Lower Back Pain

Low back pain is a leading cause of medical disability and lost work time in the United States. It is estimated that the annual cost of low back pain in terms of lost production, medical expenses and workmen's compensation benefits is in the tens of billions of dollars. In fact, four out of every five Americans can expect to experience an episode of back pain at some time in their lives. Low back pain or what has commonly been referred to as "sciatica" or "lumbago" can have many etiologies and, often, is multifactorial. Some of the more common causes are disc herniation, degenerative disc disease, osteoarthritis, malalignment including scoliosis and spondylolisthesis, osteoporotic compression fractures, trauma, tumor, infection, seronegative spondyloarthropathies and sacroiliitis.

The radiologist has a vital role in diagnosing the cause of back pain and guiding treatment. Multiple imaging examinations utilizing different equipment or modalities are available including conventional radiographs (x-rays), computerized tomography (CT), nuclear medicine, magnetic resonance imaging (MRI), myelography and discography. More recently, radiologists have become more involved in image-guided spine interventions such as epidural steroid injections, radiofrequency rhizotomy, intradiscal electrothermal annuloplasty (IDET), vertebroplasty and kyphoplasty.

Various Imaging Examinations/Modalities

Conventional Radiography

Conventional radiography is often the first imaging examination utilized to evaluate low back pain. Conventional radiographs provide an overview of the spine. Information about alignment both under static and dynamic conditions can be obtained. Conventional radiography, however, is primarily for evaluation of the bony structures with limited soft tissue discrimination and also uses ionizing radiation.

The standard conventional radiographic exam at most institutions consists of 3 views: AP, lateral and a spot film of the lumbar spine (obtained with the patient supine). At HSS, we image the patient in the standing position with axial weight load on the lumbar spine in order to demonstrate subtle evidence of disc disease and malalignment which may otherwise be undetected (view images). An uptilt coned frontal view of the sacroiliac joints and the L5-S1 disc space (which often is obscured on a routine AP film due to the normal lordosis at this level) is (view images). If clinically indicated and specifically requested by the referring physician, other, more specialized views are obtained. Oblique views of the spine are very useful for assessing the facet joints and looking for lysis (breakage) in the posterior spinal elements (oblique, lateral). If malalignment or instability is of clinical concern, flexion/extension and/or lateral bending views obtained with the patient standing or supine views while stress is applied are performed (view images).

For accurate assessment of scoliotic curvature, long cassettes (36 or 52 inches) are used so the entire spine is incorporated into the examination (view images).

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Computed Tomography(CT)

CT is a very useful tool for analyzing the alignment and morphology of the bony elements of the spine providing information about the soft tissues including the discs, ligaments, nerve roots and spinal cord. CT, both alone or in conjunction with myelography, is often the next imaging examination ordered for many patients with low back pain. With the advent of high speed CT scanners utilizing new technology, it is now possible to obtain thin slices in a short time period and software allows the information to be reformatted in different planes. Although it does afford some information about the soft tissues, CT is not as sensitive as magnetic resonance imaging and also utilizes ionizing radiation.

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Magnetic Resonance Imaging (MRI) 

MRI, with its excellent soft tissue discrimination and multiplanar imaging capabilities, has been the fastest growing new method for spine imaging. Utilizing different pulse sequences to create contrast between the tissues, information about all of the elements of the spinal column and its contents (spinal cord, nerve roots and cerebrospinal fluid) can be obtained and early disease which is radiographically occult on conventional radiographs and CT can be detected. Standard MRI protocols for spine imaging include the axial and sagittal planes (view images). At HSS, we include coronal images which enable further analysis of exiting nerve roots and assessment for areas of impingement or stenosis (view images). The coronal plane also gives information about the status of the sacroiliac joints which are often a source of low back pain.

In the post-laminectomy patient, MRI is the gold standard for assessing the cause of "failed-back syndrome" and differentiating disc herniation from post-operative scar formation (view images). Up until recently, metallic susceptibilty artifact due to surgical hardware has limited the role of MRI in such patients. At HSS, we have developed sequences to minimize this effect and can achieve sensitive and specific imaging in this patient population. (view images).

