CSF, well im-aged as low T1-weighted and high T2-weighted signal, often can be used to determine the type of pulse se-Figure 1 A,Sagittal T1-weighted MRI scan of a normal lum-bar spine i
Trang 1of the Pediatric Spine
A Jay Khanna, MD, Bruce A Wasserman, MD,
and Paul D Sponseller, MD
Abstract
Magnetic resonance is an excellent
modality for imaging pathologic
processes in the pediatric spine It
allows high-resolution views of not
only osseous structures (including
the vertebral body, spinal canal, and
posterior elements) but also
soft-tis-sue structures (including the spinal
cord, intervertebral disk, and nerve
roots) Magnetic resonance imaging
(MRI) can show these structures in
various planes using different pulse
sequences that allow optimal
char-acterization of the tissues in and
around the pediatric spine
Indica-tions for MRI in children (<18 years)
are gradually expanding as
technol-ogy improves Properly interpreting
MRI scans in these age groups
de-pends on understanding the MRI
appearance of the normal pediatric
spine anatomy at various stages of
development For entities such as
spinal dysraphism, left thoracic
curves, and juvenile scoliosis,
spe-cific recommendations can help
cli-nicians use MRI effectively
MRI Techniques
The major factors that influence the MRI appearance of various tissues are the density of protons in the tissue, the chemical environment of the pro-tons, and the magnetic field strength
of the scanner Unlike computed to-mography (CT), which produces im-ages based on the density of various tissues, MRI produces images based
on free water content and on other magnetic properties of water, yield-ing superior soft-tissue contrast
Various sequences are produced by manipulating the strength of the ra-diofrequency (RF) pulses, the inter-val between the pulses, the repetition time (TR), and the echo time (TE), that
is, the time between applying the RF pulse and measuring the signal emit-ted by the patient By manipulating these variables, the images can be weighted to emphasize the T1, T2, gradient-recalled echo, or proton den-sity characteristics of a tissue T1-weighted images allow evaluation of
anatomic detail, including that of os-seous structures, disk, and soft tissues T2-weighted images are used primar-ily to evaluate the spinal cord and to enhance lesion conspicuity Agradient-recalled echo sequence typically is used when thin axial images are needed, such as for evaluating foraminal nar-rowing in the cervical spine, because its three-dimensional acquisition al-lows for very thin sections
Standard pulse sequences for spi-nal imaging include spin echo T1-weighted images and fast spin echo (FSE) T2-weighted images The FSE technique allows acquisition of scans without prolonged imaging times Be-cause cerebrospinal fluid (CSF) is bright
on T2-weighted images and the spi-nal cord retains its intermediate sig-nal, the images maximize the contrast between CSF and neural tissue, allow-ing optimal delineation of the spinal cord and nerve roots T2-weighted im-ages are very sensitive to pathologic changes in tissue, including any
pro-Dr Khanna is Chief Resident, Department of Or-thopaedic Surgery, The Johns Hopkins Hospital, Baltimore, MD Dr Wasserman is Assistant Pro-fessor, Department of Radiology, The Johns Hop-kins Hospital Dr Sponseller is Professor and Vice Chairman, Department of Orthopaedic Surgery, The Johns Hopkins Hospital.
Reprint requests: Dr Sponseller, c/o Elaine P Henze, Room A672, 4940 Eastern Avenue, Bal-timore, MD 21224-2780.
Copyright 2003 by the American Academy of Orthopaedic Surgeons.
Magnetic resonance is an excellent modality for imaging the pediatric spine Its
suc-cessful use requires understanding both the basic physics and the sedation protocols
necessary for acquiring high-resolution images Interpreting the images accurately
depends on appreciating the differences between the normal anatomy of the
pedi-atric and the adult spine Evaluating the images requires familiarity with the
dif-ferential diagnosis of pediatric spine disease, including the most common processes
(infections, neoplasms, and trauma) as well as spinal dysraphism Despite the
ac-knowledged usefulness of magnetic resonance imaging of the pediatric spine,
con-troversies remain related to its safety in this age group and its limitations in
di-agnosing and evaluating scoliosis and tethered cord syndrome.
