Intravascular Ultrasound: Instrumentation and Technique
- Vikrant Jagadeesan, MD
- Elizabeth M. Retzer, MD
- Sandeep Nathan, MD, MSc
- Elizabeth M. Retzer, MD
Conventional angiography remains the gold standard and the most widely used invasive modality for the diagnosis and quantification of coronary artery disease. Many decades of refinement in equipment, angiographic technique, and radiographic image acquisition and processing have resulted in dramatic improvements in the overall quality and reproducibility of angiographic images. Yet angiography suffers from significant limitations because the data represent planar images of the three-dimensional vascular lumen, and the information content about the structure and composition of the diseased vessel wall is very limited. Intravascular ultrasound imaging, now more than four decades old, allows sensitive and quantitatively accurate characterization of intimal pathology and vessel wall architecture, thus serving as an important adjunct for both diagnostic angiography and percutaneous coronary intervention.
Intravascular ultrasound imaging technology
Intravascular ultrasound (IVUS) uses reflected acoustic energy to generate high resolution, tomographic images of vascular structures in vivo. Briefly, this is accomplished via radial ultrasonic emission at the distal tip of the IVUS catheter by piezoelectric crystals, which, when electrically excited, vibrate and create a local compression wave. Thus echocardiographic signals are emitted, which then bounce off acoustically reflective anatomic structures and return to the transducer. The signal is amplified, filtered, compressed, and digitized by the transducer for display. The series of images acquired as the IVUS transducer is pulled back within a vessel can also be digitally processed for longitudinal vessel image reconstruction. Modern IVUS systems employ transducer frequencies of 20 to 45 MHz, which are well above those of noninvasive probes but required to ensure high near-field image resolution. IVUS image resolution may be subdivided into axial and lateral resolution. Axial resolution refers to the ability to differentiate closely adjacent structures along the axis of the ultrasound beam (from the catheter to the periphery of the vessel); it is primarily a function of transducer frequency and ranges between 100 and 150 μm for 20- to 40-MHz probes. Lateral resolution refers to the ability to distinguish adjacent structures along the circumferential sweep of the ultrasound beam; it is influenced by the type of IVUS imaging technology used (detailed later) and ranges between 200 and 250 μm.
There are two basic imaging approaches employed in currently available IVUS systems: mechanical rotational, single-element scanning and solid-state multielement scanning ( Fig. 17.1 ). The single-element platform employs a 40- to 45-MHz distal ultrasonic transducer that is rotated by a motorized drive cable, housed within a flexible catheter. The proximal end of a catheter has an integrated electromechanical interface with the analogous elements of the IVUS console. The individual radial images obtained are assembled by a digital image array processor into a viewable cross-sectional image. This IVUS technology is susceptible to reduced image quality due to trapped microbubbles in the inadequately flushed apparatus, and acoustic shadowing from the guide wire (“guide wire artifact”), which runs outside the catheter, adjacent to the imaging element. Additionally, nonuniform rotational distortion (NURD) may occur in tortuous vessels because drive cable friction distorts the image so that it appears to be smeared across a portion or quadrant of the cross section. In solid-state, phased-array systems, multiple transducer elements are aligned radially around the distal tip of the catheter and are activated sequentially. Current 20-MHz phased-array IVUS catheters employ 64 transducer elements. Solid-state IVUS catheters may suffer from ring-down artifact where the ring of imaging closest to the catheter must be subtracted or masked before acquiring images in the vessel of interest. Current coronary IVUS catheters are available on a 0.014-inch guide-wire platform, are approximately 2.9 French and are compatible with a 5 to 6 French guide catheter. Rotational systems are available in the United States from Boston Scientific (Natick, Mass.), Volcano Corp. (Rancho Cordova, Calif.), and Infraredx (Burlington, Mass.). Phased-array, 0.014-inch, guide-wire compatible, coronary IVUS is offered in the United States by Volcano Corporation. Larger catheters (0.018 inch, compatible with 0.035-inch guide wire) are available for deeper image penetration, for use in peripheral and aortic applications.
Data from the IVUS catheter are input to either a freestanding or integrated CPU console that processes and outputs the information as motion images suitable for viewing immediately on an integrated screen ( Fig. 17.2 ). IVUS data may also be archived digitally to a hard drive, manipulated off-line, or burned to removable media. Advanced postprocessing techniques available on certain IVUS systems (described later) allow characterization of different histologic tissue types within atherosclerotic plaques and delineate the boundaries of the vascular lumen via nondirectional color flow imaging. Although the available IVUS systems vary in their technology, resolution, and potential sources of artifact as detailed previously, quality of images and ease of interpretation are fairly comparable across the current generation of systems.
