Flashcards in Chapters 6-7 Deck (70):
weakening of sound as it travels
What is the log of 1,000?
we multiply 10 by itself 3 times
What unit is used to quantify the strength of sound beams as they are created?
What do positive decibels report?
they report signal that are increasing in strength
Decibels report what?
relative changes verses measurement of absolute numbers
How many intensities does dB require?
What are the four components of decibel notation?
a relative measurement
what does -3dB mean?
1/2 of intensity
What does 10dB mean?
What do negative decibels report?
they describes signals that are decreasing in strength or getting smaller
What are the two factors attenuation is determined by?
path length and freq
The decrease in amplitude, intensity, and power as sound travels is what?
How are distance and attenuation related?
How are frequency and attenuation related?
What are the three components of attenuation?
reflection, scattering, and absorption
What is reflection?
as sound strikes a boundary, a portion of the waves energy may be redirected, or reflected, back to the sound source
What are two forms of reflection?
specular and diffuse (nonspecular)
Define specular reflection
when the boundary is smooth, the sound is reflected in only one direction in an organized manner.
True or false: specular reflections at an angle do return to the transducer
they do not
Another name for diffuse reflection
Define diffuse reflection
not smooth and the wave reflects off an irregular surface, and radiate in more than one direction
What is an advantage of diffuse reflection?
some interfaces at suboptimal angles to the sound beam can still produce reflections that will return to the transducers
the random redirection of sound in many directions
How is scattering and freq related?
When does sound scatter?
sound scatters when the tissue interface is small; that is equal to or less than the wavelength of the incident sound beam
Define rayleigh scattering
a special form of scattering that occurs when the structure dimensions are much smaller than the beams wavelength
redirects the sound beam equally in all directions
What is an example of rayleigh scattering?
the interaction of ultrasound and red blood cells
How is freq and rayleigh scattering related?
scattering is related to freq raised to the fourth power
the conversion of sound into heat
How is absorption and frequency related?
What is the attenuation coefficient?
the number of decibels of attenuation that occurs when sound travels one centimeter
What is the formula for total attenuation?
dB= atten coeff (dB/cm) x distance (cm)
How is the attenuation coefficient calculated?
How is attenuation coefficient and freq related?
What are the mediums that have the lowest attenuation to the highest?
water, blood (urine, biologic fluids), fat, soft tissue, muscle, bone and lung and air (the highest is air)
What is the half-value layer thickness?
the distance sound travels in a tissue that reduces the intensity of sound to one half its original value or 3dB
units length, cm
What are synonyms for half value layer?
penetration depth, depth of penetration and half boundary layer
What are two factors half value layer thickness depend on?
the medium, and freq of sound
What two factors play a role in a thin half value?
high freq sound and a media with high attenuation rate
What two factors play a role in a thick half value?
low freq sound and a media with low attenuation rate
the acoustic resistance to sound traveling in a medium
What is the formula for impedance?
rays= density x prop. speed
What are typical values for impedance?
1,250,000- 1,750,000 rayls
What is the relationship between attenuation and propagation speed?
What is the relationship between freq and depth of penetration?
What are the three types of angles?
What are synonyms for normal incidence?
What is oblique incidence?
occurs when the incident sound beam strikes the boundary at any angle other than 90 degrees
What is the incident intensity?
the sound waves intensity immediately before it strikes a boundary
What is the reflected intensity?
the intensity of the portion of the incident sound beam that after striking a boundary, returns back from where it came
What is transmitted intensity?
intensity that after striking a boundary continues forward in the same general direction that it was traveling
What is the formula for incident (starting) intensity?
=reflected intensity + transmitted intensity
What is the IRC?
intensity reflection coefficient= the percentage of the intensity that bounces back when a sound beam strikes the boundary between two media
Very little sound is reflected between what two boundaries?
soft tissue/soft tissue
or organ parenchyma/ organ parenchyma
What is the ITC?
intensity transmission coefficient= the percentage of intensity that passes in the forward direction when the beam strikes an interface between two media
What must happen for reflection with normal incidence to occur?
90 degrees and different impedances
When will no reflection occur?
when two medias have the same impedances
When is all sound transmitted?
when two medias have the same impedances
What is the formula for ITC?
What two physical principles apply to reflection with oblique incidence?
1. conservation of energy
2. reflection angle=incident angle
conservation of sound must equal what?
the change in direction of wave propagation when traveling from one medium to another
When does refraction occur?
1. oblique incidence
2. different propagation speeds of two media
What does Snells law refer to?
What is the relationship between the time of flight to depth?
What is the range equation?
depth (mm)=(1.54 x go return time)/2
What is the total distance for time of flight of 39 microseconds?
39/13=3, there and back is 6 cm
What is the formula for PRP for the range equation?
PRP= imaging depth (cm) x 13 ms/cm
What is the relationship between PRP and max, imaging depth?