Chapter 22 Bioeffects

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Chapter 22Bioeffects
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Bioeffects

• Measuring the sound energy produced by
  transducers
• Hydrophone
• Measures the acoustic pressure at specific
  locations within the sound beam
• Radiation force
• Acousto-Optics

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Bioeffects

• Measuring the sound energy produced by
  transducers
• Hydrophone
• A small transducer which measures a sound beams
  characteristics
• Measures the acoustic pressure at specific
  locations
• Determines a sound beams shape, period, PRP, PRF
  PD

Membrane Hydrophone
Hydrophone
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Bioeffects

• Measuring the sound energy produced by
  transducers

• Radiation force
• Incident sound wave can exert a small, measurable
  force on the object that it strikes
• If the object is a balance or a float, acting as
  a tiny scale, the beams intensity can be
  measured
• Acousto-Optics
• Utilizes a shadowing system, a Schlieren, which
  allows the shape of a sound beam to be viewed and
  the beam profiles to be measured

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Converting Sound Energy into Heat

• Devices which measure the output of ultrasound
  transducers by absorption, the conversion of
  sound energy into heat
• Calorimeter
• Thermocouple
• Liquid crystals

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• Calorimeter
• Measures the total power of the entire sound beam
  by measuring the temperature rise time of
  heating
• Thermocouple
• A tiny electronic thermometer which is inserted
  into the sound beam, measuring its temperature
• Temperature rise is related to power of the sound
  beam at a particular location
• Liquid crystal
• Sound beam strikes the crystal and is absorbed
• Change in crystal temperature causes a change in
  their color indicative of the sound beams shape
  and strength

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Liquid Crystal Temperature of the black pouch
changes when you place your thumb upon it
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Risk-Benefit Relationship

• Benefits of a treatment or test must outweigh the
  risks of the treatment or exam.
• Under controlled circumstances, bioeffects are
  beneficial. Therapeutic ultrasound is widely
  used to treat muscle injury.
• Extremely high ultrasound intensities damage
  biologic tissues
• There are no known cases of diagnostic imaging at
  standard intensities resulting in biological
  effects and/or tissue injury
• Low intensity ultrasound has no known bioeffects

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Risk-Benefit Relationship

• Dosimetry
• The science of identifying and measuring the
  characteristics of an ultrasound beam that are
  relevant to its potential for producing
  biological effects
• Research is conducted in two areas
• In Vivo (in the living)
• Research performed within the living body of a
  plant or animal
• In Vitro (in glass)
• Research performed outside the living body, in an
  artificial environment
• Results indicate that very high intensities can
  cause genetic damage and cell death

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AIUM Statement on In Vitro Bioeffects

• Approved 1997 again 2003
• Read on pg. 370
• Summarized
• In Vitro bioeffects research is important
• In Vitro bioeffects are real even though they may
  not apply to the clinical setting
• In Vitro bioeffects which claim direct clinical
  significance, without In Vivo validation, should
  be viewed with caution

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Study Techniques

• Mechanistic approach
• Begins as a proposal that a specific mechanism
  has the potential to produce bioeffects
• Searches for a relationship between cause
  effect
• Empirical approach
• Based on acquiring and reviewing information from
  patients or animals exposed to ultrasound
• Searches for a relationship between exposure
  response

Strengths Weaknesses Table 22.1, pg. 371
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Mechanisms of Bioeffects

• Thermal Mechanism
• Proposes that bioeffects result from tissue
  temperature elevation
• Rational
• As sound propagates in the body, energy is
  converted into heat
• Body core temperature is regulated at 37 C.
  Life processes do not function normally at other
  temperatures
• Thermal Index (TI)
• A predictor of maximum temperature increase under
  most clinically relevant conditions
• TIS assumes sound is traveling in soft tissue
• TIB assumes bone is at or near the focus of the
  sound beam
• TIC assumes cranial bone is in the sound beams
  near field
• Has no units
• The best estimate of in vivo tissue temperature
  elevation

