This document discusses ultrasound safety and provides guidelines for limiting fetal exposure. It describes how ultrasound can cause thermal and non-thermal bioeffects. Thermal effects are the most concerning and can cause hyperthermia in rapidly developing fetal tissues. The safety indices of thermal index and mechanical index are displayed to guide users, but do not consider all risk factors. Doppler mode poses higher risks due to greater acoustic output. Examinations should be limited in the first trimester and exposure time restricted based on safety index values to minimize risks according to ALARA principles.

1. ULTRASOUND
SAFETY
Prof. Aboubakr
Elnashar
Benha university
Hospital, Egypt
ABOUBAKR ELNASHAR

2. CONTENTS
1.INTRODUCTION
2.FACTORS AFFECTING
3.BIOEFFECTS OF US
4.SAFETY INDICES
5.HOW TO LIMIT FETAL EXPOSURE
 CONCLUSION
ABOUBAKR ELNASHAR

3. 1. INTRODUCTION
 Ultrasound
 Form of energy
 Lack of knowledge of US clinical users on
 output
 bioeffects
 safety
both in the USA and abroad
ABOUBAKR ELNASHAR

4.  US examinations/ pregnancy
 Most pregnant women
2–3
 Some countries (Egypt): 10
whether there is a cumulative dose effect?
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5.  Epidemiological studies
 No harmful effects in human fetuses.
 Most, based on information obtained with pre-1991
machines.
 Around that time, FDA allowed the acoustic output
of US machines for fetal use to be increased from
 94 to
 720 mW/cm2, a factor of almost 8.
ABOUBAKR ELNASHAR

6. ABOUBAKR ELNASHAR

7. 2. FACTORS AFFECTING SAFETY OF ULTRASOUND
1. US exposure
 The ultrasound energy or
 Total acoustic output power (w) emitted by
equipment.
2. US settings
 Type of transducer
 Depth of penetration
 Overlying layers of tissue
 alter the acoustic output to the particular target.
 E.g. US exposure to the fetus in 1st T differs
significantly between TA and TV probes.
ABOUBAKR ELNASHAR

8. 3. Tissue composition
 determines the acoustic absorption coefficients
 more proteinaceous tissue: more susceptible to
thermal injury
 higher fluid and gas content: more susceptible
to cavitational activity.
4. Tissue susceptibility
 Fetal or embryonic tissues:
 Rapidly proliferating : more susceptible to US
effects.
 Adult tissues have:
 static cell population
 safety features such as the hyperaemic reflex
{an increase in blood flow through the tissue that
carries the heat away}.
ABOUBAKR ELNASHAR

9. 3. BIO EFFECTS OF ULTRASOUND
 When performing diagnostic US
 2 major mechanisms are operative:
 Thermal
 Nonthermal.
ABOUBAKR ELNASHAR

10. 1. THERMAL EFFECTS
 Increase in tissue temp is the most worrying bioeffect
associated with diagnostic US in obstetrics
 Resulting from
 transformation of acoustic energy into heat
 an indirect effect.
ABOUBAKR ELNASHAR

11.  Thermal Effects
 Hyperthermia is teratogenic for numerous species,
including humans.
 Most at risk is CNS
{lack of compensatory growth of undamaged
neuroblasts}
ABOUBAKR ELNASHAR

12.  Temperature threshold:
 Temp increases of 1 °C are easily reached in
routine US.
 An increase of 2.5 °C and above is possible with
1h of exposure to US
 Temp elevations ≤1.5 C: no hazard, including a
human embryo or fetus, even if maintained
indefinitely.
 A 1.5°C temp elevation above the normal value is
considered a universal threshold.
 Some scientists : any positive temp differential for
any period of time has some effect.
 Fetal temp is 0.5–1.0C higher than maternal temp:
caution in a febrile mother.
ABOUBAKR ELNASHAR

13. In early pregnancy the entire fetus is within the US beam.
Gestational age of 12 w
ABOUBAKR ELNASHAR

14.  Factors affecting thermal effect
1. Type of probe
 Abdominal probe Vs vaginal probe
 Abd:
 the skin surface is close to room temp and
heat is removed by air convection
 Vag:
 tissues are at an average temp of 37 °C and
there is very little heat removal
 A fixed transducer and target:
higher than expected temp rise.
 The active fetus in 3nd &3rd T: TAS escapes
this effect.
ABOUBAKR ELNASHAR

15. 2. Type of mode:
 B-mode, M-mode & 3D US
less likely to give rise to thermal injury in routine
practice
 Doppler US
 can cause significant temp rises.
 Temp rise to above 41°C lasting for 5 mins or
more
 potentially hazardous to a fetus or embryo
 possible with spectral Doppler and colour
Doppler imaging
ABOUBAKR ELNASHAR

16. 3. Beam characteristics
 Temp elevation is proportional to the
 wave amplitude
 length of the pulse
 pulse repetition frequency.
 manipulation of any of these will alter the in situ
conditions.
ABOUBAKR ELNASHAR