J Am Acad Orthop Surg 2003;11:248-259
Trang 2cesses in which cells and the
extra-cellular matrix have an increase in
wa-ter content This pathologic change is
usually shown as an increase in
sig-nal intensity on T2-weighted images
The signal from fat may be
sup-pressed by a variety of techniques,
in-cluding chemical saturation of its
sig-nal or application of an inversion
pulse, and imaging at a short time of
inversion (TI) when there is no fat
sig-nal present (short TI recovery [STIR])
Chemical suppression typically is
used in sequences that result in high
fat signal, such as FSE T2-weighted
images or postcontrast T1-weighted
images Fat suppression is of little
val-ue for noncontrast T1-weighted
im-ages because the signal from most
pathologic lesions, whether
inflam-matory, neoplastic, or infectious, is
of-ten low and better visualized because
of contrast against the adjacent bright
fat signal Fat suppression on
post-contrast T1-weighted images of the
vertebral body is useful in adults who
have fatty transformation of marrow
Fat-suppressed images may be
par-ticularly useful for evaluating
liga-mentous injuries or lesions involving
the paraspinal tissues The usefulness
of STIR imaging is more limited
be-cause the imaging parameters are
re-stricted and cannot be optimized to
maximize contrast between adjacent
tissues of interest
Gradient-recalled echo images
ap-pear to be T2-weighted because CSF
is relatively bright; however,
paren-chymal lesions typically are more
con-spicuous on FSE T2-weighted images
The gradient-recalled echo sequence
is sensitive to local inhomogeneities
of the magnetic field, and signal loss
is exaggerated in the presence of such
inhomogeneities Field
inhomogene-ities may be caused by metallic
im-plants (eg, pedicle screws or
paraspi-nal rods), differences in the magnetic
susceptibilities of adjacent tissues (eg,
air-tissue interfaces), and paramagnetic
substances (eg, gadolinium)
Blood-breakdown products cause local field
distortions resulting in signal loss,
mak-ing this technique very sensitive for the detection of blood
Open MRI systems are being used more frequently, especially for chil-dren These systems have notably lower field strengths than do closed systems and therefore usually pro-duce studies of inferior overall qual-ity, especially of the spine However, open MRI systems allow easier access
to the sedated or otherwise compro-mised patient Young patients and pa-tients with claustrophobia have ac-cess to parents and the environment, making the procedure less intimidat-ing However, whenever possible, spinal MRI should be done using closed, 1.5-T systems
Pediatric Sedation Protocols
Sedation is often required for success-ful MRI in young children Many studies have evaluated specific seda-tion protocols.1,2The American Acad-emy of Pediatrics (AAP) has pub-lished guidelines for the elective sedation of pediatric patients,3,4but compliance with these guidelines is not mandatory The AAP has stated that careful medical screening and pa-tient selection by knowledgeable medical personnel are needed to ex-clude patients at high risk of life-threatening hypoxia.4Also, monitor-ing usmonitor-ing AAP guidelines is necessary for the early detection and manage-ment of life-threatening hypoxia.3The AAP recommends that before an ex-amination in which sedation is to be used, children from newborn to age
3 years take nothing by mouth for 4 hours and those aged 3 to 6 years take nothing by mouth for 6 hours.4
Pediatric sedation practices vary, but a few agents are common to most protocols Oral chloral hydrate is of-ten recommended for children
young-er than 18 months Howevyoung-er, its use
is controversial because of its variable absorption, paradoxical effects, and nonstandardized dosing Older
chil-dren usually receive intravenous pen-tobarbital with or without fentanyl Although studies have reported successful administration of sedatives
by trained nurses,1,2an anesthesiol-ogist’s expertise can be beneficial for patients with substantial comorbidi-ties, including cardiopulmonary dis-ease, skeletal dysplasias, neuromus-cular disease, and abnormal airway anatomy Because of the potential risks
of anesthesia and sedation in children, there is a trend toward referring those who require sedation to hospitals with pediatric anesthesiologists
An important consideration after sedation for pediatric MRI is the need for strict adherence to established dis-charge criteria, including return to baseline vital signs, level of con-sciousness close to baseline, and abil-ity to maintain a patent airway.5 Be-cause of the inherent risks of sedation, alternative techniques have been de-vised, including sleep deprivation and rapid, segmental scanning The latter permits acquisition of high-quality images without the use of se-dation
Normal MRI Anatomy
Appreciating normal MRI anatomy (Fig 1) is essential for understanding and predicting the MRI appearance
of pathologic processes.6
Adolescents and Adults
