Intracoronary Imaging
Catalin Toma
Jeff Fowler
INTRODUCTION
Cardiac imaging technologies that involve instrumentation of the heart can provide intravascular imaging dedicated to the coronary architecture or assessment of chamber and valve function. Given the limitations of angiography, intravascular imaging was developed to better visualize the arterial lumen and the vessel wall. This technology can diagnose pathologic processes, characterize the anatomy of the coronary artery, and assist with percutaneous coronary intervention (PCI). This chapter will discuss two main clinically available technologies for coronary imaging: (1) intravascular ultrasound (IVUS) and light-based optical coherence tomography (OCT) and (2) intracardiac echocardiography (ICE).
INTRAVASCULAR ULTRASOUND FUNDAMENTALS OF INTRAVASCULAR ULTRASOUND
IVUS relies on the reflection of ultrasonic sound waves in megahertz frequency range to reconstruct the radial architecture of a coronary vessel. IVUS imaging can be performed while maintaining normal blood flow through the vessel. The resolution of the image depends on the frequency of the ultrasound with higher resolution transducers generally having a better image quality. There are two fundamental types of IVUS catheters: fixed array and rotational. The fixed array system employs several circumferentially placed ultrasound emitter/receivers with no moving parts. These catheters (such as Eagle Eye, Phillips, Netherlands) operate at lower frequencies (20 MHz), are bulkier, and less deliverable than the rotational IVUS catheters; however, they are easy to set up and operate. Rotational IVUS catheters employ a single emitter-receiver element that is rotated via a flexible shaft connected to an external motorized drive unit, which also executes the pullback during image acquisition. These catheters (such as the Boston Scientific, Phillips, and Acist HDI) use higher frequency (ie, 40-60 MHz) resulting in higher resolution imaging. With increased frequency, however, there is more scatter of the blood cells and decreased lumen definition. They are smaller in caliber than fixed array IVUS catheters but require a slightly more elaborate preparation given the need for the external motorized drive unit.
CLINICAL APPLICATIONS
Diagnostic Utility: Intravascular Ultrasound
IVUS allows in vivo characterization of the atherosclerotic plaque morphology and the coronary lumen anatomy (Figure 42.1A-F). IVUS is particularly useful at detecting calcified plaque, because sound waves bounce off hardened surfaces creating distinct echoes (Figure 42.1C). IVUS has a significantly higher sensitivity for detecting vessel wall calcium than angiography.1 Softer plaque characterization is less optimal with IVUS because of the decreased resolution (Figure 42.1A). Virtual histology (VH) IVUS, based on frequency shifts, was developed to characterize plaque composition and is currently clinically available. In a prospective natural-history study of coronary atherosclerosis, IVUS predictors of nonculprit lesion-related events had a minimal lumen area less than or equal to 4 mm2, a plaque burden greater than or equal to 70%, and a radiofrequency thin-cap fibroatheroma. However, the clinical utility of these features in guiding therapy remains unclear.2
In addition to plaque characterization, IVUS data have been used to determine the hemodynamic impact of a lesion based on the lumen size. A left main artery minimal luminal cross-sectional area of less than 4.8 mm2 by IVUS identifies a hemodynamically stenosis as determined by pressure wire measurements.3 For non-left main disease, the anatomic assessment of functional significance is more variable; a recent meta-analysis of comparative IVUS versus fractional flow reserve (FFR) studies identified a minimal luminal cross-sectional area cutoff of 2.8 mm2 as hemodynamically significant.4 However, using minimal luminal area alone does not consider the length of the diseased segment, which also plays an important role in determining the pressure drop across the stenotic segment.
Procedural Guidance: Intravascular Ultrasound
The principal benefit for IVUS in the current PCI era is that of coronary procedural guidance. Historically, IVUS played a critical role in assessing stent size and placement to reduce acute stent thrombosis5 and in preventing in-stent restenosis. In the current second-generation drug-eluting stent (DES) era (ie, with thin strut/biocompatible drug-eluting polymers), IVUS guidance retains a clinical advantage over angiographic guidance. Two randomized trials demonstrated better outcomes (ie, decrease in major adverse cardiovascular events [MACEs] and target vessel failure) when DES expansion was guided by IVUS versus angiography.6,7 A large meta-analysis examining
IVUS guidance in the DES era included 31,283 patients with a 1-year follow-up demonstrating not only a significant reduction in MACE but also a favorable impact on hard endpoint, such as death and myocardial infarction.8
IVUS guidance in the DES era included 31,283 patients with a 1-year follow-up demonstrating not only a significant reduction in MACE but also a favorable impact on hard endpoint, such as death and myocardial infarction.8
 FIGURE 42.1 Lesion characterization by intravascular imaging. (A) Heterogenous lesion by IVUS with calcifications with characteristic signal dropout (arrow) and mixed intensity signal plaque suggestive of large necrotic core (asterisk). (B) OCTs higher spatial definition allows for greater detail. Calcium nodules are well-delineated (arrow), whereas lipid-rich atherosclerotic plaques have marked signal attenuation (asterisk). Thin-cap atheroma considered to be the precursor to plaque rupture can be identified by OCT at the plaque edges (arrow head). (C) The circumferential extent of the plaque calcification can be easily identified by IVUS (arrows, about 180 degree in this case); circumferential calcium is an important predictor of the need for atherectomy to ensure optimal stent implantation. (D) OCT similarly can readily detect calcified plaque (arrows), and in addition allows for an assessment of the thickness of the plaque calcium, with thicker calcium more likely to require atherectomy. (E) Although stent expansion can be readily assessed by IVUS, stent malapposition (arrow) and presence of thrombus (asterisk) is often challenging. (F) OCT is an excellent tool in detecting vascular thrombus that has a particular lobulated appearance with signal scatter (asterisk), as well as accurately assess strut malapposition (arrow). IVUS, intravascular ultrasound; OCT, optical coherence tomography. |
Procedural guidance involves vessel imaging before stenting for adequate sizing as well as after stenting to ensure adequate stent expansion and no significant complications such as stent edge dissection (Figure 42.2).
