VERTEBRAL AND PARAVERTEBRAL ABNORMALITIES
John R. Hesselink, MD, FACR
Cross-sectional imaging of the spine and spinal cord has contributed substantially to the detection and characterization of spinal diseases. MR has several advantages over CT and has largely replaced CT as the primary imaging modality. The spine and neural elements can be imaged without the injection of intrathecal contrast agents. Images can be obtained in any plane. Although cortical bone gives essentially no MR signal, the hypointense bone is contrasted against the higher signal of the adjacent soft tissues. In addition, the marrow within the vertebral bodies and posterior elements of the spine contains blood and fat, both of which produce an MR signal. As a result, any disease process that distorts the architecture of the marrow can be detected. In addition, major vascular structures, such as the aorta and vena cava, are imaged because of the phenomenon of signal void from flowing blood.
When designing pulse sequences an understanding of the basic principles of MR imaging is essential to achieve the appropriate tissue contrast (T1- or T2-weighting), spatial resolution, sufficient signal-to-noise, and an adequate number of slices. Most protocols include a T1-weighted sequence and some type of T2-weighted sequence to give a myelographic effect. T1-weighted spin-echo (SE) pulse sequences have been very popular because they provide hyperintense bone marrow, good contrast between the intervertebral disk and the hypointense CSF, and high signal-to-noise images with reasonable scan times. The myelographic effect can be accomplished with either SE or gradient-recalled echo (GRE) sequences. GRE sequences are fast and produce a reliable myelographic effect. Fast spin-echo (FSE) techniques allow enormous time savings, and if available, they have replaced conventional SE for T2-weighted imaging of the spine. FSE images have high signal-to-noise ratio and decreased magnetic susceptibility. In the sagittal plane, FSE images produce a reliable myelogram effect with much better contrast and spatial resolution compared to GRE sequences. In the cervical and thoracic regions, axial FSE scans are degraded by inhomogeneous CSF signal. The closely spaced echo train does not allow time for gradient moment nulling.
In the postoperative back and to evaluate neoplastic and inflammatory disease, gadolinium injection with T1-weighted imaging is essential to evaluate enhancing lesions. Fat-suppression is helpful to eliminate competing fat signal from bone marrow and other soft tissues.
The spine is a common site for metastatic disease. The more common primary tumors to metastasize to the spine are lung and breast carcinoma, followed by prostatic carcinoma. Any malignancy has the potential to metastasize to bone, except perhaps CNS gliomas.
Using the T1-weighted study, the normal medullary cavity should have a relatively homogeneous high-signal appearance due to the presence of marrow fat. Metastases, primary osseous neoplasm, and some hematopoietic or metabolic disorders result in replacement of the high-signal fat by lower signal substrates such as neoplastic tissue, fibrosis, or abnormally increased extracellular fluid. Varying degrees of decreased signal in the marrow cavity from osseous metastases can be seen on T1 images, depending on the amount of blastic reaction. Except for sclerotic metastases, bone metastases are hyperintense on T2-weighted images. The majority enhance with gadolinium on T1-weighted images. Fat suppression is recommended for both the T2-weighted and the gadolinium-enhanced images.
In older patients with osteoporosis and vertebral compression deformities, difficulty may arise in trying to distinguish this entity from neoplasm, but the marrow in osteoporosis maintains its fat component. In the case of metastases, as a general rule nearly all of the vertebral body fat is replaced before compression occurs. Morphology is also important in distinguishing benign from malignant vertebral collapse. Intervertebral fluid, an intervertebral vacuum cleft, wedge-shaped deformity of the vertebral body and preservation of the posterior cortical margin favor a benign process. End plate compression with herniation of disk material into the body is another sign of benign collapse. Paravertebral soft-tissue mass suggests malignancy. Acute benign fractures do not enhance initially, but after a week or two, enhancement of granulation tissue is observed. Finally, on diffusion-weighted images, benign fractures are hypointense, whereas pathologic vertebral compression fractures are hyperintense. Initial reports were very positive for diffusion weighted images distinguishing benign from pathologic fractures, however, subsequent reports have been less enthusiastic.
Except in unusual circumstances, epidural metastatic disease occurs in association with osseous metastases. Once a soft-tissue mass is identified, additional features require characterization, including definition of the caudal-rostral and paravertebral extent, identification of any additional soft-tissue involvement, and quantification of the degree of cord, cauda equina, and root compression. Epidural neoplasm on T1 images is usually slightly lower in signal than spinal cord.
