Diagnostic ultrasound ( PDFDrive )

APPENDIX
Ultrasound Artifacts: A Virtual
Chapter
Korosh Khalili, Hojun Yu, Alexander Jesurum, and Deborah Levine
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 Proile
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. 1
When one or more of these assumptions prove invalid, artifacts
result.
Velocity of Sound
he 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. he 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 relected of a relector only once, and returns directly
back to the probe at the same angle exactly to the point from
which it let the probe. If the sound waves undergo more than
one relection, then artifacts such as mirror image, reverberation,
e1