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Ch. 22 Bioeffects
Terms in this set (62)
A small needle with a piezoelectric crystal at its end.
with hydrophone the needle is placed in the ultrasound beam. It attaches to an _____and displays __________ signals received by the crystal.
oscilloscope , acoustic signals
Hydrophone can quantitate amplitude, ______, ____________and _______
period, pulse duration and pulse repetition period.
A hydrophone measures:
the pressure in a sound beam.
Calorimeter is a transducer which turns acoustic energy into heat.
When the _____ ______ gain is measured along with the time that it took to obtain the heat, _________ ______of the US beam can be calculated.
total heat, total power
Thermocouple is a
small device embedded in absorbing material. The US energy is converted into heat, and the thermocouple measures the temperature change.
the Intensity at _________ ________ are measured by a thermocouple
Biologic Effect & Safety:
bioeffects have been studied in experiments with both
in the natural setting
anything els; literal meaning: in glass****
the science of identifying and measuring the characteristics of an US field which are especially relevant to its potential for producing biological effects****
very high intensities of US causes
damage to biologic tissues
At Standard intensities, there are _________ __________cases of bioeffects or tissue injury in diagnostic imaging .
American Institute of Ultrasound in Medicine
(complies scientific information)
Food and Drug Administration
(regulates system and transducer outputs)
what is Bioeffects intensity limit ?
-100mW / Cm2 UNFOCUSED
-1W / cm2 or 1,000 mW/cm2 FOCUSED
It is Difficult to study in vivo(in living tissue) due to
Absorptions, scattering, and reflection****
exam duration has
greatest effect on patient exposeer
Output intensities :
output intensity is most important factor with regard to
Highest output intensities are used with _____.
pulsed doppler because blood cells are weak reflectors
Lowest output intensities are used with _____.
gray scale imaging because tissues are strong reflectors
MECHANISTIC approach -
-propose a specific physical mechanism that could produce bioeffects
-theoretical analysis to estimate scope of bioeffects at various exposures.
-Identify "cause -effect" relationship
-Acquire/review data from patients or animals
-correlate exposure and effects
-Identify "exposure-response" relationship
****the strongest conclusions are made when
mechanistic and empirical approaches reach the dame conclusion .
no evidence from mechanistic or empirical studies
temperature elevation via absorption resulting from interaction of biologic tissue and US.
A second mode of thermal injury may result from localized scattering of acoustic energy , especially at inhomogeneities within the medium(rayleigh scattering)
tissue BONE is
an absorber . therefor temp elevation at a tissue-bone interface is more likley
temperature elevation in fetal soft tissue is considered of potentially greater harm than in adults, thus fetal soft tissues adjacent to bone are great concern.
is related to tissue heating
Any exam that causes a temperature elevation to greater than ____ is considered potentially harmful to a fetus.
Any exam that causes a temperature elevation of less than ____ may be used without reservation.
thermal index is
a number proposed in the most recent AIUM guidlines that relates to tissue heating. Thermal index is a theoretical calculation related to the possible temperature elevation, measured in degrees centigrade,that could or may be produced by the sound beam *****
A Thermal index of 2 means
A temperature elevation of 2 degrees centigrade MAY occur
thermal index calculated assuming that the sound beam travels in soft tissue
thermal index calculated assuming that bone is at the beam's focus, usually higher number than TIS
Thermal index calculated assuming that cranial bone is located in the near field.
Nonthermal (Cavitation) Mechanism: Gaseous Nuclei -
Microbubble (Gaseous Nuclei) may be excited by US.
This takes the form of shrinking and expanding of the bubble. Potential of near total energy absorption where the nuclei exist may lead to thermal injury.****
so far , minimal evidence that ________really occurs in diagnostic US
is a number proposed in the AIUM guidlines that relates to cavitation.
Mechanical Index equation
MI = Peak Rarefaction pressure
Mechanical index is higher( more likely to produce cavitation) with:
1.peak rarefaction pressure and
2.lower freq sound
Microbubbles get bigger during the rarefactions. This expansion shears
bubble apart and creates cavitation.
Cavitation is more likely to occur in ____ than other tissues.
stable cavitation - bubbles tend to oscillate when exposed to acoustic waves of small amplitude. bubble do ______ burts. Bubbles that are a few micrometers in diameter double in size
Bubbles intercept , reradiate and
absorb acoustic energy .
stable cavitation: EFFECTS are
shear Stresses & Microstreaming in surrounding fluid
Transient Cavitation -
Bubbles expand during Rarefactions and Bubbles BURST Depends upon the pressure of the ultrasound pulse.
*also called normal or inertial cavitation.
Transient, Inertial Normal,
what is the threshold for transient cavitation
only 10 % greater than the pressure for stable cavitation.
Threshold effect - Highly localized-violent effects:
shock wave,mechanical stress
Colossal temperatures -
thousands of degrees
Bio effects -General Concepts : In vitro Studies
(outside of normal settings) -results are real;although important scientifically, "reports of in vitro studies which claim direct clinical significance should be viewed with caution" AIUM statement.)
Bio effects -General Concepts : Very High US intensities-
genetic alteration or damage which may be lethal to cells
Bio effects -General Concepts : Under controlled conditions, bioeffects
can be beneficial
Bio effects -General Concepts : Low Level US intensities -
no known effects
Epidemiology and statistics (population studies)-
Require large numbers of patients when the occurrence rate of the measure effect is small. this is the case in diagnostic imaging.
Diagnostic US has shown no adverse effects on fetal outcome or birth weight or any other measurement.
Best study is -
Limitation of Epidemiologic studies
1. often retrospective (looking at charts and information acquired in the past)
2.Ambiguities-justification for exam,gestational age, number of exams,exposure and mode
3. Other risk factors-maternal age,nutrtion, smoking, alcohol, drugs
AIUM statement on clinical saftey
Diagnostic US has been in use since the late 1950s
Potential benefits should outweigh the risk! The AIUM suggests:
Do not perform studies without reason
do not prolong studies without reason
minimize exposure time( but perform a complete diagnostic exam)
use minimum output power and highest receiver gain required to produce optimal images****
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