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Regarding ultrasound refraction: a. The angle of refraction depends on the velocities of ultrasound in the media on both sides of the boundary b. Ultrasound traversing an interface between muscle and bone will bend towards the normal c. The critical angle is the angle of incidence at which the refracted beam travels parallel to the boundary d. Refraction plays an important role in the phenomenon of acoustic enhancement behind a fluid-filled structure e. Refraction is one of the principal causes of artefacts in ultrasound imaging |
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Answer» a. True. The ratio of sines of the angles of incidence and refraction is equal to the ratio of velocities in the two tissues forming the boundary. This is known as Snell’s law. b. False. The velocity of ultrasound in bone is higher, so the angle of refraction will be larger than the angle of incidence. The beam will bend away from normal (line perpendicular to the boundary). c. True. The angle of refraction in this case equals 90°. d. False. Acoustic enhancement is an attenuation artefact. Refraction does not play a role here. e. True. It may change the apparent position of objects (misregistration) and cause critical angle shadowing at interfaces that are at steep angles to the direction of the ultrasound beam (such as the lateral margins of well-defined structures). |
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