M2: Bioeffects Flashcards
what is acoustic propagation
the effect of tissue on sound
does diagnostic US use a lower or higher intensity than therapeutic US
lower
3 ways to describe the strength of a wave
amplitude
power
intensity
these can also express the loudness or volume of a sound
formula for intensity
I = P/A or
I is inversely proportional to Amp^2
what happens if you double amplitude
you quadruple intensity
list the acoustic variables
press
density
particle motion
temperature
define amplitude
maximum variation of an acoustic variable…. also the particle displacement, velocity or acoustic pressure of a sound wave
what does amplitude indicate
the strength of the detected echo or the voltage generated in the crystal from a press wave
what determines the initial amplitude of a pulse
power output - determined by the pulser
as a sound wave travels though tissue, what is the reduction in power called
attenuation
5 mechanisms of attenuation
absorption reflection (z values) refraction (diff velocities and non perpendicular insonation) wavefront divergence scatter
does the amplitude of an echo decrease as it returns to the probe
yes
define power
measure of total energy transmitted over the cross sectional area of the beam, per unit time
formula for power
P = I x A
absolute unit of power
watt (joules/s)
relative unit of power
decibel
how many dB is 100% power
0dB
a reduction of 3 dB drops the intensity by how much
1/2 the original intensity
a reduction of 10 dB drops the intensity by how much
0.1 of original intensity
what determines how much power is produced by the transducer
pulser
power output for PW
1140 mW/cm^2
absolute unit of intensity
W/cm^2 or mW/cm^2
relative unit of intensity
decibel
is intensity constant in time or space
no
which intensity do we use when considering bioeffects
SPTA
where is the spatial average found
probe face
where is the spatial peak found
focal point
how are SP and SA related?
Formula
BUR
BUR = SP/SA
What does the BUR compare
what does the it tell us about the focusing of the probe
The near and far field
gives us an idea of how much focusing there is in the beam
highly focused beams have what kind of BUR?
weakly focused?
high - high BUR
low - low BUR
the BUR will alwasy be greater than what value
why
1
peak is always higher than the avg
a perfectly uniform beam would have a BUR of what value
1
what kind of probe has a BUR of 1
old single disc mechanical probe w/ no natural or applied focusing
factors that effect the spatial intensities in a probe
SP will increase w/ an increase in power of focusing
and decrease with increasing depth (aperture must widen to maintain constant beam width at the focus so its less intense)
SA will increase w/ an increase in power
and decrease with increasing depth due to attenuation
what is temporal avg
includes both ringing and listening phase of the pulse
what is temporal peak
highest amplitude in a pulse at any given time
how are TP and TA related
formula
duty factor
DF = TA/TP
or
DF = PD/PRP x 100
describe PD and PRP
PD = time it takes for one pulse to occur (ringing time) PRP = time from beginning of one pulse to the beginning of the next.... includes ringing and listening time (also go return time in range equation)
why are TP and PA almost the same value
b/c the pulse is so short…. TP always a bit higher than TA
in the simplified DF formula, TP should be more accurately replaced by which value
PA
DF = TA/PA
another name for SPTP
instantaneous peak
what is the PA
avg of all intensities found w/in a single pulse
in a sine graph, where is the TP found
the first amplitude peak
whats the intensity at the listening phase
0
factors that effect temporal intensities
increase in power or duty factor
what is DF
what factors increase it
% of time the crystal is ringing
increased PRF or PD
how will an increase in depth effect the TA
reduces it because you are increasing the listening time
why does CW have a higher SPTA than PW
its always ringing (DF will be 100%)
another name for SPPA
maximum intensity or time averaged half maximum
info we have on bioeffects come from what 3 sources
epidemiology
in vitro studies
in vivo or animal studies
describe an epidemiological study
study conducted over a long period of time… there is a control grp and a group who have been exposed to US
….these grps are tracked over several years to look for cause and effect
when was US power output regulated
1992
conclusions of epidemiological studies for OB
…no evidence of low birth weight, delayed speech, dyslexia and non-right handedness @ the intensities we use today
describe in vitro studies
do their findings have clinical significance
exposes cells in liquid to US
hard to say
purpose of in vitro studies
give valuable info to set thresholds for in vivo studies
have in vivo studies shown possible bioeffects
explain
yes….
