Radiographic Evaluation of Spinal Canal Tumors

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Chapter 10 Radiographic Evaluation of Spinal Canal Tumors

INTRODUCTION

Imaging of spinal canal tumors and tumor-like masses has been revolutionized by the advent of magnetic resonance imaging (MRI), which is a sensitive modality for the identification and characterization of spinal lesions. Radiologic evaluation of these lesions focuses on localizing lesions into extradural, intradural extramedullary, and intramedullary compartments. Although this approach is somewhat artificial, because many lesions transgress compartments or can be found in different compartments, it is very useful for generating an appropriate differential diagnosis for a spinal mass (Table 10-1). Clinical information (in particular patient age) is helpful to further narrow and order the differential diagnosis of spinal lesions. This chapter begins with a brief discussion of imaging techniques used in evaluating spinal masses. This is followed by a discussion of imaging characteristics of the most common lesions that occur in each of the aforementioned categories, including a description of tumors, tumor-like cysts, and non-neoplastic tumor mimics. A discussion of spinal vascular malformations is beyond the scope of this chapter.

Table 10-1 Classification of Spinal Canal Tumors and Tumor-Like Masses

Extradural Intradural Extramedullary Intramedullary
NEOPLASTIC
Metastases lymphoma (and epidural extension of other vertebral body and paraspinal lesions)
Epidural lipomatosis
Angiolipoma
Schwannoma
Neurofibroma
Meningioma
Paraganglioma
Metastases (Hemangioblastoma)
Astrocytoma
Ependymoma
Ganglioglioma
Hemangioblastoma
Metastases (Schwannoma)
NON-NEOPLASTIC
Abscess
Hematoma
Degenerative changes
Arachnoid cyst
OPLL

Arachnoid cyst
Epidermoid
Lipoma

Syrinx
MS
Transverse myelitis
Cord infarct

MS, multiple sclerosis; OPLL, ossification of the posterior longitudinal ligament

IMAGING MODALITIES

Radiographs, computed tomography (CT), CT myelography, MRI, and angiography are the modalities available for assessment of spinal canal lesions. Radiographs can be a useful screening test but are limited, demonstrating mainly the secondary effects of spinal lesions on the adjacent bony and paraspinal soft tissues. Expansion of the central canal, bony destruction and/or remodeling (including vertebral body scalloping and neural foraminal widening), thickening of the paraspinal soft tissues, and associated scoliosis are some of the findings that may be seen in the setting of an intraspinal lesion. A normal radiograph does not exclude the presence of an underlying spinal mass. CT demonstrates bony and soft tissue changes with a greater degree of conspicuity, but the intraspinal lesion itself may be subtle or impossible to detect. CT myelography, a technique whereby myelographic contrast is introduced directly into the thecal sac followed by CT imaging, increases sensitivity for detection of intraspinal disease over conventional CT. However, it is still less effective than MRI for assessing intramedullary and extradural disease, and, although equally sensitive to contrast-enhanced MRI for detection of intradural extramedullary spinal masses, it is more invasive.13 CT myelography, therefore, typically is reserved for patients who have contraindications for MRI and sometimes for those with spinal hardware that causes excessive artifact on MRI. CT myelography demonstrates expansion of the cord or mass effect on the cord by spinal lesions but is limited in its ability to characterize the underlying lesion and may be insensitive for intramedullary lesions that have only minimal mass effect. Occasionally, CT myelography is a useful adjunct to MRI for cystic lesions within the spinal canal, where it is used to assess for communication with the cerebrospinal fluid (CSF) space.4,5 In patients without contraindications, MRI is undoubtedly the modality of choice for evaluation of spinal canal lesions. It is the most sensitive and specific modality for both identifying and characterizing intraspinal lesions. Spinal angiography is reserved for vascular lesions, providing important information for preoperative planning and guiding endovascular treatment.

