Part 5-10 (up until midterm) Flashcards
What creates loudness or strength of sound
Amplitude and intensity
Attenuation is
The reduction in intensity and amplitude with increasing imaging depth
What is absorption
Sound transferred to heat
Attenuation increases with
Frequency
Path length
Attenuation is caused by
Absorption
Reflection
Scattering
Attenuation coefficient (ac)
The attenuation that occurs with each centimeter* the sound wave travels
Unit: dB/cm
If sound intensity decreases by 2 what is the attenuation
3 dB
If sound intensity decreases by 10 what is the dB
10 dB
What does the attenuation depend on?
- Type of tissue
- Length/distance of travel
- Frequency
dB vs intensity - what do you do
dB you +
Intensity you x
Attenuation coefficient for bone
ac = 6-9 heavy attenuation
Attenuation coefficient for water
ac= 0.0022 - weak attenuation
Compare attenuation in lung vs. bone vs. soft tissue
Attenuation is higher in lung/air than soft tissue
Attenuation is higher in bone than soft tissue
Describe the relationship between imaging depth and frequency
Imaging depth decreases with increasing frequency
As frequency ____ attenuation _____
As frequency increases, attenuation increases
The average attenuation coefficient for soft tissue is _____ dB/cm for each MHz of frequency
0.5 dB/cm
The slope of the TGC in soft tissue is how much? Explain
1 dB/cm * MHz
Soft tissue attenuated at 0.5 dB * MHz, but also attenuated again coming back
To compensate for both directions of attenuation, TGC must give back 1 dB/cm * MHz
What can you adjust to compensate for attenuation
Time gain compensation
What does TGC slope do?
Amplifies far field echoes to promote uniformity in liver tissue. Displayed graphically with increasing deflection to the right. The slope compensated less at the shallowest portion and more at the deepest portion of the organ
What is impedance (formula)
The density of a medium * propagation speed
Unit: Rayls
What does impedance determine
How much of an incident sound wave is reflected and how much is transmitted
Average soft tissue impedance is how many rayls
1,630,000 rayls
There is no reflection when
No reflection when two media have the same impedance with perpendicular impedance
IRC
Intensity reflection coefficient - how much of the wave is reflected
IRC = Ir/Ii x 100
ITC
Intensity transmission coefficient - how much is transmitted into second medium
ITC = It/Ii * 100
OR
ITC = 1-IRC
The larger the impedance the _____ the IRC and echo intensity
The larger the impedance the greater the IRC and echo intensity
If IRC increases, ITC _____
If IRC increases, ITC decreases
If impedance is zero, what does that mean
There is total transmission
If impedance nears 100%, what does that mean
Almost total reflection
Examples:
Bone and diaphragm
If there is a perpendicular incidence what is the incident angle
Zero
What happens with oblique incidence
With oblique incidence, the sound is refracted at a boundary between media for which propagation speeds are different
When will no refraction occur?
If propagation speeds are equal
If the angle of incidence is 0 (perpendicular)
If C1 < C2, what is the relationship between the corresponding angles?
Incident angle is less than transmission angle
Two requirements for refraction
1) oblique incidence
2) diff propagation speed on either side of medium
What does refraction cause
- Lateral position errors
2. Duplication of aorta
What two angles are always the same at a boundary
Incidence and reflection angles
When does scattering occur?
1) not smooth - rough/heterogeneous
2) small surface
Scattering depends on
- Frequency
2. Scatterers size
Explain the relationship between frequency, wavelength with scattering/reflection
Increase frequency decreases wavelength. Thus the object will be bigger than the wavelength, so there will be less scattering, more reflection, more echoes coming back to the transducer which increase the intensity of echo
Explain the condition when increasing frequency increases scattering
Rayleigh scattering - when the object, like red blood cells, are much smaller and increasing frequency causes more scattering
Explain how contrast agents work
They produce strong harmonics because air bubbles contact and expand creating non-linear propagation
To position the echo on the screen, machine needs 2 pieces of info
- Direction echo came from
- Distance to reflector/scatterer
- machine measures time travelled
What is the pulse-echo round trip time used to determine the distance to a reflector
13 us/cm
Constructive interface
In phase, Reinforce - Intensity much stronger than individual sound waves
Destructive interface
Out of phase, cancel out
Acoustic speckle
A form of acoustic “noise” - the result of constructive and destructive interference of scattered sound waves
What is HVLT
The thickness of a medium that decreases the intensity of a beam by 50% or 3dB of the initial intensity
When perpendicular bean is transmitted, what happens
The bean moves through the second medium in the same direction as the incident sound beam
What does it mean if there is no reflection at a boundary?
All is transmission, no echo, the medium impedances are equal
What is acoustic impedance? It’s symbol? It’s unit? Equation with units?
Acoustic impedance is a measure of the resistance of a medium to the transmission of sound. Symbol is z, rayls
What must the reflective/transmission coefficients add up to?
1 or 100%
What is refraction? What determines it? Why is it important?
Refraction is a change in the direction of sound as it crosses a boundary. Determined by the different acoustic velocity/impedance in the diff mediums. It contributes a great deal to the formation of our image.
What is a specular reflector? What anatomy would produce one?
Flat smooth boundary that reflects well (mirror-like) it is produced at the artery walls or diaphragm
Describe why TGC is needed
Reflectors with equal reflection coefficient will not result in echoes of equal amplitude arriving at the transducer if their travel distances are different) bc sound weakens as it travels
What’s the difference between TGC and overall gain
TGC is depth specific
Factors affecting TGC setting
Tissue composition
Attenuation coefficient
Transducer frequency
If a medium has an attenuation coefficient of 2db/cm, what would be the best choice for the slope of the TGC
4db/cm
Slope for TGC should be double the attenuation coefficient
How is speckle produced
Constructive/ destructive interference patterns within the returning echoes producing darker and brighter dots on the screen
Units for amplitude
Pascal,
N/cm2
Kg/cm3
Volts
