Ultrasound Artifacts: A Virtual Chapter

Summary of Key Points

Artifacts are common in ultrasound imaging.
Knowledge of artifacts can aid in diagnosis.
Understanding ultrasound physics allows for correction of artifacts that distort the visualized anatomy.

Chapter Outline

ASSUMPTIONS IN GRAY-SCALE IMAGING
- Velocity of Sound
- Attenuation of Sound
- Path of Sound
- Beam Profile
PROPAGATION VELOCITY ARTIFACT
ATTENUATION ERRORS
- Shadowing
- Increased Through Transmission
PATH OF SOUND-RELATED ARTIFACTS
- Mirror Image Artifact
- Comet-Tail Artifact
- Refraction
- Anisotropy
- Reverberation Artifact
GAS-RELATED ARTIFACTS
- Reverberation Artifact
- Ring-Down Artifact
- Dirty Shadowing Artifact
BEAM PROFILE–RELATED ARTIFACTS
- Side Lobe and Grating Lobe Artifacts
- Partial Volume Averaging
DOPPLER IMAGING ARTIFACTS
- Loss or Distortion of Doppler Information
- Artifactual Vascular Flow
- Tissue Vibration Artifact
- Aliasing and Velocity Scale Errors
- Spectral Broadening
- Blooming Artifact
- Twinkle Artifact
- Acknowledgment

Almost all ultrasound images contain artifacts. Many of these artifacts go unrecognized because they are contained within (and contribute to) the background noise. However, when artifacts substantially alter the signal, they become recognizable on images. Certain artifacts distort the images and must be recognized in order to improve image quality or prevent a false diagnosis. Other artifacts add useful data and thus are important for understanding the composition, anatomy, and pathology of the image being visualized. We hope you enjoy this virtual chapter, which encompasses animated illustrations of how artifacts are produced as well as multiple examples of images and videos from the text.

Assumptions in Gray-Scale Imaging

Several assumptions about the propagation of sound waves are made when ultrasound equipment maps echoes onto the image. When one or more of these assumptions prove invalid, artifacts result.

Velocity of Sound

The speed of sound throughout the tissues is assumed to be uniform at 1540 m/sec. When the waves travel through tissues that substantially alter sound velocity, propagation velocity artifacts occur.

Attenuation of Sound

Sound waves normally become fainter—that is, their intensity decreases—as they travel through tissues. The equipment assumes that this attenuation of intensity occurs at a constant rate. If the waves travel through tissues that either do not attenuate as much or attenuate more than the adjacent tissues, attenuation errors such as increased through transmission or shadowing arise.

Path of Sound

In creating the image, the equipment assumes that the generated sound wave travels from the surface of the probe in a straight line, is reflected off a reflector only once, and returns directly back to the probe at the same angle exactly to the point from which it left the probe. If the sound waves undergo more than one reflection, then artifacts such as mirror image, reverberation, or comet-tail occur. And if the direction of the beam or its echo is altered, refraction or anisotropy artifact may be produced.

Beam Profile

An assumption is made that the sound beam generated by the transducer is a narrow line. When the beam is not sufficiency narrow in the imaging plane or in the elevation plane (i.e., along the short axis of the transducer), side lobe or grating lobe or partial volume averaging artifacts may occur.

Propagation Velocity Artifact

Ultrasound image processing assumes that the speed of sound in tissue is a constant 1540 m/sec. However, when a sufficiently large structure composed of tissue that propagates speed at a different velocity is encountered, then propagation velocity artifact can occur. In the case of a fatty lesion, the slower speed of propagation (approximately 1450 m/sec) means that the echo will take longer to return to the transducer; thus the lesion is displayed deeper in the image than its true location. Some newer ultrasound machines with multibeam (spatial compounding) features and improved signal processing can minimize this artifact.

Click here to see an explanatory video of propagation velocity artifact ( ).

Attenuation Errors

When the ultrasound beam encounters a focal lesion that attenuates the sound to a greater or lesser extent than the surrounding tissues, the intensity of the beam deep to the lesion will be either weaker (shadowing) or stronger (increased through transmission) than in the surrounding tissues.

Click here to see an explanatory video of attenuation-related artifacts ( ).

FIG. A.3, As sound passes through tissue, it loses energy through the transfer of energy to tissue by heating, reflection, and scattering. Attenuation is determined by the insonating frequency and the nature of the attenuating medium. Attenuation values for normal tissues show considerable variation. Attenuation also increases in proportion to insonating frequency, resulting in less penetration at higher frequencies. 2 (See Chapter 1 , Fig. 1.9 .)

Shadowing

Shadowing results when there is a marked reduction in the intensity of the ultrasound deep to a strong reflector, attenuator, or refractor. Clean dark shadows will be seen behind calcified objects when the focal zone is at or just below the structure.

Click here to see an explanatory video of shadowing ( ).