High-frequency linear transducers provide detailed evaluation of the spinal cord and the nerve roots. Linear or sector transducers with frequencies ranging from 8 to 15 MHz are used routinely.^1,7 High-resolution hockey stick probes may be used to evaluate dimples. Sector transducers may be useful in older patients, where the acoustic window is smaller owing to progressive ossification. Dual screen images can extend the field of view to image longer sections of the spine. Panoramic views, which are now readily available on many machines, provide an extended field of view to encompass the length of the spine.⁷ Panoramic imaging is especially helpful when determining the level of the conus (Fig. 22-1) and its relationship to the lumbosacral junction.

To evaluate the spine, the patient should be placed in a position such that the acoustic window is optimized. The infant patient may be placed prone over a towel or pillow. This will round out the back, creating a slight kyphosis.⁷ A decubitus position can also be helpful because the legs can be tucked up in front of the body, increasing the splaying of the spinous processes.⁷ A warm blanket, bottle, or pacifier can be helpful during this examination because it will help keep the patient still.

The examination should include longitudinal and axial images from the craniocervical junction to the coccyx. The rounded surface of a curvilinear transducer will optimize imaging of the craniocervical junction.⁷ For the remainder of the examination, a linear transducer is placed midline, directly over the spine. An exception is made for the older child, where a parasagittal approach may diminish shadowing from the ossified spinous process.⁷ The level of the conus medullaris, as well as the position of the spinal cord in the spinal canal, is documented. During the examination, cord and nerve root motion should be detected and noted. Any cutaneous lesion should be documented and evaluated for associated tracts or masses.

The spinal canal and cord can be observed from the base of the skull to the tip of the coccyx. Scanning from a posterior approach, in the sagittal plane, the canal will be anechoic. The echogenic vertebral bodies and hypoechoic intervertebral disks border the length of the spinal canal anteriorly.⁷ Posteriorly, the canal is bordered by the hypoechoic spinous processes and the narrow echogenic epidural space. An intense linear echo, representing the arachnoid-dural layer, is seen lining the canal both anteriorly and posteriorly. Between the echogenic dura, the hypoechoic spinal cord is situated centrally to slightly anterior and is surrounded by anechoic cerebrospinal fluid (CSF) of the subarachnoid space⁸ (Fig. 22-2A, B). The subarachnoid space or thecal sac extends into the sacral region.

In the sagittal plane, the cord is a hypoechoic, tubular structure bordered by two echogenic, nearly parallel lines anteriorly and posteriorly and by a linear echogenic central canal⁸ (Fig. 22-3A). Between 1 and 3 months of age, the cervical, thoracic, and lumbar segments of the spine are 5.3 ± 0.29, 4.4 ± 0.42, and 5.8 ± 0.66, respectively.⁸ The spine is larger in the cervical and lumbar regions because of the number of nerves in these areas. The spinal cord tapers to a point and terminates at the conus medullaris between L1 and L2. The conus gives way to the filum terminale, which is surrounded by the echogenic strands of the cauda equina⁸ (Fig. 22-3B). The filum terminale can be slightly more echogenic and thicker (≤2 mm) than the surrounding nerve roots.^8,9 The roots of the cauda equina form a collection of less echogenic linear strands that move freely with changes in patient positioning and with crying. The filum terminale and nerve roots extend into the distal portion of the thecal
sac. Inferior to the thecal sac, the echogenic sacral vertebral bodies are noted coursing posteriorly toward the skin surface (Fig. 22-3C). Caudal to the sacrum are two to three hypoechoic coccygeal segments. The coccyx is hypoechoic because of its cartilaginous nature (Fig. 22-3D).

