Imaging for Lower Back Pain: An Overview

• Various Imaging Examinations/Modalities
• Special Techniques- Myelography and Discography
• Imaging of Specific Disease Entities

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. Radiologists also perform image-guided spine interventions such as facet and epidural steroid injections.

Various Imaging Examinations/Modalities

Conventional Radiography

Conventional radiography is often the first imaging examination utilized to evaluate low back pain. Conventional radiographs (x-rays) 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. If clinically indicated and specifically requested by the referring physician, other, more specialized views are obtained. Oblique and lateral views of the spine are very useful for assessing the facet joints and looking for lysis (fracture) in the posterior spinal elements. If malalignment or instability is of clinical concern, flexion/extension and/or lateral bending views obtained with the patient standing or supine can be helpful.

For accurate assessment of a scoliotic curvature, long 52" images are used so the entire spine is visualized in continuity.

Computed Tomography (CT)

CT is very useful for analyzing the alignment and morphology of the bony elements of the spine and the soft tissues, providing information about the soft tissues including the discs, ligaments, nerve roots and spinal cord. Both alone or in conjunction with myelography, CT is often ordered for patients with low back pain to obtain thin slices of the back which can be reformatted in different planes.

Magnetic Resonance Imaging (MRI)
MRI, with its excellent soft tissue discrimination and multiplanar imaging capabilities, is an excellent 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 may be radiographically occult on conventional radiographs and CT, can be detected. Standard MRI protocols for spine imaging include the axial, sagittal and coronal images. Analysis of exiting nerve roots and assessment for areas of impingement or stenosis are analyzed. 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 "failed-back syndrome" and differentiating disc herniation from post-operative scar formation. Until recently, metallic susceptibilty artifact due to surgical hardware has limited the role of MRI in such patients, however at HSS, sequences have been developed to minimize hardware interference and sensitive and specific imaging of post op pain is achievable.

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

Nuclear Medicine
Nuclear medicine is another means of imaging patients with low back pain. Although very sensitive, nuclear medicine scans have relatively poor resolution and contrast which limit specificity. With the addition of Single Photon Emission Computerized Tomography (SPECT) imaging, the sensitivity is even greater. 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.

Special Procedures- Myelography and Discography

Special procedures for the imaging of back pain include myelography and discography. 
 
Myelography involves performing a lumbar or cervical 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. At HSS, many of the surgeons require a CT myelogram for preoperative planning. As a result, we perform hundreds of these exams each year.  Read additional information about myelography.

Discography involves the imaging guided placement of a needle into the disc spaces and injection of radioopaque contrast material. It is a provocative test designed to assess the specific level of origin of back pain. The patient is awake during the procedure and is part of the examination. The paitent needs to respond if their pain is reproduced during sequential injections at various levels. This procedure, like myelography, is followed by both conventional radiographs and CT which provide morphologic information about the discs. 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. Read additional information about discography.

Imaging of Specific Disease Entities

Degenerative Disease

Degenerative disease of the spine vary 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 conventional radiographs include loss of disc height, irregularity and sclerosis of the endplates, and herniation of nuclear disc material into 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 may 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 that 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. Many of the spine surgeons at HSS prefer the added sensitivity of CT myelography for assessing thecal sac or nerve root compression, particularly in post-operative patients and patients with spinal hardware.

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. With MRI, bony endplate changes characterized as edematous and fatty and represent different stages along a continuum ending with the sclerotic endplate changes noted on conventional radiographs or CT. These earlier stages 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 be distinguished from residual or recurrent disc herniation.

Discography, although invasive, is an 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. By correlating the patient's symptoms at the time of disc injection with this information, the radiologist can help direct the surgeon to the specific level and site causing the patient's symptoms.

Osteoarthritis of the Spine

Osteoarthritis of the facet joints is another component of spinal degenerative disease that may be responsible for the patients' symptoms while contributing 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 scleroris and oblique projections, joint spaces narrowing can often be defined.

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 that help provide stability of the posterior elements. These ligaments can also hypertrophy in response to the additional stresses resulting from 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 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. 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 chemical agents.

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

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 because 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.