Diagnostic ultrasound ( PDFDrive )

1038 PART IV Obstetric and Fetal Sonography
“percolating” 66,67 with near-minimal Doppler-measured velocities,
as opposed to later in pregnancy. At about week 12 of gestation,
the plugs of the spiral arteries are “loosened” and allow for freer
blood circulation. 68,69
hus perfusion status is far from approaching that for normal
tissue levels (as assumed in the TI algorithm) for much of the
irst trimester. Only later, when “free circulation” is established
(about week 11 or 12 of gestation), does the tissue become
normally perfused, when the embryonic circulation actually links
up with the maternal circulation. 69 his absence of perfusion
may result in underestimation of the actual ultrasound-induced
temperature in early gestation. his warrants extreme caution
in irst-trimester scanning, particularly with the recent increase
in utilization of Doppler in the irst trimester. 70-73
Also, the issue of transducer heating may be particularly
relevant in the irst trimester, if performing endovaginal scanning.
72,74 A mitigating factor is motion (even very small) of the
examiner’s hand, as well as the patient’s breathing and body
movements (in obstetric ultrasound, both mother and fetus),
which tend to spread the region being heated. However, for
spectral (pulsed) Doppler studies, it is necessary to have the
transducer as steady as possible. Because the intensity and acoustic
power associated with Doppler ultrasound are the highest of all
the general-use categories, time spent scanning with Doppler
ultrasound mode is crucial. Ziskin 75 reported that average duration
of 15,973 Doppler ultrasound examinations was 27 minutes
(longest, 4 hours!). It is clear that temperature increases of 1°C
are easily reached in routine scanning. 76 Elevation of up to 1.5°C
were obtained in the irst trimester and up to 4°C in the second
and third trimesters, particularly with the use of pulsed Doppler. 77
In many clinical machines, TI values of 5 or 6 can be obtained
in Doppler mode.
MECHANICAL EFFECTS
Although efects have been described in neonates or adult animals,
because gas bubbles are not present in fetal lung or bowel, it is
assumed that the risk from mechanical efect secondary to cavitation
is minimal. 78 Several other mechanical efects do not appear
to involve cavitation, such as tactile sensation of the ultrasound
wave, auditory response, cell aggregation, and cell membrane
alteration. Hemolysis has also been reported, 79 although some
cavitation nuclei must be present for hemolysis to occur. Such
microbubbles would be provided by the introduction of ultrasound
contrast agents to the area under ultrasound examination.
However, there is currently no clear clinical indication for the
use of these agents in fetal ultrasound. 80,81 In addition, fetal
stimulation caused by pulsed ultrasound insonation has been
described, with no apparent relation to cavitation. 82 his efect
may be secondary to radiation forces associated with ultrasound
exposures. No harmful efects of diagnostic ultrasound secondary
to nonthermal mechanisms have been reported in human fetuses.
However, because of these known mechanical efects of ultrasound
in living tissues, and because pressures in Doppler propagation
are much higher than in B-mode imaging, further caution is
recommended in the use of ultrasound, particularly in the irst
trimester. 83
BIOEFFECTS OF ULTRASOUND
Animal Research
Multiple studies have shown efects of ultrasound in a wide variety
of species. 84-87 Studies of gross efects on the brain and liver of
cats showed well-deined lesions and demyelination in the brain 88
and tissue damage in the liver 89 resulting from ultrasound exposure
of a few seconds at 1 and 3 MHz, respectively. Other observed
efects include limb paralysis, as a result of spinal cord injury in
the rat, 90,91 as well as lesions in the liver, kidney, and testes of
rabbits. 92 Changes in fertility were demonstrated in male mice
ater in utero ultrasound exposure of the testes. 93 Although some
efects are likely caused by mechanical processes, very high
temperature elevations (much higher than with diagnostic
ultrasound) may be more directly involved with the tissue damage.
It took acoustic pressures generated by lithotripsy to obtain efects
in muscles, 94 as well as hemorrhage in bowel 95 and lungs. 96 hese
intensities are much higher than in diagnostic ultrasound but
are helpful in understanding the mechanisms involved with
possible bioefects of ultrasound.
Several major clinical end points for bioefects in animals
that could have direct relevance to human studies include fetal
growth and birth weight, efects on brain and CNS function,
and change in hematologic function. High-level exposures were
associated with decreased body weight at birth in exposed
monkeys compared with controls, but all showed catch-up growth
when examined at 3 months of age. 97 Decreased birth weight
ater prenatal exposure to ultrasound has also been reported in
mice 98,99 but not convincingly in rats. 100 Clear species diferences
therefore seem to exist, 101 making it diicult to extrapolate to
the human. In a report of 30 pregnancies in monkeys, half were
exposed to ultrasound. 97 he scanned fetuses had lower birth
weights and were shorter than the control group. No signiicant
diferences were noted in rate of abortions, major malformations,
or stillbirths. Moreover, all showed catch-up growth when
examined at 3 months of age. In situ intensities were higher than
routinely used in clinical obstetric imaging in the human. Studies
in mice have shown increased mortality and decreased body
weight ater in utero exposure to diagnostic ultrasound. 102,103
Gross lesions have been described in the CNS 104 and the spine 91
in mammals.
Neurologic or behavioral indings may be sensitive markers
of teratogenic efect. 84,105 Pregnant Swiss albino mice were exposed
to diagnostic ultrasound for 10, 20, or 30 minutes on day 14.5
(fetal period) of gestation and compared with sham-exposed
controls. 106 Signiicant behavioral alterations in the exposed groups
included decreased locomotor and exploratory activity and more
trials needed for learning. No changes were observed in physiologic
relexes or postnatal survival. he authors concluded that
ultrasound exposure during the early fetal period can impair
brain function in the adult mouse. 106 In another study, the same
authors found increased anxiolytic activity and learning latency
in ultrasound-treated animals. 107 Pregnant Swiss albino mice
were exposed to similar diagnostic levels of ultrasound for 10
minutes on day 11.5 or 14.5. Behavioral tests at 3 and 6 months
post partum showed more pronounced efects in the 14.5-day
than in the 11.5-day group. he authors concluded that exposure