Attenuation Flashcards
Weakening of sound beam as it travels is called
Attenuation
Attenuation is of great clinical importance in what three ways
- limit imaging depth
- has to be compensated for
- useful in diagnosis
Relative units of measurement expressing loudness (intensity) of sound waves
Bel (B)
Decibel (dB)
1 bel is equal to how many decibels
1 Bel = 10 decibel
Bel or decibel is used for virtually all loudness measurements
Decibel
Instrument output uses what to determine the power of the sound leaving the transducer
Decibels
What uses decibels to express the number of shades of gray displayed on the monitor
Dynamic range
What two things use decibels to express the amount of amplification required to optimize the returning echos
Gains and TGC
What formula is used to calculate bels
New I
Bel = Log ( —————— )
Original I
What formula used to calculate decibels
New I
dB = 10log ( ————— )
Original I
Formula for power
New P
dB = 10log ( ————— )
Original P
Formula for voltage
New V
dB = 20Log ( ————— )
Original V
What are the two rules of thumbs when dealing with decibel
- a 3 dB drop = half the original intensity
- a 10 dB drop = 0.1 the original intensity
Amount of attenuation that occurs with each cm travelled is called
Attenuation coefficient
In soft tissue the attenuation coefficient is equal to
Half the frequency
1/2 frequency
In soft tissue 0.5 dB of attenuation occurs every 1 cm per —-MHz
Per 1 MHz
What is the total attenuation formula
TA = (att Coeff) Times (the path length)
In soft tissue the formula for total attenuation is
TA = (1/2 frequency) times (path length)
The distance sound must travel in material to reduce the intensity to half the original value is called what
The half-value layer
What three things can vary attenuation
- nature of tissue (dead/alive)
- frequency of ultrasound
- depth
What are the five general reasons we have attenuation
- absorption
- reflection
- refraction
- scatter
- wave-front divergence
The conversion of sound energy into heat is called
Absorption
The dominant factor in effecting attenuation is what and it accounts for approximately how much
Absorption
80%
What are the three factors that affect absorption
- viscosity
- relaxation time on molecules
- frequency
The ease in which molecules can slide past one one another is called
Viscosity
Does the viscosity increase or decrease to provide greater resistance?
Increase
Greater resistance due to increased viscosity creates more or less friction
More friction
Having increased viscosity which creates more resistance, therefore more friction, does the attenuation increase or decrease
Attenuation increases
What is it called when sound energy is converted into heat
Friction
When a mechanical force is applied to a molecule what happens to it
The molecule will vibrate
The time it takes for a molecule to come to rest is called
The relaxation time
If molecules can’t come to rest before the next compression time, is more or less energy required to reverse its direction
Take more energy, and it produces heat
Since relaxation time is pretty constant in soft tissue what has more influence in changing the amount of absorption
Frequency
If frequency increases is there more or less time for molecules to recover during relaxation process
Less time
The molecules having less time during the relaxation process, results in more or less absorption
Results in more absorption
What are the two types of reflection
Specular
Non-specular
This type of reflection occurs when a sound beam hits a large smooth surface
Specular
A large surface is relative to —— used, and is greater then what in diameter
Relative to the frequency used
Greater then one wavelength in diameter
—— from interfaces contributes to the majority of the images
Reflection
What is an example of a specular refector
Diaphragm
Intensity of the reflected sound depends on what two things
Angle of incidence
Acoustic impedance of two media
The angle of incidence is equal to what
The angle of reflection
Reflections of sound that have a what way of incidence may not return to the probe
Non-perpendicular
Does perpendicular or non-perpendicular incidence help improve reflection
Perpendicular
How fast sound moves through a medium depends on what two things
Density and stiffness
As density decreases or stiffness increased what happens to the propagation speed
It increases
Acoustic impedance is also known as
Characteristic impedance
Describe the relationship between acoustic pressure and the speed of the particle vibrations in a sound wave (speed propagation)
Acoustic impedance
What are the units for impedance
Rayls (Z)
Impedance increase if density and velocity increase or decrease
Increase
Z values vary with different tissues in the body due to the different what two things
