Diagnostic ultrasound ( PDFDrive )

1674 PART V Pediatric Sonography
he mesenchyme lateral to the closing neural tube organizes
into somites. he dorsolateral portion of each somite will become
skeletal muscle and dermis, whereas the ventromedial portion
will become the cartilage, bone, and ligaments of the vertebral
column. he formation of the vertebrae at the caudal end of the
embryo proceeds by a diferent process with a mass of cells of
notochord, mesenchyme, and neural tissue that divides into
somites.
One can readily see how failure of completion or error in
organization at any of these levels might lead to anomalies in
spinal formation. Complete nondisjunction of the cutaneous
ectoderm and the neuroectoderm prevents mesenchymal migration
and results in a nonskin-covered posterior dysraphism such
as myelomeningocele. A focal nondisjunction results in a focal
ectodermal-neuroectodermal tract or dermal sinus tract. In
focal unilateral premature disjunction, the neural tube separates
from cutaneous ectoderm before complete neural tube closure,
and this allows access of mesenchymal cells to the neural groove
and ependymal lining. he mesenchyme diferentiates into
fat and develops skin-covered abnormalities such as lipomyelomeningoceles.
Filum terminale lipomas likely occur from
abnormal development of the caudal cell mass. he caudal cell
mass forms in close proximity to the cloaca (origin of the lower
genitourinary and anorectal structures), which explains the high
incidence of lumbosacral hypogenesis and tethered spinal cords
in patients with anorectal and urogenital anomalies. Aberrations
in gastrulation, which can interfere with primary and/or secondary
neurulation, explain diastematomyelia, neurenteric istula,
caudal agenesis, and segmental spinal dysgenesis. 10
SONOGRAPHIC TECHNIQUE AND
NORMAL ANATOMY
In general, infants are scanned in the prone position, although
it is possible to scan in a decubitus position while the infant is
fed with a bottle or even breast-fed. A much better scan will be
obtained if the caregiver is allowed to feed a struggling infant
and return to the prone position in the postprandial state. A
paciier dipped in glucose can also be used to calm the patient
during imaging. If possible, the lumbar lordosis is accentuated
by elevating the shoulders, to aid determination of vertebral
body level by deining the lumbosacral junction. 11 For an accurate
deinition of the level of the cord termination, an anteroposterior
(AP) and lateral spine radiograph using a metal marker at the
level of the end of the spinal cord by ultrasound can aid in
documentation of the level, because the estimated level on
ultrasound may be in error in the presence of spinal anomalies
such as six lumbar vertebrae. In addition, elevation of the upper
trunk and head can further distend the dependent caudal portion
of the thecal sac. An excellent alternative approach is to place a
rolled blanket under the lower abdomen in a prone position, to
widen the acoustic window by separating the spinous processes. 12
he posterior elements of the spine ossify ater birth (from
caudally to cranially). here is reduced penetration of sound in
the lumbar spine at 3 to 4 months, and in most patients the
quality signiicantly decreases ater 6 months. In the older infant
and child, parasagittal scanning can better visualize the canal
contents relative to midline sagittal scanning. 13
High-frequency linear transducers allow visualization of ine
details of anatomy, with the additional aid of saved clips and
extended–ield-of-view images, which are useful for counting
vertebral segments (Fig. 49.3). A split screen can document the
landmarks used for counting vertebral bodies and determining
the level of the conus medullaris. Movement of the spinal cord
and cauda equina with crying, respiration, and the cardiac cycle
is seen in real time and documented with saved clips (Video
49.1). he normal movement can be seen in newborns but might
not be noticeably brisk until approximately 2 postnatal months. 12
Although diferent indications for imaging may require varying
views, it is best to scan the entire back in both the longitudinal
and the transverse planes, with right and let labels on transverse
images (which can be confusing when scanning in the prone
position). his allows a thorough search for contiguity of vertebral
body rings, an assessment of contour and position of the spinal
cord, and a survey of the paraspinous musculature and overlying
skin. A standof pad or a good amount of gel can be used to
evaluate a subcutaneous mass or other structures in the near
ield.
If the acoustic window is small, as occurs during the evaluation
of the craniocervical junction, a smaller-footprint curvilinear,
sector, or small linear transducer allows scanning at the base of
the skull and through the foramen magnum. his view allows
visualization of the cisterna magna, brainstem, inferior cerebellum,
and proximal cervical cord 14-16 (Fig. 49.4).
he ine anatomic display possible with sonography has been
shown in correlative studies between ultrasound and specimen
anatomy. 17 Fig. 49.5 demonstrates the basic landmarks that should
be visible. he cartilaginous spinous processes are hypoechoic
(Fig. 49.6), and the epidural space, which contains a variable
amount of fat, is visible anterior to the posterior dura mater. he
dura is seen on longitudinal images as an echogenic line parallel
to the posterior anechoic subarachnoid space that is anterior to
the spinal cord. he cord is surrounded anteriorly by anterior
subarachnoid space and echogenic vertebral bodies. he cord is
hypoechoic, whereas the interfaces created by the fanning nerve
rootlets are echogenic.
he central echo complex in the otherwise hypoechoic cord
has been shown with ultrasound and histoanatomic correlation
to be produced by the interface between the myelinated ventral
white commissure and central end of the anterior median
issure. 18,19 Images obtained with high-frequency transducers
sometimes reveal a column of luid within the center of the
central echo complex within the conus medullaris. he ventriculus
terminalis is a persistent fetal terminal ventricle and appears as
smooth dilation of the central echo complex within the conus
medullaris. his normal inding is seen oten in the neonate and
regresses or disappears soon ater birth (Fig. 49.7). 12,20
he normal ilum terminale is visible and mobile with
cerebrospinal luid (CSF) pulsations. he center of the ilum
tends to be relatively hypoechoic compared with its bright outer
margins. he ilum terminale extends from the tip of the conus
medullaris, crosses the subarachnoid space, and inserts on the
irst coccygeal vertebral body. he ilum is surrounded by the