Performance of the IVUS procedure
IVUS may be performed as a standalone diagnostic procedure following coronary angiography or as percutaneous coronary intervention (PCI). In the latter capacity, it may be used to guide a device, assess complications, or optimize PCI results. Standard interventional techniques are employed with femoral, brachial, or radial arterial access and a suitable 5 to 6 French guide catheter advanced to the vessel of interest. Systemic anticoagulants are administered: unfractionated heparin is most frequently used (to an activated clotting time of 250 to 300 seconds), or bivalirudin may be prescribed, adjusted for weight and renal function in accordance with local practice standards. A 0.014-inch coronary guide wire is advanced well past the vessel segments to be imaged and typically, 100- to 200-mcg doses of intracoronary nitroglycerin are given to minimize coronary spasm upon advancement of the IVUS probe. The IVUS catheter is flushed, prepped, connected, and calibrated in accordance with manufacturer recommendations specific to the catheter being used, then introduced over the guide wire into the vessel and past the vessel segment to be imaged ( Fig. 17.3 and Video 17.3). Appropriate care must be taken as with any intracoronary device manipulation to minimize the risk of trauma to the vessel, thrombosis, or embolism. Image acquisition may be performed at specific regions of interest with the IVUS probe stationary in the vessel, or as a continuous pullback sequence with the catheter withdrawn through a length of vessel. Pullback recordings may be performed manually or using an external motorized pullback device that has speed settings ranging from 0.5 to 1.0 mm/sec. The longitudinal resolution is determined by the pullback speed and a user-defined digitization frame rate. The advantages of motorized pullback are that longitudinal resolution remains constant and lesion length may be accurately measured. Image acquisition is ECG gated to prevent systolic-diastolic and cyclic motion artifacts. The starting and ending points of the pullback run are typically recorded under fluoroscopy or cineangiography to facilitate ease of interpretation. The IVUS sequences are digitally archived for immediate, later, or remote viewing.
Modalities of IVUS imaging
A number of imaging modalities exist under the umbrella of IVUS. The most commonly used mode is gray-scale imaging ( Fig. 17.4 and Video 17.4). This type of imaging processes backscattered acoustic signals in real time to create a two-dimensional cross-sectional image in shades of gray. Reflection, absorption, and scattering phenomena attenuate the returning echo amplitude. Increasing gain amplifies the reflected signal during image acquisition in both the signal and noise. Dynamic range, also known as contrast resolution , refers to the ability to reconcile subtle differences in tissue density. A lower dynamic range results in images that are dominantly black or white without many intermediate gray levels. A higher dynamic range incorporates more shades of gray that reflect softer tissue structures so that tissue subtleties are highlighted. Gray-scale resolution may be influenced by several factors as reviewed in the previous section, and longitudinal resolution is dependent on pullback rate and framing speed.
ChromaFlo imaging (Volcano Corp., Rancho Cordova, Calif.) is a proprietary adjunct to gray-scale imaging; it tracks blood flow within the vessel lumen in real time. This software is based on conventional Doppler shift principles with frequency differences relating to calculated velocities. The ChromaFlo software captures up to 30 frames per second, comparing sequential axial images in real time. It discerns slight differences in the position of echogenic blood particles between successive images and denotes blood flow using a red color scheme. Demarcation of blood flow within the vessel is particularly helpful in visualizing dissection, ulceration, perforation, and side branches. It is also helpful for determining adequacy of stent deployment and apposition as well as differentiating soft plaque, thrombi, and blood speckle.
Other advanced postprocessing techniques such as autoregressive modeling, fast Fourier transformation, and wavelet analysis allow characterization of different histologic tissue types within atherosclerotic plaque. VH-IVUS (Volcano Corp., Rancho Cordova, Calif.) uses autoregressive spectral analysis of IVUS backscattered data, whereas iMAP IVUS (Boston Scientific, Marlborough, Mass.) employs a different algorithm for backscattered ultrasound frequency spectrum analysis. The four identifiable tissue types represented in VH and iMAP terminology are fibrous/fibrotic, fibrofatty/lipidic, necrotic core/necrotic, and dense calcium/calcific (see Fig. 17.4 ). These algorithms have been validated and used in multiple clinical studies to date, including the multicenter PROSPECT trial, which used VH-IVUS to estimate future atherothrombotic risk in patients who had developed acute coronary syndromes.
