Ch 3 Image Formation with Echoes

Question 1

Using focus on u/s machines improve what types of resolution?

Answer 1

-Lateral resolution (aka beam width resolution)
-Contrast resolution (b/c the focus increases the reflection strength of the returning echoes)

Question 2

What is acoustic impedance?

Answer 2

-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

Question 3

Explain the variables in the formula: z = p x c

Answer 3

z: acoustic impedance (rayls)
p: density (kg/m^3)
c: propagation speed (m/s)

Question 4

The greater the acoustic impedance mismatch b/w 2 mediums, the stronger or weaker the reflection?

Answer 4

Stronger (produces brighter echoes)

Question 5

What does perpendicular incidence mean?

Answer 5

-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

Question 6

What happens if Z2 = Z1?

Answer 6

-There would be no reflection b/c there is no acoustic impedance mismatch
-There would be 100% transmission into the 2nd medium

Question 7

What is the intensity reflection coefficient (IRC)?

Answer 7

-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)

Question 8

If the difference in the impedances increases, will this increase or decrease the IRC?

Answer 8

Increase

Question 9

When Z2 = Z1, what echogenicity will this produce on the u/s image?

Answer 9

-No echogenicity, will be anechoic
-No reflection

(ex. blood, fluid, bile)

Question 10

When Z2 is significantly greater than Z1, what echogenicity will this produce on the u/s image?

Answer 10

-High echogenicity, will be hyperechoic

(ex. plaque, specular reflectors, bones, vessel walls, heart valves, calcifications)

Question 11

When there is a small difference b/w Z2 + Z1, what echogenicity will this produce on the u/s image?

Answer 11

-Hypoechoic images
-Strong chance a mass may not be seen on the image

Question 12

Why do we use u/s gel?

Answer 12

-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)

Question 13

What does oblique incidence mean?

Answer 13

-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)

Question 14

Is perpendicular or oblique incidence better in u/s?

Answer 14

Perpendicular!

-b/c more u/s waves will be reflected back to the probe + fewer will be “scattered” away, resulting in a better image

Question 15

What 2 things is the transmission angle dependent on?

Answer 15

-Incidence angle
-Media propagation speeds

Question 16

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?

Answer 16

Greater

(same speeds = same transmission angle)

Question 17

What is refraction?

Answer 17

-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)

Question 18

Refraction causes what 2 artifacts?

Answer 18

-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)

Question 19

What 2 things are required for refraction to occur?

Answer 19

-Oblique incidence
-Different propagation speed on either side of boundary

Question 20

What do specular reflections refer to?

Answer 20

-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

Question 21

Are specular reflectors angle or frequency dependent?

Answer 21

-Angle dependent (think we must get the right angle to reflect off the lake)

-Frequency independent

Question 22

Does perpendicular or oblique incidence give us the brightest reflection strength?

Answer 22

Perpendicular = hyperechoic

(oblique = hypoechoic + less reflective)

Question 23

Give 2 examples of specular reflectors?

Answer 23

-Diaphragm
-Heart walls + valves

Question 24

What is scattering (aka back scatter)?

Answer 24

-Reflection of the sound wave that gets redirected in many different directions (they scatter everywhere)
-The reflecting surface is rough

Question 25

Is scattering angle or frequency dependent?

Answer 25

-Frequency dependent (higher frequencies create a stronger echo)

-Little to no angle dependence

Question 26

Do most of our echoes arise from scattering or specular reflectors?

Answer 26

Scattering! It allows for imaging of tissue parenchyma + organ boundaries

Question 27

Do lower or higher frequencies create a coarse or fine speckle pattern?

Answer 27

Lower: coarse
Higher: finer - structures are more defined (think higher end gets finer things)

Question 28

What is speckle?

Answer 28

-Form of acoustic noise
-Gives tissue a grainy appearance + shows the impression of tissue texture

-We must note that this speckle/grainy appearance is just an artifact due to the constructive + destructive interference, it is NOT a representative of true tissue characteristics!

Question 29

Differentiate constructive + destructive interference with speckle?

Answer 29

Constructive:
-2 waves superimpose
-Higher amplitude

Destructive:
-2 waves superimpose + partially or totally cancel each other out
-Lower amplitude

Question 30

How does constructive + destructive interference result in a speckle pattern?

Answer 30

-Scattered echoes interact with each other causing constructive + destructive interference with the sound waves

-This pattern occurs when these signals get added together + we acquire varying degrees of larger + smaller signals

Question 31

Differentiate heterogeneous + homogeneous speckle patterns?

Answer 31

Hetero:
-irregular speckle pattern
-ex. breast tissue, skeletal muscle, adipose tissue

Homo:
-uniform consistent speckle pattern
-ex. liver, myocardium

Question 32

What is rayleigh scattering?

Answer 32

-Reflecting structures that are very small (with respect to wavelength)
-Very weak amplitude/brightness

(is the weakest reflective mechanism for imaging, as it is dark/anechoic on 2D imaging)

Question 33

Is rayleigh scattering angle or frequency dependent?

Answer 33

-Frequency dependent (more back scatter with higher frequencies)

-Angle independent (b/c it is a scatter reflection)

Question 34

Give an example of rayleigh scattering?

Answer 34

Sound reflecting RBCs

(need CD to visualize blood)

Question 35

What is rouleaux flow?

Answer 35

-Clumping of slow flow RBCs
-Increases amplitude
-This can be a normal finding, or can be an indicator of a hemodynamic problem (such as embolization or thrombus formation)

Question 36

What does rouleaux flow typically look like?

Answer 36

-“Smoke” or “spontaneous contrast”
-Commonly seen in veins due to blood pooling

(ex. seen in LA or LAA in heart due to thrombus formation)

Question 37

What is the range?

Answer 37

-Distance from probe to an echo generating structure
-Can determine the exact location of each echo on its scan line

Question 38

What 2 items of info are required to determine the exact location of each echo on its scan line?

Answer 38

-Direction where the echo came from

-Distance to the reflector or scatterer where the echo was produced (the u/s machines can NOT measure distance directly)

Question 39

Depth location is based on that it takes ___ microseconds for every 1cm of round trip travel (to reflector + back to probe)?

Answer 39

13 microseconds (b/c round trip)

(6.5 microseconds of travel from probe to reflector only - single trip)

Question 40

What happens if sound travels through mediums with slightly different propagation speeds other than the assumed speed of 1540 m/s?

Answer 40

-When speed of sound is slower or faster, the u/s machine will place that reflector deeper or shallower than reality
-Known as speed error artifact

(faster = too shallow, slower = too deep)

Question 41

A broken needle with speed error artifacts is referred to as what sign?

Answer 41

The bayonet sign

Question 42

Anything man-made put into the body should be suspected for what type of artifact?

Answer 42

Speed error artifact

(ex. needles, pacemakers, mechanical valves, wires, clips)

Question 43

If propagation speed of the medium is higher than 1540 m/s, the echoes will arrive sooner + be placed where?

Answer 43

Too shallow

Question 44

If propagation speed of the medium is slower than 1540 m/s, the echoes will arrive later + be placed where?

Answer 44

Too deep