Intensity and Power Flashcards by Cee F

Measurement of Intensity

Intensity

Determining Acoustic Output

–_____

– Radiation Force _____/_____

–_____

Hydrophone
Balance, Scale
Calorimeter
Thermocouple

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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

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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

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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

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A sonographer can vary _______ by adjusting the output power/acoustic output

Intensity

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Instantaneous intensity

I = P^2/pc

Where:

I = instantaneous \_\_\_\_\_\_\_
P = acoustic \_\_\_\_\_\_\_
p = medium \_\_\_\_\_\_\_
c = acoustic \_\_\_\_\_\_\_

Intensity
Pressure
Density
Velocity

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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

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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

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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

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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

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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

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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

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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

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Intensity Descriptors

______ dependence (time)

______ dependence (area that the beam is hitting (space))

Intensity values for various operating modes

Common intensities

Temporal

Spatial

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Temporal Dependence

______ peak

Pulse ______

Temporal
Peak
Average

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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

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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

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Temporal Dependence

For pulse ultrasound, pulse ______ (PA) is the average intensity that occurs within the pulse during the pulse duration

Average

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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

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Spatial dependence

Spatial ______

Peak

Average

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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

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Spatial dependence

Intensity is measured at its peak value, spatial ______ (SP)

Intensity is measured at its average value, spatial ______ (SA)

Peak

Average

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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

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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)

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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?

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

Intensity and Power Flashcards

(76 cards)