Spine Imaging

Plain Films

Conventional radiography (CR) is the initial modality of choice for evaluation of the osseous structures of the spine after trauma, scoliosis, atlantoaxial (AA) instability, and formation/segmental anomalies. Frontal and lateral projections of the spine are typically obtained. However, depending on the indication, further views or follow-up imaging may be required. For instance, scoliosis often requires serial CR examinations to monitor evolution of the curvature. The scoliosis curvature may be influenced by patient posture—that is, sitting, standing, lying supine or bending. CR is often the initial modality for the evaluation of back pain in children and may be used to screen for congenital, traumatic, neoplastic and infectious causes of pain. Back pain with scoliosis is considered a red flag for the presence of underlying spinal pathology in children. Hence back pain with scoliosis should be investigated with a thorough clinical history, physical examination, laboratory tests, and neuroimaging including magnetic resonance imaging (MRI). Increasingly, concern over radiation exposure in children has resulted in the substitution of MRI for CR and computed tomography (CT) for the evaluation of scoliosis, formation/ segmentation anomalies and craniocervical junction (CCJ) instability. CR, however, remains an important imaging tool of the spine but must be used according to the “as low as reasonably achievable” (ALARA) principle.

Ultrasound

Ultrasound is a portable, bed-side, readily available, and radiation-free technique that is extremely helpful in the evaluation of neonates and young infants. The most common indication in the neonate is closed spinal dysraphism, which includes lesions such as dorsal dermal sinus tracts. Images are obtained in the longitudinal and transverse planes using a linear 5- to 12-MHz transducer with the patient lying prone. Dynamic real-time images can be acquired to assess movement of the lumbosacral nerve roots if there is a question of spinal cord tethering. Counting down from the 12th rib and counting up from the sacral elements will determine the vertebral level. If the vertebral level is unclear, CR may be helpful for correlation with markers. Color or power Doppler ultrasound may be used as an adjunct to characterize soft tissue masses on the skin or within the spinal canal.

Magnetic Resonance Imaging

MRI represents the modality of choice for imaging of the spine and the spinal canal contents. The strengths of conventional MRI include its unique contrast resolution, multiplanar capability, and high-resolution isotropic three-dimensional (3D) acquisitions. Diffusion tensor imaging is an advanced MRI technique that allows for studying the white matter tracts in vivo. Diffusion tensor imaging may be used when there is a suspicion of spinal cord ischemia or infarction. In addition, functional imaging, for example, of cerebrospinal fluid (CSF) flow can be measured.

The conventional MRI protocol includes sagittal T1- and T2-weighted images plus axial T2-weighted images of the entire spine (as one or more series) from the posterior fossa through the coccyx. The sequences can be performed with and without fat saturation. Fat saturation is helpful to confirm bone marrow edema or for assessment of intraspinal fatty lesions (eg, lipoma). In addition, axial T1-weighted images may be added to evaluate for filar thickening, extent of a dermal sinus, and a tethering mass (eg, lipoma) or subacute hemorrhage. Coronal T2-weighted images assist in evaluating for spinal column anomalies (eg, hemivertebra in congenital scoliosis), split-cord malformations (eg, diastematomyelia), and renal anomalies. Sagittal short tau inversion recovery (STIR) images are helpful in the evaluation of ligamentous injury after trauma and the presence of marrow edema in infectious processes. T1-weighted acquisitions after intravenous gadolinium administration, both with and without fat saturation, in sagittal and axial planes, enhance the ability of MRI to evaluate inflammation and tumor, whether intramedullary, intradural, or extradural.