MRI operates on the principles of electromagnetic fields and does not use ionizing radiation. There are certain limitations though. The bore of the magnet is small due to constraints imposed by the laws of physics, and large and claustrophobic patients may not be able to tolerate the exam. Patients with shrapnel, pacemakers or certain other surgically implanted devices may not be imaged due to possible metal motion or heating.

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Nuclear Medicine

Nuclear medicine provides another means of imaging the patient with low back pain. Although very sensitive, nuclear scans have relatively poor resolution and contrast which limit specificity. With the addition of Single Photon Emission Computerized Tomography (SPECT) imaging in different planes has become possible with increased sensitivity. Radionuclide scans are a very effective means of assessing the entire skeleton, including the spine, for the presence of metastatic disease in patients with a known primary tumor. This examination may identify an easily accessible lesion for image guided biopsy if tissue confirmation is necessary. Bone scans have also been utilized for imaging spondyloysis in the pediatric population (view images).

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Special Techniques- Myelography and Discography

Special techniques for the imaging of back pain include myelography and discography.  

Myelography involves performing a lumbar or sometimes cervical spine puncture for the purposes of introducing radioopaque contrast material into the thecal sac which is the space containing the cerebrospinal fluid surrounding the spinal cord and nerve roots. This material outlines the canal contents increasing contrast and sensitivity for detection of neural foraminal or central canal stenosis. This procedure is usually performed in conjunction with both conventional radiographs and CT (view images). At HSS, many of our surgeons require the information provided by a CT myelogram for preoperative planning and we perform hundreds of these exams each year. The technique is especially useful in those patients for whom MRI is not an option. The procedure is invasive, however, and does carry certain risks including post-procedural headache. Additional information about myelography is available.

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Discography involves the imaging guided placement of a spinal needle into the disc spaces and injection of radioopaque contrast material. It is a provocative test designed to assess the level of origin of back pain. The patient is awake during the procedure and is asked if their typical pain is reproduced during sequential injections at multiple levels. This procedure, like myelography, is followed by both conventional radiographs and CT which provides morphologic information as regards the discs (view images). In the patient with no obvious source of back pain on traditional imaging methods, discography may provide the surgeon with a probable focus. Like myelography, discography is also an invasive procedure with some limited risk to the patients. Additional information about discography is available.

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Imaging of Specific Disease Entities

Degenerative Disease

Overview:
Degenerative disease of the spine can take several different forms depending on which structures are involved. Changes restricted to the discs are referred to as degenerative disc disease or discogenic degenerative change. The formation of osteophytes or spurs along the vertebral body margins is termed spondylosis or spondolytic changes. Alternatively, osteoarthritis of the spine similar to that encountered in other joints in the body refers to degenerative changes of the facet joints between the posterior elements of the spine.

Degenerative Disease of the Discs

The characteristic findings of discogenic degenerative change on plain radiographs include loss of disc height, irregularity and sclerosis of the endplates (view images) and herniation of nuclear disc material through or along the margins of the endplates. The latter finding results in the formation of what is referred to as a Schmorl's node. When this occurs in a skeletally immature patient, the herniation may occur through the physis and the ring apophysis may not ossifiy or ossify as a separated fragment resulting in a limbus vertebral body. These findings on plain radiography are often a harbinger of anterior or posterior disc herniation which may cause compression of the spinal cord or nerve roots resulting in neurogenic symptoms. The disc, being a soft tissue structure, is not seen on plain films and cross sectional imaging (CT or MRI) is necessary for further evaluation in this scenario.

On CT many of the same features noted on conventional radiographs can be visualized along with the disc material. The mass effect caused by herniated discs on the spinal cord and nerve roots can be assessed and this information utilized for determining surgical or non-surgical therapy (view images). Many of the HSS spine surgeons at HSS prefer the added sensitivity of CT myelography for assessing the degree of thecal sac or nerve root compression, particularly in post-operative patients and patients with spinal hardware. With this technique, the nerve roots can be distinguished as filling defects in the contrast column.

