IMAGING OF THE SPINE: INTRODUCTION

It goes without saying that the spine is critical for normal human function. The spine provides height and mobility for turning and bending and protects the spinal cord and spinal nerves. Given the wide range of pathologic conditions that may affect the spine, radiologists are frequently called on for spine imaging. Recognition of normal anatomy and variants, differentiation from abnormal anatomy, and diagnosis of different pathologic conditions are the goals of spine imaging.

It is assumed in this chapter that the reader is already familiar with basic spine anatomy learned early in medical school. With such a foundation, this presentation of the imaging appearance of the spine will serve to solidify and perhaps even enhance this knowledge base.

The purpose of this chapter is to review the different techniques employed in spine imaging and to emphasize normal anatomy as depicted with these techniques. The imaging appearance of certain common lesions will also be presented. Relative advantages and disadvantages of the various imaging modalities will be reviewed within the context of an overall imaging strategy. It is not intended that the reader will be an accomplished spine radiologist after reading this chapter. Rather, it is hoped that the reader will gain basic familiarity with normal imaging anatomy and the imaging appearance of certain types of abnormalities, as well as a sense of which test might be the best to order for a given clinical circumstance.

TECHNIQUES

Prior to the advent of computed tomography (CT) in the 1970s, spine imaging consisted primarily of plain-film radiography and an adjunct test, myelography, to be discussed below. Spine imaging was revolutionized by CT and, subsequently, magnetic resonance (MR) imaging, which for the first time allowed direct acquisition of axial and sagittal (multiplanar) images. Not until the era of CT could the spinal cord itself be directly visualized. Refinements of these cross-sectional imaging techniques are ongoing, with exciting progress undoubtedly still to come. These imaging modalities have so changed the face of spine diagnosis and treatment that virtually no neurosurgeon today would undertake spine surgery without first obtaining a CT or MR imaging study. Unfortunately, however, the widespread availability of this technology, combined with unscrupulous entrepreneurs and an adversarial legal climate, has resulted in the performance of many unnecessary scans. Perhaps health care changes, malpractice reform, or both will help to reduce the number of unnecessary examinations.

This section reviews the major modalities currently employed to image the spine. The highly specialized technique of spinal arteriography, which is used principally to detect vascular malformations, is beyond the scope of this review. Nuclear medicine scanning also will not be discussed, because it is seldom used as a primary diagnostic study in the evaluation of spinal disease (though spinal metastases are frequently diagnosed with whole-body isotope bone scanning).

Plain Film

Plain films are conventional radiographs, which are commonly referred to as x-rays. They may be obtained in a frontal projection [anteroposterior (AP) or posteroanterior (PA)—the difference is insignificant in the spine], a lateral projection (side view), or an oblique projection (Figs. 13–1, 13–2, and 13–3). Plain films are most useful for the visualization of bony structures. Soft-tissue structures (everything but bone) are largely radiolucent and cannot be seen clearly on plain films unless abnormal density such as calcification is present. Although plain films depict bone anatomy quite well, certain structures may be obscured by other structures in front of or behind them. For instance, on a lateral projection, both pedicles would be superimposed on one another (Figs. 13–1B and 13–3B ). For this reason, multiple views are always obtained as part of a routine examination.

On conventional radiographs, bony structures appear white. This appearance is referred to as radiodense or simplydense. Normally mineralized bones have a recognizable radiodensity, which should always be assessed when viewing x-rays. Certain pathologic conditions (e.g., osteopenia and osteolytic metastases) can result in decreased bone density, and other conditions (e.g., osteoblastic metastases and some exotic diseases) may result in abnormally increased bone density.