The lumbar spine is more fre-quently imaged than the cervical and thoracic area in both children and adults In adolescents and adults, the lumbar spinal canal appears round proximally and triangular distally The lumbar facet joints, best visual-ized in the axial plane, are covered with 2 to 4 mm of hyaline cartilage This cartilage can be well visualized with FSE and gradient-recalled echo pulse sequences The epidural space and ligaments also should be evalu-ated carefully Epidural fat is seen as high signal intensity on T1-weighted
Trang 3images; the ligamentum flavum
shows minimally higher T1-weighted
signal compared with the other
lig-aments The conus medullaris is
usu-ally located at the L1-L2 level The
tra-versing nerve roots pass distally from
the conus medullaris and extend an-teriorly and laterally, exiting
lateral-ly underneath the pedicle and extend-ing into the neural foramen The intervertebral disk, consisting of the cartilaginous end plates, anulus
fibro-sus, and nucleus pulpofibro-sus, normally shows increased T2-weighted signal
in its central portion CSF, well im-aged as low T1-weighted and high T2-weighted signal, often can be used
to determine the type of pulse
se-Figure 1 A,Sagittal T1-weighted MRI scan of a normal lum-bar spine in a 2-year-old boy shows the rectangular shape of the vertebral bodies The conus medullaris is seen at the L1-L2
level (arrow) B, T2-weighted image shows the long, thin ap-pearance of the intervertebral disk C, Sagittal T1-weighted scan
of a normal lumbar spine in a 10-year-old girl D, T2-weighted
scan Lordosis is normal The posterior elements are well
formed, with a resultant decrease in the canal diameter E,
Sag-ittal T1-weighted scan of a normal lumbar spine in a 16-year-old girl shows dark CSF (thin arrow), the conus medullaris at the L1-L2 level (open arrow), and the basivertebral channel (arrowhead) Note the normal rectangular appearance of the vertebral bodies and the lumbar lordosis compared with the
10-year-old girl F, Sagittal T2-weighted scan shows bright CSF
(thin arrow) and a bright nucleus pulposus (arrowhead).
Trang 4quence that is being used CSF
pul-sations often create artifacts that
de-grade the image in the lumbar spine;
these artifacts must not be mistaken
for a pathologic process
The cervical spine shows a mild
lordosis on sagittal images On axial
images, the spinal canal is triangular,
with the base located anteriorly A
dark band at the base of the dens is
a normal variant that is a remnant of
the subdental synchondrosis and
should not be mistaken for a fracture
In adults, the facet joints are small and
triangular, whereas in children they
are large and flat The spinal cord is
elliptical in cross section in the
cer-vical spine There is a difference in
sig-nal between the normal gray and
white matter of the spinal cord This
signal heterogeneity should not be
mistaken for intramedullary
pathol-ogy The intervertebral disks are
sim-ilar in appearance to, but smaller
than, those seen at the thoracic and
lumbar levels An important
anatom-ic feature of the cervanatom-ical spine is the
prominent epidural venous plexus,
which is not present in the thoracic
or lumbar spine
The thoracic vertebral bodies are
relatively constant in size, and the
spi-nal caspi-nal is almost round Abundant
epidural fat is present posteriorly, but
there is less anteriorly than in the
lum-bosacral region The cord is more
round than in the cervical or lumbar
regions, and the cord segment lies
two to three levels above the
corre-sponding vertebral body The
inter-vertebral disks are thinner than the
disks in the lumbar spine The
ap-pearance of the CSF is more variable
in the thoracic spine than in the
lum-bar region because of more prominent
CSF pulsations, but on T1-weighted
images, it is commonly seen as a
re-gion of low signal dorsal to the
spi-nal cord This artifact is often most
se-vere at the apex of curves, including
the thoracic kyphosis Certain
tech-niques can minimize this artifact,
in-cluding gating to the pulse or
cardi-ac cycle
Children
Differences Between the Pediatric and the Adult Spine
The MRI appearance of the grow-ing spine is complex Substantial changes occur in the vertebral ossi-fication centers and the intervertebral disks, changing the overall appear-ance of the spine markedly,
especial-ly between infancy and age 2 years.7
In general, the vertebral ossification centers are incompletely ossified
ear-ly in childhood, and the disks are thicker and have a higher water con-tent than those in adults The spinal canal and neural foramina are larger, and there is less curvature In addi-tion, the overall signal intensity of the vertebral bodies is lower than that of the adult spine on T1-weighted im-ages because of the abundance of red (hematopoietic) marrow relative to yellow (fat) marrow in the pediatric, adolescent, and young adult spine
Full-Term Infant
In the newborn, the overall size of the vertebral body is small relative to the spinal canal, and the spinal cord ends at approximately the L2 level
The lumbar spine does not exhibit the usual lordosis and is straight The ver-tebral bodies show a markedly low signal intensity on T1-weighted im-ages, with a thin, central, hyperin-tense band that likely represents the basivertebral plexus The spongy bone of the ossification center is el-lipsoid rather than rectangular and often mistaken for disk The interver-tebral disk is relatively narrow and often contains a thin, bright central band on T2-weighted images that represents the notochordal rem-nants.6,7
