Artifacts in ultrasound imaging
The following artifacts are common in ultrasound imaging:
- The ultrasound image displays non-existing structures.
- The ultrasound image does not display existing structures.
- The ultrasound image misrepresents the echogenicity of structures.
Echogenicity is defined as the intensity of reflected sound waves. Structures with high echogenicity will reflect more ultrasound and appear brighter on the image. Structures with low echogenicity reflect less ultrasound and become darker in the image.
Some structures have very high echogenicity (e.g skeleton, calcifications, mechanical heart valves) and reflect virtually all sound waves, leaving too few waves to explore the area behind the reflector. This results in dark areas, which are referred to as acoustic shadows. Figure 1 shows acoustic shadowing bellow a gallstone.
Ultrasound waves can be reflected multiple times between dense structures (i.e structures with high echogenicity). For each reflection, a portion of the sound waves return to the transducer and produce a copy of the reflector on the image. Hence, the ultrasound image may display multiple copies of a dense structure. Such artifacts are referred to as reverberations.
Reverberations can also occur within a structure that has boundaries with high echogenicity. Then the sound waves can be reflected multiple times between the boundary layers, as illustrated in Figure 2.
Reverberations can also occur if sound waves returning to the transducer are reflected back to the tissue.
Reverberations are common when examining lung tissue; the double-layered pleura produce reverberations, which are referred to as A-lines.
Mirror image artifact
Mirror image artifacts occur under a strong reflector that acts as a mirror. Behind the mirror, a copy of a structure appearing in front of the mirror is shown. The mechanism behind mirror image artifacts is similar to that of reverberations.
Side lobe artifact
A 2D image is formed by allowing the ultrasound beam to sweep back and forth within a defined sector. The transducer registers reflections originating from the central ultrasound beam (main beam). However, some ultrasound waves may travel off-axis in so-called side lobes (Figures 5A and 5B). Ultrasound energy in side lobes is mostly dissipated in the tissue without generating significant reflections. However, when side lobes encounter strong reflectors (calcifications, pericardium, mechanical heart valves, wires, etc), they may generate significant reflections which are detected by the transducer. These reflections are interpreted as originating from the main beam. As the ultrasound beam sweeps back and forth, multiple side lobe artifacts can be generated on both sides of the true reflector. If many side lobe artifacts are generated, they may appear as a continuous structure, as illustrated in Figure 5C.
Reflection and refraction occur when ultrasound passes tissue boundaries. Tissue boundaries represent acoustic reflectors at which some of the ultrasound energy is reflected and the remainder continues through the tissues. Depending on the difference in acoustic impedance between the tissues, the angle of the ultrasound wave may be altered. This is referred to as refraction. The larger the difference in acoustic impedance the greater the refraction.
Refraction artifacts occur when ultrasound travels through tissue that behaves as a lens which causes significant refraction, directing the ultrasound to an area interrogated simultaneously by other sound waves (Figure 6). The refracted ultrasound is then reflected back to the lens, from where it is re-refracted to the transducer, resulting in a duplicate of the reflector. The duplicate will be depicted along the original path of the sound wave. Structures behind the lens may be invisible in the image; this is due to the fact that sound waves never reach them and instead they are overwritten by the duplicate.
Refraction artifacts are generally easy to recognize because they create implausible image findings, such as duplication of the ventricles or atria. Fat, pleura and pericardium are among tissues that can behave as lenses that cause refraction. Switching the image window or adjusting the angle of the transducer may remedy refraction artifacts.