After bone density is assessed, the next observation should be the alignment of the spine. A normal spine should show cervical and lumbar lordosis (anterior convexity) (Figs. 13–1 and 13–3) and thoracic kyphosis (posterior convexity). Abnormalities in alignment may result from incorrect positioning of the patient but often reflect an underlying problem. Such abnormalities may be minor such as straightening or reversal of normal cervical lordosis in the case of muscle spasm. More significant misalignments, such as scoliosis, may be either idiopathic or secondary to an underlying lesion. Major alterations in alignment, such as subluxation, may result from trauma. In assessing alignment it is important to determine whether the vertebral bodies, as well as the posterior elements (i.e., spinous processes, pedicles, and laminae), are appropriately aligned. Remember that the spinal cord rests within the spinal canal formed by the vertebral foramen of each vertebra and that the spinal cord is invisible on plain films. One must therefore evaluate where the spinal cord should be. The anterior margin of the spinal canal is the posterior aspect of the vertebral body. The posterior limit of the spinal canal can be approximated by locating on a lateral radiograph the junction of the spinous process and the laminae. Identification of the spinolaminar line also helps in the evaluation of alignment (Fig. 13–1B ).

Most anatomic features of the spine are readily identifiable on plain radiographs (Figs. 13–1, 13–2, and 13–3). Vertebral bodies, facet joints, disc spaces, pedicles, laminae, transverse and spinous processes, and the neural foramen can all be visualized. Certain anatomic areas can be seen only on specialized views. For instance, the open-mouth view facilitates visualization of the atlantoaxial (C1–2) articulation and provides an additional view of the dens (Fig. 13–1D ). This view is an essential component of a trauma workup. Oblique views allow visualization of the neural foramen in the cervical spine (lateral views are used for this purpose in the thoracolumbar spine), which transmit the paired spinal nerves (Fig. 13–1C ). As you recall, there are 8 pairs of cervical spinal nerves, 12 pairs of thoracic spinal nerves, and 5 pairs of lumbar spinal nerves. To allow for spinal nerve exit, these neural foramina are formed by the pedicles above and below (Figs. 13–1C and 13–3B ). Abnormal bony projections, known as osteophytes, are a common manifestation of degenerative spine disease and, if present within the neural foramen, may be a cause of nerve root compression. Spinal nerves also can be compressed by disc herniations, but this type of neural compression cannot be diagnosed by means of plain film alone.

Certain small bony structures, such as the cervical transverse foramen (for the vertebral artery) and the small facets for rib articulation in the thoracic spine, are not well visualized on plain radiographs. Because "soft-tissue" structures also are poorly demonstrated on plain radiographs, the intervertebral disc is not well seen with x-ray unless calcified (and therefore dense). However, the soft tissues should not be ignored. In the cervical spine, for instance, one may identify calcification in the region of the carotid artery bifurcation, which may suggest atherosclerotic vascular narrowing. In the evaluation of cervical trauma, one should always include assessment of the width of the normal soft-tissue stripe, which is anterior to the vertebral bodies (Fig. 13–1B ). This prevertebral soft-tissue stripe may become widened in cervical spine trauma (prompting a closer search for fracture) and also in certain inflammatory conditions. When reviewing thoracic or lumbar spine films, attention to the soft tissues may facilitate diagnosis of a host of conditions ranging from pneumonia and lung cancer to retroperitoneal diseases and abdominal aortic aneurysms. Therefore, it is important not to focus only on the spine when interpreting spine radiographs.

Myelography

Contrast myelography has been around since its accidental discovery in 1922, when Sicard and Forestier, intending to administer extradural Lipiodol to treat sciatica, inadvertently introduced the material into the subarachnoid space. This radiopaque oil was noted to move freely, and it was immediately recognized that with the use of fluoroscopy (real-time radiography) and conventional radiography, this procedure would be useful for diagnosing intraspinal tumors. Lipiodol quickly replaced air as the medium of choice for myelography. [Air is lucent and is therefore a "negative" contrast agent; iodinized oils such as Lipiodol and, later, the popular Pantopaque (iophendylate) are dense and therefore "positive" contrast agents.] Following Mixter and Barr's 1934 report on the syndrome of herniated intervertebral disc, myelography became a widely used test. In the 1980s, the wide availability of less toxic water-soluble agents and, finally, nonionic contrast agents such as iopamidol and iohexol made myelography a readily tolerated procedure.

