module 2: bioeffects, safety & power Flashcards
what are 3 ways in which we can describe the strength of a wave?
- amp (max variation of acoustic variable)
- power (P=IntensityXarea)
- intensity (amp squared)
whats the absolute unit of power?
watt(joule)
whats the relative unit of power
decibels
whats power determined by
the pulser
what does power default to?
100% or zero dB
output value for PW
1140mW/cm
define amplitude
the particle displacement, particle velocity or acoustic pressure of a sound wave
- maximum displacement of an acoustic variabe
intensity
a measure of the strength of a sound wave and is equal to the power unit area
whats the relationship between intensity and power?
proportional
intensity formula
i=p/a
if area decreases what happens to intensity?
increases
if area is halved wat happens to intensity?
doubles
whats the relationship between intensity and amplitude?
I is proportional to amp^2
if u double amplitude what happens to intensity
quadrupled
whats the BUR
BUR=SP/SA
do highly focused beams . have a high or low BUR?
high
what does spacial peak increase with?
power or focusing
what will cause spatial avg to increase?
increase in power
whats the duty factor
df=ta/pa
whats the pulse avg intensity
the avg of all intensities found within a single pulse
what factors effect temporal intensities?
an increase in power or duty factor
whats prf limited by?
depth
whats the DF for CW
100%
does CW have a lower or higher SPTA than PW applications
higher
whats SPTP intensity also known as
instantaneous peak
SPPA intensity is also known as….
max intensity or time averaged half maximum
what sources do we get info from about bioeffects
- epidemiology
- in vitro cell studies
- animal
whats the extensive resource for bioeffects info
the american institute of ultrasound in medicine (AIUM)
what happens in an epidemiologic study
- conducted over a long period
- ppl monitored that were exposed in utero
- purpose to look for cause n effect
- there is no evidence of insufficient low birth weight, delayed speech, dyslexia and non right handedness
animal studies
- no adverse effects observed if intensities are kept below 100mW/cm^2 for UNFOCUSED
- below 1000 mW/cm^2 for FOCUSED
two main categories for bioeffects
- thermal
- non thermal (mechanical_
there are no adverse effects when the temperature is less than….
2 degrees
how long can you scan at 6 degrees before producing potential side effects
16 mins
what do mechanical effects include
radiation force and cavitation
whats radiation force
force exerted by sound on the medium which can deform and disrupt structures
whats cavitation
production & behavior of bubbles in a liquid medium
what can cavitation be divided into
stabe and transient
whats stable cavitation
simple oscillation of bubbles that can result in streaming of liquid resulting in shear stress
ODS
output display standard
quantities displayed with ods
thermal index
mechanical index
define TI
ratio of acoustical poer produced by the transducer to the power required to raise the temp in tissue 1 degree
ti is divided into 3 categories
- TIS soft tissue
- TIB bone near focus
- TIC bone near surface for transcranial study
mechanical index
likelihood that energy used will lead to cavitation
relationship between MI and peak rarefactional pressure
proportional
what happens to MI if frequency goes up
goes down
TI & MI max exposure time
6 & 1.9
pulsed doppler output intensity
1140
what poses a higher risk for thermal effects, scanned or non scanned
non scanned
MMODE
CW
PW
what MI value has little risk
below 0.4
ALARA
as low as reasonably achievable
keep ultrasound exposure minimized while optimizing diagnostic info
