rich, whereas red thrombi contain an abundance of fibrin mixed with erythrocytes and platelets [64].

Dissections — Dissections, mostly encountered after balloon angioplasty, range in size from small surface disruptions to those that are wide and long.

Subintimal hemorrhage — Subintimal hemorrhage can be defined as a patch of red coloration within the arterial wall. It can be difficult to distinguish from red lining thrombus. Subintimal hemorrhage is usually observed after balloon angioplasty.

Luminal narrowing — Angioscopy is not a good technique for quantitative evaluation of luminal narrowing. Nevertheless, for high-grade stenosis, the coronary guidewire that passes through the stenosis may give a usable visual reference. As an example, a lumen that can contain only one or two 0.014" (0.36 mm) wires would have a minimal luminal diameter of 0.72 mm or less, and should be considered an important stenosis. This method has been evaluated by the European Working Group, and has been found to have limited value because of a large intra- and interobserver variability. However, coronary angioscopy can recognize a total occlusion and the presence or absence of narrowing. Therefore, at present, coronary angioscopy is not an ideal tool for visual estimation or automated quantification of coronary artery stenoses.

Restenosis — Restenosis with angioscopy has a white, smooth appearance [65].

Stents — Intracoronary stents can be easily visualized by angioscopy; many stents have been studied [66-68]. However, insertion of the angioscope into any stent over a wire and crossing into a side branch is not recommended, since the lens tip assembly can get trapped behind these wire crossings and dislocate the stent. The warning relates to any stent placed at any time.

Coronary angioscopy clinical applications — coronary angioscopy has been utilized to help understand and/or evaluate several different clinical settings:

●Consequences of different interventional cardiology techniques

●Cardiac transplant vasculopathy

●Pathophysiology of various coronary syndromes

Coronary stent implantation — coronary angioscopy has been used as an adjunctive technique during bare metal stent implantation, either guiding the intervention or leading to a modification of the procedure [66,69]. In one study of 15 patients, coronary angioscopy was performed to evaluate the results of stent placement. Thrombus, undetected by angiography, was found in two cases, while four patients had residual narrowing requiring redilation [66]. In an additional two patients, angioscopy was performed because of stent restenosis, which was initially thought to be due to thrombus formation. In both cases, angioscopy revealed that the restenosis was due to subtotal occlusion because of fibrotic tissue and thrombus, thereby obviating the use of thrombolytic therapy. Thus, the findings on angioscopy affected some aspect of management in over one-half of patients.

Angioscopy has also been used to evaluate the time course of neointimal coverage of bare metal stents. One study performed angioscopy before, immediately after, at seven days, and two months after stent implantation in 20 patients treated with either antiplatelet agents or anticoagulation. In the majority of patients, neointimal formation was completed by two months after stent placement [67]. In a second series of 14 patients who underwent angioscopy immediately after stent placement, at 8 to 45 days, and at 65 to 142 days, three types of neointimal layer were observed: a white layer with a cotton-like surface, a white layer with a smooth surface, and a transparent layer with a smooth surface. Approximately three months were required for the completion of coverage [70].

The time course of neointimal coverage is much slower with drug-eluting stents. This was illustrated in an angioscopic study of 37 consecutive stented lesions (15 sirolimus stents and 22 bare metal stents) [71]. The following findings were noted on angioscopy:

●Neointimal coverage was complete in all 22 bare-metal stents. In contrast, neointimal coverage was complete in only 2 of the 15 sirolimus stents and three had essentially no coverage.

●Thrombi were present in eight stented segments, none of which was seen on angiography. Thrombi were more common with incomplete neointimal coverage (5 of 13 versus 3 of 24 stents with complete neointimal coverage).

Delayed neointimal coverage with drug-eluting stents prolongs the period of high risk for stent thrombosis [72]. (See "Coronary artery stent thrombosis: Incidence and risk factors", section on 'Very late stent thrombosis'.)

Coronary bypass graft surgery — Angioscopic evaluation of saphenous vein bypass grafts (SVG) has added new information regarding the causes of graft failure and potentially effective preventative therapies. In one study, 31 SVGs were assessed and 15 had both yellow plaque and thrombi, whereas in the remaining 16 SVGs the intima was clear white. (See "Coronary artery bypass graft surgery: Graft choices".)

Cardiac transplant vasculopathy — CA has provided significant information concerning the coronary vasculopathy, which commonly occurs in cardiac allografts [73,74]. Although contrast angiography is the current "gold standard" for diagnosing allograft vasculopathy, it underestimates its presence when correlated with pathologic examination. One study, for example, compared CA with quantitative angiography and intracoronary ultrasound in 29 consecutive cardiac transplant recipients examined one to four years after cardiac transplantation [73]. Angioscopy demonstrated pigmented (yellow) or nonpigmented (white) plaques in 79 percent of patients, and coronary stenoses in 24 percent. In contrast, radiographic coronary angiography detected plaques in only 10 percent, and stenoses in only 3 percent. The frequency of plaques and stenoses with intracoronary ultrasound was similar to angioscopy. However, only angioscopy provided information about luminal surface morphology and plaque pigmentation. (See "Heart transplantation: Clinical manifestations, diagnosis, and prognosis of cardiac allograft vasculopathy".)

