Chapter 1: Physical Principles Flashcards
Sound is a pressure wave, created by a mechanical action, and is therefore called a ________
mechanical wave
produced when a vibrating source causes the molecules of a medium to move back and forth
sound wave
any form of matter: solid, liquid, or gas
medium
Sound requires a _____ to propagate; therefore it cannot travel in a vacuum
medium
When sound energy propagates through a medium, it does so in _____ waves, meaning that the molecules of the medium vibrate back and forth in the same direction that the wave is traveling.
longitudinal
Sound is a _______, _______ wave.
mechanical
longitudinal
molecules in a medium vibrate at 90 degrees to the direction of the traveling wave
transverse waves
changes that occur within a medium as a result of sound traveling through that medium
acoustic variables
The three primary acoustic variables are _____, ______, and ______.
pressure
density
particle motion
Each back and forth movement of molecules completes one _____ or one cycle of movement
wave
Each cycle consists of two parts: _______ and ______
compression
rarefaction
molecules are pushed closer together
compression
molecules are spread wider apart
rarefaction
mass per unit of volume
density
the back and forth motion of the molecules in the medium, and its units are that of distance
particle motion
how far apart objects are, and it is the measurement of particle motion
distance
may also be referred to as vibration or displacement
distance
if one variable increases the other increases
directly related
if one variable increases the other decreases
indirectly related
Parameters of sound
period
frequency
amplitude
power
intensity
propagation speed wavelength
the time it takes for one cycle to occur
Period
units of Period
us
the number of cycles per seond
frequency
Frequency units
Hs, kHz, MHz
Frequency and period are _____ related
inversely
When two reciprocals are multiplied together the result is
1
the speed at which a sound wave travels through a medium
propagation speed
Propagation speed is faster in _____
solids
Units for propagation speed
m/s or mm/us
average speed of sound in all soft tissue
1540 m/s
1.54 mm/us
Propagation speed is influenced by two properties
stiffness
density
the ability of an object to resist compression and related to hardness of the medium
stiffness
Stiffness and propagation speed are _____ related
directly
Density is _____ related to propagation speed
inversely
the length of a single cycle of sound
wavelength
distance from the beginning of a cycle to the end of that cycle
wavelength
In clinical imaging, the wavelengths measures between ___ and __ mm
0.1
0.8
Wavelength and frequency are _____ related
inversely
propagation speed divided by the frequency
wavelength
____, _____, and _____ all relate to size of strength of a sound wave.
amplitude
power
intensity
maximum or minimum deviation of an acoustic variable from the average value of that variable
amplitude
Ultrasound physics = _______ amplitude
pressure
units of amplitude
pascals
defined as the rate at which work is performed or energy is transmitted
Power
units of power
Watts or milliwatts
Power is proportional to ______
amplitude squared
If the amplitude doubles, the power ______
quadruples
defined as the power of the wave divided by the area over which it is spread, or the energy per unit area
intensity
_______ is proportional to both power and to amplitude squared
intensity
Intensity range in diagnostic ultrasound
0.01 to 100 mW/cm^2
Resistance to the propagation of sound through a medium
impedance
The amount of ______ depends on the density and the propagation speed of the medium
impedance
______ and ______ are controlling factors of propagation speed
density
stiffness
impedance units
Rayls
the product of the density of the medium and the propagation speed of sound in the medium
impedance
average speed of sound in soft tissue
1540 m/s
Assuming the beam strikes the interface at a 90 degree angle and there exists a large impedance difference between two tissues, there will be a _______
strong reflection
If the impedance between two tissues is the same there will be ______ reflection.
no
sound that is continuously transmitted
continuous wave ultrasound
material within transducers that when electronically stimulate, produce ultrasound waves
piezoelectric materials
pressure is created when voltage is applied to the material and electricity is created when a pressure is applied to the material
principle of piezoelectricity
Parameters of pulsed sound
PRF
PRP
PD
DF
SPL
number of pulses of sound produced in 1 second
PRF
Diagnostic imaging typical values PRF
1-10 kHz
The PRF changes when the sonographer changes the ______ control.
depth
If imaging depth increases, PRF ______
decreases
The time taken for a pulse to occur
PRP
the time from the start of one pulse to the start of the next pulse, and therefore, it includes the “on” and “off” times.
