Chapter 6-7

Question 1

What is attenuation

Answer 1

weakening of the sound beam due to a decrease in intensity, power, and amplitude as sound travels through the body.

Question 2

With attenuation, the weak reflections sent back to the transducer need to be amplified by the ultrasound system. How do we do this?

Answer 2

We use decibels to quantify the degree of attenuation or amplification

Question 3

What are characteristics of decibles (dB) ?

Answer 3

They are logarithmic, a relative measurement, and a ratio

Question 4

What kind of intensity do decibles require?

Answer 4

They require 2 intensities: a starting level and a level at time of
measurement

Question 5

What does a positive dB mean?

Answer 5

the signal is increasing in strength or getting larger. +3 means it doubles and +10 means is multiplies by 10

Question 6

What does a negative dB mean?

Answer 6

The signal is getting weaker or smaller. -3 means 1/2 and -10 means the signal has decreased 1/10 its original value .

Question 7

What determines attenuation?

Answer 7

Path length and frequency. More distance and more frequent will lead to more attenuation.

Question 8

What is the equation for total attenuation?

Answer 8

dB = attenuation coefficient x cm

Question 9

What is the attenuation coefficient? Does it change or remain constant?

Answer 9

The number of decibels of attenuation that occurs in 1 cm. (measured in dB/cm) It remains constant regardless of how far the sound has travelled overall.

Question 10

The attenuation coefficient does not change with depth. Is it related to frequency? What is the equation that proves this?

Answer 10

Yes it is directly related to frequency. Attenuation coefficient = frequency/2

Question 11

What 3 processes contribute to attenuation?

Answer 11

Reflection, scattering, and absorption

Question 12

What is half-value layer thickness?

Answer 12

the distance sound travels in a tissue that reduces the intensity of sound to one-half its original value

Question 13

When would the half-value layer be thin?

Answer 13

in tissues that attenuate sound greatly such as lung or bone or with high frequency.

Question 14

Half-value layer depends on what two factors?

Answer 14

medium and frequency

Question 15

What is reflection? How does it affect the beam?

Answer 15

Reflection is the portion of energy that is reflected back to the transducer. It weakens the beam that continues forward.

Question 16

What are the necessary and limiting components of specular reflection?

Answer 16

Sound must have a smooth interface larger than the beam. It must have a normal incidence and impedence mismatch. It is limited if the beam is not 90 degrees, because the wave will not return to the transducer.

Question 17

What are synonyms for when the sound strikes a boundary at 90 degrees?

Answer 17

Normal incidence, perpendicular, orthogonal, right angle

Question 18

What is impedance

Answer 18

the acoustic resistance to sound traveling in a medium

Question 19

What is the impedence equation?

Answer 19

Rayls= density x propagation speed

Question 20

Diffuse reflection (backscatter) is when a wave bounces off an irregular surface. What are the necessary componants?

Answer 20

surface is larger than the beam, radiates in more than one direction, irregular-not smooth, impedence mismatch

Question 21

If a beam is not 90 degrees, can a diffuse reflection still be produced?

Answer 21

Yes.

Question 22

Which is weaker diffuse or specular reflection?

Answer 22

Diffuse reflected beams are weaker than specular reflections

Question 23

What is scattering? When does it occur?

Answer 23

Random redirections of sound in many directions. It occurs when the boundary is small (equal or less than the beam’s wavelength)

Question 24

Scattering is directly related to which property?

Answer 24

frequency.

Question 25

What is rayleigh scattering? When would we find this clinically?

Answer 25

A special form of scattering where the bounday dimensions are much smaller than the wavelength of the beam. This is used when ultrasound imaging Red blood cells.

Question 26

How does Rayleigh Scattering redirect or reflect sound?

Answer 26

equally and organized in all directions?

Question 27

How is rayleigh scattering mathematically related to frequency?

Answer 27

Directly related to frequency ^4. When frequency x 2 = rayleigh scattering x 16.

Question 28

What is absorption? How is it related to frequency?

Answer 28

Ultrasonic energy is converted into another form of energy. Bone undergoes excessive absorption- we can't see through it. Absorption is directly related to frequency.

Question 29

What is incident intensity? What is its equation?

Answer 29

sound wave’s intensity immediately before it strikes a boundary. Incident intensity = reflected + transmitted because of conservation of energy.

Question 30

What is reflected intensity?

Answer 30

the intensity of the portion of the beam that is reflected after it strikes a boundary

Question 31

What is transmitted intensity?

Answer 31

the intensity of the portion of the beam that continues forward after striking a boundary

Question 32

What is the intensity reflection coefficient (IRC) and intensity transmission coefficient (ITC)?

Answer 32

the percentage of the intensity that bounces back when sound strikes a boundary is IRC. The percentage that continues is ITC. They add up to 100%.

Question 33

What is refraction?

Answer 33

Refraction is the change of a wave's direction when moving from one media to another.

Question 34

What are the two conditions for refraction? What happens when both are met?

Answer 34

Oblique incidence and different propagation speeds for the two mediums. If both conditions are met, the transmitted sound beam will bend.

Question 35

What is the difference between mediums 1 and 2 in Snell's law?

Answer 35

medium 1 is where the sound is currently traveling. medium 2 iis where the sound is entering.

Question 36

What happens to refraction when there is an oblique incidence but the medium speeds are the same?

Answer 36

there will not be refraction if the speeds of the media are equal.

Question 37

What happens with refraction when Speed 1 is less than Speed 2

Answer 37

The transmission angle will be greater than the incident angle. The transmitted beam is bending AWAY from normal incident

Question 38

What happens with refraction when Speed 2 is less than Speed 1

Answer 38

the transmission angle will be less than the incident angle. The transmitted beam bends towards normal incident.

Question 39

What is the echo ranging theory equation? What does this mean?

Answer 39

Distance = velocity x time. We can measure distance by knowing how fast something traveled and how long it took.

Question 40

What is go-return time directly related to?

Answer 40

Depth. The deeper the structure, the longer the go-return time

Question 41

What is the go-return time equation?

Answer 41

depth = (1.54 x go-return time) / 2

Question 41

The 13 microsecond rule applies when sound travels through soft tissue. What does it mean?

Answer 41

For every 13 microseconds of go-return time, the object is 1 cm deeper in the body.