Research applications

Mechanism of acute coronary syndromes — CA has contributed to the understanding of the mechanisms underlying acute coronary syndromes [75,76]. In one report, angioscopy was performed in 86 patients presenting with stable or unstable angina, or an acute or old myocardial infarction [75]. Thrombus was present in 100 percent of patients with an acute myocardial infarction, 90 percent with unstable angina, but none with stable angina or old myocardial infarction. The thrombus was usually occlusive in those with an acute myocardial infarction (79 percent), but not in patients with unstable angina (10 percent). (See "Mechanisms of acute coronary syndromes related to atherosclerosis".)

Evaluation of thrombi — The relative insensitivity of angiography techniques for thrombi may be explained by the inherent limitations of a radiographic contrast "lumenogram" of a tortuous, three-dimensional structure viewed in two dimensions. Detection of intracoronary thrombus by radiographic coronary angiography requires that the thrombus be large enough to displace contrast material and appear as a "filling defect" or a contrast stain in the vessel. The specificity of radiographic coronary angiography for detecting intracoronary thrombi is also weakened by the observation that not all angiographic filling defects are thrombi.

By comparison, angioscopic detection of a red thrombus is based upon color and texture discrimination and does not rely upon a "mass effect." Small quantities of red thrombus are easily seen against the white or yellow background of the vessel wall. However, the angioscopic identification of intracoronary thrombus is subjective. It requires that the operator have some experience to reliably distinguish red thrombi from other intravascular features which are red:

●Thrombi may occur as red intraluminal masses that are fixed to the vessel wall; they are distinguished from stagnant blood in the lumen by their globular, gelatinous texture and their inability to be detached from the vessel wall with a flush solution.

●Mural thrombi appear as patches of red material fixed to the surface of the vessel wall, analogous to a red carpet on a white or yellow floor. The surface texture of a mural thrombus commonly has a "woven" appearance that is not glistening or shiny. It is possible that an inexperienced observer could confuse mural thrombi with either an intramural hematoma or a vessel wall abrasion. However, intramural hematomas have the distinctive appearance of a deep bruise, with a dull, dusky, red color below the smooth glistening surface of the vessel intima. Abrasions of the vessel wall, on the other hand, appear as linear scratches or red streaks on the surface of the artery in contrast to the confluent patches of red thrombus.

●Angioscopy, by depicting both color and texture, can readily distinguish white or yellow plaque fragments protruding into the vessel lumen from red thrombus. However, the ability of angioscopy to differentiate "white" thrombus and other “white” tissue elements is limited. The angioscopic distinguishing feature of these white intraluminal masses is their shape and texture. Tissue fragments or dissection flaps usually demonstrate sharp, angular margins analogous to tattered white bed sheets on a clothesline blowing in the wind, whereas white thrombi (platelet aggregates and fibrin strands) are globular masses with fuzzy, indistinct borders.

Color differences in thrombus may reflect differences in composition due to variations in age or the presence or absence of blood flow. Red thrombi contain an abundance of fibrin mixed with erythrocytes and platelets, whereas white thrombi are platelet-rich. The color of the thrombus shifts progressively toward a pale, whitish color as organization progresses [77].

These color differences may be observed via angioscopy among patients with varying acute coronary syndromes. One series evaluated the physical appearance of the thrombus in 15 and 16 patients with unstable angina and acute ST elevation myocardial infarction, respectively [78]. Coronary thrombus was present in 93 percent of patients; the thrombi usually had a grayish-white color (ie, platelet-rich) in patients with unstable angina but was predominantly red (ie, fibrin-rich) in those with acute myocardial infarction.

Differences in plaques — The color of plaque may vary among patients with different acute syndromes. Plaques in patients with acute myocardial infarction can be classified into two color types:

●Xanthomatous plaque (yellowish), which cannot be recognized with angiography, is more common in patients with acute myocardial infarction and unstable angina than in those with old myocardial infarction and stable angina [75,79]; such plaques are found in all three major coronary arteries, suggesting that the development of these vulnerable plaques is a pan-coronary process [80]. These plaques often have an irregular intimal surface due to plaque rupture, ulceration, and intimal flap.

●Smooth, white plaques are observed in patients with old myocardial infarction and stable angina.