PRIMARY BONE LESIONS
Detection of vertebral hemangiomas by CT and plain films has been limited to those relatively large lesions that demonstrate a coarse, vertically striated trabecular pattern. On short TR/TE MR sequences, the trabeculae maintain a low-signal intensity, but the high-signal of the intervening matrix usually predominates and tends to obscure the bony trabeculae. Pathologic and chemical shift studies have shown that the matrix of a hemangioma is composed mostly of adipose tissue. On long TR/TE sequences, high-signal in the hemangioma is also seen, due to the presence of angiomatous tissue. Contrary to their appearance in the vertebral body, extraosseous components of hemangiomas have an intermediate T1 and a relatively long T2. The longer T1 is explained by the absence of fat in the extraosseous component. In many otherwise normal spines, small hyperintense foci are found in the vertebral body marrow on short TR/TE sequences. Postmortem studies of these foci have revealed small hemangiomas or intraosseous islands of fat. Distinction between fat islands and hemangiomas should be possible on T2 images, because the hemangioma maintains the high-signal but the fat becomes progressively lower in signal on longer TE sequences.
They are relatively rare skeletal neoplasms that arise from notochordal remnants in the skull and spine. About one half of chordomas occur in the sacrum, another 35 per cent in the clivus, and the remainder in the vertebrae. They are slow-growing tumors but locally aggressive and invasive. Sagittal MR images are particularly good for displaying the destroyed areas of the sacrum and the relationship of the tumor to the pelvic viscera anteriorly and the gluteal muscles posteriorly. Chordomas are slightly hypointense on T1-weighted images and hyperintense on T2-weighted images. They often have a heterogeneous internal texture owing to calcification, necrosis, and gelatinous mucoid collections.
Primary Bone Tumors
There are a host of other primary bone tumors that can occur in the spine, but they are relatively uncommon. Many of them have characteristic plain film findings, but sectional imaging studies are required to assess soft-tissue components Most bone tumors have prolonged T1 and T2 relaxation times. Even lesions that appear relatively dense on CT may have sufficient soft tissue matrix to render them hyperintense on T2-weighted images. Dense calcification and reactive bone are hypointense, but much of the calcification within the tumor matrix is invisible to MR. The presence of hemorrhage or fat alters the signal characteristics. Aneurysmal bone cysts have a distinctive feature of multiple cysts with variable signal intensities related to protein content, in some cases with fluid-fluid levels.
Paget's disease, or osteitis deformans, is an acquired disorder of unknown etiology. Normal bone is destroyed and replaced by poorly mineralized osteoid matrix. The bones most often involved are the pelvis, skull, femur, and spine, followed by other long bones and the mandible. Paget's disease results in enlarged, expanded bone that has a heterogeneous texture. The soft-tissue matrix accounts for the intermediate signal intensity on T1-weighted images and hyperintensity on T2-weighted images. The mineralized components, underestimated by MR, become more prominent in later stages of the disease.
BONE MARROW DISEASE
The vertebral body is composed of three tissues: bone, hematopoietic (red) marrow, and fatty (yellow) marrow. With aging, the bone becomes demineralized and the ratio of hematopoietic to fatty marrow changes. MR studies of patients of varying ages have revealed that the T1 relaxation time of bone marrow progressively decreases with age. The T1 shortening is due to replacement of hematopoietic marrow by fatty marrow. At birth the vertebral marrow consists entirely of hematopoietic tissue. The fatty component increases to about 15 per cent by age 10, 35 per cent by age 25, and 60 per cent by age 80.
As a result of increased marrow fat, the T2 relaxation time also decreases with age except in women over 50. The greater loss of bone mineral in postmenopausal women is postulated as the cause of the variation. Bone mineral affects the T2 relaxation time of marrow. Magnetic susceptibility effects reduce the apparent T2 of protons at bonetissue interfaces, making those protons invisible to MR. With osteoporosis (loss of bone mineral), the protons are "unmasked," resulting in an increase in spin density and T2 relaxation time.
There are many diseases that affect the bone marrow, including metastatic disease, tumors of blood cell origin, blood dyscrasias and anemias, and radiation therapy. In normal marrow there are two populations of protons (water and fat protons), and the two populations are more or less balanced. Any disease process that upsets that balance affects the MR signal. In the adult, the active red marrow of the body is found mostly in the axial skeleton. Primary and secondary marrow malignancies preferentially infiltrate areas of active hematopoiesis, so the spine and pelvis are commonly affected. Replacement of the normal marrow by abnormal soft-tissues results in increased marrow cellularity, a greater number of water protons, and prolongation of T1 and T2.
