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Potential Biological Effects of Ultrasound and It Safety

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Title: Potential Biological Effects of Ultrasound and It Safety


1
Potential Biological Effects of Ultrasound and It
Safety
  • Evans Agyei Sakyi (Diagnostic Sonographer)
  • International Maritime Hospital
  • Imaging Department

2
Objectives
  • Systematically review the biologic effects of
    ultrasound on human studies
  • Types of safety indices.
  • To appropriately weigh the risks and benefits of
    its uses especially when targeting certain organ
    tissues in the body
  • To make informed judgements about ultrasound
    safety, and in order to protect patients from
    excessive exposure.
  • Evidence of long-term adverse effects
  • Safety guidelines issued by recognized bodies
  • The current thoughts on the bioeffects of
    ultrasound

3
Introduction
  • Diagnostic ultrasound is an imaging modality that
    is useful in a wide range of clinical
    applications, and in particular, prenatal
    diagnosis.
  • From its official introduction into the medical
    world in 1942 by karl Dussick, its metamorphosis
    into todays high-tech equipment has led to a
    general trend towards increased power output and
    the potential for associated risks
  • The acoustic output of modern equipment is
    generally much greater than that of the early
    equipment and in view of the continuing progress
    in equipment design and applications, outputs may
    be expected to continue to be subject to change.
  • There is no evidence that diagnostic ultrasound
    has produced any harm to patients since it has
    been in use.
  • Investigations into the possibility of subtle or
    transient effects are still at an early stage.
    Consequently diagnostic ultrasound can only be
    considered safe if used prudently.

4
Venous/Arterial Doppler
Obstetrics
Echocardiography
  • Ultrasound is a type of mechanical energy that
    penetrates tissues as an oscillating wave of
    alternating pressure

5
Ultrasound Interaction with Tissues
6
The Revolution of ultrasound
  • Pierre Curies discovery of the piezoelectric
    effect in 1880 launched the ultrasound technology
    revolution
  • First applied in ships for depth detection and
    metallurgy for fracture identification
  • Soon thereafter medical applications appreciated
    it used

7
Primary advantage of ultrasound
  • Real-time assessment of organ/organ tissues
  • Absence of radiation
  • Decreased cost
  • Portability

8
Historical Background
  • The potential for ultrasound to produce biologic
    effects was first reported in 1917. (Langevin
    demonstrated that fish in a small tank died when
    exposed to ultrasound)
  • The thermal effects of ultrasound were used in
    1940s to cauterize tissues during surgery and to
    destroy cancerous cells in situ
  • Fry et al. examined the detrimental effects of
    focused ultrasound on neural tissue, including
    reversible and irreversible impairments in nerve
    conduction abnormalities
  • Transient (43.5s) ultrasound exposure (35W/cm2)
    caused transient conduction blockade in the
    ventral abdominal ganglia of crayfish.
  • Brief exposure to an ultrasound beam of similar
    intensity produced complete paralysis with
    destruction of neurons in the lumbar region of
    intact frogs

9
  • Intact Frog

10
Cont.
  • These facts emphasized that ultrasound produces
    important thermal effects that are capable of
    interfering with body tissues similar to the
    actions of heat.
  • The potential hazard of ultrasound depends mainly
    on four diverse yet mutually dependent factors.
  • Ultrasound exposure (total acoustic output power)
  • Target tissue composition (This determines the
    acoustic absorption coefficients more
    proteinaceous tissue is susceptible to thermal
    injury, higher fluid and gas content tissue
    susceptible to cavitational activity)
  • Tissue susceptibility (Rapid proliferating tissue
    are more susceptible to ultrasound effects, than
    static cell population)
  • Clinical settings (type of transducer used, the
    depth of penetration and overlying layers of
    tissue)

11
AIUM and NEMA consensus
  • AIUM (America Institute of Ultrasound in
    Medicine) and NEMA(National Electrical
    Manufactures Association) stated in their report
    that manufacturers of ultrasound equipment should
    provide detailed information about parameters
    including power, transmission duration, and mode-
    continuous versus pulsed.
  • This were identified as important determinants of
    adverse biologic effects in animal experiments,
    i.e.
  • Intensities (responsible for temperatures
    increase)
  • Wavelength-related pressures (responsible for
    mechanical effect)

12
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13
Standard for ultrasound Equipment
  • The Standard for Real Time Display of Thermal and
    Mechanical Indices on Diagnostic Ultrasound
    Equipment, commonly referred to as the Output
    Display Standard, was developed in 1992. (TI and
    MI)
  • The output display standard currently is the only
    information required by the food and drug
    administration to alert the clinical user of the
    potential of an ultrasound device to produce
    tissue injury.
  • TI and MI are to be displaced at the top right
    corner. The location may varying depending on the
    manufacturer.
  • Acoustic power is the primary determinant of
    thermal and mechanical indices but the ultrasound
    mode color Doppler blood flow imaging, area of
    interest, transmission frequency, pulse
    repetition frequency and focal zone also affect
    thermal and mechanical indices.

