Imaging Artifacts During Transesophageal Echocardiography

Abstract

A 54-year-old man is undergoing elective mitral valve surgery. Intraoperative transesophageal echocardiography after aortic cannulation reveals multiple linear structures inside the ascending aorta (Video 1, see Supplemental Digital Content 1, https://links.lww.com/AA/A707), which mimic an aortic dissection. Echocardiographic imaging results from the interaction between the ultrasound beam, which is generated by the piezoelectric effect at the transducer, and biologic tissue. As the ultrasound beam propagates, it interacts with different media. These interactions include reflection, refraction, and transmission. The travel time and loss of energy of the returning echo is used to generate the 2-dimensional (2D) image. In soft tissue (myocardium), the ultrasound beam travels with a speed of 1540 m/s. A structure (reflector) at a distance (D) from the transducer will generate a reflection that traveled for 2 × D/1540 seconds or D/770 seconds before returning to the transducer. The ultrasound system processes the returning echoes to generate a 2D image. Generation of the 2D image is based on the principles listed in Table 1. When these assumptions are violated, artifacts are generated.1,2Table 1: Assumptions Made During 2-Dimensional ImagingAn echocardiographic imaging artifact does not convey anatomic information. This is the case when the displayed structure is not real, or a real structure is misplaced, absent, enhanced, or attenuated. It is obvious that a thorough understanding of the mechanism of artifact generation is necessary to optimize imaging and avoid misinterpreting anatomic data, which may lead to erroneous diagnosis. In this echo didactic, we summarize the most commonly encountered imaging artifacts in 2D echocardiography (Table 2).3,4Table 2: Types of Imaging Artifacts and Their CharacteristicsMULTIPLE REFLECTIONS Multiple echoes result from violation of the “1 reflector–1 echo” principle. They range from parallel lines to continuous echoes or a duplicate image. They are typically displayed distal or lateral to a reflector, in a straight line along the path of the primary ultrasound beam. Reverberation Reverberations occur when a portion of the primary echo encounters a strong secondary reflector on its way back to the transducer. While the transducer is receiving the primary echo, the remaining ultrasound energy from the secondary reflector will travel back and encounter the original reflector a second (or third, etc.) time. Having traveled a longer time (2 × D/770 or 3 × D/770 seconds, respectively), reverberation artifacts appear in a characteristic “step ladder” fashion, which does not respect anatomic boundaries. In clinical practice, the secondary reflector is usually the ultrasound probe or the highly reflective walls of a calcified descending aorta or a pulmonary artery catheter, etc (Video 1, see Supplemental Digital Content 1, https://links.lww.com/AA/A707). A reverberation artifact from tracheal rings is frequently noted on initial insertion of the transesophageal echocardiography probe (Fig. 1). Decreasing gain and by using alternate imaging planes are effective strategies for reducing or eliminating reverberation artifacts.Figure 1: Reverberation artifact from tracheal rings during insertion of the transesophageal echocardiography probe (upper esophageal aortic views in short (left) and long (right) axis.Comet Tail (Near-Field Clutter or Ring Down) Multiple, short-path reverberations resulting from closely spaced surfaces, as is the case with aortic plaque deposits, collections of microbubbles,5,6 or components of prosthetic valves, are displayed as nearly continuous echoes and are called “comet tail” or “ring down” (Fig. 2). When this type of artifact occurs in the near field, it is referred to as “near-field clutter.” Comet tail artifacts typically occur when ultrasound interacts with closely spaced surfaces of moderate density that allow transmission of sound but reflection at the interfaces. Denser objects, however, tend to reflect the ultrasound beam leading to acoustic shadowing (see text) instead. These artifacts may overlap anatomic structures and make distal imaging impossible.Figure 2: Comet tail artifact is noted in the upper esophageal long-axis view of the aortic arch. A mirror image of the aorta is also noted distal to the actual structure.Comet tail artifacts generated at the pleural interface, also referred to as “lung rockets,” “lung comets” or “B-lines,” have recently been exploited in the emerging field of critical care ultrasound to aid in the diagnosis and management of pulmonary congestion and other pulmonary pathology.7 Mirror Image Mirror images are created by a mechanism similar to reverberations. A portion of the echoes is traveling between the near and far sides of a reflector before returning to the transducer. The result is the imaging of an identical structure, distal to the original reflector. Mirror image artifacts are particularly common when imaging the descending aorta and aortic arch in short- or long-axis views, respectively: the anterior aortic wall is duplicated in the distal image sector (“double-barrel” aorta).8 Typically, application of color Doppler to the ascending aorta will also be duplicated in a double-barrel artifact. The fluid-filled pulmonary artery catheter (Video 2, see Supplemental Digital Content 2, https://links.lww.com/AA/A708), pericardium,9 and a prosthetic mitral valve ring10 are also examples of objects duplicated in the distal part of the sector image. Linear Artifact The linear artifact is typically encountered in the ascending aorta where it can be misinterpreted as the intraluminal flap of an aortic dissection. Linear artifacts are generated when a portion of the echo from the anterior wall of the left atrium or the right pulmonary artery (Video 3, see Supplemental Digital Content 3, https://links.lww.com/AA/A709) is reflected off the transducer surface. This reverberation-type artifact will be placed