Intensity and Power Flashcards
Measurement of Intensity
Intensity
Determining Acoustic Output
–_____
– Radiation Force _____/_____
–_____
–_____
Hydrophone
Balance, Scale
Calorimeter
Thermocouple
Intensity
_____ is the amount of power divided by the area
The rate at which _____ is transmitted by the wave over a small area
Units - mW/cm^2 , W/cm^2
I = P/A
Intensity
energy
Intensity is _______ as the sound travels through a medium
Increasing intensity =
—Particle distribution in compression zone becomes more _______
—Peak acoustic pressure is _______
—-Length of particle oscillations _______
—-Maximum particle velocity _______
Reduced Dense Higher Increase Increases
Intensity
_______, _______, and acoustic _______ of an US beam are NOT affected by a change in intensity
_______ is linked to the study of potential bioeffects
_______ intensity wave is more disruptive to living tissues
Frequency Wavelength Velocity Intensity High
A sonographer can vary _______ by adjusting the output power/acoustic output
Intensity
Instantaneous intensity
I = P^2/pc
Where:
I = instantaneous \_\_\_\_\_\_\_ P = acoustic \_\_\_\_\_\_\_ p = medium \_\_\_\_\_\_\_ c = acoustic \_\_\_\_\_\_\_
Intensity
Pressure
Density
Velocity
Intensity
Frequently the pulsed-wave US is characterized by a peak _____ pressure, a.k.a., peak rarefactional pressure
US instruments produce peak rarefactional pressure amplitudes from ___-___ megapascals (50 times greater than atmosphere pressure
negative
- 5
- 5
Intensity
Free-field Conditions
Pressure in measured in _____ without reflectors or other disturbances to the US field
Pressure and corresponding intensity in water is converted to intensity in tissue by applying correction (derating) factors for attenuation
Free-field measurements do not assess:
- –_____ by tissue
- –_____ by anatomic structure
- –Production of standing _____
- –Effects of _____ bodies
water attenuation focusing waves reflecting
Determining Acoustic Output
Hydrophones
Very specialized equipment; usually used by the manufacturer
a.k.a. _____
Used in two forms:
—A small transducer element < __ mm in diameter mounted on the end of a hollow needle
microprobe
1
Determining Acoustic Output
Hydrophones
Used in two forms:
—A large _____ membrane with small electrodes on both sides
PVDF is used to make probe _____ and membrane
_____ floride has a wide bandwidth
piezoelectric
tip
polyvinylidene
Determining Acoustic Output
Hydrophones
—Receive sound from all directions without altering the sound by their presence
Period, PRP, and PD can be determined
—From these _____, _____, _____ can be calculated
Pressure _____ can be determined
_____, _____ and _____ can be calculated
frequency PRF DF amplitude wavelength SPL intensities
A radiation force ______ or radiation force ______ measures the intensity or power of the sound beam by measuring the force the sound beam exerts on the scale or balance
Balance
Scale
Determining Acoustic Output
______ is calibrated to measure total power in a sound beam through process of absorption
______ is a tiny electric thermometer placed into beam and temperature is measured; temp rise directly related to amount of power; power measured at specific area in beam
Calorimeter
Thermocouple
Intensity Descriptors
______ dependence (time)
______ dependence (area that the beam is hitting (space))
Intensity values for various operating modes
Common intensities
Temporal
Spatial
Temporal Dependence
______ peak
Pulse ______
Pulse ______
Temporal
Peak
Average
Temporal Dependence
Considerations:
Temporal Average and Peak Intensities
______ values are used to describe the intensity over time
Intensity is NOT uniform over ______; therefore, several intensities are used
Temporal
Time
Temporal Dependence
The peak values over time occur when the beam is on, temporal ______ (TP)