14
Thermal Effects
  • Ultrasound increases temperature in the focal
    area of the beam
  • The magnitude and duration of this temperature
    elevation is quantified as the thermal dose
    delivered to the tissue.

15
Measurement of Thermal Effects
  • The thermal index is defined as the ratio of the
    total system power to the power required to cause
    a 10C increase in temperature
  • Types of Thermal Indices
  • Three different thermal indices-depending on the
    structures encountered in the path of the
    ultrasound beam
  • Soft tissue (TIs)
  • Bone (TIb)
  • Cranium (TIc)

16
Biologic Consequences of Thermal Effects
  • During ultrasound propagation, a portion of the
    energy is absorbed and converted into heat, which
    could lead to a temperature increase.
  • (70 of the total temperature increase associated
    with ultrasound occurs within the first minute of
    exposure but temperature does continues to rise
    as exposure time is prolonged).
  • The relative protein content of each tissues is
    also an important determinant of absorption, and
    hence, temperature rise. Absorption coefficients
    of tissues are directly related to protein
    content, thereby providing a surrogate marker for
    potential increase in tissue temperature.
  • Absorption coefficients vary between 1(skin,
    tendon, spinal cord) and 10 (bone) dB/cm MHz
  • (Heat produces a wide variety of tissue injury
    including necrosis and apoptosis, abnormal cell
    migration, altered gene expression, and membrane
    dysfunction).

17
cont.
  • The greatest temperature increase from ultrasound
    exposure occurs in bone because of its high
    absorption coefficient.
  • Temperature also increases in tissues adjacent to
    bone
  • Ultrasound intensity and exposure duration cause
    direct increase in tissue temperature, a wider
    beam width reduces the rate and extent of
    temperature rise by permitting the energy to be
    distributed over a larger perfusion territory.
  • The use of a narrow write-zoom box increases this
    potential (Thermal hazard)

18
  • Focused beam and Wide beam

19
Mechanical Effects of ultrasound
20
  • Ultrasound energy creates mechanical forces
    independent of thermal effects, thereby causing
    biologic effects that are not related to
    temperature rise alone (nonthermal)
  • The mechanical effects results in shear forces,
    pressure changes and release of various reactive
    molecules.

21
Biologic consequences of Mechanical Effects
  • Gas-containing structures (eg. Lungs, intestines)
    are most susceptible to the effects of acoustic
    cavitation.
  • Petechial hemorrhages developed on the mucosal
    surface of the intestines after ultrasound
    exposure at or above typical diagnostic
    frequencies
  • Increased small intestinal cell apoptosis through
    a cavitation mechanism
  • Mechanical effects also occur in tissues near
    bone.
  • A combination of thermal and nonthermal effects
    are purported to be responsible for hemorrhage
    adjacent to bone.
  • These potential effect increases with acoustic
    intensity, pulse repetition frequency, and
    transducer frequency
  • Single beam modes (A-mode, M-mode and spectral
    pulsed Doppler) have a greater potential for
    non-thermal hazard than scanned modes (B-mode,
    Color Doppler)

22
Cavitation
Ultrasonic cleaning
  • Cavitation is the formation and then immediate
    implosion of cavities in a liquid that are the
    consequence of forces acting upon the liquid

23
  • Fat Reduction using the principle of cavitation
  • Ultrasonic lipolysis
  • Can achieve volume reduction of tissue
  • Causes the formation of micro-bubbles collapse,
    they affect fat cell wall and allow triglycerides
    to pass out of the cell

24
  • Cell apoptosis through cavitation mechanism

25
Determinants of Mechanical Effects
  • The interaction of ultrasound with gas bubbles or
    contrast agents causes rapid and potentially
    large changes in bubble size (cavitation). This
    may increase temperature and pressure within the
    bubble and thereby cause mechanical stress on
    surrounding tissues, precipitate fluid micro-jet
    formation, and generate free radicals.
  • Ultrasound wavelength has an important role in
    bubble formation and growth short wavelength
    ultrasound (observed at higher frequencies) does
    not provide sufficient time for significant
    bubble growth, therefore cavitation is less
    likely under these circumstances compared with
    long wavelengths
  • Acoustic cavitation (Inertial/Transient
    Noninertial/Stable)
  • Measurement of Mechanical Effects
  • Defined as the ratio of the peak rarefactional
    negative pressure adjusted for tissue attenuation
    and square root of the frequency (mechanical
    index Pr.3/vf)

