Lesion Morphology
In a normal coronary artery, an ultrasound reflection is generated at two tissue interfaces: at the border between blood and the leading edge of intima and at the external elastic membrane (EEM) located at the media-adventitia border. The resulting three-layered structure consists of the tunica intima (bright, relatively echogenic layer compared to lumen and media), media (dark, less echogenic layer compared to intima) and adventitia (bright). Tunica media has lower ultrasound reflectance due to lower content of collagen and elastin (highly reflective materials) compared to intima and adventitia. The trailing edge of the intima, internal elastic membrane (IEM), cannot always be distinguished clearly on IVUS images. Similarly, IVUS cannot detect the outer border of adventitia due to comparable echoreflectivity of adventitia and periadventitial tissue.1,2
1. J Am Coll Cardiol. 2001 April, 37(5): 1478-92
2. Circulation. 2001;103:604-616
Fibrous plaques demonstrate echogenicity intermediate between hypoechoic soft plaques and hyperechoic calcified lesions. The majority of early atherosclerotic lesions are fibrous plaques. In general, the higher the amount of fibrous tissue, the greater the echogenicity of atheroma. Very dense fibrous plaques may appear brighter than adventitia. In some cases, fibrous plaques can show sufficient echo attenuation and acoustic shadowing to be misclassified as calcifications.1
1. J Am Coll Cardiol. 2001 April, 37(5): 1478-92
Soft (echolucent), or fatty plaques are characterized by low echogenicity due to the high lipid content in mostly cellular lesions. However, areas of low echogenicity might also arise from a necrotic core within the plaque, an intramural hemorrhage or thrombus. In general, soft plaques contain minimal collagen and elastin and demonstrate lower echogenicity than adventitia.1
1. J Am Coll Cardiol. 2001 April, 37(5): 1478-92
Attenuated plaque is defined as hypoechoic plaque with deep ultrasound attenuation without calcification or very dense fibrous plaque. Histologically, attenuated plaque on IVUS correlates with a fibroatheroma containing a large necrotic core or pathological intimal thickening with a large lipid pool. Attenuated plaques identified by grayscale IVUS are associated with ST-segment elevation MI and no reflow in patients with CAD who undergo PCI.1,2
1. J Am Coll Cardiol. 2009, 2(1): 65-72
2. JACC Cardiovascular Interv. 2011, 4(5): 495-502
Calcified deposits are represented by high intensity signals (yellow dotted line) with acoustic shadowing without the passage of ultrasonic waves, since high frequency ultrasound does not penetrate calcium. As a result, IVUS can detect only the leading edge of a calcification and is not able to assess the thickness of the deposit. Calcium deposits can be classified according to their location as superficial (calcium at the intimal-lumen interface or closer to the lumen than to the adventitia), deep (calcium at the media/ adventitia border or closer to the adventitia than to the lumen), or both. In many cases, calcific deposits produce reverberations (multiple reflections appearing as concentric arcs) resulting from the oscillation of ultrasound between transducer and calcium.1,2
1. Circulation. 1995, 91(7): 1959-1965.
2. J Am Coll Cardiol. 2001 April, 37(5): 1478-92
A calcified nodule is a type of potentially vulnerable plaque accounting for approximately 2% to 7% of coronary events. Calcified nodules have distinct IVUS characteristics and can be identified by the following criteria: 1) a convex shape of the luminal surface, 2) a convex shape of the luminal side of calcium, 3) an irregular luminal surface, and 4) an irregular edge of calcium.1
1. Am J Cardiol 2011;108:1547–1551
Most plaques consist of a mixture of two or all three of the main atheroma types: fibrous, soft (fatty), and calcific. Mixed plaques can be described according to their echogenic properties as fibrofatty, fibrocalcific, etc.1
1. J Am Coll Cardiol. 2001 April, 37(5): 1478-92
Plaque rupture is defined as a lesion with a large necrotic core and a thin ruptured fibrous cap. IVUS images of plaque rupture reveal a cavity (ulceration) with a connection to the lumen; remnants of the ruptured fibrous cap can be visualized in some cases. Calcium deposits are often seen at the base of the ulcer. Plaque rupture is the IVUS finding with the second highest odds ratio for the occurrence of slow flow phenomena after attenuated plaque. Plaque rupture has been associated with distal embolization after PCI, similar to other lesion morphologies such as large plaque burden, positive remodeling, and attenuated plaque.1,2
1. J Am Coll Cardiol. 2001 April, 37(5): 1478-92
2. Am J Cardiol. 2006 Jan 1;97(1):29-33
Myocardial bridging is a common coronary anomaly, in which a coronary artery segment traverses through the myocardium. The muscle overlying the artery is termed a myocardial bridge, and the intramyocardial segment of the artery is referred to a bridged or tunneled artery. IVUS allows for objective measurement and quantification of the phasic compression of a bridged arterial segment. A characteristic echolucent band, a “half-moon” (arrows) can be visualized by IVUS between the bridged segment and epicardial tissue. Based on histopathology studies, the echolucent band identified by IVUS represents a muscle band overlying the bridged arterial segment.1
1. J Am Coll Cardiol. 2021 Nov, 78 (22) 2196-2212