Age 3 Months
At age 3 months, the osseous com-ponent of the vertebral body has in-creased and the amount of hyaline cartilage has decreased, giving the vertebral bodies a rectangular appear-ance The ossification centers begin to
gain in signal intensity, starting at the end plates and progressing centrally The neural foramina have not sub-stantially changed at this age, remain-ing relatively large and ovoid.6,7
Age 2 Years
At age 2 years, the spine has be-gun to show its normal sagittal align-ment, most likely because of weight bearing (Fig 1, A and B) The ossified portion of the vertebral body
increas-es substantially and begins to assume its adult appearance, with near-complete ossification of the pedicles and the articular processes The disk space and nucleus pulposus become longer and thinner The cartilaginous end plate has decreased in size and
is often difficult to identify The neu-ral foramen also begins to take its adult appearance as its inferior por-tion narrows.7
Age 10 Years
At age 10 years, sagittal alignment resembles that of an adult (Fig 1, C and D) Ossification of the vertebral bodies and posterior elements is
near-ly complete, with a resultant decrease
in the spinal canal diameter The ver-tebral bodies also develop concave superior and inferior contours The nucleus pulposus becomes smaller at this age and spans approximately half the disk space in the sagittal plane The neural foramina continue to nar-row inferiorly.6
The Conus Medullaris
In early fetal life, the spinal cord extends to the inferior aspect of the bony spinal column.6Because the ver-tebral bodies grow more rapidly lon-gitudinally than the spinal cord does,
by birth the conus medullaris is re-positioned in the upper lumbar spine
It is important to note the location of the conus medullaris on every pedi-atric spine MRI study (Fig 1, A and E) Aconus medullaris level below the L2-3 interspace in children older than
5 years is abnormal and indicates pos-sible tethering.8,9Saifuddin et al10
Trang 5re-viewed the MRI findings in 504
nor-mal adult spines and found that the
average position of the conus
med-ullaris was the lower third of L1
(range, middle third of T12 to upper
third of L3)
Pathologic Processes in the
Pediatric Spine
Infection
Infectious processes involving the
pediatric spine include osteomyelitis,
diskitis, and epidural and paraspinal
abscess.11-13In general, the MRI
sig-nal characteristics of infection include
a region of low T1 and high T2
sig-nal intensity in bone and soft tissue
In identifying vertebral
osteomy-elitis, MRI is more sensitive than
con-ventional radiographs or CT and
more specific than nuclear
scintigra-phy.14,15Marrow edema can be
detect-ed on precontrast, fat-suppressdetect-ed,
FSE T2-weighted images
Postgado-linium enhancement of the disk and
adjacent vertebral bodies on
postcon-trast, fat-suppressed, T1-weighted
images helps confirm the diagnosis
The specificity of MRI for infection is
higher in children than adults because
one of the primary confounders,
de-generative arthritis, is not part of the
differential diagnosis Differentiating
osteomyelitis from neoplastic disease
is a common dilemma; generally,
in-fectious processes are more likely to
cross and destroy intervertebral disks
than are neoplastic conditions
Diskitis is seen as a disruption of
the normally well-defined
disk-vertebral borders on T1-weighted
im-ages and as an increase in signal of
the disk on T2-weighted images.12On
T2-weighted images, diskitis may
obliterate the normally seen
horizon-tal cleft within the intervertebral disk
The abnormal signal seen in
infec-tious diskitis is associated classically
with surrounding soft-tissue
inflam-mation and reactive end-plate
chang-es Primary diskitis is more likely to
develop in children than adults
be-cause of the greater blood supply to the disk Secondary diskitis after dis-kography or surgery is more likely to develop in adults
Epidural abscesses are rare, but when they do develop, it is usually after sur-gery or vertebral osteomyelitis Epi-dural abscesses are diagnosed based
on the MRI findings of a collection in the epidural space and the appropri-ate clinical setting.11 Gadolinium-enhanced T1-weighted images often show a peripheral rim of enhancement that represents the abscess wall
Paraspinal abscesses occur adja-cent to the spinal column, most com-monly in the paraspinal musculature
They may be secondary to a primary infection in the spine or may arise spontaneously in the paraspinal mus-culature These abscesses may be seen
as retropharyngeal abscesses in the cervical spine, paraspinous or retro-mediastinal abscesses in the thoracic spine, or psoas abscesses in the lum-bar spine The MRI characteristics of paraspinal abscesses include a well-defined wall and peripheral
T1-weighted images
Trauma
MRI can be used to evaluate the pediatric spinal trauma victim who has an abnormal neurologic exami-nation or is unresponsive The patient