MRI is the imaging modality of choice for evaluating the bone marrow, intervertebral disks, and extradural soft tissues. Bone marrow signal intensity in children is age dependent due to its varying content of hematopoietic and fatty marrow. T1 signal intensity of hematopoietic marrow is equal to or slightly higher than muscle but less than fat. Diffuse marrow involvement in systemic disease (leukemia) with marrow replacement or localized bone marrow edema (traumatic/inflammatory) is best evaluated with T1-weighted images without fat suppression and T2-weighted images with fat suppression. For evaluation of extradural traumatic, inflammatory, or neoplastic processes (eg, ligamentous injury, disk herniation, osteomyelitis, sarcoma), sagittal T1-weighted images are performed, followed by sagittal fat-suppression T2-weighted or STIR images. Additional gadolinium-enhanced sagittal and axial fat-suppression T1-weighted images are necessary for evaluating neoplastic and inflammatory processes, as well as for assessing recurrent disk protrusion (nonenhancing) from scar (enhancing) after disk surgery. These techniques allow for the complete assessment of marrow, bony, and paraspinal soft tissue involvement and extent, as well as for spinal cord, cauda equina, or nerve root compression. Gadolinium enhancement is also necessary in the assessment of intramedullary tumors (eg, astrocytoma, ependymoma, and ganglioglioma) and intradural tumors (eg, neurofibroma and schwannoma), and for neoplastic leptomeningeal seeding (eg, medulloblastoma and germ cell tumors).

Sagittal and axial T2-weighted MRI may also be done in flexion and extension (with patient cooperation or physician supervision) to assess craniocervical instability or fixation (eg, craniocervical anomalies). Sagittal T2-weighted or STIR imaging, along with axial T2-weighted imaging and postcontrast T1-weighted imaging, is helpful to evaluate for intramedullary inflammation or degeneration, including the assessment of spinal cord atrophy. A combination of sagittal and coronal T1- and T2-weighted imaging plus axial T2-weighted imaging is performed for screening of patients with suspected spinal vascular malformation.

Additional pulse sequences may be helpful in certain circumstances. For example, hemorrhage is best demonstrated using gradient recalled echo (GRE) techniques (eg, MEDIC—a fast T2-weighted two-dimensional spoiled GRE multiecho sequence), which can be positive as early as 3 hours after trauma. High-resolution sequences (eg, a heavily T2-weighted high-resolution isotropic 3D-constructive interference in steady state [CISS] or 3D-T1 magnetization-prepared rapid gradient-echo (MPRAGE), a fast GRE sequence with fat saturation, with multiplanar reconstruction (MPR) may be helpful in the evaluation of sinus tracts, CSF leaks, or pseudomeningocele. Functional assessment of CSF movement can be achieved by using “velocity-encoded imaging” which may be useful in planning posterior fossa decompression in Chiari I malformation.

Computed Tomography

Computed tomography (CT) imaging of the spine is typically performed without the administration of intravenous contrast. Axial images are obtained followed by coronal and sagittal MPR in bone and soft tissue kernels. CT is replacing CR and becoming the standard for the emergency evaluation of spine trauma. However, due to the relatively greater radiation exposure with CT compared to CR, CT should be restricted for use to areas in question on CR or to assess for additional injury when there is evidence of osseous trauma or ligamentous injury on CR. CT continues to be the choice for assessment of localized bony abnormalities, particularly when the level is precisely defined clinically or by plain film, single-photon emission computed tomography (SPECT), or MRI (eg, diastematomyelia, spondylolysis, spinal stenosis, spondylitis, bone tumor—ie, osteoid osteoma or aneurysmal bone cyst). Axial sections with two-dimensional reformatting (coronal, sagittal, or oblique) and/or 3D reconstructions are often important in the preoperative evaluation of spine trauma, craniocervical anomalies, and congenital scoliosis, as well as for the postoperative assessment of instrumentation and fusion. CT may also be obtained (with patient cooperation or physician supervision) with the spine in flexion and extension or with right and left head turning, to evaluate for translational craniocervical instability or rotatory atlanto-axial (AA) instability or fixation, respectively. CT myelography is rarely needed except when MRI is contraindicated or spinal instrumentation does not allow adequate MRI quality. It may also assist in the more precise delineation of nerve root or other intradural lesions (eg, cysts). A water-soluble, low-osmolar, nonionic contrast material specifically approved for myelography is used because of its low toxicity and infrequent side effects. These techniques are best used in a limited and selective manner and using ALARA guidelines to minimize radiation exposure.