The most important element in preventing subsequent clinical events is achieving adequate stent expansion. This refers to achieving a minimal lumen area inside the stented segment as close as possible to the reference segment. The ULTIMATE (IVUS vs angiography-guided DES implantation) trial has put forward a relatively simple definition for optimal stent expansion (>90% of the distal cross-sectional area or >5 mm2) and demonstrated the link between achieving these parameters and decreased event rates.7 Stent expansion can be limited by the presence of heavy calcification, which can be easily identified by IVUS. An arc of calcium greater than 270 degrees (three quadrants) is considered an indication for rotational or orbital atherectomy for adequate lesion preparation prior to stenting. Lastly, stenting from “healthy to healthy vessel” or not having significant plaque burden past the stented segment is recommended. Poststenting, adequate high-pressure postdilatation based on the vessel size is essential in optimizing the PCI result. Only significant edge dissections (>3 mm) or flow limiting distal edge dissections should require additional treatment.
 FIGURE 42.2 Coronary PCI guidance using IVUS. Assessment of the vessel before intervention allows for adequate sizing, as well as evaluating the need for atherectomy to ensure proper stent expansion. After stenting and high pressure balloon dilatation, IVUS is critical in determining adequate stent expansion relative to the reference segment, as well as the presence of any flow limiting edge dissections. IVUS, intravascular ultrasound; PCI, percutaneous coronary intervention. |
Patients at high risk of PCI-related complications may derive benefit from IVUS, such as those with left main, chronic total occlusions, or long lesions.7 A number of nonrandomized studies found better outcomes for left main PCI with IVUS guidance.9 Chronic total occlusions have a higher degree of procedural complexity when treated percutaneously, and IVUS guidance has been shown to reduce target vessel failure in a randomized controlled trial.10
IVUS is particularly useful in the treatment of in-stent restenosis. Although the incidence of in-stent restenosis has decreased with the newer-generation thinner struts DES, the prevalence is relatively unchanged, because more complex patients can undergo PCI. Although traditionally in-stent restenosis was associated with neointimal hyperplasia, in the era of DES other factors are commonly found; in particular, stent
underexpansion11 and stent fracture. IVUS is particularly useful at assessing stent expansion and guiding vessel dilatation and re-stenting. The European Society of Cardiology endorses imaging for interventional guidance in cases of stent failure (Class of Recommendation IIA).12
underexpansion11 and stent fracture. IVUS is particularly useful at assessing stent expansion and guiding vessel dilatation and re-stenting. The European Society of Cardiology endorses imaging for interventional guidance in cases of stent failure (Class of Recommendation IIA).12
Another scenario where IVUS may be particularly useful is PCI in patients with a high risk of acute kidney injury. IVUS can size vessels, determine stent length, and assess poststent outcomes without contrast injections. Ultralow contrast PCI (defined as using less contrast than the glomerular filtration rate) can thus be successfully performed, including zero contrast for PCI. In a recent series using IVUS, the average contrast volume was under 10 mL, with 0% contrast-induced kidney injury in chronic kidney disease patients undergoing PCI.13
LIMITATIONS AND FUTURE DIRECTIONS: INTRAVASCULAR ULTRASOUND
Despite the compelling supportive data, the penetration of IVUS for PCI guidance remains suboptimal, primarily related to limited expertise, cost, time, and lack of reimbursement. Integrated systems have been created to facilitate rapid image acquisition and coregistration with the angiographic data to support decision-making. This will likely favorably impact utilization. Newer miniaturized IVUS elements currently in development will improve catheter delivery and quality image acquisition. Automating border detection and enhancing the user interface software will speed procedural decision-making and procedure duration. The IVUS-derived lumen geometry can also be used to compute the functional pressure drop along the lesion to assess its functional significance.14
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