IVUS technologies that are currently investigational include high-definition (HD-IVUS; Silicon Valley Medical Instruments, Fremont, Calif.) and forward-looking IVUS (FL.IVUS; Volcano Corp., Rancho Cordova, Calif.). HD IVUS employs a transducer, which may be operated at 40 or 60 MHz for maximal axial resolution. The objective of this approach is to provide near-optical resolution without the limited penetration and device complexity that comes with optical coherence tomography (OCT). FL.IVUS combines an imaging catheter with radiofrequency tissue ablation for chronic total occlusion (CTO) recanalization. FL.IVUS can be used to address the inability of conventional (cross-sectional) IVUS to acquire images distal to the transducer. FL.IVUS employs a 300-Hz spiraling ultrasonic beam that is directed forward to create 60-degree conical volumes 5 to 10 mm distal to the catheter tip. One proposed development pathway is to mate the FL.IVUS imaging catheter with a radiofrequency ablation device, allowing high-fidelity luminal catheter positioning information to assist with safe CTO recanalization.
Conclusions
Intravascular ultrasound imaging remains an important if underused adjunct to conventional coronary angiography. Recent years have seen a proliferation of IVUS imaging systems, miniaturization of IVUS catheters, and development of advanced complementary imaging technologies. Established applications of IVUS include assessing vascular anatomy, precisely quantifying lesion extent and severity, characterizing plaque, planning PCI strategy, and optimizing PCI results. Future applications may include high-resolution IVUS imaging and assessing vessels in longitudinal tissue planes.
Intravascular Ultrasound: Applications and Limitations
- Elizabeth M. Retzer, MD
- Vikrant Jagadeesan, MD
- Sandeep Nathan, MD, MSc
- Vikrant Jagadeesan, MD
Intravascular ultrasound (IVUS) was originally developed in the mid-1950s as a diagnostic tool for imaging the endocardial surfaces of cardiac chambers, but it evolved over the next several decades into a useful in vivo coronary imaging tool. The first human coronary image was generated with IVUS in 1988, and it subsequently progressed to its current role as an important adjunct to complex percutaneous coronary intervention. As detailed in the previous chapter, even when optimal image quality is attained, angiography only provides two-dimensional lumenography of what is often a complex, three-dimensional structure. Diagnostic accuracy and reproducibility of angiography can be hindered by numerous factors, including contrast streaming, motion artifacts, lesion foreshortening and eccentricity, ectasia or diffuse reference vessel disease (rendering reference vessel caliber difficult or impossible to ascertain), tortuosity, and vessel overlap. IVUS potentially remedies many of these deficiencies by generating reproducible, tomographic images of the vessel lumen, and by providing additional vessel- and lesion-specific characteristics not observable or quantifiable by conventional angiographic imaging techniques alone.
Current applications
Types of IVUS Imaging and Basics of Image Interpretation
As introduced in the preceding chapter, there are multiple IVUS imaging modes currently available, each providing valuable information about the vascular segment of interest. Gray-scale imaging, serving as the foundation technology, allows for visualization of the vessel wall and delineation of its three layers, so that vessel dimensions and remodeling can be assessed. Lesion severity; eccentricity; gross compositional and topographic features, such as presence, extent, and depth of calcification; and presence of dissections can all be determined as well. IVUS imaging fundamentally relies on the ability to distinguish adjacent tissue planes and tissue types on the basis of differences in acoustic signature. Muscular arteries such as those in the coronary tree typically display a trilayer appearance. The intima appears as a thin white stripe closest to the lumen. The central layer is the smooth muscle of the media, which appears echolucent (black); and the outer layer is collagen-rich, echodense adventitia, which is white and has a characteristic onionskin appearance. Plaque in a diseased vessel is confluent with the intima, and connective tissue surrounding a vessel is as echoreflective as the adventitia; therefore, distinct boundaries are not visualized in either case (between intima and plaque, or between adventitia and connective tissue) ( Fig. 18.1 ). The addition of nondirectional, color flow imaging (ChromaFlo, Volcano Corp., Rancho Cordova, Calif.) to gray-scale IVUS provides real-time information regarding blood flow within the vessel. This helps further delineate dissection planes, identify side branches and aids in evaluations of percutaneous coronary intervention (PCI) results and device deployment (i.e., stent strut apposition). Additionally, this application also helps distinguish blood speckle from soft plaque, thrombus, and other tissues and materials ( Fig. 18.2 ). Virtual Histology (Volcano Corp., Rancho Cordova, Calif.) and iMAP (Boston Scientific, Natick, Mass.) can be used to evaluate lesion composition, differentiating between fibrous and fibrofatty tissue, necrotic core, and dense calcium ( Fig. 17.4 ; also illustrated in the preceding chapter).