Myelography is employed most commonly to evaluate for disk herniations and to rule out spinal cord compression caused by tumor or trauma. In many parts of the United States, CT and MR imaging have all but replaced myelography. However, in many locations, myelography is still commonly performed. A myelogram is often followed by a postmyelogram CT examination, which is addressed later in this chapter.

The technique for performing myelography is simple. The patient is placed in the prone position on a fluoroscopy table. Under fluoroscopic guidance, a lumbar puncture (LP) is made with an 18- to 22-gauge spinal needle. (A fluoroscopically guided LP is much easier than an LP performed on a sick patient on the ward in the decubitus position.) Cerebrospinal fluid (CSF) is then drawn for laboratory tests, if needed, and contrast material is placed into the subarachnoid space. Once instillation of the contrast agent is fluoroscopically confirmed, the needle can be withdrawn and the patient studied. Depending on the spinal level to be examined, the patient can be standing, flat, or in Trendelenburg position. Typically, multiple views including lateral, AP, and oblique views are obtained. In the lumbar region, the cauda equina nerve roots are well visualized (Fig. 13–4A ). The conus medullaris, usually at L1–2, also can be seen. In the thoracic and cervical levels, the spinal cord can be seen as a "negative" shadow within the dense contrast, and its size and shape can therefore be evaluated (Fig. 13–4B ). Cervical spinal nerves are also well seen (Fig. 13–4B ). The presence of any lesions and their precise location relative to the dura usually can be determined on the basis of the myelographic appearance. For instance, lesions may be extradural, intradural but extramedullary (not in the spinal cord), or intramedullary (within the spinal cord).

Computed Tomography

CT utilizes x-rays to obtain images by means of multiple sources and detectors surrounding the patient in a radial fashion. This is why the patient appears to be entering a large doughnut-shaped device during the CT examination. The data obtained are processed by a computer, which then generates an image. Though sagittal reconstructions can be generated and are occasionally useful in spine imaging, the axial plane offers the highest image resolution. Once the raw data are obtained, images can be displayed with different "windows" and "level" values that take advantage of density ("attenuation" in CT lingo) differences between tissues. For instance, filming a set of soft-tissue windows allows differentiation of soft-tissue structures that are very similar in attenuation to adjacent structures (e.g., muscle and fluid). This is one of the key features of CT: whereas plain films usually cannot discriminate between different kinds of soft tissue, CT, with its superior resolution, can do just that. In the spine, CT makes it possible to discriminate between CSF, nerve roots, and ligaments, for instance. Without the administration of contrast agents, therefore, a CT examination can demonstrate the ligamentum flavum, nerve roots, epidural fat, and other structures that cannot be identified discretely on plain films (Fig. 13–5A ). We also typically film a set of bone windows, whose window and level settings are adjusted to give detailed information on bony structures (Fig. 13–5B ). On such images, little soft-tissue information is available.

CT is widely used to image the spine in the evaluation of almost all types of pathologic conditions. Most common indications include degenerative disc disease (i.e., to rule out disc herniation in patients with myelopathy or radiculopathy), suspected spinal tumors, and trauma. Assuming a normal appearance on plain films, CT is often the first study ordered in the evaluation of patients with back pain.

CT Myelography

As mentioned earlier, in patients who have undergone myelography, CT is often obtained immediately afterward (Fig. 13–6). It has been shown that a postmyelogram CT is more sensitive in the detection of pathologic conditions than is either test alone. This is particularly true for lesions within the spinal canal, such as disc herniations or tumors unassociated with a bony component. The presence of subarachnoid contrast allows dramatic visualization of the cauda equina nerve roots and spinal cord in a way that cannot be achieved with regular CT. In our institution, the vast majority of myelograms are immediately followed by CT.

MR Imaging

Since the early 1980s, MR imaging has gained widespread acceptance as the most sensitive imaging modality in the study of spine disease. Though not necessarily the first study performed, MR imaging undeniably allows visualization of intraspinal anatomy with much higher resolution than does any other modality. The ability to image directly in the sagittal plane contributes a great deal to the evaluation of the diseased spine. A description of the physics of MR imaging is beyond the scope of this chapter, and the reader is referred elsewhere for this information.