Radionuclide Imaging

Radionuclide imaging plays an important role in the evaluation of children with suspected infection. In vitro radiolabeling of leukocytes with ¹¹¹ In-oxyquinoline or ^99m Tc-HMPAO (hexamethylpropyleneamine oxime) may be helpful to localize bacterial infection. Bone scintigraphy with ^99m Tc-labeled diphosphonates ( ^99m Tc-methyl diphosphonate) and single-photon emission CT (SPECT) is frequently helpful for evaluating children with lesions of the spinal column (eg, osteoid osteoma and metastases). Fluorine 18-fluorodeoxyglucose positron emission tomography (PET) is used for infection and tumor imaging. PET (including PET-CT) is also used in the assessment of treatment response or tumor progression in childhood neoplasia.

Disorders of Primary Neurulation

From a clinical and embryologic perspective, disorders of primary neurulation can be divided into (1) open spinal dysraphism, (2) closed spinal dysraphism, and (3) dorsal dermal sinus. In an open spinal dysraphism, there is a defect in the overlying skin, and the neural tissue is exposed to the environment. In closed spinal dysraphism, the malformed neural tube is covered by mesodermal (subcutaneous fat) and ectodermal (skin) elements.

Open (Non–Skin–Covered) Spinal Dysraphism

Myelomeningocele and Myelocele.

Incomplete or defective closure of the neural tube results in a neural placode that fails to detach from the adjacent surface ectoderm, a process termed nondisjunction. Consequently, a flattened midline neural placode is either flush with the cutaneous surface (myelocele-MC) or is pushed above or dorsal to the adjacent skin (myelomeningocele-MMC). MMC accounts for more than 98% of open spinal dysraphism. The prognosis of MMC and MC is worse than that of a skin-covered spinal dysraphism. MMC and MC have a significant impact on the quality of life. All children with an open spinal dysraphism have an associated Chiari II hindbrain malformation [ Fig. 9.1 ]. It is hypothesized that the chronic leakage of CSF at the level of the neural placode during the intrauterine development of the fetal brain results in an incomplete or defective expansion of the rhombencephalic vesicle. This in turn prevents normal growth of the skull base causing a small posterior fossa with herniation of hindbrain elements into the upper cervical spinal canal. Varying degrees of central canal dilation or frank hydromyelia may be present in the spinal cord in patients with MMC or MC. These malformations can be diagnosed by ultrasound during fetal life and confirmed by fetal MRI.

Hemimyelomeningocele and Hemimyelocele.

Hemimyelomeningoceles and hemimyeloceles are rare and are associated with diastematomyelia (cord splitting), and occur when one hemicord fails to undergo primary neurulation.

Closed (Skin-Covered) Spinal Dysraphism

Lipomyelomeningocele and Lipomyelocele.

Lipomyelomeningoceles (LMMC) and lipomyeloceles (LMC) have a significantly better neurologic prognosis than open neural tube defects, presumably because the neural placode is covered and protected by skin and subcutaneous tissue. LMMC and LMC are typically not associated with Chiari II malformation. Premature disjunction of the neural tube from the adjacent surface ectoderm before closure of the neural tube results in mesenchymal elements having access to the neural groove and ependymal lining of the grove. It is hypothesized that the interaction of mesenchymal elements with the inner lining of the neural tube induces an excess production of fat. On imaging, in lipomyeloceles, the neural tissue is flush with the spinal canal; in lipomyelomeningoceles, the neural tissue is pushed outside of the spinal canal with expansion of the subarachnoid space [ Fig. 9.2 ]. Large lipomas typically cover the neural placode and may extend from the subcutaneous region into the widened spinal canal.

Dorsal Dermal Sinus

Dorsal dermal sinuses are a distinct group of malformations that are classified as an intermediate between open and closed spinal dysraphisms. They are epithelial-lined fistulae or tracts connecting the skin surface with neural tissue or meninges. They are believed to result from a focal incomplete disjunction of the neuroectoderm from the surface ectoderm. Physical examination reveals a midline dimple or pinpoint ostium and may also reveal an associated hairy nevus, hyperpigmented patch, or capillary hemangioma. CSF may intermittently leak out of the ostium. The sinus tract may represent a port of entry for infections and may result in meningitis or abscess formation. The fistulous tract is well demonstrated on thin-slice, high-resolution T1- or T2-weighted sagittal images as a T1 hypointense, T2 hypointense, or hyperintense linear streak extending from the cutaneous dimple to the spinal canal [ Fig. 9.3 ]. The malformation may result in a low-lying spinal cord (ie, below the L2-3 disk level) and spinal cord tethering.

