Understanding the developmental features of the pediatric spine and spinal cord, including embryologic steps and subsequent growth of the osteocartilaginous spine and contents is necessary for interpretation of the pathologic events that may affect the pediatric spine. MR imaging plays a crucial role in the diagnostic evaluation of patients suspected of harboring spinal abnormalities, whereas computed tomography and ultrasonography play a more limited, complementary role. This article discusses the embryologic and developmental anatomy features of the spine and spinal cord, together with some technical points and pitfalls, and the most common indications for pediatric spinal MR imaging.
Key points
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Understanding embryologic steps and developmental features is crucial for a correct analysis of MR images in pediatric patients.
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Technical requirements (ie, choice of study protocols, sequences and so forth), must be tailored to address the clinical problem in the rapidly changing environment of the growing pediatric spine.
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Clinical presentations in children with severe, function-threatening disorders can be subtle and long-standing, leading to delays in the diagnosis.
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A working knowledge of the main indications and limitations for spinal imaging in children, including pitfalls or normal variants, is fundamental.
Introduction
Imaging of the spine and spinal cord is commonly required in the pediatric age group to address a wide array of medical conditions, sometimes presenting in the emergency room. MR imaging has made the diagnosis of these disorders easier, faster, and more accurate, thereby enhancing the possibility of an early and case-tailored treatment, mainly thanks to its multiplanar imaging and tissue characterization capabilities and lack of radiation exposure. Although the MR imaging picture in patients with spinal disorders may appear complicated and puzzling even to experienced observers, a rational approach focusing on a correlation of clinical, embryologic, and neuroradiological data greatly facilitates the diagnosis in most cases. In this article, the principal indications for spinal MR imaging in the pediatric age group are discussed, along with a description of the embryologic steps that lead to the formation of the spine, the main technical issues pertaining to pediatric spinal MR imaging, and a few pitfalls or variants that may simulate disorder.
Introduction
Imaging of the spine and spinal cord is commonly required in the pediatric age group to address a wide array of medical conditions, sometimes presenting in the emergency room. MR imaging has made the diagnosis of these disorders easier, faster, and more accurate, thereby enhancing the possibility of an early and case-tailored treatment, mainly thanks to its multiplanar imaging and tissue characterization capabilities and lack of radiation exposure. Although the MR imaging picture in patients with spinal disorders may appear complicated and puzzling even to experienced observers, a rational approach focusing on a correlation of clinical, embryologic, and neuroradiological data greatly facilitates the diagnosis in most cases. In this article, the principal indications for spinal MR imaging in the pediatric age group are discussed, along with a description of the embryologic steps that lead to the formation of the spine, the main technical issues pertaining to pediatric spinal MR imaging, and a few pitfalls or variants that may simulate disorder.
Embryology
The development of the spine and spinal cord is a highly coordinated phenomenon that begins very early during gestation. It consists of several consecutive steps, which are briefly described here.
During gastrulation, the bilaminar embryonic disc, formed by epiblast (future ectoderm) and hypoblast (future endoderm) is converted into a trilaminar disc because of formation of an intervening third layer, the mesoderm. This process begins by day 14 or 15 when the primitive streak, a stripe of thickened epiblast composed of totipotential cells, appears along the midline of the inferior portion of the dorsal surface of the embryo. The primitive streak has a knoblike cranial termination called the Hensen node. Epiblastic cells start migrating toward the primitive streak and pass inward at the primitive pit, a central depression of the Hensen node, to ingress the interface between the epiblast and the hypoblast; the first cells to ingress displace the hypoblast and form the endoderm, whereas subsequent waves of epiblastic cells migrate laterally above the endoderm to form the mesoderm. Those cells migrating along the midline of the ectoderm-endoderm interface form the notochord. The notochord is the foundation of the axial skeleton and extends throughout the length of the future vertebral column. From the mesoderm surrounding the neural tube and notochord, the skull, vertebral column, and the membranes of the brain and spinal cord are developed. The notochord is required for the ectoderm to become neural ectoderm and form the neural tube.
Establishment of the neural plate marks the onset of primary neurulation. This process occurs about day 18, when the neural plate starts bending, forming paired neural folds. In the following days, these progressively increase in size and approach each other to eventually fuse in the midline to form the neural tube. According to the traditional zipper model, closure of the neural tube occurs first at the level of the fourth somite (future craniocervical junction) and then proceeds both cephalad and caudad. The cranial extremity of the neural tube (rostral or anterior neuropore) closes at day 30, whereas the caudal extremity (caudal or posterior neuropore) closes at day 31. Closure of the posterior neuropore marks the termination of primary neurulation.