17. 4. Gestational age
 Milder exposure during the preimplantation period:
 similar consequences to more severe
exposures during embryonic and fetal
development
 death and abortion or
 structural and functional defects.
ABOUBAKR ELNASHAR

18.  Recommendations
 ALARA (As Low As Reasonably Achievable)
principle: keep the exposure
 as low as possible,
 for the least amount of time possible,
 enough to get adequate diagnostic images.
 World Federation for Ultrasound in Medicine and
Biology (WFUMB)
 Temp elevation of no more than 1.5 °C above
normal physiological levels may be used
without reservation on thermal ground
ABOUBAKR ELNASHAR

19. 2. Non thermal Effects
 These are interactions between US wave and the
tissue that do not cause a significant degree of temp
increase (<1°C above physiological temp).
 Included are
 Mechanical
 acoustic cavitation
 radiation torque
 force and acoustic streaming secondary to propagation
of US waves.
 Physical (shock wave)
 Chemical (release of free radicals) effects.
ABOUBAKR ELNASHAR

20. 1. Cavitation
 The major factor in mechanical effects
 To occur, gas bubbles must be present in the
tissues.
 No gas bubbles in fetal lungs or bowel: risk from
mechanical effect is minimal
 US contrast agents can act as source of cavitation,
when injected into the body before US
examination.
 No indication for its use in fetal US
ABOUBAKR ELNASHAR

21.  US Doppler: no relation to cavitation
 No harmful effects of diagnostic US, secondary to
nonthermal mechanisms have been reported in
human fetuses.
ABOUBAKR ELNASHAR

22. 2. Acoustic streaming and torque
 = twisting or spinning forces
 US wave tend to push target tissue away from the
transducer:
 acoustic streaming in fluids
 cell distortion and lysis
 demonstrated in experimental models
 unlikely to be significant with diagnostic US in soft
tissues in vivo
{in situ adhesiveness is high}.
ABOUBAKR ELNASHAR

23. 4. SAFETY INDICES
 An on-screen display to guide the user to extent of
thermal & mechanical effect
(American Institute of Ultrasound in Medicine (AIUM)
National Electrical Manufacturers Association (NEMA).)
 Called the Output Display Standard (ODU)
 First reported in 1992.
 FDA allow manufacturers to increase power outputs
by up to 8–10 times, provided there is a display of
safety indices on the screen.
 The aim to
 keep these indices as low as possible
 While obtaining the best possible diagnostic
images
ABOUBAKR ELNASHAR

24. The TI and MI acoustic indices as demonstrated on the
monitor screen during US examination.
MI is 0.9 and the TIS, 0.1ABOUBAKR ELNASHAR

25.  Thermal index
 An indicator of the temp elevation possible at a
particular equipment setting.
 The ratio of
 acoustic power emitted by the transducer To
 acoustic power required to produce a 1C rise in
temp at a particular equipment setting
 3 subdivisions:
 Soft tissues (TIS)
 Bone (TIB)
 Adult cranial exposure (TIC).
 In obstetric:
 TIS should be used for the first 8 w
 TIB should be monitored thereafter.
ABOUBAKR ELNASHAR

26.  The acoustic power of an US scanner depends on:
1. focus,
2. pressure
3. intensity
4. scan depth
5. mode and transducer characteristics.
 Various combinations of these parameters:
varying levels of acoustic power output with
significant variations in the temp levels
ABOUBAKR ELNASHAR

27. Safety indices
 Mechanical
index (MI)
 Thermal index
 soft tissues
(TIS);
 bone (TIB)
 adult cranial
exposure or
bone (TIC)
ABOUBAKR ELNASHAR

28. B-mode ultrasound
(TIB and MI are displayed in the top right hand corner)
TIB = 0.2, MI = 1.1 ABOUBAKR ELNASHAR

29. Doppler mode.
Note the change in TIB and MI when the settings are changed
from B mode to Doppler mode
TIB = 1.4, MI = 0.55 ABOUBAKR ELNASHAR

30. Umbilical artery Doppler.
TIB (solid arrow) is displayed in the top right hand corner
TIB = 1.1, MI = 0.55
Depth: 5.8 cm
ABOUBAKR ELNASHAR

31. Umbilical artery Doppler.
TIB (solid arrow) is displayed in the top right hand corner.
Note how an increase in depth from 5.8 cm (Fig a) to 13 cm (Fig
b) almost triples the TIB (1.1 to 3.1)ABOUBAKR ELNASHAR

32.  Mechanical index
 an indicator of the Likelihood of cavitation events.
 Definition:
 „Maximum estimated in situ rarefaction pressure
or
 Maximum negative pressure (in mpa) divided by
the square root of the frequency (in mhz)
 inversely proportional to the frequency.
 MI: 0.3 is the threshold value for hges to occur in
the mouse lung.
ABOUBAKR ELNASHAR

