Imaging for Scoliosis: An Overview

Scoliosis refers to abnormal curvature of the spine, which can result from various causes. The most common form is idiopathic scoliosis, with other causes including congenital malformations.

Scoliosis can occur in the mediolateral or anteroposterior directions. In the mediolateral dimension, the curvature is rightward or leftward and is named according to the direction of convexity or concavity. In the anteroposterior direction, the curvature is referred to as kyphotic or lordotic. Most scoliosis also has a rotational component.

Conventional radiography (x-ray) is an excellent means of assessing scoliosis. Examinations at HSS are performed on long continous images usually obtained with the patient standing. Utilizing this technique, the angles can be measured and grade the deformity and aid in the assessment of the progression or resolution of curvature. In the pediatric age group, congenital anomalies of the spine contributing to the curvature can also be assessed.

Many surgeons utilize stress or lateral bending views to provide additional prognostic information for preoperative planning. In post-surgical patients, conventional radiographs also serve as a means of follow-up and assessment of the hardware.

CT and MRI do not traditionally have much of a role in the evaluation of scoliosis, but are useful for the further assessment of congenital anomalies of the vertebral bodies which may contribute to curvature. In rare cases, the scoliotic curve may be a result of a syndrome such as neurofibromatosis or a painful tumor such as an osteoid osteoma. In these cases, cross sectional imaging (CT and MRI) is invaluable. Scoliosis produces abnormal forces on the spine that can often lead to premature and severe degenerative change of the spine. CT and MRI can assess these changes as outlined in the preceding sections.

Listhesis refers to movement of one vertebral body in relationship to an adjacent vertebral body. This movement may be in either a mediolateral or anteroposterior direction. With scoliosis, lateral listhesis is not uncommon adjacent to the epicenter of the curve. Antero- or retrolisthesis can be the result of degenerative change at the level of the facet joints or a fracture (break or lysis) in the posterior elements, most commonly at the level of the pars interarticularis.

The movement of one vertebral body on another may promote degenerative change and lead to narrowing of both the central canal and neural foramina. The listhesis or "slippage" may progress over time, causing more symptoms. Conventional radiographs can often help distinguish between these etiologies. Oblique projections are necessary to visualize the pars interarticularis and facet joints optimally.

CT and MRI can provide additional information as to the etiology and effects of the listhesis. The dimensions of the central canal and neural foramina and effects on the thecal sac and nerve roots can be evaluated. Surgery is often necessary to address the effects of listhesis and stabilize the spine to prevent any further progression. Crossectional and multiplanar imaging from CT or MR examinations are utilized to plan appropriate treatment and surgical plan.

In the pediatric patient where decreased levels of radiation are desirable, MRI may be indicated. However, MRI usually requires anesthesia to reduce patient motion. Nuclear medicine also has a role in the detection of pars defects. In patients with pars defects who opt not to have surgery, imaging guided steroid injections of the pars defect often alleviate some of the symptoms.

Osteoporosis has important implications in the spine. Weakened, osteoporotic bone is more prone to the effects of even minor trauma. Crushed or collapsed vertebrae can be very painful and can lead to loss of pulmonary function as the thorax is decreased in size.

 Most vertebral compression fractures take the form of an anterior wedging deformity and do not involve the posterior aspect of the vertebral body. Fractured vertebrae can also be of the burst variety with retropulsion of the posterior vertebral body margin which can result in canal compromise. The thoracic spine is the most common location for these fractures, but lumbar compressions are not infrequent.

Conventional radiography should be the first means of evaluating these injuries, providing accurate information as to the number of vertebral bodies involved and the degree of compression. Asymptomatic compression fractures found incidentally may be followed for progression. Unfortunately, evaluation of osteoporotic bones can be limited because of poor bony detail.

CT and MRI have a role in the evaluation of compression fractures because both are more sensitive to bone detail. Information about alignment and the effects of the fractures on the vertebral canal and neural structures can be obtained on mutliplanar reformations.

MRI is particularly useful in the assessment for possible vertebroplasty and in the differentiation of malignant from benign compression fractures. Vertebroplasty and kyphoplasty are recent techniques for the treatment of compression fractures whereby cement is percutaneously injected into the vertebral bodies to relieve the symptoms and/or restore height. These procedures are most successful when the compression fracture is in the acute to subacute stages. Chronic fractures with sclerosis may be more difficult to treat and less likely to respond. MRI, with its ability to detect bone marrow edema, a feature of acute to subacute fractures, helps in the identification of appropriate candidates for these procedures.

Trauma

Fractures of the spine are more common in the cervical and thoracic region than the lumbosacral spine. Seatbelt injuries or traumatic burst fractures may occur at the lumbosacral junction. Conventional radiographs can reveal displacement or loss of height. CT or MRI are utilized to evaluate the integrity of the spinal canal and neural structures. MRI is also optimally suited for evaluation of suspected injury despite negative conventional radiographs and/or symptoms out of proportion to the imaging findings. MRI may reveal unsuspected fractures and associated soft tissue injuries that were not evident on conventional radiographs.

This elderly age group that is prone to osteoporotic compression fractures is also more susceptible to the development of malignancies and vertebral body metastases. Metastases may replace healthy bone, weakening the vertebral bodies, and increasing the risk of fracture. MRI may help distinguish these fractures by detecting the replacement of normal marrow by abnormal tumor or an associated soft tissue mass.