Because dense cortical bone has few mobile protons (which are necessary to create an MR signal), MR imaging is sometimes limited in its ability to demonstrate either osteophytes that may be a source of clinical symptoms or calcific components of other lesions. In such cases, CT with its superb depiction of bony detail may be useful as an adjunct examination. On the other hand, MR imaging is very sensitive in its ability to detect abnormalities in bone marrow. The vertebral bodies normally contain a large amount of bone marrow, and an abnormal appearance may be seen in a variety of disorders, such as anemia, infection, and metastatic disease.

MR images can be obtained with a variety of "sequences." The most commonly utilized are called spin-echo, and these can be "weighted" for either T₁ or T₂. (A thorough explanation of these parameters can be found elsewhere.) On a T₁-weighted image, normal adult (yellow/fatty) bone marrow has a "high signal" (i.e., it is hyperintense, or whitish in color), and CSF has a "low signal" (i.e., it is hypointense, or black in color). Neural tissue, such as the spinal cord or nerve roots, is intermediate in signal intensity (Fig. 13–7A ). Cortical bone, lacking mobile protons to produce a signal, is hypointense on all pulse sequences. On T₂-weighted images, marrow becomes lower in signal intensity, CSF becomes hyperintense, and neural tissue maintains an intermediate signal intensity. However, the spinal cord appears relatively lower in signal intensity, surrounded as it is by CSF with its very high signal intensity (Fig. 13–7B ). The intervertebral discs in normal individuals are typically of intermediate signal on T₁-weighted images and, because of their water content, appear hyperintense on T₂-weighted images.

Any alterations in the expected normal signal intensity for an anatomic structure should prompt a search for either a technical or a pathologic explanation for the abnormal signal. In some clinical applications, scanning after administration of intravenous gadolinium (gadopentetate dimeglumine) or other paramagnetic contrast agents can add valuable information, which may either clarify questions raised by the precontrast imaging results or permit detection of lesions that were invisible without contrast. In recent years, the use of fat suppression has increased the utility of contrast-enhanced imaging of the spine, particularly in the evaluation of lesions within the spinal canal (Fig. 13–8).

TECHNIQUE SELECTION

A great many clinical circumstances may necessitate spine imaging. The purpose of this section is to convey a sense of which techniques would be most appropriate for the given clinical setting. In some instances, the choice is clear. In others, the test to be performed is determined by the technology available, and often the decision is influenced by the preferences of the person ordering the test. In some clinical settings more than one imaging modality is acceptable as a first test. If the clinician consults with the radiologist before deciding on the initial test, unnecessary examinations may be avoided. Perhaps most importantly, however, if the clinician consults with the radiologist and conveys to him or her the clinical information, imaging often can be tailored to home in on the most likely site or type of abnormality. Still, general guidelines can be established to help decide which imaging test is appropriate. What follows is a brief outline providing general imaging recommendations for common clinical problems related to the spine. Only rarely is a particular test the only useful one for a suspected abnormality. In many cases, any of several tests would be useful as a baseline examination, with the understanding that additional imaging might be required to answer all clinical questions.

Trauma

Plain films provide the best initial examination for the evaluation of spine trauma. In a potentially unstable patient, they are obtained readily and often yield an immediate diagnosis. For further characterization of complex fractures, for conditions in which plain films would be inadequate (e.g., the cervical thoracic junction), or when additional information is required (e.g., to rule out canal compromise by a bone fragment), CT is frequently performed. CT is the best imaging study to evaluate complex spine fractures. In certain circumstances, such as suspected spinal cord contusion or transection or hemorrhage within the spinal canal, MR imaging is indicated. It is also useful in evaluating the patient with delayed onset of neurologic dysfunction after trauma to rule out myelomalacia (softening) of the spinal cord or post-traumatic syrinx.