Plasmacytoma and Multiple Myeloma
These neoplasms of plasma cells commonly involve the spine. Plasmacytoma is a solitary lesion that destroys a vertebra and frequently breaks through the cortex to produce a soft tissue mass. Multiple myeloma causes widespread skeletal destruction and is associated with anemia, hypercalcemia, renal impairment and increased susceptibility to infections. Both lesions are osteolytic and often lead to vertebral collapse. If the soft tissue components enter the epidural space, cord compression and a myelopathy may result. Signal characteristics can be identical to metastatic disease, however, multiple myeloma usually exhibits more diffuse involvement of the bone marrow.
Diskitis and Osteomyelitis
Spine infections involve the vertebral body and adjacent disk interspace. Bacterial infections (Staphylococcus and Streptococcus) are most common and are spread by a hematogenous route. Risk factors include intravenous drug abuse, diabetes mellitus, immunocompromised states, and recent surgical procedures on or near the spine. Tuberculous and fungal infections result from systemic spread of pulmonary infections.
The early diagnosis of spinal diskitis and osteomyelitis has important clinical implications. Diskitis and osteomyelitis have a typical appearance on T1- and T2-weighted images. On short TR/TE sequences, the earliest signs of infection include loss of signal in both the disk space and the end-plate marrow of the contiguous vertebral bodies. There is also disruption of the low-signal end-plate band separating the marrow cavity from the nucleus pulposus of the disk. The bacteria produce a proteolytic enzyme that destroys the disk material, so narrowing of the disk space also occurs in the early stages. On long TR/TE images, there is an increase in signal in the disk and in the contiguous vertebral body marrow cavities that may appear more extensive than the changes seen on the T1-weighted studies. Tuberculous and fungal infections have a propensity to extend beneath the longitudinal ligaments to involve multiple vertebral levels. Since these organisms don’t produce any proteolytic enzyme, the disk space is preserved during the early stages of infection. Gadolinium-enhanced T1-weighted images with fat suppression are the most sensitive for detecting spinal infections and for delineating the extent of disease.
While these signal alterations are also seen in the marrow of patients with primary or metastatic neoplastic disease, as a rule tumor does not breach the end plate. Also, although extensive involvement of the marrow cavity may be present with neoplasia, including vertebral destruction and collapse, the disk space often remains intact or is only minimally affected. Based on the morphologic features of vertebral diskitis and osteomyelitis, a more accurate diagnosis of spinal infection can be made with MR imaging than conventional plain film radiography, nuclear medicine, and computed tomography. When early diskitis and osteomyelitis go undetected and untreated, epidural or paravertebral abscess may develop, requiring surgical drainage.
Most epidural abscesses are associated with diskitis or osteomyelitis, however, isolated infections of the epidural space can occur. The diagnosis of epidural abscess can be a challenge for both the clinician and radiologist. Patients may present with back pain or radicular pain. Fever and leukocytosis may be mild. Early diagnosis and prompt therapy are critical for favorable patient outcomes.
The imaging findings can be quite subtle on plain T1 and T2-weighted images. During the cellulitis stage, the first sign of infection is thickening of the epidural tissues, which is initially isointense on T1-weighted images and moderately hyperintense on T2-weighted images. When liquefaction occurs, the abscess cavity becomes hypointense and more hyperintense on T1 and T2-weighted images, respectively. Detection of the infectious process is easier on gadolinium-enhanced scans. The inflamed tissues are very vascular and enhance with gadolinium. On both the T2-weighted images and the enhanced T1-weighted images, fat suppression increases the contrast between the infectious process and normal tissues. The abscess cavity does not enhance, and appears as thin linear region of hypointensity surrounded by the enhancing cellulitis on sagittal images. The abscess cavity has an oval configuration on axial images.
These tumors arise from the sympathetic nervous system and occur in children. Two thirds originate in the adrenal medulla and the remainder in the paravertebral sympathetic plexus. A prominent soft-tissue mass is often found at the time of presentation. Of major concern is the potential of these tumors to extend through the adjacent neural foramina to enter the epidural space and compress the cord. MR can accurately assess both the paraspinal and intraspinal components so that myelography is no longer necessary in these patients. On T2-weighted images neuroblastoma is hyperintense relative to the adjacent tissues in the mediastinum and epidural space. T1-weighted scans give good contrast between epidural tumor and the thecal sac. Neuroblastoma also has the potential to spread to the bone marrow.
Another common tumor of the paravertebral region is lymphoma. It usually starts in the periaortic lymph nodes and extends from there to involve adjacent structures. Like other paraspinal tumors or infections, lymphomas have ready access to the epidural space via the neural foramina. Osseous involvement is also possible. MR is gaining in importance for staging of lymphomas.