Ch 2b Attenuation

Question 1

What is attenuation?

Answer 1

-The decrease in sound energy as the sound wave propagates

-Reduction in intensity + amplitude as sound travels

-Sound waves weaken as they propagate in a medium. Attenuation is this decrease in intensity, power + amplitude as sound travels.

(think the weakening of sound as it propagates/travels)

Question 2

The farther the sound travels, the greater or weaker the attenuation?

Answer 2

Greater - causing the beam to become weaker

(distance + attenuation are directly related)

Question 3

Attenuation is compensated by what diagnostic instruments?

Answer 3

TGCs + 2D Gain

Question 4

What units is attenuation in?

Answer 4

dB (decibels)

Question 5

Attenuation encompasses what 3 things?

Answer 5

-Absorption (dominant factor)
-Reflection
-Scattering

Question 6

A higher or lower frequency probe leads to greater attenuation?

Answer 6

Higher

(attenuation + frequency are directly related)

Question 7

List 2 factors that would cause more attenuation + 2 factors that would cause less?

Answer 7

More:
-longer distances
-higher frequencies

Less:
-shorter distances
-lower frequencies

Question 8

What is absorption?

Answer 8

The conversion of sound energy into heat, as sound passes through tissue

Question 9

Higher viscosity, dense mediums + higher transmitting frequency cause an increase or decrease in absorption?

Answer 9

Increase

Question 10

If the frequency of the wave is increased, will the amount of energy lost to heat increase or decrease?

Answer 10

Increase

(ex. rubbing hands together many times per second creates more heat than only rubbing hands a few times per second)

Question 11

Do different mediums attenuate sound differently?

Answer 11

Yes!

Soft mediums:
-ex. soft tissue
-low attenuation

Hard mediums:
-ex. bone
-high attenuation

Question 12

What happens to the penetration of sound waves when we adjust our frequency?

Answer 12

Increase frequency: decrease penetration
Decrease frequency: increase penetration

(inversely related)

Question 13

List the 3 key concepts of absorption?

Answer 13

-Beam frequency
-Nature of the attenuating medium (based on the viscosity)
-Distance/length of sound travel

Question 14

What substance tends to have lower viscosity + low densities than tissue?

Answer 14

Fluids - very low absorption

Question 15

Fluids give rise to which artifact?

Answer 15

Posterior enhancement

(ex. cysts, amniotic fluid, bladder + blood pool)

Question 16

What is posterior enhancement + why is it important?

Answer 16

-Anything posterior to the medium will appear brighter (hyperechoic) than adjacent tissue

-Is important b/c aids in the detection of cystic masses

Question 17

Why does sound not penetrate through posterior shadowing?

Answer 17

Due to the calcific changes of the medium

Question 18

Another term for posterior/acoustic shadowing?

Answer 18

Complete absorption

Question 19

What happens when we scan through bone?

Answer 19

-There is complete absorption + no echoes posterior (it is anechoic)
-Posterior shadowing is seen under the bone

Question 20

List an advantage + disadvantage to posterior shadowing?

Answer 20

Advantage:
-we can differentiate b/w tissues

Disadvantage:
-can’t view structures posterior to highly attenuating mediums

Question 21

What is dB?

Answer 21

-Used to quantify attenuation
-Can help compare sound intensities before + after attenuation
-Helps describe the relationship b/w measured sound levels

Question 22

Describe what 3, 6 + 10 dB decrease corresponds to in regards to intensity?

Answer 22

3 dB: 50% reduction intensity (half pizza)
6 dB: 75% reduction intensity (quarter pizza)
10 dB: 90% reduction intensity (slice pizza)

(0 dB = entire pizza is there, no reduction)

Question 23

As attenuation increases, the attenuation coefficient increases or decreases?

Answer 23

Increases

(directly related)

Question 24

As path length increases, attenuation increases or decreases?

Answer 24

Increases

Question 25

What is the attenuation coefficient for soft tissue?

Answer 25

0.5

Question 26

What does TGC stand for?

Answer 26

Time gain compensation

Time: based on depth
Gain: to increase the amplitude
Compensation: to make an adjustment or compensate

Question 27

What does overall 2D gain do?

Answer 27

-Increases the voltage amplitude of returning signals
-Creates brighter returning signals for the entire FOV (at all imaging depths)
-Measured in dB

Question 28

What is TGCs used for?

Answer 28

-Compensates for the effect of attenuation on an image
-Gives us the ability to amplify signals from various depths
-Improves strength of signals from farther scanning depths

(overall - it increases the amplitude based on depth due to the increasing amount of attenuation)