In the axial plane, the spinal cord appears as an oval to round hypoechoic structure located within the spinal canal, with an echogenic circumferential border and echogenic dot centrally. The cord is surrounded by CSF, which is contained by the echogenic dura surrounding the canal (Fig. 22-4A). Posteriorly, the spinous process is noted centrally as a small hypoechoic circle. The echogenic vertebral arches are seen posterior and lateral on both sides of the canal. The cord diameter is larger in the cervical region, narrows through the thoracic segment, and then enlarges again near the conus. As the spinal cord tapers to the conus, the cord diameter diminishes to a small hypoechoic circle. The filum terminale appears as a slightly thicker, round, echogenic, centrally located nerve arising from the tip of the conus (Fig. 22-4B). The nerves of the cauda equina appear as smaller echogenic dots surrounding the conus and filum (Fig. 22-4C). Because of these surrounding nerves, it can be difficult to differentiate the filum from the cauda equina.⁸ As the thecal sac tapers distally, the sacral vertebral bodies appear as echogenic round structures in the far field, which gives way to the hypoechoic round cartilaginous bones of the coccyx just below the skin surface.

Ascertaining the exact level of the vertebral column allows determination of normal or abnormal levels of the position of the conus medullaris and exact localization of
intraspinal abnormalities. As stated previously, the spinal cord should terminate between L1 and L2. Determination, however, can be difficult.¹⁰ In the earlier reports of sonography use in the spine, methods used to determine the exact level in the spinal cord included palpation of the end of the lower ribs (said to indicate the level of L2). Another method involves determining the supracristal line by connecting the top of the palpated iliac crest (supposed to transect the L3 to L4 space). A more objective method of determining the level of the conus is to count vertebrae up from the sacrum or down from the thoracolumbar junction. Generally, there are five sacral vertebrae and five lumbar vertebrae. Starting at S5 (the last echogenic structure in the sacrum), count cephalad five vertebrae to the lumbosacral junction (LSJ) (S1/L5) and then five more vertebrae to the thoracolumbar junction (T12/L1) (Fig. 22-5). The count could be confirmed by locating T12 by following the last rib to the junction with the spine and counting the lumbar vertebrae caudally. Another way to confirm the level of the conus is to visually identify the LSJ and count upward.⁷ The LSJ is identified by the transition from the relatively straight line of the lumbar vertebral bodies to the gentle kyphosis of the sacrum.¹⁰ Because there can be variants in the bony anatomy, if the level of the conus is uncertain, an X-ray may be needed to confirm the level. Using sonographic guidance, a radiopaque marker can be placed on the skin, over the tip of the conus, and an X-ray of the spine obtained.¹¹ The X-ray will confirm any variant in the bony anatomy and the level of the conus.

Spinal dysraphism refers to an array of spinal abnormalities caused by inadequate or improper fusion of the neural tube (NT) early in fetal life.¹³ The incidence of spinal dysraphism is 0.5 to 0.8 per case per 1,000 births.¹³ Most commonly, the defect occurs in the lower spine, although any part of the spine may be affected. The spectrum of spinal dysraphism can be categorized into two major groups. An Open spinal dysraphism (OSD) is characterized by neural tissue exposed without skin covering. A closed spinal dysraphism (CSD) is a skin-covered spinal abnormality. Owing to the obvious defect, open dysraphisms, like myeloceles and MMCs, are easily classified.

The second group, closed dysraphisms, can present with a skin-covered subcutaneous mass or various cutaneous markers. CSDs presenting with a subcutaneous mass include lipomyelocele/lipomyelomeningocele and myelocystocele. Closed dysraphisms without a subcutaneous mass-like tethered cord, spinal lipoma, diastematomyelia, or a dorsal dermal sinus (DDS) can present with various cutaneous markers that can indicate an underlying abnormality. These cutaneous markers are typically located in the midline lumbosacral region and include hair tufts, sacral dimples or pits,
pigment changes, hemangiomas, and skin tags.¹⁴, ¹⁵, ¹⁶ and ¹⁷ Nearly all of the previously mentioned anomalies are associated with a tethered cord (Fig. 22-8A-C).