Back Pain

Back pain is one of the most common medical complaints. Though most cases are caused by muscle strains and the like, persistent severe pain, pain associated with sciatica (a shooting pain down the leg), or neurologic findings such as weakness, decreased sensation, or abnormal reflexes should prompt a search for an underlying structural abnormality. The most common pathologic conditions are related to bony degenerative disease (osteoarthritis) or intervertebral disc abnormalities. As with trauma, plain films are a good place to start. Though disc herniations (extrusion of the nucleus pulposus beyond the annulus fibrosus) are not visible on plain films, degenerative changes are generally quite apparent, and any unsuspected lesions such as compression fractures or metastatic disease (both of which are common in older patients) may be detected.

For patients with a suspected herniated disc, MR imaging is generally considered the most sensitive examination. CT is still a good examination for the detection of disc herniation and, when combined with intrathecal contrast (CT myelography), it is still a widely used imaging modality. Although MR imaging is not essential for detecting disc abnormalities, it is more sensitive and is especially useful for detecting other pathologic conditions that might mimic disc herniation, such as lesions of the conus medullaris or metastatic disease. A possible exception to the use of MR imaging as a first-line cross-sectional imaging procedure in degenerative spine disease is for patients suspected of having foraminal nerve impingement by an osteophyte. Osteophytes are small, sharp projections of bone that occur in patients with osteoarthritis and they may impinge on the spinal cord or nerve roots. Such osteophytes in the cervical spine may be difficult to detect with MR imaging. However, it is not always possible to differentiate clinically between patients who have disc herniations and those whose nerves are compressed by osteophytes. All in all, MR imaging is the best test to order for these patients. Occasionally, a CT examination may be needed in addition to answer specific questions.

Myelopathy

In patients who are suspected of having a myelopathy (a true cord syndrome as opposed to radicular symptoms), MR imaging is unequivocally the first study to be employed. MR imaging is the only imaging procedure that allows direct visualization of the spinal cord and it is effective for diagnosing or excluding primary spinal cord lesions such as infarct, tumor, hemorrhage, or inflammatory conditions (e.g., multiple sclerosis or transverse myelitis).

Congenital Spine Lesions

A variety of congenital lesions may affect the spine. Plain films may be useful to survey the spine, but ultimately MR imaging is the modality of choice. Though bony defects may be imaged suboptimally, disorders of the spinal cord or nerve roots can be readily identified on MR images.

Metastatic Disease

If metastatic disease in the spine is suspected, plain films are an economical, easy way to rule out bony metastases. Unfortunately, plain films do not demonstrate such abnormalities until a significant amount of destruction has taken place. MR imaging, on the other hand, is quite sensitive to replacement of normal bone marrow by tumor and can establish the diagnosis much earlier. Gadolinium-enhanced MR imaging is also the best choice if spread of tumor to the subarachnoid space (carcinomatous meningitis or leptomeningeal carcinomatosis) is suspected clinically.

EXERCISE 13-1: DEGENERATIVE SPINE DISEASE

Clinical Histories:

Case 13-1. A whiny 45-year-old neuroradiologist presents with low-back pain. A coned-down lateral plain film of the lumbar spine is shown in (Fig. 13–9).

Case 13-2. A 58-year-old man presents with right-sided L5 radiculopathy. A myelogram was performed, and an oblique view demonstrating the right-sided nerve roots is displayed in (Fig. 13–10).

Case 13-3. A 53-year-old woman presents with neck and right arm pain. Plain films of the cervical spine were ordered, and a lateral film is shown in (Fig. 13–11).

Questions:

Radiologic Findings:

13-1. In this case, Fig. 13–9 shows a subtle anterior displacement of the L5 vertebral body relative to S1, known asspondylolisthesis. (D is the correct answer to Question 13-1.)

13-2. In this case, in Fig. 13–10 an extradural defect is seen at and below the L4–5 disc space and the right L5 nerve root does not fill. These changes are most likely caused by a disc herniation. (A is the correct answer to Question 13-2.) Note normal filling of the right L4 nerve root (arrowheads).

