Applied Radiology

Dr. Gerscovich is Professor of Radiology and Associate Director of Abdominal Imaging and Ultrasound, and Dr. McGahan is Professor of Radiology and Director of Abdominal Imaging and Ultrasound at University of California Davis Medical Center, Sacramento, CA. Dr. McGahan is also a member of the editorial board of this journal.

The use of ultrasound for examination of the spinal canal in infants and children is not new. It was first described by several authors in publications in 1983. ^1-3 With the advent of widespread use of magnetic resonance imaging (MRI), sonography of the spine of the young child has often been forgotten or may be used as a secondary modality. Magnetic resonance imaging has become the gold standard for examination of the spine in the adult and pediatric population for a number of clinical situations. ^4,5 However, ultrasound has been recommended as a valuable diagnostic tool in the evaluation of the lumbar spine in infants and is "recommended as the primary imaging modality in those patients." ⁶

Ultrasound examination of the lumbar spine in infants or young children has several advantages. First, recent technical innovations in sonographic imaging, especially the improved nearfield resolution, allow exquisite anatomical detail of the spinal canal. Furthermore, ultrasound uses no radiation, requires no infant sedation when examining the spine, is performed rather quickly, and is relatively inexpensive compared with MRI. In this article, we will present an overview of the use of ultrasound of the spinal canal in infants and young children.

Clinical indications

Ultrasound is most often utilized in the newborn to check for tethering of the spinal cord and associated abnormalities. Infants with various midline back abnormalities, including hemangiomas, lipomas, a hairy patch, or a dimple may be screened with ultrasonography. ^7,8 Other abnormalities such as bony anomalies of the lumbosacral spine or neurogenic bladder may be associated with a tethered cord. Ultrasound may also be useful in examining cases of spinal dysraphism including myelomeningocele and may be used in the follow-up of these cases to check for cord tethering after repair. ⁹

Technique

The technique for examination of the spinal cord in neonates is quite simple. A high-resolution linear array (7.5 MHz or greater) probe is used. Patients are brought into a quiet room and are often fed prior to the examination. A small towel may be placed under the abdomen of a young child to help separate the posterior neural elements for better sonographic visualization of the spinal canal (figure 1). In newborns, the spinal canal may be examined in the midline longitudinal plane or the transverse plane. In older children, either slight angulation or scanning slightly lateral is needed for scanning in the longitudinal plane, to avoid shadowing from the ossification centers of the posterior spinous processes.

Normal anatomy

The cord itself is hypoechoic. It exhibits a central echogenic line, which extends for the length of the cord and is visualized easily on both longitudinal and transverse imaging (figure 2). Originally, this central echo complex was thought to represent the central canal of the cord. However, work by Nelson et al ¹⁰ showed that this central echogenicity is produced by interfaces between the myelinated ventral white commissures and the central end of the anterior median fissure. The normal cord smoothly tapers distally at the conus medullaris and is surrounded at this point by the fairly echogenic nerve roots of the cauda equina. One of the important elements in examination of the newborn spinal canal is determination of the caudal extent of the tip of the spinal cord. Surprisingly, studies of the fetus in the third trimester and young children have fairly similar results. In general, in the neonate or young adult, the conus level should basically be at L2 or higher. Rarely, it may extend to the L2-3 or L3 level. ^11-13 This level should remain fairly constant throughout childhood. Thus, determination of the conus level is of the utmost importance. Extension of the spinal cord below the L2-3 region is usually associated with tethering of the same.

The location of the distal cord can easily be determined with ultrasound by recognizing that there is usually a normal lordotic transition of the lumbar vertebral bodies to the sacral vertebral bodies. This lumbosacral transition demarcates the L5-S1 interface. It may be recognized on longitudinal scans by two intersecting lines. These lines should be drawn from a line tangent to the lumbar vertebral bodies that intersect the line drawn tangent to the sacral vertebral bodies. ¹⁴ In the newborn, usually S5 is the lowest ossified vertebral body that can be recognized. Thus, counting upwards from S5 will be another method to cross-check the exact vertebral level (figure 3). Further confirmation of the exact level of the conus medullaris can be obtained in transverse imaging. In transverse imaging, the top of the iliac crest is usually identified when scanning through the L5 vertebral body. When scanning laterally to the L2 vertebral body the tip of the 12th rib usually can be identified. ¹¹ If there is any doubt of the location the conus medullaris by ultrasound, a metallic marker can be placed over the conus and this can be verified by a plain radiograph. This is required only infrequently.