MRI of the spine involves greater technical challenges than imaging of the brain. The presence of bone housing the spinal canal creates large susceptibility gradients. The small size of the spinal canal and spinal cord limits spatial resolution. In addition, CSF pulsation introduces motion artifacts. MR techniques used in screening for spinal lesions include at least a T1- and T2-weighted sequence. Addition of a gradient echo (GRE) sequence is useful to evaluate for blood products. Intravenous gadolinium improves detection of intradural extramedullary disease and helps to characterize and delineate intramedullary lesions.3,6 Evaluation in both the sagittal and transverse planes is important to confirm true findings and exclude artifacts. The role of these standard sequences in evaluating spinal masses is described in Table 10-2. Examples of the normal appearance of the spine when imaged with these standard sequences are shown in Figs. 10-1 to 10-3.

Table 10-2 Standard MRI Sequences for the Evaluation of Spinal Tumors

T1-weighted image (T1WI)
Spin echo (SE)
Excellent morphologic detail, especially cord size; dark CSF; important information about cysts, fat, and blood
T2-weighted image (T2WI)
Fast spin echo (FSE)
FSE has mostly replaced spin echo T2; sensitive for identification of intramedullary tumor and edema; hyperintense CSF; susceptible to CSF motion artifacts, especially in axial plane in cervical spine
Gradient echo (GRE) Sensitive to magnetic susceptibility artifact from hemorrhage; often used in axial plane in cervical spine instead of FSE T2 to reduce CSF pulsation artifact
T1-weighted + gadolinium Useful for characterization and accurate assessment of borders of a lesion; distinguishes tumor from adjacent edema, and differentiates solid tumor from tumor cysts and syrinx; fat suppression improves contrast between enhancing lesion and background, distinguishes fat from hemorrhage, and is particularly important in extramedullary lesions

CSF, cerebrospinal fluid.

Some newer pulse sequences have been investigated for use in spinal imaging with variable results. These include fluid-attenuation inversion recovery (FLAIR), short tau inversion recovery (STIR), diffusion weighted imaging (DWI), diffusion tensor imaging (DTI), and MR spectroscopy (MRS) (Table 10-3). FLAIR is a heavily T2-weighted sequence that nulls signal from CSF and thus may increase conspicuity of hyperintense lesions in close proximity to CSF. Despite its utility in brain imaging, FLAIR has been less successful in the spine and has shown to be less sensitive than standard T2-weighted sequences in assessment of cord lesions.710 STIR (a fat suppressed T2-weighted sequence) is commonly used to assess for vertebral body involvement in the setting of trauma or infection but also may have useful application in detecting intramedullary disease. Several studies have demonstrated increased sensitivity of STIR for cord disease in multiple sclerosis (MS) relative to other T2-weighted sequences.10,11 However, STIR is limited by reduced signal to noise ratio and greater sensitivity to motion than standard T2-weighted sequences.8 DWI is an MR technique that is sensitive to the random motion of water molecules in tissues over microscopic distances (5–20 microns) and can generate a map of the average apparent diffusion coefficient (ADC). Reduced ADC is seen in acute cerebral stroke and aids early detection of infarcts, improving accuracy over conventional MRI.12 DWI also has become an invaluable technique for assessing other intracranial diseases, including applications in infection, tumors, and cysts. DWI in the spinal cord and spinal canal has been more difficult to implement than in the brain because of technical factors, in particular susceptibility from the vertebrae and limited spatial resolution. Although DWI is not currently routinely used for spinal cord imaging at most institutions, DWI techniques have been developed that are practical for use in the clinical setting.13,14 DTI, an application of DWI that measures diffusion anisotropy and thus can be used to trace white matter tracts within the brain and spinal cord using statistical methods, is of great interest in spinal cord tumor imaging. DTI demonstrates a high degree of anisotropy in the normal spinal cord, which is disrupted by spinal cord tumors, a feature that may be helpful to depict the full extent of spinal cord lesions more accurately than conventional imaging and perhaps eventually map out involvement of key spinal cord white matter tracts.15,16 MRS is a technique that can obtain biochemical information about tissues being studied by creating a spectrum of metabolites within a region of interest. Currently, a number of technical factors preclude use of MRS in the spine, but future advances may make this technique both possible and useful in spinal cord imaging.8

