Physics review Test 2 Flashcards
What happens to sound waves as they propagate in a medium?
They weaken.
What does attenuation mean ?
The loss of energy. Decrease in intensity, power and amplitude.
What two factors is attenuation determined by ?
Path length
Frequency of sound.
How are distance and attenuation related?
They are directly related.
How are frequency and attenuation related?
They are directly related.
What is attenuation measured in ?
Db
The more attenuation you have, the _______ the distance and the higher the _______.
Longer, frequencies.
The less attenuation you have, the _______ the distance and the lower the ________.
shorter, frequencies.
What three processes contribute to attenuation ?
reflection, scattering, absorption.
As a sound strikes a boundary, a portion of the wave’s energy may be redirected or _______.
Reflected
Name two forms of reflection.
specular & diffuse
What type of reflection is smooth, and reflected only in one direction in an organized manner?
Specular reflection.
Name one limitation for a specular reflector.
once the wave is slightly off axis, the reflection does not return to the transducer.
When a wave reflects off an irregular surface, it radiates in more than one direction, this form of reflection is called ________.
diffuse reflection or backscatter.
What is an advantage of diffuse reflections ?
interfaces at sub optimal angles to the sound beam can still produce reflections that will return to the transducer.
What is a disadvantage of diffuse reflections?
they have a lower strength than specular reflections.
What is scattering in ultrasound ?
the random redirection of sound in many directions.
What scatters more, higher frequency sound beams or lower?
Higher frequency.
How is scattering related to frequency?
directly related.
When does Rayleigh scattering happen?
occurs when the structures dimensions are much smaller than the beam’s wavelength.
In clinical imaging, the interaction of ultrasound and red blood cells results in ___________.
Rayleigh scattering
Mathematically, scattering is related to frequency raised to the ______ power.
fourth
what type of reflection sends sound back to the transducer, and is organized?
specular reflection.
What type of reflection sends sound back to the transducer but is disorganized?
diffuse or backscatter.
What type of reflection sounds sound in all directions, and is organized?
Rayleigh scattering.
What type of reflector sends sound in all directions and is disorganized?
Scattering.
What is the third and most sizeable component of attenuation ?
absorption
what is absorption?
occurs when ultrasonic energy is converted into another energy, such as heat.
How do you report attenuation, in ultrasound?
by calculating attenuation coefficient
What is the attenuation coefficient ?
is the number of decibels of attenuation that occurs when sound travels one cm.
What are the units of attenuation coefficient?
db/cm
What is the equation for total attenuation?
total attenuation= attenuation coefficient (db/cm) * distance (cm)
What is half-value layer thickness?
is the distance that sounds travel in a tissue that reduces the intensity of a sound to one-half its original value.
Whats another way that half-value layer thickness can be described ?
depth of a tissue that results in 3 db of attenuation to the intensity.
The half- value layer is _____ for tissues that attenuate sound a great deal, such as lung or bone.
thin
high frequency sound results in a _____ half-value layer.
thin
Lower frequency sound results in a _____ half-value layer.
Thick
In clinical imaging half- value layers ranges from ___ to ___.
0.25 to 1.0 cm.
What other names are half value layer known by
Penetration depth, depth of penetration, half-boundary layer.
The half -value layer thickness depends on what two factors?
the medium and the frequency of sound
What is impedance ?
Acoustic resistance to sound traveling in a medium.
How do you calculate impedance ?
Z= P*C
Reflection of an ultrasound wave depends upon what differences?
the difference in acoustic impedance’s of the two media at a boundary.
Impedance is determined by the ________ only.
Medium
Acoustic impedance is also called __________.
characteristic impedance
Impedance is reported in the units of _________.
rayls
What does normal incidence mean ?
That the incident sound beam strikes the boundary at exactly 90 degrees.
What does oblique incidence mean ?
That the incident sound beam strikes the boundary at any angle other than 90 degrees.