33.  Mechanical bioeffects
 in humans
 not reported from currently used diagnostic US
 In animals:
reported: raising the concern that there is potential
for similar injury in humans.
 MI Should be less than 1.9.
ABOUBAKR ELNASHAR

34.  Limitations of the thermal and mechanical indices
 do not consider factors such as
 Duration of examination
 Patient temp
 Presence of contrast agents.
 There is probably an underestimation of temp rise
by the thermal Index.
 Not perfect
 The most practical measurements available.
ABOUBAKR ELNASHAR

35.  Recommendations:
 Mechanical index
 ≥0.3: minor damage to neonatal lung or
intestine is possible
 ≥0.7 have a propensity for cavitation injury,
especially with use of contrast agents.
 Thermal index
 ≥0.7 the overall exposure to embryo or fetus
should be restricted to less than 60 mins
ABOUBAKR ELNASHAR

36. Maximum recommended exposure times for an embryo or
fetus
(British Medical Ultrasound Society.)
ABOUBAKR ELNASHAR

37.  Doppler effects on the fetus in the first trimester
 Doppler is different
 1. Acoustic Output is much higher in Doppler
than in B-mode:
 34 mW/cm2 for the ISPTA in B-mode versus
 1080 mW/cm2 for spectral Doppler
 35-fold difference.
 2. Dwell time (duration of exposure) is
important:
 The average duration was 27 min (the longest
4 h!).
ABOUBAKR ELNASHAR

38. Very high TI (5.7) may be obtained in Doppler mode
(not an actual clinical examination).
Note that this is a general obstetrics settingABOUBAKR ELNASHAR

39.  Precautions
1. Clear indication
2. Limit time and acoustic output
 Excellent, diagnostic images can be obtained at
low outputs, as defined by the TI values of 0.5
or even 0.1.
 Therefore, the switch-on default should be set
up such that a low acoustic output power is
initiated for each new patient, when starting an
examination.
 Only if images are not satisfactory from a
diagnostic standpoint, should the output be
increased
ABOUBAKR ELNASHAR

40. 3. To have the transducer as steady as possible.
{blood vessels or heart valves are small in comparison to the general
organ or body size being scanned and even small movements will have
more undesired effects on the resulting image}.
4. Using Doppler to “listen” to the fetal heart should be
discouraged and replaced by M-mode assessment.
 If Doppler is used, it is sufficient to “hear” 3–4
heart beats and thus limit the exposure
ABOUBAKR ELNASHAR

41. Doppler velocimetry in the
umbilical artery.
(a) TIB is 2.4.
(b) TIB is 0.4 and the image is
equally diagnostic
ABOUBAKR ELNASHAR

42.  3D/4D ULTRASOUND
 Characteristics
 short acquisition time and post processing
analysis: decreased exposure.
 TI and MI, acoustic output are comparable to
the TI during the B-mode scanning
ABOUBAKR ELNASHAR

43. 5. HOW TO LIMIT FETAL EXPOSURE AND SAFETY
STATEMENTS
1. Perform US only with a clear indication
2. keep exposure to a minimum power and time,
compatible with an adequate diagnosis
 application of the ALARA principle
3. Watch the TI (and, to a lesser degree) the MI on-
screen
4. Begin your exam with a low power output and
increase only if necessary.
ABOUBAKR ELNASHAR

44. 3-D acquisition with 3 orthogonal planes and reconstructed
volume. The output power is determined by the acquisition plane
(in general plane A), since the 2 other planes (B, C) and the
reconstructed volume are computer-generated. In this
acquisition, TIS was 0.5 ABOUBAKR ELNASHAR

45. 5. Pulsed Doppler (spectral, power, and color
flow imaging) ultrasound
1. should not be used routinely, may be used for
clinical indications such as to refine risks form
trisomies.
2. When performing Doppler US
 TI should be less than or equal to 1.0
 Exposure time should be kept as short as
possible
 usually no longer than 5–10 min and not
exceed 60 min.
ABOUBAKR ELNASHAR

46. CONCLUSIONS
 The early fetal period is a time of increased
susceptibility to external factors, such as
hyperthermia, a recognized teratogen, with CNS
being most at risk.
 Bioeffects of US may be secondary to 2 major
mechanisms:
1. thermal (indirect, resulting from conversion of
acoustic energy into heat)
2. non-thermal (also known as mechanical, direct
effects caused by bubble cavitation and other
mechanical phenomena).
ABOUBAKR ELNASHAR

47.  The application of safety indices and on-screen
display is important.
 To limit exposure and potential harmful effects,
 use US only when indicated,
 keep the exam as short as possible,
 at lowest possible output for diagnostic accuracy
(ALARA principle)
 keep TI and MI below 1.
 Diagnostic US is safe in pregnancy
 both for the mother and fetus
 no substantiated long-term effects have been
demonstrated.
ABOUBAKR ELNASHAR

48. Thanks
ABOUBAKR ELNASHAR