Module 2 : Bioeffects Flashcards
what are the three waves we can describe the strength of a wave
- amplitude
- power
- intensity
other than strength what else can amplitude, power, and intensity express
- loudness/ volume of sound
what are the 4 definitions of amplitude
- particle displacement
- particle velocity
- acoustic pressure of a sound wave
- maximum variation of an acoustic variable
what does amplitude indicate about an echo
- indicates the strength of the echo or voltage induced in a crystal by a pressure wave
what are the 4 acoustic variables
- pressure
- density
- particle motion
- temperature
what 4 questions should you ask yourself about amplitude
- what determines the initial amplitude of a pulse
- as the pulse travels through the medium what is the reduction in amplitude called
- what are the 5 mechanisms that cause attenuation
- does the amplitude of the echo decrease as the echo returns to the transducer
what determines the initial amplitude of a pulse
- the pulser (output power)
as the pulse travels through the medium what is the reduction in amplitude called
- attenuation
what are the 5 mechanisms that cause attenuation
- ABSORPTION
- reflection
- refraction
- wave front divergence
- scatter
does the amplitude of the echo decrease as the echo returns to the transducer
- yes
what is the definition of power
- measure of the total energy transmitted summed over the cross sectional area fo the beam per unit time
power equation
power = intensity x area
what is the absolute unit of power
- the watt (joules)
what is the relative unit of power and intensity
- the decibel (db)
what is a decibel in relation to a bel
- it is 1/10 of a bel
what does 0dB equate too
- 100% power
what does a 3dB loss equate to
- 1/2 the original power
what is power determined by
- the pulser
what is the definition of intensity
- measure of the strength of a sound wave
- equal to the power per unit area
what is the intensity equation
intensity = power / area
what is the absolute unit of intensity
mW/cm^2 or W/cm^2
how would you describe the relationship between intensity and power
- it is proportional
if there is an increase in power then what happens to the intensity
the intensity increases
how would you describe the relationship between area and intensity
- it is inversely proportional
if the area decreases what happens to the intensity
- it increases
how is intensity related to amplitude
- it is proportional to the square of the pressure amplitude
a small change in amplitude will do what to the intensity
- result in a very very large change in intensity due to the square exponent
if the amplitude is doubled what happens to the intensity
- it is quadrupled
is intensity constant in space and time
- no
what are the four different expression of intensity
- spatial average (SA)
- spatial peak (SP)
- temporal average (TA)
- temporal peak (TP)
where is the spatial average found
- at the transducer face
where is the spatial peak found
- found at a the focal point where the beam area is the smallest
how are spatial peak and spatial average related
- by the Beam Uniformity Ratio BUR
what is the Beam Uniformity Ratio equation
- BUR = SP / SA
what does the BUR tell us about the beam
- tells us the amount of focusing there is
highly focused beams have a high or low BUR
- high
weakly focused beams have a high or low BUR
- low
what number will the BUR always be over and why
- the BUR will always be over 1
- the spatial peak will always be higher than the averages
what would the BUR be in a perfectly uniform beam
- 1
what are the two factors that effect the spatial intensities
- spatial peak will increase with an increase in power or focusing
- spatial average will increase with an increase in power
what is the temporal average
- over the time the pulse is generated it includes both the ringing and listens phase of the pulse
what is the temporal peak
- the highest amplitude in the pulse at any given time
- instantaneous peak
how are the temporal average and temporal peak related
- related by the Duty Factor DF
what is the Duty Factor DF equation for TP and TA
- DF = TA / TP
what is the Duty Factor
- it is the percent of time the sound is on / the probe is ringing
Duty Factor Equation
DF = PD/PRP x 100
what should the temporal peak be replaced by and why
- the pulse average PA
- the pulse is so short that the TP almost equals the PA
what is the duty factor equation for TA and PA
- DF = TA / PA
what is the pulse average
- pulse average intensity is Thea average of all intensities found within a single pulse
what are the two factors that effect temporal intensities
- increase in power
- increase in duty factor
when is the duty factor increase
- if the PRF or PD is increased
what is the duty factor for continuous wave CW
- 100% because the crystal is always ringing
- usually has a higher SPTA than PW
combined intensities from highest to lowest
- SPTP
- SPPA
- SPTA
- SATP
- SAPA
- SATA
why don’t we use SPTP for bioconsiderations
- not fair to use this because it is such a short part of the pulse its very fast
what combined intensity do we use for bioconsiderations
- SPTA
what is the SPTP intensity also known as
- instantaneous peak
what are the three sources that we use for information on effects of ultrasound
