Ch 3 Image Formation with Echoes Flashcards
Using focus on u/s machines improve what types of resolution?
-Lateral resolution (aka beam width resolution)
-Contrast resolution (b/c the focus increases the reflection strength of the returning echoes)
What is acoustic impedance?
-A measure of the resistance to sound traveling within a medium
-Determines how much of an incident sound wave is reflected back + how much is transmitted into the 2nd medium
Explain the variables in the formula: z = p x c
z: acoustic impedance (rayls)
p: density (kg/m^3)
c: propagation speed (m/s)
The greater the acoustic impedance mismatch b/w 2 mediums, the stronger or weaker the reflection?
Stronger (produces brighter echoes)
What does perpendicular incidence mean?
-A perpendicular direction of travel of the incident u/s wave to a boundary b/w 2 media
-The incident sound wave may be reflected back or transmitted into the next medium, depending on the impedances of the 2 media at the boundary
What happens if Z2 = Z1?
-There would be no reflection b/c there is no acoustic impedance mismatch
-There would be 100% transmission into the 2nd medium
What is the intensity reflection coefficient (IRC)?
-Another way to calculate the acoustic impedance mismatch via intensity
-It is the fraction of the incident intensity that is reflected
(IRC = reflected intensity / incident intensity)
If the difference in the impedances increases, will this increase or decrease the IRC?
Increase
When Z2 = Z1, what echogenicity will this produce on the u/s image?
-No echogenicity, will be anechoic
-No reflection
(ex. blood, fluid, bile)
When Z2 is significantly greater than Z1, what echogenicity will this produce on the u/s image?
-High echogenicity, will be hyperechoic
(ex. plaque, specular reflectors, bones, vessel walls, heart valves, calcifications)
When there is a small difference b/w Z2 + Z1, what echogenicity will this produce on the u/s image?
-Hypoechoic images
-Strong chance a mass may not be seen on the image
Why do we use u/s gel?
-B/c there is an enormous acoustic impedance mismatch with air + the matching layer of the probe
-The gel provides transmission of sound as it eliminates the layer of trapped air that would reflect sound + prevent the entrance of sound into the body
(the gel avoids the mismatch b/w the air + tissue)
What does oblique incidence mean?
-Oblique direction of travel of the incident u/s wave that is NOT perpendicular to the boundary b/w 2 media
-Reflected sound does NOT return to the probe, instead it travels off in another direction
(contributes to attenuation)
Is perpendicular or oblique incidence better in u/s?
Perpendicular!
-b/c more u/s waves will be reflected back to the probe + fewer will be “scattered” away, resulting in a better image
What 2 things is the transmission angle dependent on?
-Incidence angle
-Media propagation speeds
If the propagation speed through the 2nd medium is greater than the 1st medium, will the transmission angle be greater or smaller than the incidence angle?
Greater
(same speeds = same transmission angle)
What is refraction?
-Change in direction of sound when it crosses a boundary
-The “bending” of a wave at an interface b/w 2 media
(think of the pencil in a glass of water in bright sunlight, making it look bent)
Refraction causes what 2 artifacts?
-Lateral position artifact (where 1 side is the true reflector + the other is a false image - structure looks duplicated)
-Refractive/edge shadowing (2 dark lines on either side of a structure)
What 2 things are required for refraction to occur?
-Oblique incidence
-Different propagation speed on either side of boundary
What do specular reflections refer to?
-Mirror like reflections (ex. reflection off a smooth, calm lake)
-They occur from a surface that is large + smooth
-The angle of incidence always equals the angle of reflection
Are specular reflectors angle or frequency dependent?
-Angle dependent (think we must get the right angle to reflect off the lake)
-Frequency independent
Does perpendicular or oblique incidence give us the brightest reflection strength?
Perpendicular = hyperechoic
(oblique = hypoechoic + less reflective)
Give 2 examples of specular reflectors?
-Diaphragm
-Heart walls + valves
What is scattering (aka back scatter)?
-Reflection of the sound wave that gets redirected in many different directions (they scatter everywhere)
-The reflecting surface is rough