13-3. In this case, Fig. 13–11 shows disc space narrowing and osteophytes are seen at the C5–6 and C6–7 disc spaces. (C is the correct answer to Question 13-3.)

Discussion:

Degenerative osteoarthropathy may affect different parts of the spine. When the facet joints are involved, the result is often bony osteophytes, which may project into the neural foramen or spinal canal and compress neural structures. When the disc space is affected, bony changes in the vertebral body endplate can occur. In addition, the intervertebral disc itself may be affected, and disc herniation can occur as a result. Differentiation between disc bulge (less clinically important, usually in the midline, with no significant compression of cord or thecal sac) and actual herniation (larger, off-midline, with possible compression of nerves or thecal sac) is not always possible. Treatment decisions must be based on clinical as well as radiologic data.

In Case 13-1 (author's spine), the spondylolisthesis of L5 over S1 is a result of a defect in the pars interarticularis. This is the place between the superior and inferior articular facet of a given vertebra (Figs. 13–3C and 13–12A ). Spondylolysis, as this defect is known, is usually caused by a chronic stress fracture, though rarely it can be congenital or acute. If, as is commonly the case, the spondylolysis is bilateral, the vertebral body is essentially disconnected from the posterior elements and this allows the anterior slipping, or spondylolisthesis, shown in Fig. 13–9. This entity is included here because it is quite common, and because it predisposes to premature degenerative disease. In older patients, spondylolisthesis can be secondary to degenerative disease in the absence of a pars defect, and this "nonlytic" form is known as pseudospondylolisthesis or degenerative spondylolisthesis. When present, the spondylolysis defect is readily identified on oblique lumbar plain films, as a "broken neck on the Scottie dog" (Fig. 13–12B ). The lysis defect is also readily detected on CT (Fig. 13–12C ) though it may superficially resemble a facet joint.

Disc herniations are a common medical problem. Though they can usually be diagnosed with noninvasive CT or MR imaging, myelography is still employed in some places to diagnose disc herniations. In Case 13-2, Figure 13–10 shows an extradural defect, seen as an area of low density distorting the lateral aspect of the thecal sac, deviating the nerve roots. This is the typical appearance of a herniated nucleus pulposus (HNP) on myelography. We see the effect of the disc rather than the actual disc. On a CT study, the actual herniated disc can be visualized (Fig. 13–13A ). Most of the myelographic filling defect can be seen to be below the L4–5 disc space, secondary to inferior migration of disc material. This helps explain why the patient had an L5 radiculopathy. The right L4 nerve root (arrowheads in Fig. 13–10) had already exited and would be unaffected by an L4–5 HNP unless it was far lateral (Fig. 13–13B ). As previously mentioned, MR imaging is excellent in detecting disc herniations and eliminates the need for painful, invasive procedures such as myelography (Fig. 13–13B, C ).

Osteophytic ridging is a common manifestation of degenerative bone disease and in the cervical spine may cause myelopathy (if the cord is compressed) or radiculopathy (if a nerve root is compressed). In Case 13-3, Fig. 13–11 shows marked narrowing and osteophyte formation at C5–6 and C6–7. An oblique radiograph is useful in demonstrating the foraminal compromise that can result if osteophytes occur in that location (Fig. 13–14A ). Myelography can demonstrate effacement of nerve roots (Fig. 13–14B ). CT, with or without intrathecal contrast material, is excellent in depicting foraminal stenosis caused by osteophytes (Fig. 13–14C ). As mentioned earlier, MR imaging may be limited in its ability to depict subtle bony abnormalities such as foraminal compromise, though utilization of specialized techniques has resulted in improved detection with MR imaging.

EXERCISE 13-2: NEOPLASTIC SPINE DISEASE

Clinical Histories:

Case 13-4. A 39-year-old man presents with leg pain and weakness. A prior lumbar spine MR examination was normal. A thoracic myelogram is shown in Fig. 13–15.

Case 13-5. A 70-year-old woman presents with a5-year history of back pain and recent onset of paresthesia in the groin and inner thighs (saddle distribution) (Fig. 13–16).

