Ch. 6 Physics Flashcards
A measurement tool used to report attenuation changes, relative changes of sound beams and ratio
decibel notation
A mathematical construct or method of rating numbers
logarithm
Represents the number of 10s that are multiplied to create a number
log
The number of decibels of attenuation that occurs when sound travels one centimeter
attenuation coefficient
How sound weakens as it travels through the body
attenuation
Is there a relationship between attenuation and speed
no
Describe US signals decreasing in strength
negative decibels
Describe US signals increasing in strength
positive decibels
When sound strikes a boundary and a portion of the wave’s energy may be redirected back to the sound source
reflection
When sound waves reflect off an irregular boundary + radiate in more than one direction
diffuse reflection
When the US beam hits a smooth boundary and the sound is reflected in only one direction
specular reflection
Distance and attenuation are
directly related
Intensity and attenuation are
directly related
The number of decibels of attenuation that occurs when sound travels one cm
attenuation coefficient
When US energy is converted to another energy form (i.e. heat) and is the most sizeable component of attenuation
absorption
The distance sound travels in a tissue that reduces intensity by 1/2
half-value layer thickness
Random redirection of sound waves in many directions
scattering
When a sound beam is reflected equally in all directions
Rayleigh scattering
The process of sound energy being extracted from a wave by absorption, scattering and reflection
attenuation
Greater than 90 degrees
obtuse angle
The acoustic resistance to sound traveling in a medium
impedance
Exactly 90 degrees
right angle
Less than 90 degrees
acute angle
Incident intensity=
reflected intensity + transmitted intensity
The intensity of a sound beam that returns after striking a boundary
reflected intensity
The intensity of a sound beam that continues forward after striking a boundary
transmitted intensity
Sound wave intensity before it strikes a boundary
incident intensity
When incident angle of sound beam strikes a boundary at any angle other than 90 degrees
oblique incidence
IRC% + ITC%=
100%
% of US intensity that bounces back when beam hits a boundary between 2 media
intensity reflection coefficient (IRC)
Amount of ultrasound intensity reflected at a boundary in soft tissue
less than 1%
% of US intensity that continues when beam strikes interface between 2 media
intensity transmission coefficient (ITC)
Amount of ultrasound intensity transmitted at a boundary in soft tissue
> 99%
US beam strikes a tissue boundary at a 90 degree angle is called
normal incidence
If an ultrasound beam strikes a tissue boundary at any angle other than 90 degrees it is
oblique incidence
When the transmission angle is greater or less than the incident angle what happens
refraction
Quantification of the physics of refraction
snell’s law
Occurs only if these two conditions are present - oblique incidence and different prop speeds. Causing “bending” of the sound beam
refraction
With oblique incidence, the reflection angle equals
incident angle
When media on either side of a boundary have different impedances and the sound beam is at a 90 degree angle to the tissue boundary
normal incidence
Units for measuring impedance
rayls
Units of intensities
w/cm squared & time and space
As the path length increases, the attenuation coefficient of US in soft tissue
Remains the same
Rayleigh scattering is related to frequency to the
4th power
Scattering is directly related to
frequency (increased frequency=increased scattering)
The two forms of reflection
specular and diffuse
the half-value layer thickness is determined by
frequency and medium
Thin half-value = media with
high attenuation rate
Refraction only occurs if these 2 conditions are present
oblique incidence and 2 medium with different prop speeds
List 3 processes that contribute to attenuation
reflection, scattering and intensity
If attenuation causes a reduction in the intensity of a sound beam by 1/2 its original value, it is a
-3dB change
Every 3dB increase means that the increase will be
double the intensity
The attenuation coefficient __________________ as path length (depth) increases
stays the same
Attenuation coefficient in blood is _____ than in soft tissue
lower
What 3 things have the highest attenuation coefficient
air, lung and bone
Acoustic impedance is calculated by multiplying the _____ by the _____ at which the sound travels in a medium
density and prop speed
An ultrasound wave travels through soft tissue, hitting a boundary at a right angle. The 2 media have the same impedance. What percent of the sound beam has been refracted
0%