The temporal ______ (TA) value averages the intensity during on and off time
Peak
Average
Temporal Dependence
For pulse ultrasound, pulse ______ (PA) is the average intensity that occurs within the pulse during the pulse duration
Average
Temporal Dependence
Related to the Duty Factor; aka: Duty ______
The DF is to time what the SP/SA factor is to ______
Unit less
Max = 1.0 (100%) = CW
Min = 0.0 (0%)
DF = PD/PRP
Cycle
Space
Spatial dependence
Spatial ______
Spatial ______
Peak
Average
Spatial dependence
Considerations:
Spatial average and peak intensities
Intensities are important for discussing bioeffects
Intensity is NOT uniform over ______; therefore, several intensities are used
______ values are used to describe intensity as it relates to distance or space across the beam
Space
Spatial
Spatial dependence
Intensity is measured at its peak value, spatial ______ (SP)
Intensity is measured at its average value, spatial ______ (SA)
Peak
Average
Spatial dependence
Related to the beam uniformity ratio or coefficient; aka: BUR, BUC, or SP/SA factor
The SP/SA factor is to space what the DF is to ______
BUR = SP/SA
Unitless
Minimum value of 1.0
If SA decreases —- BUR ______
Time
Increases
Temporal and Spatial dependence
By combining spatial and temporal values, six intensities can be measured (see chart in PowerPoint)
(See chart in PowerPoint for six intensities)
Intensity Values for Various Operating Modes
______ (intensity) values for various modes are found in equipment operator’s manual
Typical SPTA Intensity values include:
Gray Scale = 1 - ___mW/cm^2
M-mode = 70-___mW/cm^2
PW Doppler = 20 - ___mW/cm^2
Color Scale = 10 - ___mW/cm^2
SPTA 200 130 290 230
Common Intensities
______ - is the lowest of all intensity measurements used in diagnostic US
______ - has the highest values of all intensity measurements used in diagnostic US and is the intensity that exists at the spatial peak when the sound beam is on
SATA
SPTP
Common Intensities
______ - Spatial peak, temporal average intensity is used to describe pulsed US intensities and to measure biological effects
______ average intensity at the position of the spatial peak
Intensity that is referred to when discussing data concerning biological effects
Equal to SATA x SP/SA
SPTA
Time
Common Intensities
______ - Spatial Peak, pulse average is the intensity averaged over the duration of the pulse
The spatial ______ pulse average (SPPA) and spatial ______, pulse average (SAPA) are used only in pulsed US
SPPA
peak
Average
Common Intensities
Im - maximum intensity is the average intensity over the most intense half cycle of the pulse; its values are similar to the ______ intensity
(less than the SP)
SPTP
Common Intensities
For CW US, the following intensities are equal:
SPTP = \_\_\_\_\_\_ SATP = \_\_\_\_\_\_
The TP is always equal to the TA for CW due to the continuous generation of cycles
SPTA
SATA
Power
The rate at which work is performed
______ transmitted per unit time, summed over the entire cross-sectional area of the beam
Until
—(mW) - Measured in Watts
— Joules/sec
Energy
Power
Typical Power Values for various operating modes
Gray scale = 1 - _____ mW
M-mode = 1 - ____ mW
PW Doppler = 2 - ____ mW
Color scale = 2 - ____ mW
20
5
20
20
Power
Power / Intensity / Amplitude
Power and intensity are ______
Power and intensity are proportional to ______ squared
If you double the amplitude the power or intensity will increase ______ times
Proportional
Amplitude
Four
Logarithms
Logarithms are especially useful as they compress a large range of numbers into a smaller range.
In diagnostic US the range of signal ______ (strength) which are reflected from the body is enormous relative to the range of amplitudes that can be displayed on a monitor and interpreted by the human eye.
Definition : the power to which a base must be raised to get the desired number
Amplitudes
Logarithms
So what?