Case 13-6. A 63-year-old man presents with severe upper neck pain not responding to anti-inflammatory medication (Fig. 13–17).

Case 13-7. A 65-year-old man presents with back pain. A CT bone window is shown in Fig. 13–18.

Questions:

Radiologic Findings:

13-4. In this case, the patient has a lower thoracic primary spinal cord astrocytoma. (D is the correct answer to Question 13-4.) The cord is normal inferiorly but is seen (in Fig. 13–15) to get wider toward the middle of the image. The contrast column on either side of the lesion is narrowed, most noticeably on the patient's right. This lesion has caused a "block" to the flow of contrast. Subsequent postmyelography CT (Fig. 13–19A ) confirmed the spinal cord enlargement. An MR image demonstrated the tumor (Fig. 13–19B ) within the spinal cord.

13-5. In this case, the plain film (Fig. 13–16) shows a large destructive mass replacing most of the lower sacrum. (Bis the correct answer to Question 13-5.) Notice how normal bone disappears below the midsacrum. A CT showed a large destructive mass with areas of calcification (Fig. 13–20).

13-6. In this case, the plain film (Fig. 13–17) shows that the body of C2 has been destroyed (lytic destruction). (D is the correct answer to Question 13-6.)

13-7. In this case, the CT image (Fig. 13–18) shows multiple small areas of lytic bony destruction. This is characteristic of either multiple myeloma or metastatic disease. (C is the correct answer to Question 13-7.)

Discussion:

Unfortunately, the spine may be involved by tumors of various types. In Case 13-4, the diagnosis was primary spinal cord glioma. Most of these are either astrocytomas or ependymomas. As with this patient, the diagnosis may be elusive for some time while other diseases such as disc herniation are ruled out. This patient even had a normal lumbar MR examination several months prior to the myelogram. While the thoracolumbar junction is usually visualized on a lumbar MR imaging study, this tumor (at T10) was just missed. A thoracic MR examination would certainly have made the diagnosis, but the patient's doctor ordered a myelogram. Spinal cord tumors are generally very difficult to treat. The more malignant ones, usually astocytomas, are associated with a poor prognosis. Ependymomas, because they are less infiltrative and more readily resectable, are associated with a much better prognosis.

Primary bony tumors also may affect the spine. A variety of benign bone tumors and cysts may be encountered. In the sacrum, giant cell tumor is the most common benign tumor. The most common primary sacral malignancy is chordoma. This is the diagnosis in Case 13-5. Chordomas develop from remnants of the embryonic notochord and represent 2% to 4% of primary malignant bone tumors. The sacrum is the most common site for chordoma, accounting for 50% of these lesions. The skull base accounts for 35% and other vertebrae account for 15%. Typical presentation of sacral chordoma is low-back pain, paresthesias, or rectal dysfunction. Figure 13–16 shows the typical radiographic appearance of expansile, lytic destruction. On CT (Fig. 13–20), a large soft-tissue mass with internal calcifications is characteristic.

By far the most common type of spinal tumor is metastatic disease, with lung and breast being the most frequent primary sites. Virtually any tumor may metastasize to the spine. In general, certain tumors tend to result in osteoblastic or dense metastases, and prostate adenocarcinoma falls in this category. Other primary malignancies, such as those in the lung and breast, tend to have osteolytic, destructive spine metastases. The patient in Case 13-6 had lung carcinoma, and Figure 13–17 represents a hematogenous spread of tumor to the C2 vertebral body. Metastatic disease may affect the spine by other mechanisms. Tumors adjacent to the spine may grow directly into it (Fig. 13–21A, B ). This may occur in lung carcinoma and lesions such as neuroblastoma or lymphoma (with retroperitoneal/ paraspinal lymphadenopathy). Finally, the spinal canal may be affected by spread of malignant neoplasm. Rarely, a metastatic lesion may occur in the spinal cord itself, usually as a terminal event. Metastatic disease may occur in the subarachnoid space by two methods. First, an intracranial malignancy (i.e., glioma, medulloblastoma) can seed the subarachnoid space and "fall" into the spine. These are known as "drop" metastases. Hematogenous spread to the subarachnoid space may occur in non-CNS primary tumors. Such involvement is known as leptomeningeal carcinomatosis or carcinomatous meningitis (see Fig. 13–8), and is associated with a very poor prognosis.

