Chapter 24 Flashcards
Hydrophone is similar to
a small hypodermic needle with a tiny piece of piezoelectric material attached to its end
_ is a microprobe
hydrophone
We want to know the output of the machine because_
the majority of sound energy that goes into the patient stays there. We need to know if there are potential bioeffects that are going to harm the patient in the process.
Hydrophone: a wire connects _ to _
the PZT to an oscilloscope
Hyrophone is placed in the _ created by _
sound beam created by the transducer
A voltage from the hydrophone relates to the _ and is displayed in the _
sound beams pressure.
oscilloscope
Hydrophone: The acoustic pressure is measures at
specific locations within the sound beam
By moving the hydrophone to numerous locations while measuring the acoustic pressure, one can determine
the sound beams shape
How do you determine the sound beams shape?
By moving the hydrophone to numerous locations
while measuring the acoustic pressure
Hydrophone also measures _
period, PRP, PRF, and pulse duration
Hydrophones may be calibrates, that provides a known relationship between _
the acoustic pressure signal and the voltage created by the PZT.
In general, transducer output is lowest when
performing _ and highest when performing _
gray scale imaging, doppler
Another form of hydrophone is constructed from
a very thin membrane of piezoelectric plastic.
Hydrophone: Only _ is
pressure sensitive
a very small area in the
center of the membrane
_ force : A transducer’s sound beam creates a very
small force on any target that it strikes.
Radiation
Radiation force: the target can be _ or _ and act as a _
a balance, a float, scale to measure the force from the sound beam
Radiation force: the measured force relates to _
the power in the beam
Radiation force AKA
feedback microbalance
Acousto-Optics are based on
the interaction of sound and light
A shadowing system that
allows us to view the shape of a sound beam in a
medium.
Schlieren
Absorption is
the conversion of sound energy
into heat
Three devices measure the output of
ultrasound transducers by absorption
- Calorimeter
- Thermocouple
- Liquid crystal
Calorimeter measures
the total power in a sound beam through the process of absorption
Calorimeter: The sound beam is directed into the calorimeter where the sound energy is
converted into heat
Calorimeter: the sound beams total power is calculated by
measuring the temperature rise and the time of heating.
Tiny electronic thermometer
Thermocouple
A dab of absorbing material is placed on the
thermocouple and it is inserted into _ and _ is measured
the sound beam, temperature
Thermocouple: the temperature rise is related to
the power of
the sound beam at the particular location
where the device is positioned.
Certain liquid crystals change color based on
their temperature
Liquid crystals: when a sound beam strikes the crystals, the sound energy is _
absorbed
Liquid crystals: the change in crystal temperature causes a changes in their color, providing insight into _
the shape and strength of the sound beam.
The benefits to the patient must outweigh
the risks of
the exam
Low intensity ultrasound has _ bioeffects
no known
_ ultrasound intensities damage biologic
tissues
Extremely high
there are _ cases of diagnostic imaging at standard intensities in the
absence of contrast agents resulting in biological effects and tissue injury.
no known
Under_ circumstances, bioeffects are
beneficial. Example, _
controlled.
Therapeutic ultrasound for muscular injury
The science of identifying and measuring the
characteristics of an ultrasound beam that are relevant to its potential for producing biologic effects
Dosimetry
Dosimetry
The science of identifying and measuring the
characteristics of an ultrasound beam that are relevant to its potential for producing biologic effects
Bioeffects research may be conducted in two broad
areas:
in vivo and in vitro
In vivo
within the living body of an animal or a plant
In vitro
outside the living body/in an artificial
environment
n vitro research indicates that _ intensities can cause genetic damage and cell
death.
very high
AIUM Statement on In Vitro
Bioeffects
In vitro bioeffects research is important
In vitro bioeffects are real even though they may not apply to to the clinical setting
In vitro bioeffect research that claims direct clinical significance (without in vivo validation) should be viewed with caution
There are two techniques used to study bioeffects:
Mechanistic approach
Empirical approach
_ approach begins as a proposal that a
specific mechanism has the potential to produce
bioeffects. Based on that proposal, a theoretical
analysis is performed to estimate the scope of the
bioeffects at various exposure levels. Searches for a relationship between cause and effect.
