Approach Considerations
The patient must be anticoagulated, usually with heparin, before inserting the guidewire into the coronary artery. Unless it is contraindicated, image acquisition should be performed after administrating intracoronary nitroglycerin to avoid catheter-induced spasm.
Standard coronary interventional techniques and equipment (guiding catheter and 0.014-inch angioplasty guidewire) are used for catheter delivery for intracoronary ultrasound examination. The catheter is placed distal to the segment of interest. Subsequently, the operator retracts the transducer, either manually or with a motorized pullback device. During pullback, images are obtained and recorded digitally for analysis.
Motorized pullback devices allow withdrawal at a constant speed (between 0.25 mm/s; most frequently 0.5 mm/s), which is essential in serial studies. Motorized pullbacks permit length and volumetric measurements and provide uniform and reproducible image acquisition for multicenter and serial studies. However, inadequate examination of important regions of interest can occur because the transducer does not remain for long at any specific site in the vessel.
Normal Arterial Appearance
A standard intravascular ultrasound image consists of 3 main components: catheter, lumen, and arterial wall. An ultrasound reflection is generated at a tissue interface if an abrupt change in acoustic impedance occurs. In the normal artery, 2 such interfaces are usually observed: one at the border between blood and the leading edge of the intima and a second at the external elastic membrane, which is located at the media-adventitia border. In patients younger than 40 years, the reported normal value for intimal thickness is typically between 0.15 and 0.25 mm. [22] Most investigators use 0.25 to 0.50 mm as the upper limit of normal.
(See the videos below.)
Quantitative Measurements
Border identification
Recognizing that all ultrasound techniques, including intravascular ultrasonography, require measurements to be performed at the leading edge of boundaries, never the trailing edge, is important. In muscular arteries such as the coronary arteries, frequently 3 layers exist. [23] The innermost layer consists of a complex of intima and internal elastic membrane. This innermost layer is relatively echogenic as compared to the lumen and media. The trailing edge of the intima (which would correspond to the internal elastic membrane) cannot always be distinguished clearly. Moving outward from the lumen, the second layer is the media. The third and outer layers consist of the adventitia and periadventitial tissues, respectively. The boundary separating the true adventitia from surrounding perivascular tissues is less well defined on intravascular ultrasonography images.
Lumen measurements
Lumen measurements are performed using the interface between the lumen and the leading edge of the intima. Generally, the leading edge of the innermost echogenic layer should be used as the lumen boundary. The following basic measurements can be recorded depending upon the operator’s preference:
-
Lumen cross sectional area (CSA): The area bounded by the luminal border
-
Minimum lumen diameter: The shortest diameter through the center point of the lumen
-
Maximum lumen diameter: The longest diameter through the center point of the lumen
-
Lumen eccentricity: 1 [(Maximum lumen diameter – minimum lumen diameter)/maximum lumen diameter]
-
Lumen area stenosis: (Reference lumen CSA – minimum lumen CSA)/reference lumen CSA
The reference segment used should be specified as proximal, distal, largest, or average.
Further special measurements facilitated by intravascular ultrasonography are as follows:
-
External elastic membrane measurements
-
Atheroma measurements
-
Calcium measurements
-
Stent measurements (a postdeployment minimal stent area of 5 mm2 predicts increased likelihood of angiographic restenosis)
-
Remodeling
-
Length measurements
Qualitative Assessment
Atheroma morphology
Although intravascular ultrasonography cannot be used to detect and quantify specific histologic contents, certain image patterns can be very useful in estimating the morphology and content of the atheroma.
Soft (echolucent) plaques refer to the acoustic signal that arises from low echogenicity instead of plaque’s structural characteristics. Although a zone of reduced echogenicity generally results from high lipid content in a mostly cellular lesion, it could also result from a necrotic zone within the plaque, an intramural hemorrhage, or a thrombus. [23, 24]
Fibrous plaques have an intermediate echogenicity between soft (echolucent) atheromas and highly echogenic calcific plaques. [22] Generally, the greater the fibrous tissue content, the greater the echogenicity of the tissue.
Ultrasound imaging is more sensitive than fluoroscopy for coronary calcification detection. [25] Large calcifications may be associated with lesion stability. In contrast, microcalcifications are frequently found in lipid-rich necrotic core areas of unstable plaques and may not be well reflected in intravascular ultrasonography images. [26]
Plaques frequently contain more than one acoustical subtype.
A thrombus is usually recognized as an intraluminal mass, often with a layered, lobulated, or pedunculated appearance. [27] However, in vitro studies have revealed limitations of intravascular ultrasonography in the diagnosis of thrombi (sensitivity of 57% and specificity of 91%), considerably inferior to a conventional angiogram. [28] Intravascular ultrasonography is still unreliable in differentiating acute thrombi from echolucent plaques because of the similar echogenicity and texture of lipid-laden tissue, loose connective tissue, and stagnant blood.