is first evaluated with conventional radiographs, which may be normal, even in a child with a neural deficit
Although CT allows for better eval-uation of osseous detail and displaced fractures, MRI provides improved evaluation of nondisplaced fractures because of its ability to detect marrow-signal abnormalities
Spinal cord injury without radio-graphic abnormality (SCIWORA) is
a well-defined entity seen in the pe-diatric age group.16,17The character-istic hypermobility and ligamentous laxity of the pediatric bony cervical and thoracic spine predispose children
to this type of injury.16The elasticity
of the bony pediatric spine and the
relatively large size of the head allow the musculoskeletal structures to de-form beyond physiologic limits, which results in cord trauma followed by spontaneous reduction of the spine.16
As with other types of spinal cord injuries, the most important predic-tor of outcome is the severity of neu-rologic injury A patient with a com-plete neurologic deficit after SCIWORA has a poor prognosis for recovery of neurologic function The role of MRI
in SCIWORA syndrome is to define the location and the degree of neural injury, rule out occult fractures and subluxation that may require surgi-cal intervention, and evaluate for the presence of ligamentous injury T2-weighted images typically show in-creased signal in the cord, vertebral body, or ligaments The increased T2 signal in the cord is compatible with edema and can range from a partial, reversible contusion to complete transection of the cord
Two other traumatic entities can oc-cur in children, usually as the result
of participation in sports The first is acute disk herniation This is often a fracture with a hingelike displacement
of fibrocartilage and slipping of the entire disk with vertebral end-plate fracture rather than extrusion of a disk fragment from the nucleus, as is seen
in adults.18Such avulsion fractures are often occult on conventional radio-graphs and are better detected with
CT and MRI.18Axial MRI scans dem-onstrate the fracture fragment as an area of low signal intensity protrud-ing into the spinal canal, and sagittal images demonstrate a low signal in-tensity region in the shape of a Y or
7 on all pulse sequences.18
The second entity is a spondylo-lysis as a cause of back pain in young athletes MRI, however, is not the op-timal method for evaluating spondy-lolysis CT offers increased spatial res-olution and the ability to accurately define the osseous defect, whereas ra-dionuclide imaging can demonstrate increased radiotracer activity in the region of the defect
Trang 6MRI is the modality of choice for
evaluating neoplasms in and around
the pediatric spine.19An effective and
commonly used approach is to
clas-sify the lesion as extradural,
intra-dural-extramedullary (Fig 2), or
in-tradural-intramedullary (Fig 3) With
this anatomic classification system,
the primary role of the MRI
exami-nation is to define the location of the
suspected neoplasm, which is best
achieved with axial and sagittal
T1-and T2-weighted images Once the
lesion has been classified, the
T2-weighted images can be used to
char-acterize the lesion further
Specifical-ly, the degree of surrounding edema
and tissue infiltration and the
pres-ence or abspres-ence of a cystic component
can be determined Next,
postgado-linium enhancement images should
be compared with unenhanced
T1-weighted images The final step in
ob-taining a diagnosis is to correlate the
imaging findings with the patient’s
age and other criteria to narrow the
differential diagnosis
Spinal Dysraphism
Spinal dysraphism is a general term
used to describe a wide range of
anomalies resulting from incomplete
fusion of the midline mesenchyma,
bone, and neural elements The
os-seous abnormalities consist of defects
within the neural arch with partial or complete absence of the spinous pro-cesses, laminae, or other components
of the posterior elements MRI has been shown to be the best modality for evaluating spinal dysraphism.20,21
A classification system has been proposed for evaluating a patient with a suspected spinal dysraphism (Table 1).21The differential diagnosis can be narrowed to one of three types:
spinal dysraphism with a back mass either covered or not covered with skin, or with no back mass The final
diagnosis then can be made based on the lesion’s MRI characteristics Myelomeningocele is the most common form of spinal dysraphism (Fig 4) It usually presents in the lum-bosacral region (although it can be seen at higher levels) as a back mass not covered with skin The mass may
or may not be covered by lepto-meninges containing a variable amount of neural tissue The sac her-niates through a defect in the poste-rior elements of the spine The spinal cord usually contains a dorsal cleft,
Figure 2 A schwannoma in an 8-year-old boy A, Sagittal T1-weighted MRI scan shows an
intradural-extramedullary mass impressing on the anterior cervical cord at the C5 level
(ar-row) B, Axial T2-weighted image shows the lesion herniating through the right C5-C6
neu-ral foramen (arrows).