PRP
If PRP increases, PRF ______
decreases
the time during which the sound is actually being transmitted, the “on” time
Pulse Duration
The duration of “on” time depends on:
how many cycles there are in each pulse
period of each cycle
equal to the number of cycles in the pulse multiple by how long each cycle lasts
pulse duration
A ____ PD is preferred for imaging
short
The _____ or _____ layer reduces the long “ring” of a vibrating crystal to two or three cycles per pulse.
backing
damping
The backing layer helps improve _____ resolution.
axial
the percentage of time that sound is actually being produced
duty factor
PD/PRP =
duty factor
If the PRP is short, the DF will be _____
greater
In continuous wave, the DF is:
100%
In clinical imaging with pulsed wave, the DF will be:
1% or less
the length of a pulse
spatial pulse length
depends on the wavelength of each cycle and the number of cycles in each pulse
SPL
equals the number of cycles in the pulse multiplied by the wavelength
SPL
If the wavelength increases, the SPL ______
increases
If the number of cycles in the pulse increases, then SPL ______
increases
Shorter SPLS mean ______ pulse durations
shorter
Parameters determined by the sound source
period
frequency
amplitude
power
intensity
pulse duration
duty factor
pulse repetition period
pulse repetition frequency
Parameters determined by the medium
propagation speed
impedance
Parameters determined by the sound source and the medium
wavelength
spatial pulse length
decrease in the amplitude, power, and intensity of the sound beam as sound travels through tissue
attenuation
3 mechanisms of attenuation
absorption
reflection
scattering
used when evaluating two intensities, such as powers or amplitudes
decibels
If intensity or power doubles = change by ___ dB
3
If intensity or power decreases by half = change by ___ dB
-3
conversion of sound energy to heat
absorption
greatest contributor to attenation
absorption
The amount of attenuation that occurs as sound travels is related to the ____ of the beam
frequency
rate at which sound is attenuated per unit depth
attenuation coefficient
equal to 1/2 of the frequency in soft tissue
attenuation coefficient
total amount of sound in dB that has been attenuated at a given depth
total attenuation
If length increases, attenuation ____
increases
Average rate of attenuation in soft tissue is
0.7 dB/cm/Mhz
describes the depth at which sound has lost half, or -3dB of its intensity
half-intensity depth
Soft tissue half intensity depth is equal to:
6/f
dividing line between two different media
interface
occur when sound impinges upon a large, smooth reflector at a 90 degree angle
specular reflections
one in which the size of the reflector is larger than the wavelength of the incident beam
large specular reflector
Which reflectors are highly angle dependent?
specular reflectors
If the sound strikes the transducer at an _____ angle, the reflection will not return to the transducer.
oblique
one in which their size is smaller than the wavelength of the incident beam
nonspecular reflector
scattered sound in many different directions
backscatter
_______ reflectors are not angle dependent
nonspecular
constructive and destructive interactions
acoustic speckle
Higher frequency transducers = ______ intensity of scatter
intensity
very small reflectors; scatter sound equally in all directions
Rayleigh scatterers
As the frequency increases, intensity of the scatter increases proportional to:
the fourth power of frequency
When is reflection formed?
normal incidence
two media have two different impedances
synonyms for normal incidence
orthogonal
right angle
perpendicular
90 degrees
No change in impedance =
no reflection
Angle of reflection =
angle of incidence
Two types of oblique angles
obtuse
acute
percentage of sound transmitted at an interface
intensity transmission coefficient
equal to 1 minus the IRC
intensity transmission coefficient
the percentage of sound reflected at an interface
intensity reflection coefficient (IRC)
IRC + ITC
1
If impedances are the same, then there is no reflection, and ITC =
100%
The transmitted beam angle is equal to ______
incident angle
When sound strikes an interface with an oblique angle of incidence, the transmitted beam angle will equal the incident angle only if the ______ are identical
propagation speeds
If there is oblique incidence and a propagation speed mismatch, the transmitted angle will be different from the _____ angle
incident
If the propagation speed of medium 2 is less than the propagation speed of medium 1, then the angle of transmission will be ____ than the angle of incidence
less
describes the angle of transmission at an interface based on the angle of incidence and the propagation speeds of the two media
Snells law
______ and _____ refer to where the beam was measured.
Spatial average
Spatial peak
SP intensity is measured where?
at the center of the beam
SA intensity is average intensity across ______
face of entire beam
SP/SA factor
beam uniformity ratio
ratio of center intensity to average spatial intensity
Beam Uniformity Ratio
intensity measured at the highest intensity, or peak, of the pulse
Temporal peak
highest of all temporal intensities
Temporal peak
average of all of the intensities during both transmission and the listening period
Temporal average
lowest of all temporal intensities
Temporal average
measured only during beam transmission
Pulse average
PA x DF
TA
lowest of intensities
SATA
highest of intensities
SPTP
used to measure output intensity
hydrophone, or microprobe