_____ are good units for comparing the relationships between various measured sound levels and the threshold of human hearing
Used for measuring _____, _____, _____ range, and _____
Units Attenuation (loss of signal strength) = _____ = dB
When US waves travels through a medium (tissue) they LOSE _____/weaken
may be annotated as -dB
Decibels attenuation output dynamic gain Decibels strength
Logarithms
Decibels involve logarithms (log)
RULES OF THUMB:
3 dB of _____ = intensity reduction of 1/2
3 dB of _____ = intensity increase x 2
attenuation
gain
Logarithms
Initial intensity x intensity ratio (of 6dB) = remaining intensity
100 mW/cm^2(1/4) = 25 mW/cm^2
Formula
Attenuation
The reduction in the intensity (_____) of an US beam as it travels through a medium, the weakening of sound as it propagates
Encompasses _____, _____, and some say _____
The depth of penetration becomes less as frequency is _____, and the ability to observe deep-lying structures is forfeited (axial resolution)
amplitude absorption scattering reflection increased
Attenuation
_____ limits imaging depth and must be compensated for
Attenuation is affected by
_____ - directly
_____ - directly
_____ Type (Soft tissue)
attenuation
frequency
depth
medium
Attenuation
Loss of intensity by attenuation in different media
_____ = 0.9995
_____ = 0.96
_____ = 0.87
_____ Tissue = 0.83 - 0.89
_____ = 0.01
_____ = 0.0001
Water Blood Fat Soft Skull Lung
Attenuation
_____ are good units for comparing relationships between various measured sound levels and the threshold of human hearing
Used for measuring _____, _____ range, and _____
_____ involve logarithms
Log of a number = number of tens that must be multiplied together to result in that number
decibels output dynamic gain decibels
Attenuation
Level (dB) = 10 log base 10 (I/I base O)
I = intensity at point of interest
I base O = Original or reference intensity
___ dB of attenuation = intensity decrease to 1/2 the original intensity
___ dB of attenuation = intensity decrease to 1/4 the original intensity
___ dB of attenuation = intensity decrease to 1/10, or 0.1 the original intensity
___ dB of attenuation = intensity decrease to 1/100, or 0.01 the original intensity
3
6
10
20
Attenuation
Attenuation Units = Decibels = dB
for ST:
1/2 x frequency(MHz) x pathlength(cm)
or
Attenuation coefficient x pathlength
as frequency increases, attenuation _____
as pathlength increases, attenuation _____
as attenuation coefficient increases, attenuation _____
increases
increases
increases
so WHAT’S AN ATTENUATION COEFFICIENT
AC(db/cm) = Attenuation dB/Separation between two points (cm)
The attenuation per unit _____of sound travel
The attenuation for each _____ of sound travel
Numerical values that express how different materials will attenuate an US beam per path length
Different materials have _____attenuation coefficients
length
cm
different
SO WHAT’S AN ATTENUATION COEFFICIENT?
The average attenuation coefficient for soft tissue is one-____ the frequency in MHz
half
Attenuation- Half-Value Layer
1/2 x cm x frequency
The amount (depth in cm) of material required to reduce the intensity by _____ of its original value
A half-value layer results in a ___-dB reduction in intensity
half
3
Absorption
The only process whereby sound energy is dissipated in a medium.
_____ (conversion of sound to heat) is normally the dominant contribution to attenuation (in ST)
Other modes of interactions (_____ ,_____, _____ and _____) decrease beam intensity by redirecting its energy
absorption reflection refraction scatting divergence
Absorption
Strongly dependent on _____
-Rate of absorption is _____ related, if frequency doubles, absorption _____
Related to the viscosity and _____ time of the medium
- As viscosity increases, absorption _____
- The longer the relaxation time, absorption _____
frequency doubles relaxation increases increases
Regions of lower pressure and density are called
rarefactions
The rate at which energy passes through a unit area is called
intensity
As sound travels, the reduction in amplitude and intensity of the wave is called
attenuation
If beam power increases, intensity _____
increases
If the beam area decreases, the intensity _____
increases
Attenuation increases with increasing _____
frequency
Amplitude and intensity are indicators of the sound wave’s _____
strength
The attenuation coefficient for soft tissue using a 10 MHz transducer is _____
5 dB/cm
Impedance is equal to density multiplied by _____ _____
propagation speed
The unit for impedance is _____
rayl
What does 3 dB of attenuation mean?
one half the original intensity
The proportion of intensity to amplitude is _____
squared
Attenuation encompasses (3)
absorption, scattering, and reflection
What units are used to quantify attenuation?
dB
Intensity is equal to the power of a wave divided by the _____ over which the power is spread.
area
Amplitude is the maximum variation that occurs in an acoustic _____
variable
_____ is the dominant factor contributing to attenuation of ultrasound in soft tissue.
absorption
For each centimeter of distance, the pulse round-trip travel time is equal to _____
13 microseconds
Amplitude and intensity describe the strength of sound.
TRUE or FALSE
true
Cycle, period, and frequency are examples of acoustic variables.
TRUE or FALSE
false
Sound is a mechanical longitudinal wave.
TRUE or FALSE
True
Stiffer media have higher sound speeds.
TRUE or FALSE
true
Imaging depth increases with increasing frequency.
TRUE or FALSE
false
Attenuation increases with an increase in wavelength.
TRUE or FALSE
false
The impedances of the media determine how much of the incident sound wave is reflected and transmitted into the second medium.
TRUE or FALSE
true
The average attenuation coefficient for soft tissue is 1.0 dB/cm for each megahertz of frequency.
TRUE or FALSE
false
Attenuation limits imaging depth
TRUE or FALSE
True
Sound having a frequency of 10,000 Hz or higher is called ultrasound.
TRUE or FALSE
false