Figure 3 An astrocytoma in a 6-year-old boy A, Sagittal T1-weighted MRI scan shows an intradural-intramedullary lesion within the spi-nal cord at the T3-T5 levels (arrow) B, Sagittal T2-weighted image shows the partially cystic nature of the lesion C, Axial T2-weighted
image confirms that the lesion (arrow) is within the center of the spinal cord.
Trang 7is splayed open, and is often tethered
within the sac.21Progressive
scolio-sis is seen in 66% of patients with
my-elomeningocele, Arnold-Chiari type
II malformation in 90% to 99%,
di-astematomyelia in 30% to 40%, and
80%.22Scarring can occur at the
sur-gical site after sac closure, so it is
im-portant to monitor these patients for
signs and symptoms of tethered cord
syndrome
Of the entities presenting with a
skin-covered back mass in the
pres-ence of spinal dysraphism,
lipomen-ingocele is the most common.6,21The
lipomeningocele consists of
lipoma-tous tissue that is continuous with the
subcutaneous tissue of the back and
also insinuates through the
dysraph-ic defect and dura and into the
spi-nal caspi-nal The spispi-nal cord often
con-tains a dorsal defect at the level of the
lipomatous tissue and may be
teth-ered at this level The essential MRI
feature of this lesion is that the
li-pomatous tissue follows the signal
characteristics of subcutaneous fat on
all pulse sequences, including
fat-suppressed pulse sequences
Occult spinal dysraphism
pre-sents without a back mass
Diastema-tomyelia is characterized by a
sagit-tal splitting into two segments of the
spinal cord, conus medullaris, or
filum terminale, often in the thoracic
or lumbar spine The dural tube and arachnoid are undivided in approx-imately half these patients; clinical findings are rare, and surgery is not indicated In the remaining patients, the dural tube and arachnoid are completely or partially split at the level of the spinal cord cleft, which results in tethering of the cord and subsequent clinical symptoms Coro-nal T1- and T2-weighted images best define the sagittal split in the
cord; the findings should be con-firmed on axial images
Another entity often seen in pa-tients with spinal dysraphism is sy-ringohydromyelia, or a syrinx (Fig 5)
A syrinx is a longitudinal cavity
with-in the spwith-inal cord that may or may not communicate with the central ca-nal Attempts to explain the etiology include developmental, traumatic, in-flammatory, ischemic, and pressure-related causes Sagittal MRI scans show a linear, low T1 and high T2
sig-Table 1 Classification of Spinal Dysraphism
Back mass not covered with skin Myelomeningocele
Myelocele Back mass covered with skin Lipomyelomeningocele
Myelocystocele Simple posterior meningocele
No back mass (occult) Diastematomyelia
Dorsal dermal sinus Intradural lipoma Tight filum terminale Anterior sacral meningocele Lateral thoracic meningocele Hydromyelia
Split notochord syndrome Caudal regression syndrome
(Adapted with permission from Byrd SE, Darling CF, McLone DG, Tomita T: MR
im-aging of the pediatric spine Magn Reson Imim-aging Clin North Am 1996;4:797-833.)
