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RefractionMRI Resource Directory:<br> - UltraSound Physics -
 
The refraction is the change of the sound direction on passing from one medium to another. In ultrasound, refraction is due to sound velocity mismatches combined with oblique angles of incidence, most commonly with convex scanheads. When the ultrasound wave crosses at an oblique angle the interface of two materials, through which the waves propagate at different velocities, refraction occurs, caused by bending of the wave beam.
See also Refraction Artifact, Acoustic Shadowing, Acoustic Mismatch, and Duplication Artifact.
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    • Rarefactional Pressure
    • Refraction Artifact
    • Acoustic Impedance
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Ultrasound artifactsOpen this link in a new window
   by rad.usuhs.mil    
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Refraction ArtifactInfoSheet: - Artifacts - 
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Different sound velocities in tissue are causing refraction artifacts. With convex elastomer lens transducers, sound beam refraction at the skin interface can alter the transducer's focusing characteristics and beam profile, cause element to element nonuniformity, and cause phase changes in the acoustic wave. These cumulative refraction induced errors degrade the image quality through distortion and loss of resolution. Because the amount of refraction is proportional to the velocity mismatch, the greater the mismatch, the greater the refraction.
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Acoustic MismatchInfoSheet: - Artifacts - 
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Acoustic mismatch arise at the boundary between two different media where reflection and refraction occurs.
See also Snells Law.
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Acoustic ShadowingInfoSheet: - Artifacts - 
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Through diffraction and refraction on intersections edge acoustic shadowing can be created. The acoustic shadowing artifact is the loss of information below a dense object because the majority of the sound energy was reflected back by the object.
Shadowing artifacts occur if decreasing of the echo amplitude is not exponential with penetration depth caused by inhomogeneous tissue layers and fluid or air-filled regions. Bone, air, foreign bodies and calcification stop the transmission of sound waves producing a 'sonic shadow' which is a dark region distal to the echogenic obstructing region. This artifact occurs also in objects like e.g. prosthetic valves.
See also Boundary Layer, and Half-Value Layer.
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 Further Reading:
  Basics:
Ultrasound artifactsOpen this link in a new window
   by rad.usuhs.mil    
  News & More:
Ultrasonographic fata morganaOpen this link in a new window
2003   by ndt.oxfordjournals.org    
Ultrasound compounding with automatic attenuation compensation using paired angle scans(.pdf)Open this link in a new window
June 2006   by mi.eng.cam.ac.uk    
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Duplication ArtifactInfoSheet: - Artifacts - 
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ReductionMRI Resource Directory:<br> - Artifacts -
 
Duplication artifacts can be created through diffraction and refraction on interfaces, also if the acoustical impedances of tissue is too much different and the ultrasound is reflected multiple on tissue layers, where the detected echo does not come from the shortest sound path.
See also Mirror Artifact.
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 Further Reading:
  Basics:
Ultrasonographic fata morganaOpen this link in a new window
2003   by ndt.oxfordjournals.org    
  News & More:
Sonographic Duplication Artifact of the Spinal Cord in Infants and ChildrenOpen this link in a new window
2004   by www.jultrasoundmed.org    
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Related Searches:
 • Rarefactional Pressure
 • Refraction Artifact
 • Acoustic Impedance
 • Diffraction
 • Reflection
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