1. Correct Answer: A. Ultrasound propagates in a straight line in the direction of the central ultrasound beam.

Rationale: As the transducer emits an ultrasound wave and awaits the returning ultrasound wave to reconstruct an image, certain assumptions are being made with respect to wave propagation: (a) ultrasound propagates in a straight line in the direction of the central beam; (b) a given structure will reflect the beam only once; (c) only structures located within the intended path of the beam will generate reflections back to the transducer; (d) the position of a structure along the scan line is directly proportional to the travel time of the transmitted wave. These assumptions, however, are not always correct, and when they are not, typical ultrasound artifacts may appear. All clinicians who use ultrasonography must be fluent in ultrasound physics and artifacts so as to generate correct image interpretations.

1. Bertrand PB, Levine RA, Isselbacher EM, Vandervoort PM. Fact or artifact in two-dimensional echocardiography: avoiding misdiagnosis and missed diagnosis. J Am Soc Echocardiogr. 2016;29:381-391. doi:10.1016/j.echo.2016.01.009

2. Weyman AE. Principles and Practice of Echocardiography. Lea & Febiger; 1994.

3. Physics and instrumentation. In: Feigenbaum H, Armstrong WF, Ryan T, eds. Feigenbaum’s Echocardiography. 6th ed. Lippincott Williams & Wilkins; 2005:11-45.

2. Correct Answer: B. Refraction is determined by differences in acoustic impedance between two tissues.

Rationale: Ultrasound waves traveling through biologic tissue obey the physical laws of reflection and refraction (Snell’s law). The boundary of two tissues with different acoustic impedance can act as a specular (mirror-like) reflector if significantly larger than the wavelength of the ultrasound waves. A portion of ultrasound wave energy is reflected with reflection angle equal to the angle of incidence. Another portion will be transmitted with a refraction angle dependent on the magnitude of difference in acoustic impedance between both tissues (Figure 15.18).

1. Bertrand PB, Levine RA, Isselbacher EM, Vandervoort PM. Fact or artifact in two-dimensional echocardiography: avoiding misdiagnosis and missed diagnosis. J Am Soc Echocardiogr. 2016;29:381-391. doi:10.1016/j.echo.2016.01.009.

2. Physics and instrumentation. In: Feigenbaum H, Armstrong WF, Ryan T, eds. Feigenbaum’s Echocardiography. 6th ed. Lippincott Williams & Wilkins; 2005:11-45.

3. Weyman AE. Principles and Practice of Echocardiography. Lea & Febiger; 1994.

3. Correct Answer: D. are due to the assumption that a given structure will reflect the ultrasound beam only once.

Rationale: A reverberation artifact occurs when a reflected ultrasound wave on its way back to the transducer encounters a reflector in its path that reflects a portion of this returning energy back to the first reflector. A portion of sound energy that was not interrupted by the closer reflector returns to the transducer as expected and the first reflector’s structure is mapped accurately. The portion of sound energy that makes a second round trip to the first reflector and back to the transducer will have had a longer travel time. Due to the assumptions of wave propagation (see Question 15.2), the transducer interprets this reflected structure as being at a further distance from the transducer and thus maps a structure below the first reflector (at a distance below first reflector equal to the distance between first and second reflector). In clinical practice, the second reflector is often the ultrasound transducer itself—generating an artifact at a distance twice that of the first reflector (Figure 15.19). During the cardiac cycle, the motion of the reverberation artifact parallels that of the true structure but with a greater (typically double) amplitude. Multiple reflections between the two reflectors are possible, causing multiple reverberations with gradually diminishing intensity. Indeed, as ultrasound energy decreases with each additional round trip, each reverberation occurs at progressively weaker signal intensity than the true structure (Figure 15.19).

1. Bertrand PB, Levine RA, Isselbacher EM, Vandervoort PM. Fact or artifact in two-dimensional echocardiography: avoiding misdiagnosis and missed diagnosis. J Am Soc Echocardiogr. 2016;29:381-391. doi:10.1016/j.echo.2016.01.009.

2. Feldman MK, Katyal S, Blackwood MS. US artifacts. Radiographics. 2009;29:1179-1189. doi:10.1148/rg.294085199.

3. Kremkau FW, Taylor KJ. Artifacts in ultrasound imaging. J Ultrasound Med. 1986;5:227-237. doi:10.7863/jum.1986.5.4.227.

4. Correct Answer: C. Moving the transducer to an alternative imaging window

Rationale: Decreasing the image gain and using alternative imaging planes to avoid potential reflectors in the near-field are the most common strategies for recognizing and reducing/eliminating reverberation artifacts. Color Doppler (at normal or decreased scale) can be helpful to demonstrate that flow is not affected by the artifactual structure. The basic recognition of reverberation artifacts comes from appreciating structures at a “double distance” (compared to the probe-to-structure difference) exerting parallel motion while not respecting anatomic boundaries.

