Key words
ultrasound - safety - recommendation
General Information
Diagnostic ultrasound has been widely used in clinical medicine for many years with
no proven deleterious effects. Biological effects (such as localized pulmonary capillary
bleeding) have been reported in mammalian systems at diagnostically relevant exposures,
but the clinical significance of such effects is not fully known [1]. Consequently, diagnostic ultrasound can be considered safe only if used prudently.
Ultrasound examinations should be performed only by competent personnel who are trained
and updated on safety matters.
It is important that ultrasound devices are CE- (European Conformity) approved and
appropriately maintained. The range of clinical applications is becoming wider, the
number of patients undergoing ultrasound examinations is increasing and new techniques
with higher acoustic output levels and new pulse emission sequences are being introduced.
It is therefore essential to maintain vigilance to ensure the continued safe use of
ultrasound.
Available safety information during clinical scanning
Available safety information during clinical scanning
Ultrasound produces heating, pressure changes and mechanical disturbances in tissue.
Diagnostic levels of ultrasound are capable of producing temperature rises that may
be hazardous to sensitive organs and the embryo/fetus. Biological effects of non-thermal
origin have been reported in animals [2], but to date, no such effects have been demonstrated in humans, except when a microbubble
contrast agent is present.
The ALARA-Principle
The Thermal Index (TI) is an on-screen guide for the user regarding the potential
for tissue heating. The Mechanical Index (MI) is an on-screen guide with respect to
the likelihood and magnitude of non-thermal effects. Users should remain aware of
both indices while scanning, especially when changing scan modes, and should adjust
the machine controls to keep them as low as reasonably achievable (ALARA principle)
without compromising the diagnostic value of the examination [3]. Where low values cannot be achieved, examination times should be kept as short
as possible [4].
Adapt your Pre-sets
The scanner pre-sets should be set so that the default power for a given examination
is at the minimum level necessary. In obstetric applications, this default power should
result in a TI no higher than 0.7. The output should be increased during the examination
only if this is necessary to produce a satisfactory diagnostic result. Some modes
are more likely than others to produce significant acoustic outputs, and when these
modes are used, particular care should be taken to regularly check the TI and MI Indices.
Doppler mode
Spectral pulse-wave Doppler and Doppler imaging modes (color flow imaging and power
Doppler imaging), in particular, can produce more tissue heating and hence higher
thermal indices, as can B-mode techniques involving coded transmissions. During scanning,
the ALARA principle should be followed [5]
[6].
Elastography mode
In general, a longer acoustical pulse sequence is needed in elastographic mode (but
not in conventional strain imaging) than for B-mode imaging. Since the algorithms
used to calculate TI for most imaging modes are not appropriate for elastography,
it should be noted that the TI and MI displayed for these applications may represent
an underestimate of the temperature rises and mechanical effects to be expected. In
acoustic radiation force impulse (ARFI), the temperature has its maximum at the focus,
and significant temperature rises may occur if bone lies in the beam. The scanning
times should be kept short, especially when exposing vulnerable tissues [7].
Harmonic imaging mode
Tissue harmonic imaging mode can sometimes involve higher MI values. Users should
be aware of the MI value displayed during scanning.
3 D/4 D mode
Three-dimensional imaging does not introduce any additional safety considerations,
particularly if there are significant pauses during scanning to study or manipulate
the reconstructed images. However, four-dimensional (real-time three-dimensional)
scanning involves continuous exposure, and users should guard against the temptation
to prolong examination times unduly in an effort to improve the recorded image sequence
beyond that which is necessary for diagnostic purposes.
Ultrasound Exposure During Pregnancy
Ultrasound Exposure During Pregnancy
The embryo/fetus in early pregnancy is known to be particularly sensitive. In view
of this and the fact that very little information is currently available regarding
possible subtle biological effects of diagnostic levels of ultrasound on the developing
human embryo or fetus, care should be taken to limit the exposure time and the TI
and MI indices to the minimum commensurate with an acceptable clinical assessment,
particularly when the TI exceeds 0.7. In this case the exposure time has to be reduced
and only TIs less than 3.0 are permitted.
Based on scientific evidence of ultrasound-induced biological effects to date, there
is no reason to with-hold diagnostic scanning during pregnancy, provided it is medically
indicated and is used prudently by fully trained operators. This includes routine
scanning of pregnant women.
Exposure of sensitive organs in the case of Doppler or Cardiotocography
Temperature rises are likely to be greatest at bone surfaces and adjacent soft tissues.
With increasing mineralization of fetal bones, the possibility of heating sensitive
tissues such as brain, eye and spinal cord increases. The TI value has to be monitored
during these scans and must be less than 1.0. The exposure time has to be reduced
if the lung or intestine is scanned at MI values above 0.3. Extra vigilance is advised
when scanning such critical fetal structures at any stage in pregnancy and the ALARA
principle should be applied.
However, Doppler ultrasound examinations should not be used routinely in the first
trimester of pregnancy. The power levels used for fetal heart rate monitoring (cardiotocography
– CTG) are sufficiently low, so that the use of this modality is not contraindicated
on safety grounds, even when it is to be used for extended periods [4]
[8].
Safety Considerations when Scanning Neonates or the Eye
Safety Considerations when Scanning Neonates or the Eye
Particular care should be taken to reduce the risk of thermal and non-thermal effects
during cardiac, pulmonal and cranial investigations of neonates. When scanning the
neonatal brain, the TI should never exceed 3.0 and the duration of ultrasound exposure
if TI is > 0.7 should be restricted. However, there is experimental evidence that
transducer self-heating can lead to a significant temperature rise at the skin surface,
and so scanning times and exposure levels should be kept as low as possible [9].
In ophthalmic applications the TI could greatly underestimate the actual temperature
rise in the eye because of the inappropriate underlying tissue model. Therefore, scanning
has to be done with very low exposure levels compared to other diagnostic investigations.
To reduce the risk of bioeffects, it is prudent to use TI < 1, and MI < 0.23 for eye
scanning [10].
Safety information concerning Ultrasound Contrast Agents
Safety information concerning Ultrasound Contrast Agents
These usually take the form of stable gas-filled microbubbles, which can potentially
produce cavitation or microstreaming, the risk of which increases with the MI, scanning
time and lower frequency. Data from small animal models suggest that microvascular
damage or rupture is possible. Caution should be considered for the use of ultrasound
contrast agents in tissues where damage to the microvasculature could have serious
clinical implications, such as the brain and the eye [10]
[11]
[12]. As in all diagnostic ultrasound procedures, the MI and TI should be continually
checked and kept as low as possible.
It is possible to induce premature ventricular contractions in contrast-enhanced echocardiography
when using high MI values and end-systolic triggering. Users should take appropriate
precautions in these circumstances.
The use of contrast agents is still off-label in pregnancy, renal assessment and in
intravenous application in pediatric patients. Specifically for the assessment of
focal liver lesions in pediatrics, the application of a contrast agent has been approved
[13]
[14].
Intravenous and intracavity application of ultrasound contrast agents should be avoided
24 hours prior to extra-corporeal shock wave therapy [15]
[16]. Since a rare allergic response to these agents has been seen (1:10 000, [14]), it is recommended that an “allergy kit” be readily available at all times during
these procedures.
Current safety statements are available on EFSUMB’s website, while only a few have
been published in Ultraschall in der Medizin/European Journal of Ultrasound or in
the EFSUMB Newsletter [17]
[18].