shown fetal weight reduction, postpartum mortality, abnormalities, lesions, etc… but also shown good bioeffects like wound repair enhancement and tumor regression
US intensity threshold for an unfocused probe that is considered safe
< 100 mW/cm^2
US intensity threshold for a focused probe that is considered safe
< 1000 mW/cm^2 (1 W/cm^2)
see table on pg 34
/
2 main categories of bioeffects
thermal
non-thermal/mechanical
which mechanism of attenuation accounts for the majority of this process
absorption (80%).. the conversion of sound into heat
tissue must not be heats more than how many degrees in order to avoid bioeffects
> /=2 degrees
an increase in temp of what value is considered significant
2 degrees
what effect can an increase of 4 degrees have on a fetus
can kill it
if tissue is heated b/w 2-6 degrees, what becomes an important consideration
exposure time
a 6 degree increase in temp (for a fetus) will not cause any bioeffects if exposure time is under how many minutes
16 mins
2 types of mechanical bioeffects
radiation forces
cavitation:
describe radiation forces
the force exerted by sound on the medium which can deform and disrupt structures (think of debris in a cyst)…. this force can cause flow in absorbing fluids leading to tearing (shear forces)
describe cavitation
2 types
production and behavior of bubbles in a liquid medium
2 types: stable cavitation (we like this) transient cavitation
describe stable cavitation
the oscillation of the bubbles (resonating) in a liquid medium that can result in the streaming of liquid which can cause enough stress to tear the structure if theres too much stable cavitation
describe transient cavitation
when a bubble in a liquid medium collapses and produces shock waves that can cause localized extremely high temp and can emit light in clear fluids (sonoluminescent)
what year was the output display standard displayed (ODS) on the US screen
what is it
1992
give you info about the potential or bioeffects based on the power you’re using the scan
2 ODS displayed
Thermal index (TI) Mechanical Index (MI)
what is the TI
How much you are heating the tissue… TI of 1 means that you may increase the temp of the tissue by 1 degree
are TI and MI assumed values
yes
3 categories of TI
divided based on diff tissue scanned and their absorption rates (will have different density and compressibility)
TIS - soft tissue, most common
TIB - for bone near the focus, used for OB scanning
TIC - for bone near the surface, used for transcranial
what is MI
when is it especially import to consider this number
number that represents the likelihood that cavitation will occur…. MI is proportional to the peak rarefractional press (highest amplitude) so if the press doubles so will the MI
contrast US
how are frequency and MI related
inversely… but you would need a big change in frequency (quadruple) to see this effect
how can we reduce MI
reduce the power
when can MI and TI be underestimated
when scanning a large fluid collection…. but we can usually use less power in this situation
what is the TI and MI max
TI: 6
MI: 1.9
since PW doppler has an SPTA of 1140, what becomes an important consideration when using it
exposure time
2 types of applications of US
scanned and non-scanned
what are some non-scanned US application
CW, PW and M-mode
why do non-scanned US applications have a higher risk for thermal effect?
since we are repeatedly insonating the sound beam in the same spot and its not swept across the face of the probe like 2D scanning… CW has the highest risk
when might we see non-thermal bioeffects when scanning below our acceptable threshold of 1000mW/cm^2
what are some of these effects
when gas bodies are present in circulation (contrast US)
PVCs
micovascular leaks w/ petechiae
glomerular capillary hemorrhage
local cell killing
MI must be below what value when doing contrast US
0.4
how do we follow ALARA in US
only scanning when medically necessary, not for entertainment or other purposes (eg learning the sex of the fetus, pictures of the fetus, commercial purposes)
what are the 4 restricted acts the US technicians perform
OB scans
EV
Contrast
IVs
Another name for transient cavitation
Collapse cavitation
