Diagnostic ultrasound ( PDFDrive )

CHAPTER 35 The Fetal Spine 1221
F
T
F
T
A
B
FIG. 35.6 Normal Spinal Cord. (A) Posterior longitudinal and (B) posterior transaxial (transverse) sonograms of a normal spinal cord. Note the
normal position of the cord (arrows) and ilum terminale (T) in the dependent portion of the spinal canal. Cerebrospinal luid (F) is seen between
the anterior aspect of the spinal cord and the anterior wall of the spinal canal.
medullaris is usually lower than normal. For earlier pregnancy
(13-18 weeks’ gestation), the conus medullaris may be normally
as low as L4. At term, the conus is normally at or above L2. 21
SCANNING TECHNIQUES
In clinical practice, the most useful scan planes to assess the
posterior neural arches are posterior transaxial (Fig. 35.7), lateral
transaxial (Fig. 35.8), lateral longitudinal (coronal) (Fig. 35.9),
posterior longitudinal (sagittal) (Fig. 35.10), and posterior angled
transaxial (Fig. 35.11). he posterior angled transaxial scan plane
is useful to visualize the pedicles and laminae simultaneously.
Because the laminae course caudal to the transaxial plane, which
contains the centrum and pedicles, only the angled scan plane
can depict the pedicles and laminae simultaneously in their
entirety.
he detection rate of spina biida at 18 to 20 weeks’ gestation
may be 80% or less during routine screening ultrasound, 22 because
the accuracy of ultrasound depends on the skill and experience
of the operator. he accuracy of referral centers performing
detailed targeted imaging because of a suspected NTD or high
maternal serum alpha-fetoprotein (MS-AFP) level is close to
100%.
A detailed sonogram of the fetal spine may be requested for
several reasons: previous suspicious ultrasound, family history
of NTD, or raised serum or amniotic luid AFP. To enhance
detection of spina biida, a detailed protocol should be consistently
followed. he irst step in assessing for spina biida is scanning
the head, because most fetuses with open spina biida have signs
of a Chiari II malformation in the brain at 16 to 22 weeks’
gestation. hese signs include obliterated cisterna magna (banana
sign), concave frontal bones (lemon sign), and dilated lateral
cerebral ventricles. 23,24 he sensitivity of the banana sign for open
spina biida is close to 99%, and false-positive diagnoses are rare,
although the lemon sign may occur in 1% to 2% of normal
fetuses.
he next step is to determine the position of the fetal spine.
he scan plane is placed perpendicular to the long axis of the
fetal spine, either posterior transaxial or lateral transaxial (see
Figs. 35.7 and 35.8). he sonographer should scan from one end
of the spine to the other while maintaining the scan plane
perpendicular to the spine (Video 35.1). his is repeated several
times. In the process, the sonographer builds up an impression
of the 3D structures of the spine. he scan plane should then
be repositioned parallel to the long axis of the fetal spine to
obtain posterior longitudinal and lateral longitudinal views. he
sonographer then examines all levels of the spine in posterior
transaxial, lateral transaxial, lateral longitudinal, and posterior
longitudinal scan planes (Video 35.2). his may not be possible
in a short time because of fetal position, but this usually changes
enough in 30 to 45 minutes at 16 to 22 weeks to obtain all scan
planes. If the spine cannot be visualized optimally, a repeat scan
can be performed at a later gestational age. In both axial and
longitudinal scan plans, it is important to visualize the skin
covering the fetal back by ensuring that amniotic luid is interposed
between the fetus and the uterine wall.
Three-Dimensional Ultrasound
hree-dimensional ultrasound imaging has shown promise in
evaluating normal fetal structures and in providing additional
information in abnormalities of many fetal structures, including
the spine, hand, foot, and face. 25-34 Bony structures can be visualized
with maximum-intensity projection methods (see Fig. 35.9D).
In evaluation of spinal abnormalities, 3D ultrasound is most