Attenuation Flashcards
What does attenuate mean?
To weaken or reduce in strength.
What is the decrease in intensity, power, and amplitude of sound as it travels through a medium called?
Attenuation
Attenuation is determined by what two factors?
Path length
Frequency of sound
What are the units of attenuation?
Negative dB, since it’s a decrease of intensity.
How are distance and attenuation related?
They are directly related.
The farther a sound travels, the greater the attenuation and the weaker the beam becomes.
How are frequency and attenuation related?
They are directly related.
Attenuation is greater in higher frequency sound than in lower frequency sound.
Attenuation is measured in decibels and reported as a _____ _____ not an _____ _____.
Relative change
Absolute value
What 3 processes contribute to attenuation?
Reflection
Scattering
Absorption
There will be more attenuation with:
_____ _____ and _____ _____.
Longer distances
Higher frequencies
There will be less attenuation with
_____ _____ and _____ _____.
Shorter distances
Lower frequencies
What happens when sound strikes a boundary?
A portion of the energy may be redirected, or reflected, back to the sound source.
How does reflection affect the portion of the sound wave that continues in the forward direction?
It is weakened
When is reflection more likely to occur?
When the dimension of the boundary is large; that is, more than a few wavelengths of the sound.
What two forms of reflection are created in soft tissue?
Specular and Diffuse
What determines which form of reflection is created?
Which form occurs depends on the nature of the interface that a sound wave strikes. (ex: smooth or flat)
When do you get specular reflections and how are they reflected?
Smooth or flat boundaries create specular reflections, reflected in only one direction in an organized manner.
If your transducer is off axis, how does that affect specular reflection?
The reflection does NOT return to the transducer.
What is it called when a wave reflects off an irregular surface in more than one direction?
Diffuse reflection, or backscatter
What is an advantage of diffuse reflections?
Interfaces at suboptimal angles to the sound beam can still produce reflections that will return to the transducer.
What is a disadvantage of diffuse reflections?
Backscattered signals have a lower strength than specular reflections.
What is scattering?
The random redirection of sound in many directions.
When does sound scatter?
When the tissue interface is small.
(Equal to or less than the wavelength of
the incident of sound beam).
How is scatter related to frequency?
Directly related.
High frequency = more scatter
What is Rayleigh scattering?
A special form of scattering that occurs when the structure’s dimensions are much smaller than the beam’s wavelength. Ex: RBC’s
How does Rayleigh scattering affect the sound wave?
It redirects the sound wave equally in all directions.
How does increasing frequency affect
Rayleigh scattering?
The occurrence of Rayleigh scattering increases dramatically with higher frequencies.
Mathematically, Rayleigh scattering is related to frequency how?
Frequency raised to the fourth power.
When frequency doubles, Rayleigh scattering is
16 times greater. (2x2x2x2 = 16)
What is the most sizable component of attenuation?
Absorption
When does absorption occur?
When ultrasonic energy is converted into another energy form, such as heat.
How is absorption related to frequency?
Like scattering, absorption is directly related to frequency. As a result of absorption, higher frequency waves attenuate more than lower frequency waves.
How are the attenuation coefficient (dB/cm) in soft tissue and frequency (MHz) related?
Directly related.
Specifically, the attenuation coefficient is one-half the frequency. 0.5 dB/cm/MHz
For each cm depth, there will be: ______
For each MHz of frequency, there will be: ______
- 5 dB attenuation
- 5 dB attenuation
How would you choose which probe to use if you are doing an echo on a big person vs. a carotid scan?
If you have to go deeper, you choose the probe with the least attenuation (lower frequency).
Ex: 2.5 MHz for echo vs. 7.5 MHz for carotid
What is the tradeoff for using a lower frequency probe?
Poorer resolution
This is an example of what type of reflection?
Diffuse reflection, or backscatter
This is an example of what type of scatter?
Rayleigh scatter
These are examples of what types of
ultrasound waves?
a) Continuous
b) Gated continuous
c) Pulsed