The posterior neuropore, that is the caudal extremity of the primary neural tube, corresponds to the 32nd somite (ie, the future third sacral metamere). The segment of the spine and spinal cord caudad to somite 32 is formed by secondary neurulation. This embryologic step begins immediately after completion of primary neurulation and proceeds until approximately gestational day 48. During secondary neurulation, the tail bud, a mass of cells deriving from the caudal portion of the primitive streak, lays down an additional part of the neural tube caudad to the posterior neuropore. This cord segment differs from the one formed by primary neurulation in several ways. Although the primary neural tube results from an upfolding of the lateral borders of the neural plate that join at the midline, the secondary neural tube is formed by an infolding of the neural plate, creating an initially solid medullary cord that subsequently becomes cavitated. The fate of the secondary neural tube is to undergo an incompletely understood process of regression, degeneration, and further differentiation, called retrogressive differentiation. This process results in the formation of the tip of the conus medullaris, which contains the lower sacral and coccygeal cord metameres, and the filum terminale, a fibroconnectival structure practically devoid of neural elements. The conus medullaris contains a focal expansion of the ependymal canal called the terminal ventricle, representing the remnant of the lumen of the secondary neural tube.
The development of the vertebral column proceeds simultaneously with that of the neural tube. At first, the paraxial trunk mesoderm is unsegmented. As development proceeds, epithelial spheres, called somites, are formed and undergo maturation in a cephalocaudal gradient. This maturation leads to dissociation of the epithelial somite, forming the dermatome (dorsal), the myotome (intermediate), and the sclerotome (ventral). The dermatome is located underneath the surface ectoderm. It gives rise to dermal cells for the dorsal moiety of the body. The myotome gives rise to all striated muscle fibers of the body. The sclerotome differentiates into cartilaginous cells of the vertebrae, cells of the intervertebral discs and ligaments, and cells of the spinal meninges. Furthermore, the somite gives rise to endothelial cells. The sclerotome is first located ventrally, and then it spreads to enwrap the entire neural tube forming at its dorsal face the so-called dorsal mesoderm, which will insinuate itself between the neural tube and the surface ectoderm after disjunction. On a next step of differentiation, the sclerotomes divide in half horizontally; the bottom half of one fuses with the top half of another to form the vertebrae. Notochordal remnants between the vertebrae become the nucleus pulposus within the intervertebral disc.
Imaging protocols
Imaging of the spine and spinal cord in the pediatric age group is best accomplished with MR imaging in almost all cases, whereas other modalities play a complementary role in selected indications. Sonography can be used as a valid imaging modalities in newborns and small infants but is limited by the degree of ossification of the neural arches of the vertebral columns other than by individual operator expertise. Computed tomography (CT) offers a detailed depiction of the structure of bone, but its use must be weighed against radioprotection issues; in principle, CT should be reserved for the elucidation of specific features and should always be tailored to the minimum possible field of view so as to minimize unnecessary radiation exposure.
A significant issue in pediatric MR imaging in general is the capability of small patients to cooperate long and well enough to obtain quality imaging studies. In general, children may be sufficiently cooperative at age 5 years, although specific conditions, such as acute illness or psychomotor delay, may change this. Younger or severely ill children typically require sedation, which is administered differently according to individual center protocols. Imaging during spontaneous sleep with a feed-and-wrap technique is a viable option in the neonatal period; in our experience, unsedated children weighing 5 kg or less can be studied with this technique, with a yield of greater than 80% technical success. Availability of dedicated rooms for the preparation of patients and subsequent awakening greatly improves the chances of success for imaging small infants without sedation.
Recent technical advancements regarding MR imaging equipment have had a great impact on the possibilities of spinal imaging in children as well as in older age groups. The use of multichannel phased-array coils and applications combining multiple images into a single full field of view have greatly improved the visualization of the entire spine in the sagittal plane, making it possible to acquire whole-spine imaging, including the craniocervical junction and sacrococcyx, in a reasonable amount of time. Scanners of 1.5 T remain the most widely available for clinical MR imaging of children. However, 3-T units have been increasingly used in several centers. Advantages of 3-T MR imaging compared with lower-field scanners include better image quality thanks to a higher spatial and contrast resolution, and improved clinical efficiency thanks to a higher temporal resolution. However, the scanners are more expensive, and various artifacts caused by field inhomogeneity, susceptibility, vascular pulsation, and chemical shift are exaggerated. However, technical adjustments may significantly counteract these setbacks.
