Diagnostic ultrasound ( PDFDrive )

APPENDIX Ultrasound Artifacts: A Virtual Chapter
e13
Refraction
As sound moves between tissues with diferent propagation
velocities, such as from muscle to fat, it changes its direction at
their interface as a result of refraction. he physical properties of
refraction are explained by Snell’s law, as demonstrated in Figs.
A.23 12 and A.24. he change in the direction of the sound waves
results in violation of the assumption made by the ultrasound
equipment that the incident and relected waves travel in a
straight line. herefore, with refraction, structures insonated
by the bent path of sound waves are artifactually translated to
appear as if the sound waves had traveled along a straight path.
If refraction is suspected, it can be minimized by increasing
the scan angle so that sound waves travel perpendicular to the
interface.
Click here to see an explanatory video of refraction from the
anterior abdominal wall (Video A.10).
FIG. A.23 Refraction. Refraction can be due to tissues with different
propagation velocities. When sound passes from soft tissue with
propagation velocity (C 1 ) of 1540 m/sec to fat with a propagation velocity
(C 2 ) of 1450 m/sec, there is a change in the direction of the sound wave
because of refraction. The degree of change is related to the ratio of
the propagating velocities of the media forming the interface (sinθ 1 /sinθ 2
= C 1 /C 2 ) 2 . Note that when the incident angle is changed to zero degrees—
that is, the incident beam is perpendicular to the interface (θ 1 = 0°)—the
artifact disappears.
FIG. A.24 Refraction. Image of a straw in water. Refraction has
occurred owing to the difference between air and water, causing an
offset in the appearance of the straw. Note also how the straw appears
larger in the water because of distortion of the image.