In most patients, MRI is the gold standard for assessment of disc degeneration and herniation and a reliable means of preoperative assessment. Excellent soft tissue contrast allows the radiologist to distinguish the different compartments of the spinal canal, identify individual nerve roots and assess the degree of central canal stenosis and neural foraminal narrowing (view images). With MRI, bony endplate changes other than sclerosis have been characterized. Most radiologists believe that these edematous and fatty endplate changes represent different stages along a continuum ending with the sclerotic endplate changes noted on plain film or CT (view images). These earlier stages can often serve as a marker for developing degenerative disease and have been correlated with symptoms. MRI is also the modality of choice for assessing the need for minimally invasive therapy including epidural steroid injections. In the post-operative patient, MRI with gadolinium is effective for discerning the different etiologies of the so-called "failed-back syndrome". With this method, scar tissue can often be distinguished from residual or recurrent disc herniation (view images).

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Discography, although invasive, is another effective means of assessing disc degeneration. The normal disc has a soft central nucleus pulposus and a tough outer annulus fibrosus which keeps the nucleus contained. The discs act as shock absorbers. Early disc degeneration often begins with tears or rents in the annulus and escape of nuclear material into surrounding spaces. Although MRI is often sensitive to these early changes, the clinicians can be faced with a situation where the imaging findings do not seem to correlate with the patient's symptoms. In these cases, the provocative discogram may help guide the surgeon to the appropriate level. The distribution of contrast within a disc provides useful information about disc morphology including specific sites of annular tear or degeneration (view images). By correlating the patient's symptoms at the time of disc injection with these findings, the radiologist can help direct the surgeon to the source of the patient's symptoms.

Osteoarthritis of the Spine

Osteoarthritis of the facet joints is another component of spinal degenerative disease which may be responsible for the patients' symptoms as well as contribute to spinal canal stenosis or neural foraminal narrowing. The changes are often referred to as hypertrophic degenerative change. One of the dominant features is formation of new bone including osteophytes. On conventional radiography, degenerative change of the facet joints can present as increased sclerosis in this region. On oblique projections, the joint spaces themselves and any narrowing can often be defined (view images).

CT is an excellent imaging examination for more detailed anatomy of the facet joints and the effect of hypertrophic degenerative change on the central canal and neural foramina. In addition, CT can provide information about associated changes of the ligamentum flavum, the ligaments along the inner margin of the facet joints which help provide stability of the posterior elements. These ligaments can also hypertrophy in response to the additional stresses as a result of degenerative changes of the spine. The facets and ligamentum flavum form a portion of the posterior margin of both the central canal and the neural foramina and they can contribute to narrowing in both locations and compression of both the thecal sac and exiting nerve roots. These relationships and changes can be identified on CT (view images). This information is important to the surgeon in planning the procedure appropriate to the individual. Oftentimes, the facet joints themselves which receive innervation from the medial branch of the nerve roots can be symptomatic. In these cases, the clinician may opt to inject the facet joints with steroids or ablate the median nerve utilizing radiofrequency or chemically.

MRI is another excellent means of assessing the facet joints and ligaments (view images). Many of the same changes noted on CT can be detected with MRI. However, MRI is more sensitive in the detection of synovial cysts related to the facet joints. The facet joints, like other synovial joints, have a joint capsule and synovial lining. Oftentimes, in degenerative joints, cysts which are continuous with the joint space can form. These cysts are space-occupying lesions which can contribute to central canal stenosis or neural foraminal narrowing (view images). When requested, the radiologists perform CT or fluoroscopically guided aspirations and steroid injections of these cysts (view images).

Acute Disc Herniation

Though many disc herniations are related to gradual degeneration and deterioration of the discs, other disc herniations can be acute in nature, precipitated by an event with markedly increased pressure in an otherwise normal disc space. In these cases, conventional radiography may serve for an initial screening, but is often non-contributory as the herniation is unrelated to degenerative changes. CT and MRI are more effective means of assessing these patients as described in the preceding discussion. MRI may also reveal associated ligamentous or soft tissue trauma.

Malalignment - Scoliosis and Listhesis

Malalignment in the spine has two major forms. The first usually involves a long segment or the entire spine and is referred to as scoliosis. The second occurs at as specific level between two vertebral bodies and is termed listhesis.