- epidemiology
- in vitro cell studies
- animal studies (invivo)
what is an epidemiological study and what is the time line of it
- people are monitored over the course of several years that were exposed to ultrasound in utero
- conducted over a long term
what is the purpose of an epidemiological study
- look for cause and effect
what is an in vitro study
- exposes macromolecules, membran transport systems, cells, or clumps of cells suspended in liquid to ultrasound
- it is difficult to say that an effect on cells will have a direct clinical significance
- the results of in vitro studies can give valuable information to set thresholds for in vivo studies
what have in vivo studies shown
- potential for bio effects exists
what 6 adverse effects have in vivo studies shown
- fetal weight reduction
- postpartum mortality
- fetal abnormalities
- tissue lesions
- hind limb paralysis
- blood flow stasis
what two more positive effects have been reported from in vivo studies
- wound repair enhancement
- tumor regression
what does the official AIUM statement say about bio effects
- there are no adverse effects observed if the intensities are kept below a certain threshold
what is the highest safe intensity level for an unfocused probe
- 100 mW/cm^2
what is the highest safe intensity level for a focused probe
- 1000 mW/cm^2 (1W/cm^2)
what are the two main categories of bio effects
- thermal
- non thermal (mechanical)
what is an important thing to remember about thermal bioeffects
- attenuation is primarily due to absorption which is the conversion of sound to heat
do adult of fetal tissues handle temperature increases better
- adult tissues
no adverse effects occur when the temp increases is less than what number
- 2ºC
what other temperature range is considered safe as long as exposure time is reasonable
- between 2-6ºC
the higher the exposure time leads to what
- more chance of adverse effects at higher temperatures
- 6ºC increase in temp will not produce effects if exposure time is under 16 minutes
what two categories are included in mechanical effects
- radiation force
- cavitation
what is radiation force
- force exerted by the sound on the medium which can deform and disrupt structures
what can radiation force cause
- can cause flow in absorbing fluids resulting in shear forces
what is cavitation
- production and behaviour of bubbles in a liquid medium
what are the to divisions of cavitation
- stable and transient
what is stable cavitation
- simple oscillation of the bubbles that can result int eh streaming of liquid resulting in shear stresses
what is transient cavitation
- when the bubble actually collapses producing shock waves
- the shock waves can result in localized extremely high temperatures and has even emitted light in clear fluids
what is the standard set in 1992 for real time scanning called
- the Output Display Standard (ODS)
what 2 quantities are displayed by the ODS
- thermal index TI
- mechanical index MI
what is the thermal index
- ratio of acoustic power produced by the transducer to the power required to raise the temperature in tissue 1ºC
- assumed value
what are the three other TI categories
- TIS
- TIB
- TIC
TIS
- TI for soft tissue and is the MOST COMMON
TIB
- TI for bone near the focus and is used for OB
TIC
- TI for bone near the surface as in the case of a transcranial study
what does the mechanical index represent
- likelihood that the energy used will lead to cavitation
what is the MI proportional to
- peak rarefaction pressure so if the pressure doubles the MI doubles as well
what is the MI inversely proportional to
- proportional to the square root of the frequency so the frequency would have to quadruple for the MI to halve
is there a chance for MI and TI to be underestimated and if yes then in what circumstance
- yes it can be
- in presence of a large fluid collection but these situations usually allow for less power
what is the max TI
6ºC
what is the max MI
1.9
cardiac SPTA limit
430 mW/cm^2
peripheral vascular SPTA limit
20 mW/cm^2
ophthalmic SPTA limit
17 mW/cm ^2
fetal and other (abd, peds, SS, neonatal, brain) SPTA limit
94 mW/cm^2
typical output intensity SPTA for b scan
18.7 mW/cm^2
typical output intensity SPTA for m mode
73 mW/cm^2
typical output intensity for SPTA for PW
1140 mW/cm^2
typical output intensity SPTA for color
234 mW/cm^2
what are the two different applications of ultrasound
- scanned
- non scanned
what are the 3 non scanned applications of ultrasound
- CW
- PW
- m mode
what do the non scanned applications of ultrasound pose risk of
- highest risk of thermal effects with W having the most potential due to the fact they transmit repeatedly in a straight line
what other non thermal adverse effects can occur
- when gas bodies are present in the circulation contrast agents in the blood stream pose a concern since these findings were confirmed at intensities below the accepted threshold
- when MI is below 0.4 there is little apparent risk
what 4 adverse effects have been found from scanning the gas bodies
- induction of premature ventricular contractions
- microvascular leakage with petechiae (micro bleeds)
- glomerular capillary hemorrhage
- local cell killing
ALARA
- balancing between benefit and risk
- As Low As Reasonable Achievable