Figure 4 A myelomeningocele in a 6-year-old girl A, Sagittal T1-weighted MRI scan shows a low-back mass contiguous with the contents
of the spinal canal (arrows) B, T2-weighted image shows that the mass is filled with high-signal-intensity fluid, compatible with CSF (ar-rows) C, Axial T1-weighted image confirms that the mass communicates with the spinal canal through a defect in the posterior elements
(arrows).
Trang 8nal intensity within the parenchyma
of the spinal cord
Gibbs artifact, or truncation
arti-fact, can mimic a syrinx on sagittal
images (Fig 6) Gibbs artifact is seen
on sagittal T1- and T2-weighted
im-ages as a linear region of altered
sig-nal intensity in the center of the
spi-nal cord Thus, it is important to
evaluate serial axial T1- and T2-weighted images to confirm findings
Gibbs artifact results from not using
a sufficiently high spatial frequency for sampling data It can be corrected
by using a higher-resolution matrix
Chiari Malformations
Chiari malformations are seen
fre-quently in patients with spinal dys-raphism Chiari type I malformations consist of cerebellar tonsillar ectopia,
in which the cerebellar tonsils extend below the level of the foramen mag-num The common measurement for the degree of herniation of the ton-sils below the foramen magnum is 5
mm Mikulis et al23reported a
vari-Figure 5 A large syrinx involving the entire spine in a 2-year-old boy A, Sagittal T1-weighted MRI scan shows the syrinx to be largest
at the level of the lower thoracic spine (arrows) Axial T1-weighted (B) and T2-weighted (C) images confirm that the syrinx is located within
the center of the spinal cord.
Figure 6 A 5-year-old girl had a history of
neck and arm pain A, Sagittal T2-weighted
MRI scan shows a long linear region of high signal intensity within the center of the cer-vical spinal cord (arrow) This finding can
easily be mistaken for a syrinx B, Sagittal
T1-weighted image also suggests low signal intensity in the same region but fails to show a syrinx, demonstrating normal cord
anatomy C, Axial T2-weighted image also
demonstrates normal anatomy These find-ings are compatible with a Gibbs artifact.
Trang 9ation by age in the upper limit of
nor-mal: 6 mm in the first decade of life,
5 mm in the second and third
de-cades, and 3 mm by the ninth decade
In Chiari I malformations, the
brain-stem is spared and the fourth
ventri-cle remains in its normal location
Chiari I malformations are
associat-ed with syringohydromyelia,
cranio-vertebral junction anomalies, and
basilar invagination Chiari II
malfor-mations are more advanced and
con-sist of downward displacement of the
brainstem and inferior cerebellum into
the cervical spinal canal, with a
de-crease in size of the posterior fossa
Tethered Cord Syndrome
Tethered cord syndrome is seen in
a substantial number of patients with
spinal dysraphism, especially those
who have undergone surgical closure
of the defect.24,25During fetal life, the
spinal cord extends to the
sacrococ-cygeal level Because of the rapid growth
of the vertebral column after birth, the
cord ascends to the L1-L2 level in the
newborn During the formation of a
spinal dysraphic defect such as
my-elomeningocele, the open neural
el-ements often attach to the peripheral
ectoderm, resulting in spinal cord
teth-ering After surgical closure of the sac,
there is a tendency for the spinal cord
to become adherent at the repair site
As the child grows, this adherence may
tether the cord and prevent cephalad
cord migration, with eventual
symp-toms Thus, in patients with spinal
dys-raphic and related conditions,
includ-ing myelomeninclud-ingoceles, myeloceles,
lipomeningoceles, and
diastematomy-elia, tethered cord should be ruled out
as the potential cause of any
deteri-oration in neurologic function
MRI has been proposed as the
ini-tial, and possibly only, imaging study
for a patient with a suspected
teth-ered spinal cord.9 Sagittal images
should be evaluated to determine the
level of the conus medullaris (Fig 7)
A conus level below the L2-L3
inter-space in children older than 5 years
is abnormal and an indication of
pos-sible tethering.8,9In addition, the teth-ered cord is often displaced posteri-orly in the spinal canal Other findings include lipoma or scar tissue within the epidural space and increased thick-ness of the filum terminale.9Although MRI can determine whether a spinal cord is anatomically tethered, these findings should be correlated with the patient’s symptoms and serial phys-ical examinations before surgphys-ical re-lease is considered