The intimal hyperplasia characteristic of early in-stent restenosis often appears as tissue with very low echogenicity, at times less echogenic than the blood speckle in the lumen. Appropriate system settings are critical to avoid suppressing this relative nonechogenic material. The intimal hyperplasia of late in-stent restenosis often appears more echogenic.
Evaluating dissections and other complications after intervention
Intravascular ultrasound is commonly used to detect and direct the treatment of dissections and other complications after intervention. Intravascular ultrasonography allows for detailed classification of dissection and assessing the severity of the dissection. Additionally important characteristics of a dissection, such as the presence of a false lumen, the identification of mobile flap(s), the presence of calcium at the dissection border, and dissections in close proximity to stent edges are identified distinctly by intravascular ultrasonography.
Unstable lesions and ruptured plaques
Although no definitive features define a plaque as vulnerable on intravascular ultrasonography, a hypoechoic plaque without a well-formed fibrous cap is presumed to represent potentially vulnerable atherosclerotic lesions. [29]
In patients studied after an acute coronary syndrome, ultrasound imaging may reveal ulceration, often with remnants of the ruptured fibrous cap evident at the edges of the ulcer. Various other appearances are common, including fissuring or erosion of the plaque surface.
Unusual lesion morphology
Intravascular ultrasonography can be used to characterize unusual lesion morphology, such as aneurysms, pseudo-aneurysms, and true versus false lumens.
Ambiguous lesions
Intravascular ultrasonography lends itself to the identification of technically difficult lesions, such as intermediate lesions of uncertain stenotic severity or ostial stenosis and disease at branching sites. The application of intravascular ultrasonography in these circumstances depends on the interventionist. Intravascular ultrasonography can also be helpful for delineating lesions involving tortuous vessels and left mainstem lesions. Intravascular ultrasonography has a particularly important role in defining areas with intraluminal filling defects, angiographically hazy lesions, sites with plaque rupture, and lesions with local flow disturbances. Intravascular ultrasonography can be used to determine vein graft morphology in situ as well.
In-stent Restenosis
Of particular note is the role of intravascular ultrasound in establishing the mechanisms of in-stent restenosis. Restenosis after balloon angioplasty is largely driven by concentric geometric remodeling, with neointimal hyperplasia playing a lesser role. However, in-stent restenosis is almost exclusively the consequence of exuberant neointimal proliferation. [30, 29]
Intravascular ultrasonography measurement of minimum stent area is recognized as the most powerful predictor of long-term patency and clinical outcomes. [29] Intravascular ultrasonography observations were instrumental in the development of strategies to treat in-stent restenosis, including intracoronary brachytherapy. Most importantly, they led to the development of drug-eluting stents. Drug-eluting stents have dramatically reduced neointimal proliferation and the incidence of in-stent restenosis. [31]
Special Disease Considerations
Assessment of transplant vasculopathy
Posttransplantation coronary artery disease is the leading cause of death beyond the first year after cardiac transplantation, with a reported incidence of nearly 20% per year. [32] Intravascular ultrasonography not only allows assessment of early plaque accumulation before luminal stenosis develops in comparison to other diagnostic methods, but studies have also demonstrated an association between the severity of disease by ultrasound and clinical outcomes. [33] Patients with more severe disease on intravascular ultrasonography studies had an increased incidence of death, myocardial infarction, or retransplantation. [34, 35]
Aortic, carotid, and peripheral vascular disease
The role of intravascular ultrasonography in noncoronary vascular disease is expanding. The various unique limitations of traditional angiography in defining the anatomy of peripheral vessels can be circumvented by intravascular ultrasonography. [36] With routine angiography, often angulated or orthogonal views are not possible and foreshortening is not apparent. Heavy calcification in arteries deep within the thorax, abdomen, pelvis, and thigh can obscure the vessel borders even after digital subtraction. Intravascular ultrasonography can provide additional details on the lesion in such cases.
Nevertheless, several intravascular ultrasound artifacts or limitations are yet to be resolved, including catheter obliquity, limited spatial resolution in very large vessels, and severe acoustic shadowing by calcific plaque.
-
A still intravascular ultrasonography image demonstrating a normal coronary artery.
-
An intravascular ultrasonography run from a segment of normal coronary artery.
-
An intravascular ultrasonography video highlighting the appearance of a branch from the main stem coronary artery. Note the coronary artery branch appearing at the 1'o clock position of the screen.
