high risk ultrasound is just ultrasound for high risk mom. ultrasound is
quite safe.
Ultraschall Med. 2012 Jun;33(3):215-7. doi: 10.1055/s-0032-1312759. Epub
2012 Jun 14.
[Obstetrical ultrasound: can the fetus hear the wave and feel the heat?].
[Article in German]
Abramowicz JS, Kremkau FW, Merz E.
Abstract
"Fetuses can hear ultrasound and the sound is as loud as a subway train
entering a station." This statement originates in a single report in a non-
peer reviewed journal, despite its name 1, of a presentation at a scientific
meeting by researchers who reported measuring the sound intensity in the
uterus of pregnant women and being able to demonstrate the above. This was
later published in a peer-review journal 2 probably not very widely read by
clinicians or the general public. From time to time, the popular press or
various pregnancy-related websites repeat the assertion or a worried
pregnant patient inquires about the truthfulness of this statement. A second
, oft-quoted concern is that ultrasound leads to heating of the amniotic
fluid. These two assertions may be very concerning to expectant parents and
merit scientific scrutiny. In this editorial, we shall examine the known
facts about the physical properties of ultrasound as they relate to these
two issues. Diagnostic ultrasound employs a pulsed sound wave with positive
and negative pressures and the Mayo team, quoted in the New Scientist,
predicted that the pulsing would translate into a "tapping" effect 1.
According to their report, they placed a tiny hydrophone inside a woman's
uterus while she was undergoing an ultrasound examination. They stated that
they picked up a hum at around the frequency of the pulsing generated when
the ultrasound is switched on and off. The sound was similar to the highest
notes on a piano. They also indicated that when the ultrasound probe was
pointed right at the hydrophone, it registered a level of 100 decibels, as
loud as a subway train coming into a station. Sound levels in decibels are
defined for audible frequencies with the reference level being the threshold
for hearing at a given frequency. Although the operating frequencies used
in sonography are inaudible, it is possible for the pulsing rate (pulse
repetition frequency, PRF) to be heard, thus falling in the audible range. A
previous report had hinted at similar phenomena 3.Ultrasound is a pressure
wave with a frequency beyond (ultra) that detectable in the human auditory
system. The human ear can discern sound at roughly 20 - 20 000 cycles (hertz
) per second. The frequencies of diagnostic ultrasound are roughly 1 - 10
megahertz (MHz) or 1 000 000 to 10 000 000 cycles per second. It is a form
of energy and, as such, may have effects in tissues it traverses. Any
consequences occurring in living tissues secondary to an external influence
are called biological effects or bioeffects. This term does not imply damage
or harm. The two major mechanisms for bioeffects are thermal and non-
thermal. Thermal effects are secondary to ultrasound energy being converted
into heat in the tissue (indirect effect of ultrasound) and non-thermal
effects are secondary to the alternating positive and negative pressures
generated by the wave (direct effect). The definition of moderately loud
sound is 60 - 70 dB (2 × 10-3-2 × 10-2 Pa), defined as high urban ambient
sound, normal conversation at 1 m, or living room music 4. In comparison,
quiet conversation is 40 dB, a railway diesel engine passing at 45 mph at
100 feet is 80 - 85 dB and a rock band is 110 dB 4. There have been a few
publications describing harm to fetuses exposed to elevated levels of
ambient noise, particularly industrial noise 567, specifically in the
aircraft and textile industries, but while there have been reports of
impaired hearing in infants who were exposed to ultrasound in the womb,
several rigorous studies have disproved that notion 891011. Furthermore, a
study of fetuses exposed in utero to vibroacoustic stimulation 12 and a
recent study of fetuses exposed to noise generated during an MR exam of the
pregnant women 13 showed no ill effect on the auditory system. There have
been some reports of being able to hear a "hum" during transcranial
ultrasound. This may be the pulse-repetition frequency (PRF), but, if so, it
would be described as a higher pitch, and probably not a "hum". To our
knowledge, this phenomenon has not been investigated. Although the report
mentioned above suggested that diagnostic ultrasound is detectable at
measurable levels in the uterus, there is no independently confirmed, peer-
reviewed, published evidence that the fetus actually hears the PRF, responds
to it or is harmed by it."The fetus cannot regulate its own body
temperature, so amniotic fluid can reach very high temperatures over long
periods" 14. Does this statement reflect a real risk? What does it mean if
this statement is scientifically true? The fear is, of course, that this
will raise the temperature of the fetus. Thermally induced teratogenesis has
been demonstrated in many animal studies, as well as several controlled
human studies 1516. A temperature increase of 1.5 °C above the normal value
has been suggested as a universal threshold 17. It is important to note
that diagnostic ultrasound was not the source of the temperature elevation
in any of these studies. Some believe that there are temperature thresholds
for hyperthermia-induced birth defects (hence the ALARA [as low as
reasonably achievable] principle), but there is some evidence that any
positive temperature differential for any period of time has some effect, in
other words there may be no thermal threshold for hyperthermia-induced
birth defects 18. In experimental animals the most common defects are
microcephaly with associated functional and behavioral problems 17,
microphthalmia and cataracts. There are reports on the effects of
hyperthermia and measurements of in vivo temperature induced by pulsed
ultrasound but not in humans 192021. Temperature increases of 1 °C are
easily reached in routine scanning 22. Elevation of up to 1.5 °C can be
obtained in the first trimester and up to 4 °C in the second and third
trimesters, particularly with the use of pulsed Doppler 23. When the
ultrasound wave travels through tissue, its intensity diminishes with
distance (attenuation). In completely homogeneous materials, the signal
amplitude is reduced only by beam divergence and absorption (conversion of
sound to heat). However, biologic tissues are non-homogeneous and further
weakening occurs due to scattering. The issue of temperature increase in the
amniotic fluid is based on the fact that the energy of the ultrasound waves
is partially converted to heat in the tissue traversed by the waves.
Tissues with a high absorption coefficient (such as bone) will produce a
high conversion rate while the conversion will be lower in tissues with low
absorption. Fluids have very low absorption characteristics and, therefore,
the risk of temperature elevation in the amniotic fluid is minimal. The only
available study on the topic did not demonstrate any increase in
temperature in the amniotic fluid when performing diagnostic ultrasound,
both in grayscale anatomic imaging (sonography) and Doppler ultrasound 24.
ConclusionWhile ultrasound is a sound wave which can produce mechanical
effects and temperature elevation in tissues that it traverses, the risk to
human fetuses when using diagnostic ultrasound appears to be minimal if
certain rules are followed, such as performing a scan when medically
indicated, and observing the ALARA principle (using the lowest output power
consistent with acquiring the necessary diagnostic information and keeping
the exposure time as low as possible for accurate diagnosis).
© Georg Thieme Verlag KG Stuttgart · New York.