Density and stiffness
Does acoustic impedance depend on frequency
NO
Does there need to be a larger or smaller difference between the interface of two media to have a bigger reflection
Larger difference
The amount of sound that reflects at the interface is called
Intensity reflection coefficient
If we know how much sound is reflected we can then calculate what
The amount of sound transmitted
The amount of sound transmitted is called the
Intensity transmission coefficient
If more sound is transmitted then less sound must be reflected yes or no
Yes
As the differences between two interfaces increase more or less sound will be reflected
More
If the impedances between the interface is equal what happens
No reflection
Reflection occurs when there is what at an interface
A difference in impedance (Z values)
Is it possible for 2 tissues to have different sound velocities but have the same Z values
Yes it sure is
Looking at the sound returning to the probe from an interface is what
Reflection
Dealing with the sound that is transmitted across the interface
Refraction
Sound obeys which law
Snell’s law of optics
What happens to sound when the velocities differ across the interface and the angle of incidence is non-perpendicular
Refraction
If velocities across the interface are equal then what kind of refraction occurs
No refraction occurs
If velocity of first medium is greater then second what kind of refraction occurs
Refract (bend) towards the normal
If velocity of first medium is less then second what kind of refraction occurs
Refract (bend) away from normal
What type of reflection occurs when first medium is less then second and the angle reaches a critical value
Total internal reflection
Is it possible to have different velocities but have the same Z value
Yes
This only occurs when velocities are different across an interface and there in non-perpendicular incidence
Refraction
Does refraction relate to Z values
No
Non-specular is called
Scatter
When sound interacts with interfaces that are small and rough what occurs
Scatter
A small interface is small in comparison to
Smaller then a wavelength
Scatter is typical from what kind of media
Heterogenous
What is an example of heterogenous media
Cells
Suspended particles
Is scatter dependent or independent of sound direction
Independent
Is scatter responsible for external or internal texture of organs
Internal
What happens to the incident beam in scatter
It breaks up into many different echos
The resulting echos in scatter must equal what
The incident beam (100%)
Can the echos direction be predicted
No impossible to predict
The amount of scatter depends on what two things
Frequency
Reflector size
If the frequency is higher what happens to scatter
Higher frequency greater amount of the scatter
If the reflector is smaller what happens to the scatter
Smaller reflector greater amount of scatter
When the sound is directed back to its origin it is called
Backscatter
Is the backscatter responsible for the image we see
Yes
Because scatter is random there is a potential for what
Brightness non-uniformities
Dark and bright spots
What is the cause of brightness non-uniformities
The result of interference patterns from echos that have undergone multi-path scattering
The phenomenon of brightness non-uniformities is called
Acoustic speckle
To minimize acoustic speckle what techniques are used
Persistence also called frame averaging
What is the specific type of scatter that occurs when sound interacts with red blood cells called
Rayleigh scatter
Why do we not see blood flow at faster velocities
Red blood cells dimensions are much smaller then wavelengths so the scatter is weak
The spreading of sound beam as it travels is called
Divergence
As area increases what happens to intensity
Intensity decreases
As sound beam diverges the intensity of the beam weakens which adds to the overall attenuation of the beam but is called what
Wave front divergence
Define attenuation
The weakening of sound as it travels
How many decibels of sound are required to reduce the initial intensity by half
3 dB
Define half value layer
How many centimetres it takes to cut the intensity in half
List the five means that sound is attenuated in soft tissue
Scatter Wave front divergence Absorption Refraction Reflection
What happens to attenuation when viscosity of a medium is increased
Attenuation increases, because there is more friction meanin more absorption
What happens to attenuation if a mediums relaxation time is increased
Attenuation increases because more energy is needed to move the vibrating particle in the direction of the sound beam
What is required for reflection of sound to occur at an interface
A difference in Z values (acoustic impedance)