twice the distance from the transducer and appears as an intra-aortic linear structure that is the mirror image of the atrial-aortic or pulmonary-aortic interface, respectively. Linear artifacts are particularly likely to occur when the ascending aorta is dilated (diameter >5.0 cm, or atrial diameter <0.6 of aortic diameter)11 so that the artifact will “fit” within the aortic lumen. The presence of similar blood flow velocities on both sides of the presumed flap and motion parallel to the aortic wall are suggestive of a linear artifact rather than an aortic dissection flap.12 This motion can also be seen with M-mode.13 MISPLACED REFLECTIONS Misplaced reflections occur when objects located lateral to the ultrasound beam are incorrectly imaged as having originated within the path of the ultrasound beam. Side Lobe The ultrasound beam comprises a primary beam (from which all reflections are presumably generated) and multiple diverging side beams. The ultrasound energy of the side beams is dissipated to neighboring tissues and typically produces weak reflections that are ignored by the ultrasound system. When side beams encounter strong reflectors, however, the system may interpret their echoes as having originated from the primary beam. This results in misplaced reflections.14 As the transducer is rapidly oscillating while scanning the area of interest, multiple such artifacts are generated. Side (or grating) lobe imaging artifacts are linear echoes, displayed in an “arc-like” fashion at a radial distance from the transducer on both sides of the true object that can traverse anatomic borders.4 Side lobe and grating lobe artifacts frequently occur in the setting of a highly reflective wire or catheter (Fig.3, Video 4, see Supplemental Digital Content 4, https://links.lww.com/AA/A710).Figure 3: A side lobe artifact and acoustic shadowing artifacts (laterally and distally, respectively) due to calcium deposit in the anterior wall of the aortic arch (upper esophageal long-axis view). Note enhancement of the aortic wall.MISSING REFLECTIONS It is assumed that ultrasound attenuation occurs at a constant rate due to reflection, absorption, and scattering. Attenuation artifacts are generated distal to the ultrasound beam path when, in otherwise homogenous tissue, focal areas of increased or decreased attenuation are encountered. Acoustic Shadowing Acoustic shadowing is one of the most common causes of missing structures during echocardiographic examination. Reflection of ultrasound waves is a function of the difference in acoustic impedance (the product of density and propagation speed of ultrasound in tissue) between adjacent tissues traversed by the ultrasound beam. When the “mismatch” in acoustic impedance is great, a significant proportion of the ultrasound beam is reflected and only a smaller proportion is transmitted distally. Strong reflectors (structures with high density), such as prosthetic valves, areas of calcification, pacemaker wires or catheters, decrease the intensity or even completely block the distal propagation of the ultrasound beam. This can obscure distal imaging and lead to the phenomenon of acoustic shadowing. Acoustic shadows are typically detected as hypoechoic or anechoic areas in otherwise uniform tissue (Figs. 3 and 4). This artifact may obscure imaging of the right or left ventricle after prosthetic aortic or mitral valve placement, respectively.Figure 4: Acoustic shadowing is noted distal to a prosthetic mitral valve in this midesophageal 4-chamber view. Note the enhancement of the septum.When shadowing occurs, switching to an alternate imaging plane so that the strong reflector is in the far sector can facilitate imaging of distal structures (i.e., switching from midesophageal views to transgastric 2-chamber or transgastric long-axis views when a mitral prosthesis obscures the myocardial walls). Increasing the time-gain compensation on the ultrasound machine can improve imaging of the hypoechoic (but not anechoic) areas of the sector display. Enhancement Enhancement artifacts are the opposite of acoustic shadowing. They occur when the ultrasound beam encounters a weak reflector or is weakly attenuated by an object in its path. Since the returning echoes distal to such weakly attenuating structures are of higher amplitude than other structures at similar depth, the system incorrectly displays them as areas of increased echogeneity (Figs 3 and 4). Decreasing time-gain compensation in areas of enhancement on the 2D image can reduce the appearance of enhancement artifact. Enhancement is typically encountered at the boundaries of the ventricular wall and the pericardium. An understanding of imaging artifacts is critical for the novice echocardiographer. Failure to recognize these entities can lead to erroneous interpretation of the ultrasound image and significantly impact clinical decision making. Teaching Points Most artifacts occur distal or next to a reflector and cross anatomic borders. Reverberations are false images caused by the back and forth traveling of an echo. They may show as horizontal or perpendicular lines, or even as a duplicate of a reflector. A strong reflector may be displayed inside a distal structure such as a dilated aorta and mimic a dissection flap. The artifact will be displayed at twice the distance of the reflector from the probe. Color Doppler can be particularly helpful in assessing whether a displayed structure is true or an artifact. Alternative imaging planes should be used when a strong reflector causes distal shadowing. Adjusting time-gain compensation can help reduce the effects of shadowing and enhancement artifacts. DISCLOSURES Name: Anup Pamnani, MD. Contribution: This author helped analyze the data and write the manuscript. Attestation: Anup Pamnani approved the final manuscript. Name: Nikolaos J. Skubas, MD, FASE. Contribution: This author helped analyze the data and write the manuscript. Attestation: Nikolaos J. Skubas approved the final manuscript. This manuscript was handled by: Martin J. London, MD.