Spinal MR imaging should include high-resolution sagittal T1-weighted and T2-weighted images covering the whole spine; in case of indications to the study of a specific segment of the spine, it is also useful to include a whole-spine sagittal view to obtain a panoramic appraisal, to rule out coexisting abnormalities and to correctly number the vertebral levels. Short-tau inversion recovery (STIR) is also extremely useful to detect subtle signal intensity abnormalities of the spinal cord as well as of the osteocartilaginous spine; a coronal acquisition offers the advantage of also scrutinizing the paravertebral regions. Axial sequences on either T1-weighted or T2-weighted imaging are used to study specific regions based on the clinical indications or findings on sagittal images; axial T2-weighted images across lesion areas help to determine the cross-sectional extent of the spinal cord involvement, which is an important element in the differential diagnosis. Optimal slice thickness for these sequences should be 3 mm. High-resolution heavily T2 weighted images, obtainable with different technical modalities (eg, constructive interference in the steady state or driven equilibrium [DRIVE]), provide an exquisite depiction of cord/root/cerebrospinal fluid (CSF) interfaces and are particularly useful to look for subtle structural abnormalities, such as those found in the context of spinal dysraphisms. Whenever indicated, postcontrast images should be acquired in the 3 planes of space; fat-suppression techniques are useful in the study of the spinal compartment, especially regarding the characterization of vertebral lesions. Advanced MR imaging modalities, such as diffusion-tensor imaging, are not yet fully incorporated into clinical practice for spinal studies in children because of significant technical issues and challenges, and are not described in this article.
Normal findings and pitfalls
Correct interpretation of spinal MR imaging studies in the pediatric age group requires a firm knowledge of several peculiar features related to the normal growth of the pediatric spine, as well as of a few pitfalls that can be mistaken for disorder.
The normal position of the conus medullaris ( Fig. 1 ) is a common element of discussion, especially regarding the possibility of a cord tethering abnormality (discussed later). This position is influenced by the phenomenon of relative ascent of the conus medullaris, which is caused by disparity in the growth of the vertebral column relative to the spinal cord such that, as gestation progresses, the conus medullaris occupies a progressively higher position with respect to the vertebral levels. The period of maximum ascent occurs between 9 and 16 weeks’ gestation. There have been various studies in the literature assessing the normal position of the conus both in children and adults; among these, Kesler and colleagues showed in a population of children aged 0.4 to 17 years with brain tumors, studied for the exclusion of leptomeningeal spread, that the tip of the conus was on average at the lower third of L1 and the mode of the distribution was at the L1-2 disc space, with no conus ending below the midbody of L2. Using ultrasonography in neonates, Hill and Gibson showed that the mean position of the conus was midway between the L1-2 disc and mid-L2 body, ranging from T12-L1 to L3, with the modal position being L1-2. Thus, it can be supposed with a good degree of certainty that ascent of the cord after birth is minor. The current position of the International Society of Pediatric Neurosurgery regarding this matter is that the conus medullaris occupies its adult level, most commonly opposite or cranial to the L1-2 disc space, by birth or at most within 2 months after birth, whereas any conus medullaris lying caudal to the midbody of L2 is to be considered abnormally low and therefore potentially tethered.
Using MR imaging, the assessment of the vertebral level at which any finding occurs (including the position of the conus medullaris) must be done by means of a sagittal sequence that includes the whole spine, from the craniocervical junction to the coccyx. Such images are easily obtainable on state-of-the-art scanners using modern technology, at no significant additional scanning time expense. Some features, especially the presence of transitional vertebrae (more frequently at the lumbosacral junction) or other vertebral deformities (such as unsegmented bars, butterfly vertebra, or hemivertebrae), may impair correct numbering. Inclusion of a coronal scan to count the posterior costal arches may add confidence, although costal malformations may also occur in combination with vertebral anomalies.
In newborns, the normal appearance of the vertebrae and intervertebral discs is markedly different than that of older children and adults (see Fig. 1 ). On T1-weighted images, the central vertebral body has a biconvex shape and appears hypointense because of the prevalent composition in hematopoietic bone marrow; in contrast, the vertebral endplates are still cartilaginous, and appear hyperintense; thus, there is a hyperintense structure, composed of the 2 opposed vertebral endplates and the interposed intervertebral disc, which lies between 2 adjacent vertebral centra. This appearance may be puzzling to unexperienced observers. On T2-weighted images, the disc stands out as markedly hyperintense as opposed to the hypointense appearance of the vertebral body.
