Physics Exam #1 Flashcards
what is wavelength? what’s parameters? what’s wavelength equation?
wavelength = propagation velocity / frequency
it is a distance so it is in mm
*or any other distance related unit
what is period? what’s parameters? what’s equation for period?
period equals 1 over the frequency
5 Hz = 1/5Hz = 1 / 5 = .2 seconds
what is frequency? what’s parameters?
it is a rate so needs a _______per________.
ex. cycles per second.
cycles per second is a Hertz, or Hz. Hz is the unit in US.
how does wavelength, period, and frequency affect the wave?
what’s difference between rarefactions and compressions?
what determines propagation speed of sound in the medium?
what’s propagation velocity for soft tissue
how do the properties of the medium affect the speed of sound: density, stiffness, compressibility, bulk modulus…
define density
define stiffness
define compressibility
define bulk modulus
explain: what is propagation through a medium and what determines the speed?
speed of sound order of various media
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Air, Lung, Fat, Water, Liver, Blood, Kidneys, Muscle, Skull
what is attenuation
attenuation is
defined as a decrease in wave
amplitude (or intensity) due to
the mechanical interaction with
the medium
represents a transfer
of energy through absorption and
scattering represented by
reflection and refraction.
what is attenuation coefficient for soft tissue
the degree of sound beam attenuation in
a tissue is usually given in decibels per
centimeter (dB/cm)
Soft Tissue Attenuation Rate:
0.5dB/cm/MHz
attenuation coefficient is one half of the frequency (0.5)
Attenuation coefficient is the amount of attenuation per
centimeter per MHz
Attenuation coefficient is the # of decibels
of attenuation that occurs when sound
travels one centimeter.
what affect does increasing the frequency of a sound wave have on attenuation?
Attenuation coefficient is the # of decibels
of attenuation that occurs when sound
travels one centimeter.
For each centimeter that sound travels more
energy is lost from high frequency sound than
from low frequency sound.
Its units are dB/cm
what is the biggest factor in attenuation?
absorption
the foundation for US is
reflection
high frequency =
high attenuation
low frequency =
low attenuation
The angle that a sound wave
approaches the boundary
between two different types of
mediums helps determine
whether __________, ___________, or _________ will occur.
transmission, reflection, or refraction
what is the angle of incidence?
The angle at which a
sound wave approaches a boundary
what is the angle of transmission?
The angle at which it continues on
past the boundary
what is the angle of reflection?
The angle at which the sound wave
reflects back to the transducer (after
striking the boundary)
“normal incidence” of an angle is exactly equal to how many degrees
90 degrees
The angle at which the
wave strikes the
boundary determines
the behavior (of the pulse)
Oblique incidence
occurs when
the incident sound beam strikes
the boundary at any angle other
than 90º
If reflection occurs the incident
angle equals the
reflected angle, WHATEVER IS NOT TRANSMITTED MUST BE REFLECTED
The strongest echoes are produced when the
angles of incidence approach the angle of reflection.
define acoustic impedance
property of tissues that influences the strength or amplitude of reflected echoes in a quantity
Z=ρc
ACOUSTIC IMPEDANCE
The acoustic impedance of a
material is equal to the product of
the medium’s density and its
speed of sound
(Acoustic impedance is a measure of the ease or difficulty with which
a sound wave propagates through a particular medium)
the unit for acoustic impedance is called
1 Rayl = kg/m²sec
2 media have identical impedances…
NO or LITTLE reflection
2 media have substantially different impedances…
LARGE reflection
explain acoustic impedance in a paragraph
Lets take 100 (we will call this our “incident”
marbles) marbles throw them at a paper wall,
while the marbles are rolling through the air
there is no reflection. Air is homogenous
therefore there is no change in impedance,
no reflection
Now we will throw them at a hard paper wall
hard enough that 20 of the marbles come
back toward us (Our reflected marbles) and
80 go through the paper. (Our transmitted
marbles) This represents a difference (a
mismatch) in impedance relative to the air
incident Intensity is (marble idea approach here)
the sound waves
intensity immediately
before it strikes a
boundary
Reflected Intensity (marble idea approach here)
is the
intensity of the portion of
the incident sound beam
that after striking a
boundary returns back
where it came from.
transmitted intensity (marble idea approach here)
is the intensity of the
portion of the incident
beam that after
striking a boundary
continues forward in
the same direction that
it was traveling
acoustic impedance media from smallest to highest
air
lung
fat
water
liver
blood
kidney
muscle
skull bone
air
lung
fat
water
liver
blood
kidney
muscle
skull bone
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Reflection Coefficient
% REFLECTED =
(Z2-Z1) ² X 100
(Z2+Z1) ²
if the answer is 10% reflected also means
(90% was transmitted)
what is the need
for a coupling medium such as
gel between the transducer and
the patient
The complete reflection at air to (other medium) interface is almost at 100%
very large acoustic impedance
The % of the intensity that bounces
back when a sound beam strikes the
boundary between two media is the
intensity coefficient
A greater % of the wave is reflected
when sound strikes a boundary such
as between soft tissue and bone or
between soft tissue and air
Reflection and Transmission are
both reported as percentages
acoustic impedance can be caused by
change in speeds of sound, a
change in densities or both
refraction is
(2 things must happen for something to be considered refraction)
the
“bending” of the wave at a interface
of two media
- oblique incidence
- different propagation speeds of the two media
Snells Law
The rule that governs and predicts the
amount of refraction is called
helps to predict the magnitude of the
refracted wave. It predicts direction of the
transmitted beam
The transmission angle will be
greater than the incident angle
when the speed of Medium 2 is
greater than the speed of Medium 1
diffraction
causes the US beam to diverge or
spread out as the waves move farther from the
sound source
power equals
amplitude squared
ex. 2x the power = 4x the amplitude
just take the power value and square it for amplitude
strength of wave are all proportional with this acronym
IPA
intensity, power, amplitude
diffuse reflection.. aka…? what kind of surface?