Table 10-3 Potential Sequences for Evaluation of Spinal Tumors

FLAIR CSF-suppressed T2-weighted sequence; intramedullary lesions may be less conspicuous on FLAIR than on FSE T2; CSF pulsation artifact accentuated
STIR Fat-suppressed T2-weighted sequence; intramedullary lesions shown to be more conspicuous; particularly useful in assessment of trauma and infection; lower signal to noise than FSE T2; sensitive to motion.
DWI Currently limited in spine because of motion, spatial resolution, and susceptibility; in vivo application has been performed in spinal cord infarcts and MS
DTI Application of DWI that measures diffusion anisotropy and may be used to trace white matter tracts; potential use in imaging spinal cord tumors, but not yet feasible in the clinical setting
MRS Demonstrates spectrum of metabolites in region of interest; currently limited by spatial resolution and susceptibility artifacts and cannot yet be performed adequately in spine

MS, multiple sclerosis.

EXTRADURAL LESIONS

Extradural lesions, which by definition are outside of the dura, usually arise from the osseous spinal structures, intervertebral discs, and surrounding soft tissues (including vessels, nerves, fat, and paraspinal muscles). These lesions cause focal displacement of the thecal sac and its contents away from the mass. Radiographs may demonstrate secondary effects of the lesion on the bony structures, including bone destruction or scalloping, expansion of the central spinal canal, and paraspinal extension. CT myelography shows extrinsic compression of the thecal sac and decrease in the ipsilateral CSF space. MRI shows the dura draped over the mass and may show an epidural fat cap (Box 10-1).17 The most common epidural masses are degenerative, including disc protrusions, osteophytes, and synovial cysts. Infection and hematomas may present as epidural masses. Of neoplastic lesions, metastases to bone with extradural extension are the most common.17,18 Imaging of lesions of the vertebrae (with and without epidural extension) was discussed in the previous chapter. Here we discuss epidural lesions that were not previously mentioned.

EPIDURAL LIPOMATOSIS

This is a rare condition manifested by excessive deposition of unencapsulated fat in the epidural space, resulting in cord and cauda equina compression with neurologic symptoms. Most cases are seen in association with long-term exogenous steroid use or conditions in which there is elevated endogenous production of steroid (such as in Cushing’s disease). Cases may occur as a result of morbid obesity or with no cause evident.19 In approximately 60% of cases, involvement of the mid to lower thoracic spine is seen with fat predominantly posterior to the cord. The remaining 40% of cases involve mainly the lower lumbar region with fat surrounding the circumference of the thecal sac (Box 10-2). A combination of thoracic and lumbar involvement is sometimes seen.20 CT and MR demonstrate increased epidural fat and diminished subarachnoid space, with tapering of the thecal sac and a Y-shaped configuration seen in the transverse plane (Fig. 10-4).21,22 Epidural lipomatosis does not enhance nor does it cause bony erosion.17,20 Fat suppression is helpful on MRI to confirm the presence of adipose tissue (Box 10-2).

ANGIOLIPOMA

Angiolipomas are benign tumors composed of adipose and vascular elements that are uncommon in the spine. They are almost always epidural, but intramedullary angiolipomas have been reported.23 Lesions usually extend over more than two vertebral body segments and may manifest as either a focal or infiltrating mass.24,25 On CT, angiolipomas usually demonstrate low attenuation relative to the cord with scattered soft tissue reticulation.26 On MRI, lesions are hyperintense on unenhanced T1-weighted images (T1WI) because of fat content and are hyperintense on T2-weighted images (T2WI) (Box 10-3). Mild, heterogeneous enhancement is seen on post-gadolinium fat-saturated images (Box 10-3).24