The normal sagittal curvature of the spine is also significantly dependent on axial load and, as such, on age ( Fig. 2 ). In newborns, the cervical lordosis, thoracic kyphosis, and lumbosacral lordosis are barely visible. The cervical lordotic curve begins to appear as the newborn starts to sustain the head (first 3 months of life); the thoracic kyphosis and lumbar lordosis become progressively more prominent as the child learns to sit, crawl, and eventually stand and walk. In parallel to these events, the thickness of intervertebral discs is uniform in newborns and young children, and the posterior annulus gently and uniformly bulges posteriorly, simulating the appearance of a protrusion. Unlike on sagittal planes, the spine on coronal planes should be straight, and any positioning error should be corrected before scanning proceeds. In the presence of axial load (such as on posteroanterior radiographs), any deviation greater than 10° in a frontal plane is consistent with a definition of scoliosis, whereas smaller deviations are sometimes termed spinal asymmetries. However, evaluation of the degree of scoliosis is more difficult in the absence of axial load (such as on spinal MR imaging), and caution is advised.
Several other physiologic findings may be prone to misinterpretation. Among common features, injection of perimedullary veins on postcontrast scans can be misinterpreted as a pathologic leptomeningeal enhancement on sagittal scans; recognition of the normal position of the veins on axial planes usually clears the way ( Fig. 3 ). Pulsation artifacts generated by CSF in the perimedullary subarachnoid spaces may be prominent, especially in small infants because of their higher cardiac pulsation rate, and may sometimes be mistaken for mass lesions ( Fig. 4 ). Normal features may also include visibility of the ependymal canal or terminal ventricle on high-resolution sagittal scans, which should not be mistaken for true hydrosyringomyelia ( Fig. 5 ).
Malformations
Congenital malformations of the spine and spinal cord (spinal dysraphisms) are usually diagnosed prenatally, at birth, or in early infancy; however, some may be discovered in older children or adults. MR imaging has made the diagnosis of these disorders easier, faster, and more accurate both in the fetal period as well as postnatally, thereby enhancing the possibility of an early and case-tailored treatment, mainly thanks to its multiplanar imaging and tissue characterization capabilities. Classification of spinal dysraphisms requires a balanced correlation of clinical, neuroradiological, and embryologic information. Use of classification schemes may prove helpful in making a diagnosis in everyday clinical practice.
Spinal dysraphisms are categorized into open spinal dysraphisms (OSDs) and closed spinal dysraphisms (CSDs). The OSDs are characterized by exposure of nervous tissue to the environment through a congenital defect in the child’s back; CSDs are covered by skin, although cutaneous birthmarks, such as angiomas, dimples, overgrowing hair, dyschromia, and dystrophy, are often present. Myelomeningocele accounts for most cases of OSD. Clinically, myelomeningoceles are characterized by external exposure and supraelevation of an abnormal, unneurulated segment of the spinal cord (the placode), whereas in the rare myelocele the placode is not extruded ( Fig. 6 ). Associated features of OSDs include the Chiari II malformation (in which there is hindbrain herniation through the foramen magnum), hydrocephalus, and complications of myelomeningocele repair.
The CSDs are more heterogeneous than OSDs. Some are not clinically evident at birth, and patients are brought to medical attention later in infancy when neurologic complications ensue. Clinical examination is significantly helpful to restrict the differential diagnosis. A critical factor is the presence of a subcutaneous mass on the patient’s back. In almost all cases, such mass involves the lumbar or lumbosacral level and is composed of a subcutaneous lipoma. When the lipomatous tissue extends into the spinal canal through a posterior bony spina bifida and attaches to the cord within the spinal canal, a lipomyelocele is diagnosed; conversely, expansion of the subarachnoid spaces with extrusion of the cord-lipoma junction in a posterior meningocele defines a lipomyelomeningocele ( Fig. 7 ). Other entities presenting with a subcutaneous mass in the lumbosacral region are meningoceles and terminal myelocystoceles. These entities are extremely rare, especially the latter. While meningoceles are herniations of a CSF-filled meningeal outpouching, terminal myelocystoceles are characterized by herniation of a hydromyelic cavity that involves the terminal portion of the cord into a meningocele. The CSDs with a tumefaction involving the cervical or thoracic spine are exceedingly rare and are represented by the so-called spectrum of nonterminal myelocystoceles.
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