Diagnostic ultrasound ( PDFDrive )

1032 PART IV Obstetric and Fetal Sonography
sonographic examination. In the past decade, sotware and
hardware have allowed for fetal MR images to be obtained in
about 400 milliseconds. his allows for fetal imaging to be
performed without maternal or fetal sedation. he ease of
performing these examinations and the superb contrast resolution
aforded by T2-weighted MRI have popularized the use of this
imaging tool to improve prenatal diagnosis.
here are no known biologic risks from MRI. he MR procedure
is not believed to be hazardous to the fetus. 43-55 No delayed
sequelae from MR examination have been encountered, and it
is expected that the potential risk for any delayed sequelae is
extremely small or nonexistent.
However, most studies on fetal risk have been performed
at 1.5 Tesla. Heating has been shown in animal studies at
3.0 Tesla. 56 herefore when possible fetal studies should be done
at 1.5T.
Gadolinium is the contrast typically used for MRI, but it is
not recommended for fetal examination. Gadolinium crosses
the placenta and appears within the fetal bladder soon ater
intravenous administration. he contrast is excreted from the
fetal bladder into the amniotic luid, where it is then swallowed
and potentially reabsorbed from the gastrointestinal tract. Because
of this reabsorption, the half-life of gadolinium in the fetal circulation
is not known. 57 his drug has been shown to have adverse
efects on the fetus in animal models. Gadopentetate dimeglumine
has been shown to impair development slightly in rats (at
2.5 times the human dose, 0.1 mmol/kg), and in rabbits (at 7.5
times the human dose). 58,59 It is considered a pregnancy category
C drug, meaning that it should be given only if potential beneit
outweighs the risk; animal studies have revealed adverse efects,
but no controlled studies have been performed in humans. 58
herefore we do not use contrast for fetal examinations at our
institution.
CONCLUSION
Ultrasound is a readily available, noninvasive, and safe means
of evaluating fetal well-being, determining gestational age, and
assessing the intrauterine environment. It is an indispensable
tool for the practice of obstetrics. Ultrasound is also a screening
test, yielding results that must be interpreted and integrated in
a knowledgeable manner. As with physical examination, the
ultrasound study is most helpful when performed in a consistent
and reproducible fashion, carefully documenting positive and
negative indings important in clinical decision making. he
information gained from routine obstetric ultrasound may provide
reassurance, guide therapy, or identify a pathologic condition
that merits further investigation.
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