irregular surface, sound travels back to transducer
aka BACKSCATTER
more hypoechoic image
specular reflection
specular means mirror, even surface reflection. more hyperechoic image
scattering aka
speckle
what is Rayleigh scattering
scattering of the sound wave in all directions, occurs with very small structures ex caused by RBC’s
attenuation rate low to high… start with WATER!
Water Brings Beautiful Life… Fill My Beer Last
Water, Blood, Brain, Liver, Fat, Muscle, Bone, Lung, (Air)
Repetitious waves are a _____ transfer of _______.
cyclical, energy
cyclical means
periodic or repetitive
the major classification of waves are
electromagnetic and mechanical
A _____ wave needs a medium to propogate
mechanical
A vacuum is the absence of a _______.
medium
Two classifications of mechanical waves are
transverse and longitudinal
In a _____ wave, particle motion is perpendicular to the wave direction.
Transverse… particle motion is perpendicular or TRANSVERSE to the wave direction
In a _____ wave, the particle motion is back and forth in the same direction as the wave propagation
longitudinal
Sound is a _______, mechanical wave
longitudinal
A longitudinal wave has areas of _____ and compression.
rarefaction
An area of rarefaction is
where the particles are farther apart than the normal state, low pressure
_______ is a way of specifying how fast a wave is cyclically varying
frequency
The unit for frequency is _____ which is an abbreviation for cycles per _____.
Hertz, second.
Frequency has units which are the inverse of _____, or one divided by seconds, also called _____.
time, Hz.
The number of times longitudinal wave reaches maximum compression and rarefaction per time is called the
frequency
the reciprocal of frequency is the
period
The period has units of
time
The _____ is the time between cycles of peak compression in a longitudinal wave such as sound.
period
a shorter period means a higher
frequency
propagation velocity is a measure of
how fast a wave will travel
the unit for propagation velocity is
m/s
frequency and wavelength have an _____ relationship.
inverse
changes in acoustic variables are the result of the mechanical interaction of the acoustic wave and the
medium
sound, human audible range
20 Hz to 20 kHz
ultrasound… “ultra” means
above
so any ultrasound is “above” human hearing
Diagnostic Sound
2 MHz - 12 MHz
elasticity is
ability of a solid object to return to its original shape after distortion by a force
compressibility is
how much the volume of the material changes for a given distorting force (pressure)
stiffness is
opposite of elasticity or compressibility. not much compression here. it is inelastic. “in” meaning “not.” so not elastic.
bulk modulus is
ratio… stress:strain.
stiff materials have high bulk modulus. it is the inverse of compressibility. Ex: a low compressibility material would have a high bulk modulus.
it the amplitude is DECREASED by a factor of 3, what happens to the power?
take the value 3 and square it… because it says DECREASE… you put it in fraction form.
It is 1/9th the power.
intensity equals
power divided by beam area
if the beam area is doubled, what happens to the intensity?
it is 1/2 the original intensity
small units… deci, centi, milli, micro
deci 1/10
centi 1/100
milli 1/1000
micro 1/1,000,000
big units… deca, kilo, mega
deca 10
kilo 1000
mega 1,000,000
Transmit power, attenuation
receiver gain, and compression
all measured in
decibles or dB
any parameter which results in a change of POWER.
the simplified rule of 3dB
with each 3dB gain there must be an
additional doubling of power delivered.
Thus, a 3dB gain requires twice the
power, a 6dB gain requires four times
the power, and a 9dB gain requires
eight times the power.
example
if the original intensity produced at
some point in the beam of the
transducer is 20mW/cm², a 3dB
increase in the output of the machine
results in a new intensity of 40
mW/cm², another 3dB increase
results in an intensity of 80 mW/cm²
When intensity increases, ten-
fold the relative change is +10
dB
When the intensity is reduced to ½ its
original value, the relative change is
-3dB
When the intensity is reduced to 1/10
its original value, the relative change
is -10dB
When dB increase by +3 dB,
the intensity increases by a
factor of 2
When the dB increase by
+10db, the intensity
increases by a factor of 10
10 dB increase =
sound energy is increased by a factor of 10
10 dB decrease =
sound energy is decreased by a factor of 10
3 dB increase =
double
3 dB decrease =
half
in terms of INTENSITY, an increase of power by a factor of 2 is
equivalent to ______dB?
3 db
an increase by a factor of 10 would be 10 dB
an increase by a factor of 20 would be 100 dB
an increase by a factor of 30 would be 1000 dB
a decrease by a factor of 1/2 would be -3dB
a decrease by a factor of 1/10 would be -10dB
a decrease by a factor of 1/100 would be -20dB
etc
a gain ratio of 1 would affect the dB by how much?
0 dB
For every positive 10dB
change this means that the
intensity will
increase 10x
ultrasound travels very slow in air due to
air is very low in STIFFNESS
Speed of sound in soft tissue
1540 m/s
154,000 cm/s
1.54 km/s
1.54mm/μs= 0.154cm/μs
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