The differences in plaque composition are associated with varying degrees of stability. Xanthomatous plaque is likely to have a high concentration of cholesterol at its base and the fibrous cap overlying the lipid core may be thin. Plaque with high concentration of lipid and a thin fibrous cap may be easily cracked by increased shear force at the level of the stenosis and by acute changes in coronary tone or exercise [81]. By comparison, white plaques are less likely to rupture since the increased fibrous content over the lipid core provides stability. (See "Mechanisms of acute coronary syndromes related to atherosclerosis".)

Identification of vulnerable plaque — Since yellow plaque is more common in acute coronary syndromes compared with stable angina, the presence of yellow plaque on angioscopy in patients with stable angina may predict the future occurrence of an acute coronary event. This hypothesis was examined in a study of 157 patients with stable angina pectoris who underwent CA; 39 had yellow plaques and 118 had white plaques [82]. At 12 months, acute coronary events, confirmed by angioscopy to be the result of thrombus arising from the ruptured culprit plaque, were much more frequent in patients with yellow plaques (28 versus 3 percent). The increase in risk was primarily due to glistening yellow plaques (69 versus 8 percent in those not glistening). Angioscopy can also be used to assess potential therapies to reduce the risk of vulnerable plaque. In a study testing the use of atorvastatin for the stabilization of vulnerable plaque [83], therapy changed lesions from yellow to white, suggesting plaque stabilization. To make potential analysis of vulnerable plaque more accurate, quantitative measurement of its color has also been attempted [84]. (See "Mechanisms of acute coronary syndromes related to atherosclerosis".)

Fibrinolytic therapy — Thrombus overlying disruption in the fibrous cap of an atherosclerotic plaque is considered to be the cause of acute myocardial infarction and unstable angina. Since fibrinolytic therapy is not effective in all patients with these findings, angioscopy has been used to help understand those factors that determine whether such therapy will be beneficial. In one study, for example, the efficacy of fibrinolytic therapy in 25 patients with acute myocardial infarction and 10 with unstable angina was correlated with the type of plaque observed with angioscopy [85]. Red thrombi were found in all patients with an acute infarction, but in only two with unstable angina; the remaining eight patients had white thrombi. Compared to the group with red thrombi, fibrinolytic therapy was significantly less effective in patients with white thrombi (25 versus 69 percent).

Limitations of coronary angioscopy — Although the technology of CA has been available for more than 15 years, it is used infrequently and mainly as a research tool, rather than an adjunct to routine clinical care. The reasons for its limited use are several:

●CA is a somewhat cumbersome technique requiring significant increases in the time of the case.

●CA can only be safely utilized in coronary arteries at least 2.5 mm in diameter, which limits its use in smaller vessels.

●The requirement to deploy a proximal occluding balloon causes unnecessary trauma to a proximal segment of the vessel that may contribute to future restenosis. It also results in temporary occlusive coronary ischemia that may be painful and even detrimental to the patient.

●Many patients with existing collateral coronary circulation produce enough back-flow of blood distal to the occluding balloon that a clear blood-free image cannot be obtained.

●A complication called "wire-trapping," which results from a loop formation of the guidewire between the two monorail wire channels of the angioscope, may occur. It can occur when the fiber bundle is withdrawn and the coils of the guidewire catch on the tip of the angioscope. This can potentially create dissection or result in an unwanted loss of the guidewire from the vessel. It can be avoided with careful fluoroscopic monitoring during catheter manipulation. When wire-trapping occurs, it may be corrected by extending the fiber bundle first and then withdrawing it carefully under fluoroscopic control.

●With the availability of intracoronary ultrasound and optical coherence tomography, the added information provided by CA is not necessary in many instances.

SUMMARY

●Introduction – The intravascular imaging techniques of optical coherence tomography (OCT), intravascular ultrasound (IVUS), and coronary angioscopy (CA) are used to provide information above and beyond that of radiograph coronary angiography. (See 'Introduction' above.)

●IVUS clinical applications – The principal clinical applications for IVUS are guiding coronary interventions and clarifying ambiguous or indeterminate radiograph angiography. (See 'IVUS clinical applications' above.)

●OCT clinical applications – No clinical indications for OCT imaging are established. Our experts believe that OCT can be of value in some patients to evaluate the presence and extent of coronary atherosclerosis, the degree of coronary lumen narrowing, and the results of percutaneous coronary intervention. (See 'OCT clinical applications' above.)

●CA clinical applications – Although there are no established indications for CA, our experts believe that it may be useful for understanding and/or evaluating several different clinical settings, such as consequences of different interventional cardiology techniques, cardiac transplant vasculopathy, and the pathophysiology of various coronary syndromes.

ACKNOWLEDGMENT — The UpToDate editorial staff thank Dr. Evelyn Regar for her past contributions as an author to prior versions of this topic review.