Disorders of Secondary Neurulation

Secondary neurulation is responsible for formation of the lowest part of the spinal cord including the conus medullaris and filum terminale. Secondary neurulation disorders include: (1) fibrolipoma of the filum terminale, (2) tight filum terminale syndrome, (3) caudal regression syndrome (CRS), and (4) sacrococcygeal teratoma (SCT).

Fibrolipoma of the Filum Terminale

Fibrolipomatous thickening of the filum terminale is referred to as a filar lipoma. However, filar thickening may be exclusively fibrous. Filar thickening, with or without fat, is a frequent normal finding on autopsy studies (4%–5%) and is only relevant if spinal cord tethering occurs. Tethered-cord syndrome is a clinical syndrome characterized by progressive neurologic abnormalities in the setting of traction on a low-lying spinal cord and conus medullaris. On imaging, a filar lipoma appears as a hyperintense strip or dot of signal on sagittal and axial T1-weighted images, respectively, within a thickened filum terminale [ Figs. 9.4 and 9.5 ]. Axial T2-weighted imaging is generally required to demonstrate fibrous thickening of the filum.

Tight Filum Terminale

Tight filum terminale syndrome is characterized by hypertrophy and shortening of the filum terminale. This condition causes tethering of the spinal cord and impaired relative ascent of the conus medullaris during development. The conus medullaris is low lying relative to its normal position, above the L2-L3 disk level [ Fig. 9.6 ].

Caudal Regression Syndrome

CRS is a spectrum of anomalies that have in common partial absence of lower vertebral elements and caudal spinal cord. CRS is associated with anorectal, lower-limb, and genitourinary malformations. In addition, CRS may be seen as part of the OEIS association (omphalocele, exstrophy, imperforate anus, spinal defects). The characteristic imaging findings are lack of the most distal segments of the spinal cord and matching musculoskeletal elements. Typically the spinal cord abruptly terminates with a club- or wedge-shaped inferior border [ Fig. 9.7 ]. The sacrum and coccyx are typically absent. The malformation can be categorized into two types depending on position and configuration of the conus medullaris: high and abrupt (type I) or low and tethered (type II).

Sacrococcygeal Teratoma

SCTs are the most frequently encountered large, congenital tumors in neonates and arise from the pluripotent cells of the caudal cell mass. SCTs occur more often in girls than in boys (3 : 1 ratio). SCTs are classified as types I to IV depending on the location of the mass. In type I the lesion is exclusively outside the pelvis, whereas in type IV it is entirely inside the pelvis. Prenatal ultrasound and fetal MRI allow identification and characterization (cystic vs. solid) of SCT [ Fig. 9.8 ]. Postnatal MRI is essential in the preoperative diagnostic workup to delineate the extension of tumor into the pelvis and degree of displacement or infiltration of pelvic structures.

Anomalies of Notochordal Development

Anomalies of notochordal development represent a complex dysraphism. These malformations may partially overlap with disorders of secondary neurulation (eg, CRS). These include the following entities: (1) split notochord syndrome, also known as diastematomyelia; (2) neurenteric fistula and (3) segmental disorders of notochordal development resulting in the previously described spectrum of CRSs; and (4) segmentation and formation anomalies of the vertebral column.

Diastematomyelia

The splitting of the spinal cord into two hemicords is referred to as diastematomyelia. The hemicords may be separated by either a membranous septum or a bony spur. Ectodermal cells that are encoded to glide between the ectoderm and endoderm do not remain in the midline but form two more laterally positioned hemi-notochords, which in turn induce the formation of two separate neuroectodermal layers resulting in two hemicords. Each hemicord has one ventral and one dorsal nerve root and one central canal. In addition, the two hemicords may either be located either within a single shared dural sac or in two distinct dural sacs [ Fig. 9.9 ]. Diastematomyelia is associated with formation and segmentation anomalies of the vertebral bodies. It is a skin-covered defect with associated cutaneous stigmata in 50% to 70% of patients. Up to 90% of patients have symptoms related to spinal cord tethering. The diagnosis should be made using fetal ultrasound or MRI. Postnatal MRI evaluates the spinal cord anatomy in detail. CT is especially helpful in the identification of a bony spur, if present.