1. Bertrand PB, Levine RA, Isselbacher EM, Vandervoort PM. Fact or artifact in two-dimensional echocardiography: avoiding misdiagnosis and missed diagnosis. J Am Soc Echocardiogr. 2016;29:381-391. doi:10.1016/j.echo.2016.01.009

5. Correct Answer: C. Lacking well-demarcated borders

Rationale: One central principle to recall for all forms of artifact is that true structures cannot pass through cardiac or vascular walls and are typically well defined, unlike the indistinct borders of artifacts. True structures are seen in multiple imaging views, whereas artifacts typically cannot be reproduced from alternative probe positions (e.g., a reverberation artifact mimicking a thrombus in the left atrium in parasternal imaging windows cannot be reproduced in apical imaging windows). In addition, unlike true anatomic structures, artifacts will not accelerate or disturb surrounding color Doppler flow in any way. Table 15.1 summarizes the typical differences between true structures and artifacts, which can aid in the investigation of uncommon echocardiographic findings and offer clues toward a correct interpretation.

1. Bertrand PB, Levine RA, Isselbacher EM, Vandervoort PM. Fact or artifact in two-dimensional echocardiography: avoiding misdiagnosis and missed diagnosis. J Am Soc Echocardiogr. 2016;29:381-391. doi:10.1016/j.echo.2016.01.009

6. Correct Answer: A. the transducer emitting ultrasound energy outside of the central ultrasound beam.

Rationale: As the ultrasound transducer aims to focus the emitted ultrasound energy within a central ultrasound beam, small amounts of energy inevitably get emitted in other directions as well. These may form so-called “side lobes” of ultrasound energy that propagate off-axis. These small portions of ultrasound energy emitted in “side lobes” are mostly dissipated in the tissue without relevant reflections. However, when this side lobe energy is reflected by a strong reflector (wires, calcifications, pericardium) in its path, these reflections are interpreted by the transducer as originating from the central beam. As the transducer scans the imaging window by sweeping in a radial direction, numerous side lobe artifacts can be generated on both sides of the true reflector. When the true reflector is echodense and wide, these multiple side lobe images can overlap and visually merge, producing a linear arc-like artifact at a radial distance of the transducer (Figure 15.20).

1. Feldman MK, Katyal S, Blackwood MS. US artifacts. Radiographics. 2009;29:1179-1189. doi:10.1148/rg.294085199

2. Laing FC, Kurtz AB. The importance of ultrasonic side-lobe artifacts. Radiology. 1982;145:763-768. doi:10.1148/radiology.145.3.7146410.

3. Lu JY, Zou H, Greenleaf JF. Biomedical ultrasound beam forming. Ultrasound Med Biol. 1994;20:403-428. doi:10.1016/0301-5629(94)90097-3.

4. Pamnani A, Skubas NJ. Imaging artifacts during transesophageal echocardiography. Anesth Analg. 2014;118:516-520. doi:10.1213/ANE.0000000000000084.

7. Correct Answer: C. at a similar distance from the probe than the true structure.

Rationale: A refraction artifact, also called a “lens artifact” or “twin artifact,” is the false duplication of an object behind a structure that acts as a wave refractor and thus behaves as a lens. Ultrasound waves directed through the “lens” are refracted toward the respective cardiac object and then re-refracted back to the original direction of transmission on the return acoustic path, resulting in a duplicate image of this object along the original direction of the beam, at similar distance from the probe (Figure 15.21). These artifacts mostly occur in subcostal and parasternal imaging planes, with costal cartilage, fascial structures and fat, and pleural and pericardial surfaces acting as the medium inducing refraction of the ultrasound beam. Structures behind an ultrasound lens may not be visible in that plane because the sound beam never reaches them and instead they are “overwritten” by the duplicated image of a nearby structure. Adjusting the probe to avoid the lens or using alternative imaging windows are strategies to avoid the double image and assess the structures that were shadowed.

1. Buttery B, Davison G. The ghost artifact. J Ultrasound Med. 1984;3:49-52. doi:10.7863/jum.1984.3.2.49.

2. Ozeke O, Ozbakir C, Gunel EN. Double mitral valve imaging. J Am Soc Echocardiogr. 2010;23:340 e1-e2. doi:10.1016/j.echo.2009.08.017.

3. Spieker LE, Hufschmid U, Oechslin E, Jenni R. Double aortic and pulmonary valves: an artifact generated by ultrasound refraction. J Am Soc Echocardiogr. 2004;17:786-787. doi:10.1016/j.echo.2004.04.003.

