Attenuation Flashcards
What is attenuation?
The weakening of sound beams as it travels
Why is attenuation important? 3
- Limits our imaging depth
- Must be compensated for
- Can be useful for diagnosis
What are the units we will use to measure loudness (intensity level) of sound waves?
Bels (B) and decibel (dB)
What is the unit conversion for Bels and decibels?
1 Bel = 10 decibels
Why do we normally use dB for virtually all loudness measurements?
Because dB is a rather large unit.
Instrument output uses decibels to determine what?
The power of the sound leaving the transducer
Dynamic range uses decibels to do what?
Express the number of shades of grey displayed on the monitor.
Gain and TGC uses decibels to express what?
The amount of amplification required to optimize the returning echoes
What is the formula used to calculate bels?
Bel = log (new I/ original I)
What is the formula to calculate decibels?
DB= 10log(new I/ original I)
The bel and decibel formula can be used to calculate what?
Power and voltage as well
DB= 10log ( new P/ original P)
DB = 20log( New V/ Original V)
What is the rule of thumb for dealing with decibel?
- A 2 dB drop = 1/2 the original intensity
- A 10 dB drop = 0.1 of the original intensity
What is the attenuation coefficient? 2
The amount of attenuation that occurs with each one centimetre travelled.
- In soft tissue 0.5 dB of attenuation occurs for ever one centimeter travelled per 1 MHz
- In soft tissue attenuation coefficient is equivalent to 1/2 frequency
What is the formula for attenuation coefficient (TAtt)?
TAtt = Att. Coef x path length ( CM)
In soft tissue what is the formula for total attenuation (TAtt)?
TAtt = (1/2)f x path length (cm)
What is half-value layer?
The distance sound must travel in a material to reduce the intensity to half its original value
What is the half value intensity numerically?
Equivalent to 3 dB
Attenuation can vary with what three things?
- Nature of the tissue
- Frequency of the ultrasound
- Depth
What are the five general reasons we have have attenuation?
- Absorption
- Reflection
- Refraction
- Scatter
- Wave-font divergence
What is absorption in attenuation?
Conversion of sound energy to heat and is the dominant factor of attenuation accounting for 80%
What is three factors that effect absorption in attenuation?
- Viscosity
- Relaxation time on molecules
- Frequency
What is viscosity in terms of absorption?
Viscosity is the ease in which molecules can slide past one another
What does having a increased viscosity do for attenuation?
Greater resistance, more friction and thus increased attenuation
What is friction in terms of absorption?
Sound energy
In terms of absorption when a mechanical forces is applied what happens?
The molecule will vibrate
What is relaxation time?
The time it takes to come to rest
If molecules can’t come to rest before the next compression phase, what happens?
More energy is required to reverse its direction. This produces heat
If relaxation time in soft tissue relatively constant, what influences the amount of absorption?
The changes in frequency
In terms of absorption if frequency is increased then there is less what?
Time available for molecules to recover during the relaxation process. This results in more absorption
What are the two types of reflection?
- Specular
- Non- Specular
Specular reflection occurs when?
A sound beam hits a large, smooth surface
What is a large surface relative to?
The frequency used and is greater than one wavelength in diameter
Reflection from interfaces contributes to what?
The majority of our image
What is a good example of a good Specular reflector?
The diaphragm
The intensity of the reflected sound depends on what two things?
- Angle of incidence
- Acoustic impedance of two media
For a reflection the angle of incidence will equal what?
The angle of reflection
If reflections of sound that have non-perpendicular incidence what might happen?
The sound may not return to the probe
Perpendicular incidence helps improve what?
Reflection
Acoustic impedance is the other factor that determines what?
The intensity of reflected sound
Density and stiffness determines what?
How fast sound moves through a medium
As density decreases or stiffness increases what happens?
Speed increases
What is the impedance (z) formula?
Impedance(z) = Density(P) x Velocity(c)
Acoustic impedance describes what relationship?
Acoustic pressure and the speed of particle vibrations in a sound wave
What is the units of impedance?
Rayls (z)
What does z increases with what?
Density or velocity
Z values vary with the different tissues in the body since what?
There are differences in the density and the stiffness
Acoustic impedance does not depend on what?
Frequency
The larger the Z value difference at the interface of 2 media what happens?
The bigger the reflection
What is the intensity reflection coefficient?
The amount of sound that reflects at an surface
What is the Intensity reflection coefficient (IRC) formula?
IRC= reflected intensity (Ir) / incident intensity (Ii)
Or
IRC = ((Z2- Z1)/ (Z2+Z1))^2
What is the Intensity transmission coefficient (ITC)?
ITC = 1 - IRC
** the more sound reflected the less transmitted
If the impedance is equal across the interface what kindof reflection happens?
No reflection will occur
What does refraction deal with?
The sound that is transmitted across the surface
What is refraction?
What happens to sound when the velocities differ across the interface and the angle of incidence is non-perpendicular
What is the formula for snell’s law of optics?
(Sin0i/sin0t) = (Vi/Vt)
What does Snell’s law of optics mean for refraction?
Sound can bend across an interface due to a difference in media velocity
If we know the velocity of tissue across the interface and the angle of the sound that comes in what can we do in terms of refraction?
We can calculate how much it will bend
If the velocities across the interface are equal what happens?
No refraction will occur
If the velocity of the first medium is greater than the second, what happens?
The sound will bend towards the normal
What happens if the velocity of the first medium is less than the second?
Sound will bend away from the normal
What is total internal reflection?
A special type of reflection occurs when V1 is less than V2 and the angle reaches a critical value
What is scatter?
The result of sound interacting with interfaces smaller than a wavelength and rough
Where does scatter typically happen?
From heterogeneous media and is independent of sound direction
Scatter is responsible for what?
Internal texture
Scatter results in what changes to our sound beam?
The incident sound beam breaking up into many different echoes, in as many directions as possible
Echoes during scatter have intensities that are what?
Intensities that are a fraction of the incident beam
What is backscatter?
When sound is directed back to its origin
The amount of scatter depends on what?
Frequency and reflector size
In terms of scatter what happens if we have higher frequency?
The greater the amount of scatter
What happens when the reflector is smaller in terms of scatter?
The greater the amount of scatter
Backscatter is responsible for what?
The parenchyma we see
Because scatter is random there is potential for what?
There is the potential for brightness non-uniformities
What is acoustic speckle?
Interference patterns from echoes that have undergone multi-path scattering
What does modern equipment do in terms of acoustic speckle?
Utilizes techniques to help minimize acoustic speckle
What is Rayleigh scatter?
There is a specific type of scatter that happens when sound interacts with red blood cells
How does RBC’s dimensions affect sounds?
They have dimensions that are smaller than a wavelength of sound. This type of scatter is very weak and is why we do not see blood flow at faster velocities
What is wave front divergence?
The divergence spreading off the sound beam as it travels. The relationship between intensity and divergent area where the intensity decreases as the divergent area increases.
In terms of wave font diverergence, as the sound beam diverges the intensity of the beam weakens and adds what?
Overall attenuation of the beam
What happens to the amount of absorption if there is an increase in relaxation time?
It increases