EPIDURAL HEMATOMA

Extravasation of blood into the epidural compartment of the spine may be caused by trauma or instrumentation but also can occur spontaneously, especially in the setting of anticoagulation or coagulopathy (Box 10-4). An epidural hematoma is rarely the sequelae of hemorrhage from a spinal vascular malformation.27,28 MR and CT myelography demonstrate an elongated, often multisegment, extra-axial mass encasing or displacing the thecal sac.20 MRI signal characteristics depend on the age of blood products and are usually heterogeneous. Hyperintensity on T1WI, heterogeneity with foci of low signal on T2WI, and susceptibility demonstrated with GRE are helpful features in making the diagnosis (Fig. 10-5).29,30 Enhancement may be seen, especially along the margins of an epidural hematoma (Box 10-4).20,30

EPIDURAL ABSCESS

Epidural spinal infection with abscess formation usually results from hematogenous spread of disease primarily to the intervertebral disc, typically from infection of the skin, urinary tract, or respiratory system. Direct spread from paraspinal infection also may occur but is less common. Predisposing factors include diabetes mellitus, trauma, alcoholism, and intravenous drug abuse (Box 10-5). Epidural anesthesia and analgesia also are associated with increased risk. Pyogenic organisms (most often Staphylococcus aureus) are the most common pathogens.31,32 MRI is the imaging tool of choice and should include the entire spinal axis to exclude multiple sites of involvement.32 The posterior epidural space may be more commonly involved than the anterior, and the abscess can extend over multiple segments.33 Abscesses are classically isointense to hypointense to the cord on T1WI, hyperintense on T2WI and STIR images, and demonstrate rim enhancement (Fig. 10-6). Homogeneously or heterogeneously enhancing phlegmon may often be seen in some infections without frank abscess formation (Figs. 10-7 and 10-8).34 Associated findings of spondylodiscitis, including vertebral body endplate erosion and disc space narrowing, are helpful in some cases in making the diagnosis. High signal on DWI with reduced ADC has been demonstrated in spinal epidural abscesses and also may be helpful to confirm the diagnosis.35

image image image image image image image image image image image image

Fig. 10-8 A 20-year-old man with chronic granulomatous disease and atypical infection with Aspergillus, affecting the thoracic and lumbar spine. A, Sagittal T1WI of the thoracic spine. Heterogeneous low signal is seen within the T8 through T10 vertebral bodies without evidence of bony destruction. Sparing of the disc space, as noted here, is seen with atypical infections or neoplasms. Epidural and anterior paraspinal soft tissue is seen at the affected levels with encroachment on the neural foramina. B, Sagittal T1WI lateral to A. The paraspinal component of the mass is demonstrated in this image. C, Sagittal T2WI of the thoracic spine. The affected vertebral bodies are mildly heterogeneous and hyperintense. D, Sagittal post-gadolinium T1-weighted fat saturated image of the thoracic spine. The affected vertebral bodies and the associated paraspinal and epidural phlegmon enhance avidly. E, Sagittal post-gadolinium T1-weighted fat saturated image lateral to D. F, Axial T2WI at the T10 level. The high signal vertebral body and epidural and paraspinal soft tissue are shown. Rib and pleural involvement is also present and is appreciated better in the axial than the sagittal plane. G, Sagittal T1WI of the lumbar spine. Pathological fracture of the L4 vertebral body is present with retropulsion into the spinal canal. A small amount of epidural and paraspinal soft tissue is seen extending above and below the L4 vertebral body. H, Sagittal T2WI of the lumbar spine. High signal is seen extending through the fractured L4 vertebral body. A small amount of high signal also is seen in the L3 vertebral body. I, Sagittal post-gadolinium T1-weighted fat saturated image of the lumbar spine. The affected vertebrae, as well as the epidural and paraspinal involvement, enhance.