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Thermal Mechanism

• Empirical Findings relationship between
  exposure response
• Serious tissue damage occurs from prolonged
  elevation of body temperature
• Tissue heating is related to the output
  characteristics of the transducer and the
  properties of the tissues
• A 2 4 rise in testicular temperature can cause
  infertility
• A combination of temperature exposure time
  determine the likelihood of harmful bioeffects
• With higher temperature, shorter exposure times
  will produce harmful effects
• No confirmed bioeffects have been reported for
  temperature elevations of up to 2 C above
  normal, for exposures of less than 50 hours
• Maximal heating is related to the beams SPTA
  intensity

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Thermal Mechanism

• Empirical Findings continued
• Fetal tissues appear less tolerant of tissue
  heating than adult tissues. Numerous fetal
  defects resulting from temperature elevation have
  been documented. None have been observed at
  temperatures less than 39 C.
• A greater amount of acoustic energy is absorbed
  by bone than by soft tissue. Circumstances
  wherein ultrasound strikes fetal bone deserve
  special attention.
• Mechanistic Data Relationship between cause
  effect
• Theoretical models appear to correlate with
  experimental data even though
• The ultrasound beam is quite complex
• Diagnostic equipment is diverse
• Tissue characteristics are different

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Cavitation Mechanism

• The interaction of sound waves with microscopic,
  stabilized, gas bubbles in the tissues
• 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
• Stable
• Transient

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Cavitation

• Stable cavitation
• Occurs at lower MI levels
• Gaseous nuclei tend to oscillate (expand
  contract)
• Bubble does not burst
• Bubbles intercept and absorb much of the acoustic
  energy
• Fluids surround the cells undergo microstreaming
  and the cells are exposed to shear stresses

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Cavitation

• Transient cavitation
• Occurs at higher MI levels
• Aka, inertial or normal cavitation
• Bubble bursting
• Localized, violent effects
• Colossal temperatures
• Shock waves (enormous pressures)
• Effects are not considered clinically important
  since they are highly localized and affect few
  cells
• Pressure threshold for transient cavitation is
  only 10 higher than that required for stable
  cavitation

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Mechanical Index (MI)

• Related to the likelihood of harmful bioeffects
  from cavitation
• Unitless
• Related to two terms
• Peak negative pressure
• Frequency
• Greater likelihood of cavitation bioeffects, and
  a higher MI with
• Additional negative pressure
• Lower frequency

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Mechanical Index - Summary
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Epidemiology

• The branch of medicine associated with population
  studies
• Empirical exposure-response method
• Many studies deal with in utero fetal exposures
  to ultrasound
• Characteristics evaluated include
• Birth weight
• Length
• Head circumference
• Congenital abnormalities
• Apgar scores
• Hearing
• Infection

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Epidemiology

• Limitations
• Often retrospective
• Data ambiguity
• Indications for exam
• Number of scans
• Technique exposure time
• Risk factors other than exposure to ultrasound
  may precipitate a bad outcome in the fetus
• Maternal age
• Environmental factors
• Poor nutrition
• Smoking
• Alcohol drug abuse

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Epidemiology

• Best studies are
• Prospective
• Randomized
• Prospective studies (forward looking)
• More desirable than retrospective studies
• Protocols are established
• Specific information is obtained
• Randomized studies
• Creates two groups of patients
• One group exposed to ultrasound one is not
• Groups are otherwise similar, e.g., both groups
  are pregnant women
• Advantage
• The presence of other risk factors which could
  negatively affect a fetal outcome are taken into
  account

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Clinical Safety Prudent Use

• Conclusions of the AIUM include
• No confirmed harmful bioeffect from exposure to
  diagnostic ultrasound has ever been reported
• It is possible that bioeffects may be identified
  in the future
• The benefit 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

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Training Research

• The conclusions of the AIUM include
• 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

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Safety

• Electrical Precautions
• Proper electrical grounding
• Routine mechanical inspection to assure proper
  physical status
• Greatest risk arises from a cracked transducer
  housing, which may also degrade image quality
• Overall Safety Considerations
• Do not perform studies without valid medical
  justification
• Do not prolong studies without valid medical
  justification
• Minimize patient exposure. ALARA

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