Mechanistic
Mechanistic approach
begins as a proposal that a
specific mechanism has the potential to produce
bioeffects. Based on that proposal, a theoretical
analysis is performed to estimate the scope of the
bioeffects at various exposure levels. Searches for a relationship between cause and effect.
_ approach – based on the acquisition and review of information from patients or animals exposed to ultrasound. The research seeks a relationship between the exposure to ultrasound and the effects of that exposure
Empirical
Empirical approach
based on the acquisition and review of information from patients or animals
exposed to ultrasound. The research seeks a
relationship between the exposure to ultrasound
and the effects of that exposure.
Mechanistic appraoch strenght:
Broad exposure range can be evaluated
Mechanistic approach weaknesses:
uncertainty about assumptions, are other mechanisms involved?, is the bioeffect clinically significant?
Empirical approach strenghts:
no need to undersand mechanisms, biological significance is obvious
Empirical approach weakness:
No need to understand mechanisms, species differences may alter results
The best studies are made when _
empirical and mechanistic approach agree.
Two important bioeffects mechanisms are:
- Thermal
2. Cavitation (nonthermal)
_ proposes that bioeffects result from tissue
temperature elevation.
thermal mechanisms
The rationale for studying thermal effects are:
As sound propagates in the body, energy is
converted into heat.
Core temperature is regulated at 37° C. Life
processes may not function normally at other
temperatures.
tissue temperatures are
elevated routinely in our daily lives (with or without)
adverse effects.
without
The thermal index is a useful predictor of
maximum
temperature increase under most clinically relevant
conditions.
Thermal indices are the best measurement or estimate of _
in vivo tissue temperature
Thermal index is reported in three forms:
TIS
TIB
TIC
TIS
soft tissue thermal index, assumes that sound is
traveling in soft tissue
TIB
bone thermal index, assumes that bone is at or
near the focus of the sound beam
TIC
cranial bone thermal index, assumes that cranial
bone is in the sound beam’s near field
Thermal Mechanism –
Empirical Findings: Serious tissue damage occurs from
prolonged
and excessive elevation of tissue temperature
Thermal Mechanism –
Empirical Findings: A _ rise in testicular temperature can
cause infertility.
2° to 4°
Thermal Mechanism –
Empirical Findings: Tissue heating is related to
the output
characteristics of the transducer and the
properties of the tissues.
Thermal Mechanism –
Empirical Findings: A combination of _ determine the likelihood of harmful bioeffects.
temperature and exposure
time
Thermal Mechanism –
Empirical Findings: Maximal heating is related to the beam’s _ intensity
SPTA
Thermal Mechanism –
Empirical Findings: SPTA: The current FDA regulatory limit is
720 mW/cm^2
Thermal Mechanism –
Empirical Findings: No confirmed bioeffects have been reported for
temperature elevations of up to _ above
normal for exposures of _
2° C. less than 50 hours
Thermal Mechanism –
Empirical Findings: Temperature elevations are greater with _ than _
Doppler
gray scale
Thermal Mechanism –
Empirical Findings: Fetal and neonatal tissues appear (more or less) tolerant of tissue heating than adult tissues
less
Bone absorbs (more or less) acoustic energy than soft tissue.
more
The temperature rise in soft tissues near bone is significantly _ than in other
locations.
higher
Circumstances where ultrasound strikes _ deserve special attention.