Other anatomical detail may be visualized with ultrasound, including the dentate ligaments in the thoracic cord, extending laterally from the cord (figure 4), and the echogenic interfaces of the dorsal and ventral cord. ¹⁵ Color Doppler ultrasound may also demonstrate the anterior spinal artery and the epidural veins surrounding the cord (figure 5). Usually, the spinal cord and cauda equina oscillate synchronously with cardiac motion. This oscillation is an important observation, as it may be absent with tethering of the spinal cord (figure 6). In the newborn, the whole spinal canal and cord can be visualized up to the foramen magnum (figure 7).

Pathology

Tethered cord--As previously described, tethering of the cord is a general term to describe low ending of the spinal cord usually due to a variety of etiologies. The ultrasound finding of a tethered cord includes the distal cord lying below the L3 level (figure 8), the cord often being eccentrically displaced dorsally, and cord oscillations being decreased or absent. ¹⁶ Spinal tethering may be associated with a number of abnormalities such as dermal sinus, lipoma, diastematomyelia, thick filum terminale, and bony abnormalities. Ultrasound may be helpful to identify these abnormalities. For instance, a dermal sinus can be observed sonographically as a hypoechoic band extending from the spinal canal to the cutaneous tissues of the back. Lipomas are recognized as well-demarcated echogenic masses in the spinal canal or subcutaneous soft tissues (figure 9). These masses may be associated with a lipomyelomeningocele with the subarachnoid space and nerve rootlets bulging into the subcutaneous tissues of the back, under an intact skin. ¹⁷

Diastematomyelia--Diastematomyelia is a split of the spinal cord in two halves (figure 10). Usually there is a bony spicule or fibrous band in the area of the spina bifida. This defect may or may not be covered by intact skin. The two portions of the cord may be best demonstrated sonographically in transverse section. ¹⁸ There may be associated spina bifida in these patients. There also may be concurrent hydromyelia. ¹⁹ The ultrasound appearance of the split spinal cord is pathognomonic in these cases.

Open neural tube defects--Prenatally, sonography is important for recognizing the findings of the Chiari Type II malformation. The most commonly sonographically recognizable intracranial abnormalities are the "banana" sign or the "lemon-shaped" head. ²⁰ In detection of open neural tube defects, Budorick et al ²¹ have emphasized that the cranial abnormalities associated with the Chiari Type II malformation are much more obvious than the spinal abnormalities. However, postnatally ultrasound is excellent for examining open neural tube defects occurring in the lumbosacral spine.

Spina bifida is the general term to describe open forms of spinal dysraphism that result from failure of the posterior neural pores to close. These defects can be from fairly simple to complex. Meningoceles are cystic lesions that result from protrusion of the meninges through the spinal defect (figure 11). Myelomeningoceles also contain abnormal central nervous system tissue in the defect. These lesions are usually large and covered by a membrane of meningeal origin and malformed spinal cord substance. ¹⁵ Ultrasound may be very useful in the newborn to identify the level of the distal cord as well as the size of the defect and the neural elements that are contained within the defect.

Postoperative repair--Ultrasound may be helpful to identify abnormalities that can occur after surgical repair of myelomeningoceles. The cord may retether in these abnormalities due to adhesions. Sonographically, this may be observed as the cord being tightly adhered to the posterior wall of the distal spinal canal (figure 12) with decreased oscillations. Other persistent abnormalities can be observed in patients after surgical repair, including epidermoid and dermoid tumors resulting from epithelial tissue that was not removed at the initial operation, or increasing hydromyelia. A thick filum terminale or diastematomyelia can be seen after cord-release surgery. Ultrasound may be helpful to recognize these abnormalities. ^16,22

Summary

Ultrasound has a number of uses for evaluation of the spinal cord in the infant and young child. Ultrasound of the spinal cord is fairly quick and easy to perform and may be useful in detecting many abnormalities. It has a number of advantages over other imaging techniques as it requires no infant sedation, is performed quickly, and provides exquisite detail. AR

Acknowledgment

Special thanks to Deborah Hoang for her assistance in the preparation of the manuscript.