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Understanding Imaging Artifacts
Introduction
Imaging artifacts encompass patterns in the image that seem to suggest the presence of structures that are in reality not present. They may in fact relate to both the appearance of nonexistent structures, as well as the concealing of existing structures. Artifacts are mostly caused by physical interactions between the imaged tissue and ultrasound itself that are more complex than assumed by the ultrasound system; however, they can also result from malfunctioning ultrasound equipment or their inadequate settings, as well as from interference caused by other electronic equipment. Imaging artifacts can occasionally encumber an echocardiographic examination (particularly for novices), and the knowledge of these occurrences should thus be used to minimize their effect. In this chapter, the most common artifacts have been categorized according to the authors’ discretion, although they recognize that alternative ways of classifying these artifacts are possible, as no standard nomenclature exists and reference to certain artifacts in the literature is limited ( Table 7.1 ).
TABLE 7.1
Types of Artifacts Encountered in Echocardiography and Potential Approaches to Their Resolution
| Types of Artifacts | Resolution Approach |
|---|---|
| B-mode Artifacts | |
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| Doppler Artifacts | |
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| 4D Echocardiography Artifacts | |
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) 
) 4D, Four-dimensional; 2D, two-dimensional.
B-Mode Artifacts
Shadowing (Attenuation) and Dropout Artifacts
Shadowing (i.e., attenuation) artifacts obscure certain (segments of) underlying structures. When imaging structures with mechanical properties (i.e., mass density and/or compressibility) substantially differ from soft tissue (e.g., metals such as used in prosthetic valves or air/contrast bubbles), very strong reflections will occur, resulting in little or no transfer of ultrasound energy to more distal regions, as (almost) all energy will have been reflected. This will manifest as a strong reflection in the area of the reflector, followed by an “acoustic shadow” that represents a signal void ( Fig. 7.1 , yellow dashed arrows, 
).
This shadowing will affect not only the two-dimensional (2D) image but also the color Doppler signal, which could be, for example, highly relevant in the assessment of regurgitant jets in the setting of prosthetic heart valves. Heavily calcified tissue is a similarly strong reflector; resorting to alternative scanning windows can mostly circumvent such artifacts. For more information on negative shadows, refer to Box 7.1 and Fig. 7.2 .
BOX 7.1
Negative Shadows
Although shadowing is typically seen as a darker zone (i.e., signal void) distal to a highly reflecting (most common) or absorbing (less common) structure, the opposite can also occur in that a structure is attenuating less than what is assumed by the scanner’s automatic time gain compensation (refer to Chapter 1 ). As a result, the distal echo signals are overamplified by the scanner, resulting in a brighter zone distal to the low-attenuating structure, which is referred to as a “negative” shadow. A typical example is that of a cyst, as the liquid in the cyst is attenuating the ultrasound wave less than soft tissue, while the scanner automatically corrects for attenuation, assuming it is imaging soft tissue only. An example of such a negative shadow following a cyst in the liver is given in Fig. 7.2 . In cardiac imaging, negative shadows do not normally occur.
Similarly, superficial structures leading to notable attenuation of ultrasound may significantly impair its penetration. As a result, the ribs or lung tissue can diminish the ability to image underlying structures, giving rise to “dropout” artifacts that typically occur at some phases of the respiratory cycle. Such artifacts can be reduced or avoided by scanning at different intervals of the respiratory cycle (i.e., breath hold), and occasionally only by choosing another transducer position ( Fig. 7.3 , 
, ). A dropout artifact is thus similar to a shadowing artifact that occurs very near the transducer, thereby causing part of the image to become invisible.
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