SYNOVIAL CYST

Synovial cysts are almost always associated with degenerated facet joints and usually do not present a diagnostic dilemma. The appearance of a round, sharply marginated, posterolateral, extradural cystic mass communicating with the facet joint is characteristic (Fig. 10-9). MRI signal intensity of these lesions varies depending on the protein content and the presence of calcification or hemorrhage (Fig. 10-10). Mild rim enhancement may be seen (Fig. 10-11). These lesions may be difficult to detect on CT unless calcification, intracystic gas, or hemorrhage is present (Box 10-6).36,37

EXTRADURAL ARACHNOID CYST

An extradural arachnoid cyst (EAC) is a congenital or acquired CSF-filled outpouching of arachnoid that protrudes through a dural defect (Box 10-7). Most EACs are found in the lower thoracic spine, posterior or posterolateral to the cord.5 MRI demonstrates a long-segment extradural mass that follows CSF signal characteristics on all sequences. CT myelography may show communication with the CSF space.17,40

INTRADURAL EXTRAMEDULLARY LESIONS

Intradural extramedullary lesions arise inside the dura but outside of the spinal cord or cauda equina. These lesions tend to displace the spinal cord and enlarge the ipsilateral subarachnoid space with a sharp interface between the undersurface of the mass and the CSF space (Box 10-8). Nerve sheath tumors (schwannomas and neurofibromas) and meningiomas account for the vast majority of intradural lesions. Paragangliomas and CSF-disseminated metastases are relatively rare neoplasms of the intradural space. Tumor-like cysts, including arachnoid cysts, epidermoids, and neurenteric cysts, are non-neoplastic lesions that may be seen in the intradural space and may cause symptoms such as spinal cord or nerve root compression. Lipomas are considered congenital developmental lesions and are associated with a tethered cord but may be an incidental finding and may not present until adulthood.

SCHWANNOMA

Schwannomas are grossly encapsulated neoplasms of the nerve sheath in the peripheral nervous system that typically arise from the dorsal sensory root. They account for approximately 30% of all spinal tumors and 70% of intraspinal tumors and occur with fairly uniform distribution throughout the spine.18 Most lesions are sporadic with peak incidence between 30–60 years of age (Box 10-9). Lesions are usually solitary unless associated with an underlying genetic condition such as neurofibromatosis type 2 (NF-2), schwannomatosis, or Carney syndrome.20 Lesions are generally benign, although malignant degeneration may occur.41 Classically, schwannomas are well-circumscribed, lobulated or dumbbell shaped, enhancing spinal masses that often show cystic degeneration and/or hemorrhage. Approximately 70–75% of lesions are intradural, 15% are completely extradural, and 15% are a combination of intradural and extradural.17 Intradmedullary schwannomas have been reported but are rare (<1%).42,43 On CT, lesions are usually isodense to the spinal cord with lower density cystic changes, adjacent bony remodeling (enlarged neural foramen and expanded canal), and either peripheral or solid enhancement.20,44 Calcification is rare. On MRI, lesions are typically isointense to the cord on T1WI, with the exception of rare melanotic schwannomas, which are hyperintense.45,46 Most lesions are bright on T2WI, with some lesions demonstrating a “target” appearance with a bright rim and dark center. Enhancement is usually intense and may be homogeneous, heterogeneous, or peripheral (Figs. 10-17 to 10-19).41,44,45,47 Melanotic schwannomas are a rare variant of schwannomas composed of melanin-producing cells with features of Schwann cells. These lesions have an increased incidence of local recurrence after resection as well as an increased likelihood of metastasis compared with typical schwannomas and require careful long-term follow-up. Melanotic schwannomas also may be challenging to differentiate from malignant melanoma, which has a much worse prognosis.48

NEUROFIBROMA

Neurofibromas are unencapsulated neoplasms of the nerve sheath. They are less common than schwannomas and have a peak incidence between 20–30 years of age. Most lesions are sporadic, solitary tumors (Box 10-10). However, multiple neurofibromas (especially those of the plexiform type) are typically seen in NF-1. Lesions may be localized, diffuse, or plexiform in morphology.20 Imaging characteristics may be indistinguishable from schwannomas, especially for the localized types. However, hemorrhage and cystic degeneration are less common. A target appearance of peripheral hyperintensity on T2WI with central low to intermediate signal intensity has been described as more common in neurofibromas than in schwannomas.49,50

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