Multiple myeloma is a disseminated malignancy caused by a proliferation of plasmacytes, typically occurring in the middle-aged and elderly, with a slight male predominance. The spine may be affected primarily or secondarily, and bone pain caused by pathologic compression fracture is the most common symptom. Plain films may be normal early in the course of the disease or show only mild osteopenia. Later, multiple, small, lytic, "punched-out" lesions may be seen. CT is very sensitive, and Figure 13–18 shows the typical CT appearance of multiple myeloma. The findings, however, would be indistinguishable from those of small lytic metastases of other origin, and for this reason, metastases and myeloma are often mentioned together in the context of multiple small lytic bony lesions. MR imaging of multiple myeloma may have different appearances, but the typical pattern would be multiple, small foci of decreased signal intensity replacing the normal hyperintense bone marrow on T₁-weighted images (Fig. 13–22).

EXERCISE 13-3: SPINE TRAUMA

Clinical Histories:

Case 13-8. A 23-year-old woman was involved in a motor vehicle accident (Fig. 13–23).

Case 13-9. A 21-year-old quadriplegic woman had a motor vehicle accident 4 weeks ago (Fig. 13–24).

Case 13-10. A 38-year-old woman presents with slowly progressive upper extremity and upper trunk sensory deficits 3 years after a motor vehicle accident (Fig. 13–25).

Questions:

Radiologic Findings:

13-8. In this case, Fig. 13–23 shows a compression fracture of the L2 vertebral body with kyphotic angulation. (E is the correct answer to Question 13-8.)

13-9. In this case, the sagittal T_l-weighted MR image (Fig. 13–26) shows a complete subluxation of C6 on C7 and a complete transection of the cervical spinal cord at that level. In all likelihood this patient will never regain use of her legs or have any normal neurologic function below C6. (D is the correct answer to Question 13-9.)

13-10. In this case, the sagittal T₁ -weighted MR image shows a low signal abnormality within the cervical spinal cord from C6 to T1. This is a typical appearance of syringomyelia or syrinx. (A is the correct answer to Question 13-10.)

Discussion:

Spinal trauma is a major medical problem, usually caused by motor vehicle and occupational accidents. Accurate and complete diagnosis is essential to maintain spine stability and ensure preservation of neurologic function. As mentioned previously, plain films should be obtained initially, and this often makes the diagnosis. However, additional imaging tests are often necessary to fully evaluate a case of spine trauma. For instance, in Case 13-8, there was clinical concern that the spinal canal was compromised. Small bony fragments within the spinal canal may not be visible with plain film alone. For this reason, CT was performed (Fig. 13–26A,B ). This allowed a better appreciation of the extent of the fractures and ruled out neural compression. An example of spinal canal compromise is shown in Fig. 13–26C .

In severe trauma, the spinal cord may be affected. Contusions may occur with or without fracture/ subluxation, and MR imaging would be required for diagnosis. In a severe fracture/subluxation, the spinal cord can be completely transected. In Case 13-9, the patient was known to have a severe C6–7 subluxation, but because of obesity, plain film and CT imaging were very limited. In this case, only MR imaging was able to demonstrate the full extent of her spinal cord injury.

Rarely, patients who have recovered from an acute spinal injury experience a delayed onset of neurologic symptoms, occurring 1 to 15 years after the trauma. This suggests the possibility of delayed post-traumatic syrinx (Case 13-10). Symptoms include pain upon coughing or exertion, sensory disturbances, or motor deficits. MR imaging is essential for diagnosis. The condition is sometimes amenable to surgical shunting. Syringomyelia can also be idiopathic or can be secondary to certain congenital or inflammatory conditions. Imaging often cannot distinguish among different possible etiologies, and history is important.

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