26
Safety Standard (bmus)
  • Medical ultrasound imaging should only be used
    for medical diagnosis
  • Ultrasound equipment should only be used by
    people who are fully trained in its safe and
    proper operation. This requires
  • An appreciation of the potential thermal and
    mechanical bio-effects of ultrasound
  • A full awareness of equipment settings
  • Examination times should be kept as short as is
    necessary to produce a useful diagnostic result
  • The operator should aim to stay within the
    recommended scan times (especially for obstetrics
    examinations)
  • Output levels should be kept as low as is
    reasonably achievable whilst producing a useful
    diagnostic result
  • Scans in pregnancy should not be carried out for
    the sole purpose of producing souvenir videos or
    photographs
  • Freeze frame or cine loop should be used to
    reviewed and discussed images without continuing
    the exposure.
  • Endo-cavity probes (e.g. vaginal, rectal or
    oesophageal probes) should not be used if there
    is noticeable self heating of the probe when
    operating in air.

27
Known biologic Effects
  • Cellular Effects of Ultrasound
  • Facilitate an influx of calcium ions in
    fibroblasts probably due to mechanical effect on
    ion channels
  • Causes efflux of intracellular potassium ions
    (Acoustic microstreaming)
  • Thrombus formation after ultrasound-induced
    endothelial damage
  • Repetitive ultrasound exposure reduced leukocyte
    production in monkeys in utero
  • A decrease in somite numbers was noted when
    embryo cultures were exposed to ultrasound for
    15min at 400C (Non-thermal mechanism form of
    injury)

28
Cont.
  • Genetic Effects of Ultrasound
  • It remains unclear whether ultrasound contributes
    directly to genetic aberrations. Chromosomal
    aberrations, enhanced sister chromatid exchange,
    and other mutations has been investigated
    extensively as possible consequences of
    ultrasound exposure, but whether these actions
    lead to meaningful physiologic consequences is
    controversial.

29
Cont.
  • Fetal Effects of Ultrasound
  • In a large randomized controlled trial from
    Helsinki, 9000 women were randomly divided into
    groups. The women in one group were scanned at
    16-20weeks whereas the women in the other group
    were not. Comparing the results from these groups
    revealed 20 miscarriages in the scanned group and
    none in the controls.
  • Multiple ultrasound exposure in utero  was
    associated with a small increase in the incidence
    of low birth weight compared with a single
    exposure, but this difference was not
    statistically significant, and was eliminated as
    the children developed. The authors subsequently
    followed the growth, development, and behavior of
    the children for another 8 yrs. They reported a
    delay in language and speech development at 1yr
    in ultrasound-exposed children, but no other
    significant differences were observed between
    groups. This finding was most likely related to
    parenting and not to ultrasound exposure per se 
    because the difference was not observed during
    later development. The results of these
    epidemiologic studies clearly requires
    qualification because ultrasound devices
    available then had lesser acoustic output. The
    studies were also performed before output display
    standard was established.
  • Research from other bodies have concluded that
    there are insufficient evidence of a direct
    causal link between ultrasound exposures in utero
    and subsequent biologic consequences in neonates
    and children.

30
Cont.
  • Neural Effects of Ultrasound
  • Fry et al. demonstrated that focused ultrasound
    is capable of causing reversible suppression of
    neural transmission
  • Ultrasound exposure to the lumbar plexus causes
    hind limb paralysis in experimental animals (Hind
    limb paralysis was observed at room temperature
    after a 4.3s ultrasound exposure (35W/cm2) to the
    lumbar area, but more prolonged exposure duration
    7.3s was required to produce similar neurologic
    damage to cooler temperature (1-20C))
    Histologic analysis revealed neuronal and myelin
    destruction in the spinal cord and axonal
    degeneration, chromatolysis, pyknosis with intact
    mesenchymal structures and clumping of myelin in
    the peripheral nerves and cauda equina

31
Cont.
  • Ocular Effects of Ultrasound
  • Focused, higher intensity ultrasound was used for
    destruction of intraocular lesions (intraocular
    tumors)
  • Prolonged exposure also produces cataracts
  • Transient chemosis, conjunctival injection,
    corneal clouding, lens opacities, reduction in
    intraocular tension, or permanent destruction of
    the ciliary body were also reported after focused
    ultrasound exposure

32
Cont.
  • Pulmonary Effects of Ultrasound
  • Ultrasound-induced lung haemorrhage has been
    widely reported in experimental animals (example
    of acoustic cavitation), but perphaps rather
    surprisingly, humans do not appear to be
    susceptible to this form of nonthermal injury.