8. Correct Answer: A. There are no cardiac abnormalities noted in this image.

Rationale: The echo image shown in Figure 15.1 shows no structural abnormalities. However, there is evidence of a mirror artifact more distal to the posterior pericardium-lung interface (Figure 15.22). A mirror artifact can present below a strong reflective surface that acts much as a mirror, producing a duplicate image behind the mirror of real structures that are located in front (more proximal) of the mirror. In dynamic images, mirrored structures move in the opposite direction from the mirror as do the real structures. The mechanism is similar to that of a reverberation: ultrasound waves hitting a strong reflector are reflected (angle of reflection = angle of incidence) toward objects closer to the transducer than the reflector. These intervening objects reflect the waves back to the strong reflector, which in turn sends them back to the transducer (Figure 15.22). Due to the assumption of wave propagation—that all the returning sound comes from objects in the initial direction of the sound beam—the scanner displays these objects below the strong reflector, at a distance equal to the distance between strong reflector and the true intervening objects. Mirror artifacts can be identified in two-dimensional images as a copy of structures located above a strongly reflective surface.

1. Adams MS, Alston TA. Echocardiographic reflections on a pericardium. Anesth Analg. 2007;104:506. doi:10.1213/01.ane.0000255056.78259.3c.

2. Bertrand PB, Verhaert D, Vandervoort PM. Mirror artifacts in two-dimensional echocardiography: don’t forget objects in the third dimension. J Am Soc Echocardiogr. 2015. doi:10.1016/j.echo.2015.07.025.

3. Scanlan KA. Sonographic artifacts and their origins. AJR Am J Roentgenol. 1991;156:1267-1272. doi:10.2214/ajr.156.6.2028876.

9. Correct Answer: A. Reverberation artifact

Rationale: M-mode echocardiography can be particularly useful in determining whether an image finding has independent motion (suggestive of a true structure) versus identical motion that is a copy or mirror of a true structure (suggestive of artifact). This is most relevant in reverberation artifacts, where the artifact and true structures are located on the same scanning line relative to the ultrasound probe. Figure 15.23 shows an example of a linear structure in the ascending aorta that on M-mode echocardiography shows identical motion compared to the anterior aortic wall and is therefore most likely a reverberation artifact. If the linear structure were a real structure, like a dissection flap, it would be expected to exhibit its own independent motion and not completely mimic the anterior aortic wall. Note also that the long R-R interval in the M-mode tracing reinforces this conclusion because there is again no motion independent of the anterior aortic wall.

10. Correct Answer: B. Side lobe artifact

Rationale: Figure 15.2 shows a calcified and strongly reflective sinotubular junction, resulting in an arc-like side lobe artifact on both sides of the junction—a finding that can sometimes be misinterpreted as a type A aortic dissection. See also Answer 15.6 and explanation of the side lobe mechanism in Figure 15.20.

1. Laing FC, Kurtz AB. The importance of ultrasonic side-lobe artifacts. Radiology. 1982;145:763-768. doi:10.1148/radiology.145.3.7146410.

2. Pamnani A, Skubas NJ. Imaging artifacts during transesophageal echocardiography. Anesth Analg. 2014;118:516-520. doi:10.1213/ANE.0000000000000084.

11. Correct Answer: D. Reverberation artifact.

Rationale: The echodensity in the ascending aorta is linear and lacks well-demarcated borders. Moreover, the motion of this structure ( Video 15.2) is similar to the posterior aortic wall closer to the probe. As such, this echodensity represents a reverberation artifact, in which ultrasound waves are reflected twice in between the posterior wall of the right pulmonary artery (closest to the echo probe) and the posterior wall of the ascending aorta. See also Answer 15.3 and explanation of the reverberation mechanisms in Figure 15.19.

M-mode echocardiography could be useful to evaluate the identical motion pattern of the artifact relative to the pulmonary artery and aortic wall. Adequate recognition is essential to avoid misinterpretation of these echo findings as a type A aortic dissection.

1. Appelbe AF, Walker PG, Yeoh JK, Bonitatibus A, Yoganathan AP, Martin RP. Clinical significance and origin of artifacts in transesophageal echocardiography of the thoracic aorta. J Am Coll Cardiol. 1993;21:754-760. doi:10.1016/0735-1097(93)90109-e.

12. Correct Answer: B. Refraction artifact

Rationale: The visualized structure in the left atrium is a “twin” or duplicate image of the mitral valve at a similar distance to the probe, that is, a refraction artifact. Video 15.3 is most helpful to appreciate the duplicate motion of the mitral valve. This artifact occurs due to the presence of a “refractor” in the near field, most commonly rib cartilage. Repositioning the transducer to avoid the refracting tissue is most effective in eliminating the duplicate image.

1. Ozeke O, Ozbakir C, Gunel EN. Double mitral valve imaging. J Am Soc Echocardiogr. 2010;23:340 e1-e2. doi:10.1016/j.echo.2009.08.017.