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), magnetic resonance imaging (MRI), computerized tomography (CT), myelography and discography. Radiologists also perform image-guided spine interventions, such as facet blocks and epidural steroid injections.
Conventional radiography is often the first imaging examination utilized to evaluate patients with low back pain. Conventional radiographs (x-rays) provide an overview of the spine. Information about spinal alignment can be obtained under static and dynamic conditions. Conventional radiography, however, is primarily for the evaluation of the bony structures. It also uses ionizing radiation, which needs to be limited whenever possible.
The standard conventional radiographic exam of the spine consists of 2 views of the spine: AP (frontal) and lateral, obtained with the patient supine (lying). At HSS, we image the patient in the standing position, which provides an axial load on the spine, in order to optimize the assessment of spinal alignment. If requested by the referring physician, additional views of the spine are obtained, including oblique views to evaluate the spinal posterior elements and a lateral spot view of the lumbosacral junction. If malalignment or instability of the spine is of clinical concern, flexion/extension and/or lateral bending views may also be obtained with the patient standing or supine.
For accurate assessment of scoliosis, long 52" views of the entire spine are obtained.
Magnetic Resonance Imaging (MRI)
If a clinician needs additional information to direct therapeutic decisions, MRI, with its excellent soft tissue discrimination and multiplanar imaging capabilities, is the optimal imaging exam to evaluate the spine in most clinical situations. Utilizing different pulse sequences to create contrast between the tissues, information about all the elements of the spinal column and its contents (spinal cord, nerve roots and cerebrospinal fluid) can be obtained. Spinal diseases which may be not be detected with x-rays or CT may be detected with MRI. Standard MRI protocols for spine imaging include axial, sagittal and coronal images. Sagittal and axial images provide comprehensive information about the disc, nerve roots and the facet joints and can be used to assess spinal stenosis. The coronal plane provides additional information about spinal alignment and the sacroiliac joints, which may be a source of low back pain.
In the post-operative patient, MRI is the gold standard for assessing the "failed-back syndrome" and differentiating a disc herniation from post-operative scar formation. Until recently, metallic susceptibility artifacts from surgical instrumentation limited the role of MRI in these patients. However at HSS, new MRI sequences have been developed to minimize hardware interference which results in the improvement of the diagnostic capabilities of MRI.
MRI operates on the principles of electromagnetic fields and does not use ionizing radiation. Due to the electromagnetic fields certain patients can not be imaged with MRI, for example patients with pacemakers, certain surgical brain clips and some surgically implanted devices.
Computed Tomography (CT)
CT is very useful for analyzing the alignment and morphology of the bony elements of the spine and to assess soft tissues. CT is the optimal exam to assess the integrity of a spinal fusion and to evaluate traumatic osseous deformities. Alone or in conjunction with myelography, CT is often utilized to evaluate patients with spinal stenosis. With the recent advances in CT technology a CT exam is completed very quickly and CT images can be reformatted in different planes. 3D models of the spine can also be created. CT uses ionizing radiation to create its images and radiation dosage at HSS is critically monitored during all examinations.
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 radio-opaque contrast material into the thecal sac. The thecal sac is the tube in the spinal central canal that contains the spinal cord and nerve roots. After the introduction of contrast material the patient can be placed in the erect position in order to evaluate the spinal contents with the patient weight-bearing. Flexion and extension views are also obtained which may provide additional useful information in a patient with spinal stenosis. Myelography is usually performed in conjunction with both conventional radiographs and CT. At HSS, a CT-myelogram may be used as part of the preoperative planning in patients with stenosis. Myelography is also particularly helpful in the evaluation the spinal canal and the thecal sac in post-operative patients with spinal instrumentation. Read additional information about myelography.
Discography involves the image guided placement of a needle into the disc space and injection of radio-opaque contrast material into the disc. It is a provocative test designed to assess the specific level of a patient’s back or leg pain. The patient is awake during the procedure and the patient needs to respond if their pain is reproduced during the injection of the discs. Discography, like myelography, is followed by both conventional radiographs and CT which provide morphologic information about the discs. Discography may be utilized if MRI and CT does not provide the reason for a patient’s back or leg pain. Like myelography, discography is an invasive procedure with some limited risk to the patients. Read additional information about discography.
Degenerative disease of the spine is a multifactorial clinical condition. Degeneration may involve the disc and the adjacent vertebral bodies and is referred to as degenerative disc disease. A disc herniation typically occurs in a disc that is degenerated. Arthritis of the facet joints (the small joints that are located at each disc space posterior and lateral to the spinal central canal) may also be the source of back pain. Spinal stenosis may result from degeneration of the disc or facet joints.
Degenerative Disc Disease
The characteristic findings of degenerative disc disease on conventional radiographs include loss of disc height and irregularity and sclerosis of the adjacent vertebral body endplates. A Schmorl's node, which is the herniation of disc material into the vertebral body, may also be detected with radiographs. In a skeletally immature patient, herniated disc material may extend through the rim of the vertebral body and may result in displacement of a bone fragment, referred to as a limbus vertebra. The exact condition of the disc can not be determined with plain films and cross sectional imaging, such as MRI or CT, would be necessary to further evaluate the disc and to determine if a disc herniation is present.