33
recommended exposure times at different index
values for different applications (Bmus)
  • Obstetric examination
  • TIs should be monitored for scans during the
    first 10 weeks after LMP.
  • TIB should be monitored for scans following 10
    weeks after LMP
  • TI up to 0.7 no time restriction, but observe
    ALARA
  • TI up to 1.0 maximum exposure time of an embryo
    or fetus should be restricted to no more than 60
    minutes
  • TI up to 1.5 maximum exposure time of an embryo
    or fetus should be restricted to no more than 30
    minutes
  • TI up to 2.0 maximum exposure time of an embryo
    or fetus should be restricted to no more than 15
    minutes
  • TI up to 2.5 maximum exposure time of an embryo
    or fetus should be restricted to no more than 4
    minutes
  • TI up to 3.0 maximum exposure time of an embryo
    or fetus should be restricted to no more than 1
    minute
  • TI gt 3.0 scanning of embryo or fetus is not
    recommended, however briefly

34
Cont.
  • Neonatal scanning
  • MI gt 0.3 possibility of minor damage to neonatal
    lung or intestine. Restrict exposure time as much
    as possible
  • TI use Tis for all transcranial and spinal
    scanning using the time limits given for
    obstetrics
  • ABDOMINAL, PERIPHERAL VASCULAR AND OTHER SCANNING
  • Use TIB with less restrictive time limits than
    those for obstetric scanning for example
    unrestricted time limit with ALARA for TIB lt 1.0
    TIB gt 6.0 is not recommended.
  • FETAL HEART MONITORING
  • This modality is not contraindicated on safety
    grounds even when used for extended periods due
    to low acoustic power levels

35
  • Eye scanning
  • TI gt 1.0 eye scanning is not recommended other
    than as part of a fetal scan.
  • TRANSCRANIAL ULTRASOUND EXAMINATIONS
  • TIC should be monitored.
  • TIC gt 3.0 is not recommended.
  • USE OF CONTRAST AGENTS
  • MI gt 0.7 risk of cavitation exists if a contrast
    agent containing microspheres is used and there
    is a theoretical risk of cavitation without the
    use of contrast agent. The risks increase with MI
    above this threshold

36
Conclusion
  • Dr. John Steed, head of obstetrics and
    gynaecology at the Virginia Commonwealth
    University School of Medicine said that
    although there is no proof that ultrasound is
    damaging, we used to think that about X-rays
    (Indeed, X-rays were vigorously promoted for
    viewing the baby in the womb, and other
    radiological procedures but it was many years
    later that research showed that X-ray exposure
    caused cancer in the children who were exposed as
    babies).
  • The use of higher intensity ultrasound combined
    with longer duration of exposure, may unmask
    detrimental effects and awareness of the possible
    biologic consequences of ultrasound and the
    factors associated with their occurrence may
    permit the clinician to balance optimal
    visualization and the risk of ultrasound-related
    complications. However, it should be borne in
    mind that the greatest danger in diagnostic
    ultrasound is misdiagnosis.

37
References
  • Is ultrasound safe? The Obstetrician
    Gynaecologist Authors Jolly J, Cooke I, Love M.
    20068222-227
  • Potential Adverse Ultrasound-related Biological
    Effects A Critical Review. Hariharan Shankar,
    M.B.B.S. Paul S. Pagel, M.D., Ph.D. November
    2011
  • Guidelines for the safe use of diagnostic
    ultrasound equipment, BMUS
  • Who says ultrasound is safe? AIMS Journal 2004/5.
    Vol 16, No 4
  • Bioeffects_Paper_July_2015
  • Fetal Thermal Effects of Diagnostic Ultrasound
    Jacques S. Abramowicz, MD, Stanley B. Barnett,
    MSc, PhD, Francis A. Duck, PhD, Peter D. Edmonds,
    PhD, Kullervo H. Hynynen, MSc, PhD, Marvin C.
    Ziskin, MD
  • Clinical Ultrasound (Third Edition), Hazel C.
    Strarritt, Francis A. Duck, 2011

38
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