Biomedical Engineering Reference
In-Depth Information
2.2
Three-Dimensional Ultrasound Imaging
of the Fetus
2.2.1
Conventional Ultrasound Imaging of the Fetus
Ultrasonography, or two-dimensional (2D) ultrasound, has been widely used
in obstetrics, since fetal assessment by using fetal images is indispensable for
modern practice, and ultrasound is the only imaging modality proved to be
safe for a fetus. More than one million babies are born in Japan every year
and almost all of them undergo ultrasound examination before birth. Two
approaches, transabdominal and transvaginal, are available in ultrasound ex-
amination for a fetus. The transabdominal approach is easy to perform, and is
used for fetuses mainly after 12 weeks of gestation. The transvaginal approach
is not easy to perform but resolution of the image near the vagina is high
because a high frequency is applied in a transvaginal probe. The transvaginal
approach is used for fetuses before 12 weeks of gestation.
A gestational sac (GS) is seen in the uterus by transvaginal ultrasound in
the middle of 4 weeks of gestation (Fig. 2.6). This finding usually excludes the
possibility of ectopic pregnancy. A fetus can be depicted at the end of 5 weeks
of gestation as a tiny circle. Multiple pregnancy is diagnosed at this stage
by counting the number of GS's or fetal images. Morphological abnormalities
of a fetus cannot be depicted, because it is too small at this stage, but its
heart can be seen beating. Fetal biometry is strongly recommended in the
first trimester to confirm or determine the gestational age (Fig. 2.7). But an
Fig. 2.6.
A gestational sac (
arrow
) in the uterus (
arrowheads
) in the middle of the
fourth week of gestation