Spontaneous coronary artery dissection (SCAD) is a rare cause of ACS with an approximate incidence of 0.07-1.1%. Typically, SCAD presents in younger patients without conventional risk factors for coronary artery disease and is more common in women younger than 50 years of age. The most common triggering factors are pregnancy, hormone therapy, and emotional stress. SCAD is a spontaneous, non-traumatic, and non-iatrogenic separation of the coronary artery wall by intramural hemorrhage, which can occur with or without an intimal dissection (tear). The false lumen with intramural hematoma can propagate antegrade and retrograde leading to compression of the lumen. Depending on the degree of the occlusion, SCAD can result in ischemia or ACS. A false lumen is usually parallel to the true lumen and does not communicate with the true lumen for a portion of the vessel length. A true lumen can be identified by the presence of all three layers of the vessel wall (intima, media and adventitia) and the side branches.1,2
1. J Am Coll Cardiol. 2001 April, 37(5): 1478-92
2. Catheter Cardiovasc Interv. 2014 Dec 1;84(7):1115-22
Artifacts
A guidewire artifact is a phenomenon in which ultrasound waves from an IVUS catheter are reflected by a guidewire resulting in the appearance of “sparkling dots” and characteristic shadows behind them. Whenever a mechanically scanned IVUS catheter is used, this will occur around the catheter in the image. The example shows a left main trifurcation lesion with three guide wire artifacts corresponding to guidewires in the LCX, LAD and RI. The image behind the catheter (outside the vessel) will be lost due to the artifacts.
Reverberation is an artifact represented by secondary, false echoes generated by the same structure. It is usually observed as concentric arcs at duplicated distances. If there is a reflector with strong acoustic impedance in the vessel wall, the returned echo may be reflected again by the ultrasonic probe, transmitted back, and returned from the reflector again. This phenomenon occurs in calcified lesions with a smooth surface, especially after rotational or orbital atherectomy, stent struts, etc. Reverberation from the leading edge of calcium can be used to predict the thickness of the deposit. In a multimodality intravascular imaging study, IVUS calcium with a smooth surface and reverberations was thinner by OCT compared to IVUS calcium with an irregular surface without reverberations.1
1. JACC Cardiovasc Imaging. 2017 Aug;10(8):869-879
Calcium can produce reverberations (multiple reflections) resulting from oscillation of ultrasound signal between the transducer and smooth surface of the calcific deposit. Rotational atherectomy selectively ablates calcified plaques resulting in luminal dilation. As a result of polishing the surface of the calcification, a higher rate of reverberations can be also observed post-RA compared to pre-RA. Maximum reverberation number, angle, and length increased after RA indicating calcium modification even though there was no significant decrease in the calcium angle and length.1
1. Int J Cardiovasc Imaging . 2018 Sep;34(9):1365-1371.
Non-uniform rotational distortion (NURD) is and results from mechanical binding of the drive cable that rotates the transducer. This motion artifact has smeared appearance and leads to a distortion of the underlying plaque. Unique to rotational systems, NURD can occur due to vessel tortuosity, tortuous shape of the guide catheter, instability of catheter engagement, manufacturing variances of the hub or drive shaft, overtightening of a hemostatic valve, kinking of the imaging sheath, or use of guiding catheters with small lumens.1
1. J Am Coll Cardiol. 2001 April, 37(5): 1478-92
Air bubble artifact is caused by improper catheter flushing with saline. In a mechanically scanned IVUS catheter, the presence of gaseous components such as air around the probe can cause artifacts in the area close to the probe due to the differences in acoustic impedance. Proper catheter preparation that includes complete flushing of air bubbles in the catheter with saline helps prevent the artifact in majority of cases. If air artifact occurs inside the body, the catheter should be removed before flushing and reintroduce to the coronary artery upon successful completion of flushing to prevent an air embolism. Air embolism might lead to slow flow or no reflow.1
1. J Am Coll Cardiol. 2001 April, 37(5): 1478-92
Radiofrequency noise appears as alternating radial spokes or random white dots in the far field. The interference is usually caused by electrical equipment in the cardiac catheterization laboratory. When the cause is identified, the artifact can be avoided by turning off or moving the equipment.
The artifact is caused by extraneous beams originating from a strong reflecting surface such as metal stent or calcification. They appear as bright rounded lines displayed over hypoechoic structures adjacent to hyperechoic structures. Side lobe echoes originated from metal stent struts might be mistaken for stent struts protruding into the lumen, potentially interfering with lumen area measurements and the assessment of stent apposition.
Ring-down artifacts are usually observed as bright halos of variable thickness surrounding the catheter, which obscure the near field imaging. The artifact can be electronically subtracted from the image in solid-state catheters, however it can limit the ability of the system to image the areas adjacent to the surface of the catheter. Microbubbles within the protective sheath may be responsible for the artifact in a mechanical catheter system. Repeated saline flush might help to reduce near-field ring-down.