MRI is the gold standard for assessment of disc degeneration and herniation and the effects of the degenerative process on the neural elements. MRI provides information as to the severity of the disc degeneration as well as to detect if a disc herniation is present. Degenerative disc disease may be associated with endplate degeneration which may lead to spinal stenosis. MRI is the optimal exam to determine the extent and severity of spinal degeneration. Vertebral body endplate degeneration may be detected by the presence of edema or fat within the endplates. Endplate degeneration represents a continuum of different stages of degeneration beginning with bone edema and ending with the endplate sclerosis. MRI is the most comprehensive exam to evaluate patients with degenerative disc disease and is frequently ordered prior to performing minimally invasive therapy, such as epidural steroid injections. In the post-operative patient, MRI with gadolinium is effective in determining the etiology of persistent or new pain. With MRI it is easy to differentiate residual scar tissue from a recurrent disc herniation.
In patients with degenerative disc disease CT is the optimal imaging technique to visualize the bony proliferation and soft tissue calcification that may accompany disc degeneration. The bony proliferation may result in stenosis of the central spinal canal or the neural foramina and the compression of nerves. In the cervical spine CT-myelography may be useful in the assessment of spinal cord compression.
Discography, although invasive, is an additional means to assess disc degeneration. The normal disc has a central nucleus pulposus and a tough outer annulus fibrosus which contains the nucleus pulposus. The disc acts as shock absorber and helps to distribute the loads placed on the spine. Disc degeneration often begins with changes in the chemical composition of the nucleus pulposus and the development of fissures in the annulus. Disc material may herniate through the fissures and impinge the nerve roots. Although MRI may detect these morphologic changes it can not determine if the degenerated disc is a source of pain. Provocative discography may help to clarify the source of a patient’s pain. The distribution of contrast within the disc provides useful information about disc morphology including specific sites of annular fissures or herniated disc material.
Osteoarthritis of the Spine
Osteoarthritis of the facet joints is another component of spinal degenerative disease that may be the cause of a patient’s symptoms and may contribute to spinal central canal or neural foraminal stenosis. The facet joints contain a layer of articular cartilage which may degenerate, similar to other joints in the body. The facet joint contains a synovial lining and a capsule. Degeneration of the facet joints may be associated with bony proliferation along the margins of the joint. Conventional radiographs only provide a limited assessment of the facet joints. MRI or CT is needed to determine the severity of the facet arthritis and its effect on the adjacent nerves. If isolated facet arthritis is a cause of a patient’s back pain a selective block of the nerve that innervates the facet joint may alleviate a patient’s pain.
CT provides more detailed anatomy of the facet joints and the effect of hypertrophic degenerative changes of the facet on the central canal and neural foramina. In addition, when facet arthritis contributes to spinal stenosis, the CT is used as part of preoperative planning to determine the extent of the surgical decompression.
MRI is also an excellent means of assessing the facet joints and ligamenta flava, the ligaments along the medial margin of the facet joints. Degeneration and hypertrophy of the ligamenta flava, which may accompany facet degeneration, may lead to spinal stenosis and nerve root compression. With facet degeneration synovial cysts may originate from the facet joint and extend into the central spinal canal or neural foramina. The cysts may compress the nerve roots and cause leg pain. MRI is the optimal imaging technique to detect synovial cysts and to determine if they are compressing the neural elements. When requested, the radiologist can aspirate or rupture a synovial cyst to alleviate a patient’s symptoms.
Almost all disc herniations are the result of degeneration of the disc. With the development of annular fissures in the disc it is possible for disc material to herniate beyond the periphery of the disc and impinge the spinal nerves. Most patients do not relate a specific traumatic event that occurred when they first experienced their pain. Conventional radiography, while frequently ordered when patients present with symptoms of a disc herniation, provides little useful information as to the cause of a patient’s symptoms. MRI is the optimum exam to assess the condition of the disc and whether a disc herniation is also present. Occasionally with a major traumatic event, such as a motor vehicle accident or a serious fall, it is possible to develop a disc herniation along with a fracture of the adjacent vertebral body. In these cases CT may provide additional information to the MRI exam as to the extent and severity of the osseous deformity. CT is also helpful in the assessment of calcification or ossification in chronic disc herniations, particularly in the thoracic spine.
Spinal malalignment - Scoliosis and Listhesis
Malalignment of the spine has two major forms. The term scoliosis is used to describe the overall curvature of the spine. A curvature of the spine of at least 10 degrees is required to meet the criterion for a mild scoliosis. Listhesis refers to the alignment of two adjacent vertebral bodies in the spine. Anterior displacement of the cephalad vertebral body with respect to the caudad vertebral body is called an anterolisthesis. Posterior displacement of the cephalad vertebral body with respect to the caudad vertebral body is called a retrolisthesis. And a lateral displacement of the cephalad vertebral body with respect to the caudad vertebral body is called a right or left lateral listhesis. Abnormal spinal alignment may lead to asymmetric loads on the spine and spine degeneration.
Department of Radiology and Imaging
Hospital for Special Surgery
Reviewed by Richard J. Herzog, MD, FACR