Controversies in MRI of the Pediatric Spine
MRI of the pediatric spine remains controversial in several conditions, in-cluding scoliosis and tethered cord
syndrome, as well as with spinal in-strumentation Safety is also a concern
Scoliosis
The use of MRI imaging in scoli-osis is primarily to detect intraspinal abnormalities, which are more fre-quently associated with uncommon curve patterns such as left thoracic curves, an abnormal neurologic ex-amination, or young age at pre-sentation.26-30Recently, Do et al26 con-cluded that MRI is not indicated before spine arthrodesis in a patient with an adolescent idiopathic scoli-osis curve pattern and a normal phys-ical and neurologic examination One area of particular controversy
is back pain in the presence of scolio-sis In a retrospective study of 2,442
Figure 7 A 14-year-old boy had a history of lipomeningocele After surgical resection, bowel
and bladder dysfunction and new lower-extremity paresthesias developed A, Sagittal
T2-weighted image shows the conus medullaris extending to approximately the L4 level and the filum terminale extending to the S1 level (arrow), compatible with tethered cord
syn-drome B, Axial T2-weighted image at the L4 level shows the cord located posteriorly within the thecal sac (arrow) C, Axial T2-weighted image at the L5 level shows the placode (thin
arrow) with a right-side nerve root (thick arrow) coursing anteriorly and laterally.
Trang 10patients, Ramirez et al31found that a
left thoracic curve or abnormal result
on neurologic examination best
pre-dicted an underlying pathologic
con-dition They found a significant
asso-ciation between back pain and age older
than 15 years (P < 0.001), skeletal
ma-turity (P < 0.001), postmenarcheal
sta-tus (P < 0.001), and history of injury
(P < 0.018) The authors concluded that
it is unnecessary to perform extensive
diagnostic studies on every patient with
scoliosis and back pain MRI should
be reserved for patients with
infan-tile or juvenile scoliosis, left thoracic
curves, or abnormal neurologic
find-ings Because coronal views are
espe-cially useful in evaluating patients with
scoliosis, they should be a part of the
routine imaging protocol
Tethered Cord Syndrome
The rate of MRI in tethered cord
syndrome remains controversial
When MRI demonstrates a tethered
cord, a choice between surgical and nonsurgical treatment must be made
Although anatomic tethering of the cord is detected easily on MRI, indi-cations for surgery depend on the clinical history and results of serial physical examinations
Imaging in the Presence of Implants
MRI of the spine in the presence
of instrumentation is generally safe but is limited by the image artifacts the implants produce The pulse se-quence used for imaging titanium produces less degradation from arti-fact because it is less ferromagnetic than stainless steel (Fig 8).32,33Thus, titanium may be the better choice of implant in a patient who may require follow-up with MRI However, with appropriate imaging techniques, clin-ically useful information can be ob-tained safely in the presence of both types of implants.34Specialized pulse
sequences such as the metal artifact reduction sequence (MARS) can help reduce the degree of tissue-obscuring artifact produced by spinal hardware and improve image quality compared with conventional T1-weighted spin-echo pulse sequences.35
MRI Safety
MRI may be contraindicated in pa-tients with ferromagnetic implants, materials, or devices because of the risk of implant dislodgement, heat-ing, and induction of current.36 Shel-lock et al36reviewed and compiled the results of more than 80 studies and described the ferromagnetic qualities
of 338 objects, including 30 ortho-paedic implants, materials, and
devic-es They found that most orthopaedic implants are made from nonferro-magnetic materials and therefore are safe for MRI procedures Another concern is that of safety within the MRI suite Areas surrounding and
Figure 8 A 6-year-old boy had a history of high-grade astrocytoma.
A,Anteroposterior radiograph 6 weeks after resection, multilevel laminectomy, and posterior spinal arthrodesis from T4 to L3 with titanium pedicle screws,
hooks, and rods B, Midline sagittal
postgadolinium T1-weighted MRI scan allows visualization of the canal con-tents with minimal artifact from the
pedicle screws (arrows) C,
Parasag-ittal postgadolinium T1-weighted im-age shows a rod (thick arrow) and pedicle screw (thin arrow) Neither
obscures the MRI scan D, Axial
post-gadolinium T1-weighted image also shows the pedicle screws (arrows) and
a patent spinal canal.