fetal bone
Thermal Mechanism –
Mechanistic Data: Theoretical models appear to correlate with
experimental data even though:
The ultrasound beam is quite complex
Diagnostic equipment is diverse
Tissue characteristics are different
Nonthermal Mechanism consist of
cavitation and radiation force
Radiation force –
exerted by a sound beam on tissues. Sheer stresses and streaming of fluids
can distort or disturb biologic structures
Cavitation –
the interaction of sound waves with microscopic gas bubbles in the tissue
Bubbles =
gaseous nuclei, found in tissues –
different from contrast agents
The only cavitation bioeffect identified at
intensities typical of diagnostic ultrasound are
in tissues with
a well defined population of
stabilized gas bodies, such as lung
Two forms of cavitation exist:
Stable and transient
At lower MI levels, _ occurs
Stable cavitation
Stable Cavitation: The gaseous nuclei tend to
oscillate (expand and contract)
Stable Cavitation: Bubbles that are a few millimeters in diameter might _ but do not _
double in size,
burst
Stable Cavitation: The bubbles intercept and absorb much of the
acoustic energy
Stable Cavitation: The fluids surrounding the cells undergo
microstreaming and the cells are exposed to shear stresses.
_ may occur with higher MI levels
transient cavitation
Bubble bursting
transient cavitation
Transient cavitation AKA
inertial or normal cavitation
_ produces highly localized, violent effects including:
Colossal temperatures
Shock waves (enormous pressures)
Transient cavitation
Are destructive effects or transient cavitation considered clinically important? Why or why not?
No, they are highly localized and only affect a few cells
Stable or transient? Oscillating bubble, microstreaming and shear stresses, lower MI
Stable
Stable or transient? AKA normal or inertial, bursting bubble, shock waves and very high temperatures, higher MI
Transient
A calculated number related to the likelihood of harmful bioeffects from cavitation. Best indicator
Mechanical index
MI is realted to 2 sound wave characteristics:
Peak rarefaction pressure and lower frequency
Greater likelihood of cavitation bioeffects and a
higher MI with:
Additional negative pressure
Lower frequency
A branch of medicine associated with
population studies
Epidemiology
an exposure response method
which utilizes clinical surveys
Empirical
When the occurrence of rate of bioeffects is
small, population studies require_.
many subjetcs.
The smaller the effect, the _ it is to detect.
Harder
Many epidemiologic studies deal with in utero
fetal exposures to ultrasound because:
A large percentage of pregnant women in the
U.S. are scanned
Ultrasound is routinely used during normal
pregnancies.
Harmful effects have the potential to affect the
fetus for life.
Epidemiologic data indicate that ultrasound exposure (is or is not) associated with adverse fetal outcome.
is not
Epidemiology limitations:
Retrospective – information is obtained from old medical records.
Ambiguities may exist in the data, such as
justification for the exam, gestational age,
number of scans, technique, and exposure time.
Risk factors other than exposure to ultrasound
may precipitate a bad outcome in the fetus.
(environmental factors, poor nutrition, smoking,
alcohol, or drug abuse)
The best epidemiologic studies are:
Prospective and randomized
Prospective
forward looking
Randomized
creates 2 groups of patients, one group exposed to u/s and one group that is not.
The conclusions of the AIUM include:
No confirmed harmful bioeffects from exposure
to diagnostic ultrasound have ever been reported
It is possible that bioeffects may be identified in the future
The benefits to the patient outweighs the risks
It is appropriate to use diagnostic ultrasound
prudently to provide benefit to the patient
It is inappropriate to use diagnostic ultrasound in
a non-medical setting for entertainment.
No confirmed bioeffects on patients or sonographers
have been found with the use of diagnostic ultrasound
Experience with diagnostic ultrasound may differ from research and training, due in part to longer research exams and greater exposure.
When used without direct medical benefit to the
patient, the subject should be informed how the
research study differs from standard diagnostic
procedures.
Precautions such as _
should always be taken to avoid electrical
hazard
proper electrical grounding
The greatest risk to the patient arises from
electrical shock from a cracked transducer
housing.
_ is the primary determinant of patient exposure.
Exam time
Use the _ output
power and _ amplification to optimize
image quality.
minimum, maximum
No bioeffects are associated with MI below
.4
tissue heating
thermal mechanism
Radiations force and cavitation
Nonthermal mechanisms
Streaming of fluid
motion of fluid due to radiation force