Tumors of the spine may result in pain or neurologic symptoms. They involve the bony structures, soft tissues or both and may be primary or metastatic.

The most common primary benign bone tumor of the spine is the hemangioma. Other less common benign tumors include osteoid osteomas, osteoblastomas and giant cell tumors. Perhaps the most common primary malignant tumor of the spine is the chordoma. Some of these tumors have characteristic appearances on conventional radiographs. Hemangiomas have a striated appearance which has prompted the description, "corduroy" vertebral body.

Osteoid osteomas have a predilection for the posterior elements of the spine and present as a sclerotic pedicle associated with painful scoliosis. Chordomas are tumors of the midline that occur most commonly in the sacrum.

Most of these tumors will require further evaluation with either CT or MRI in order to both help in diagnosis, and assess the effects on the spinal canal and nerve roots. These examinations may be of particular value in confirming the diagnosis of hemangioma and avoiding unnecessary biopsy. On CT, this lesion has vertical struts of bone with interspersed fat and on MRI, it is classically high signal on both T1 and T2 weighted images.

Primary tumors within the canal are often neurogenic in origin, e.g. meningiomas, neurofibromas, lipomas and ependymomas. The space-occupying effects of these tumors may be visualized on conventional radiographs with widening of a neural foramen or posterior scalloping of a vertebral body alerting the radiologist to the possibility of a tumor. In most cases, CT and MRI will be required to confirm or exclude the tumor as well as provide diagnostic and pre-operative information.

The most common non-primary malignant bone tumor in the spine is a metastasis. Conventional radiographs are an insensitive means of evaluating metastatic disease, particularly for lytic lesions. The most comprehensive means of evaluating for metastatic disease to bone is a radionuclide nuclear medicine bone scan. Lesions in the spine noted on bone scan can be further evaluated with CT or MRI. If tissue diagnosis is required, biopsy can be performed utilizing imaging (usually CT) guidance.

Spinal infections are relatively uncommon. In the past, one of the most common spinal infections was tuberculosis. Cases of tuberculous spondylitis still occur, but with much less frequency. The most common organism now is staphylococcus aureus. The population predisposed includes post-operative patients and immunocompromised patients, including those with HIV. In men, the prosthatic venous plexus has connections to the paravertebral plexus and genitourinary infections may result in spondylitis.

On conventional radiographs, early infection may not be apparent. With progression, loss of disc height, endplate erosion and paravertebral soft tissue mass may be evident. When there is suspicion of infection, MRI should be ordered. On MRI, infection is suspected when there is abnormal signal involving both the disc space and adjacent vertebral bodies. Soft tissue swelling can be differentiated from abscess formation and epidural extension of infection detected. Gadolinium is administered to demonstrate enhancement at sites of infection. If the diagnosis is in question or a sample is required for culture prior to initiating therapy, a biopsy of the vertebral body, disc, or adjacent soft tissues can be performed utilizing CT guidance.

Sacroiliitis of various etiologies may result in low back pain. The sacroiliac joints can be involved with osteoarthritis and infection. In addition, certain of the arthritides have a predilection for involvement of the SI joints including ankylosing spondylitis, psoriasis, reactive arthritis and the arthritis associated with inflammatory bowel disease.

The sacroiliac joints can be evaluated on conventional radiographs, especially utilizing specialized uptilt and oblique views. In early stages of inflammatory disease, erosions and sclerosis can be detected. With many of the systemic inflammatory conditions, including ankylosing spondylitis, endstage disease results in complete fusion of the SI joints.

With ankylosing spondylitis in particular, associated changes of the spine can be noted on conventional radiographs. These include the formation of thin, vertically oriented, symmetric paravetebral bony excresences called syndesmophytes resulting in the characteristic bamboo spine. Other changes can include subtle erosions and sclerosis of the vertebral body margins.

CT is of limited value in most cases of sacroiliitis unless infection is suspected and imaging guided aspiration of the joint is required. CT may delineate the extent of osteophytosis in cases of osteoarthritis.

Nuclear medicine bone scans may be positive in advanced cases of sacroiliitis and specialized radionuclide-labeled white blood cell scans may aid in detection of infection. MRI is the most sensitive modality for the detection of early disease. Inflammation of the SI joints will manifest on MRI as abnormal bone marrow edema along both the iliac and sacral margins of the joints.

Imaging in the postoperative patient is made more difficult by the presence of surgical hardware and scar tissue. With the advent of high speed thin-section CT scanning, detailed imaging with reduction of metal artifact has become possible. The addition of myelographic contrast may also be advantageous in the postoperative patient. At HSS, MRI protocols have been developed to help minimize the susceptibility artifact from hardware and provide diagnostic information in these patients.

Imaging Guided Spine Interventions

Imaging guided injection of anesthetic solution, steroid or both, can be performed for diagnostic and therapeutic purposes. These injections are made into a common epidural space surrounding the thecal sac and the central nerve roots or around a particular nerve root. The needle is positioned using fluoroscopic or CT guidance and needle placement confirmed with the injection of a small amount of radioopaque radiographic contrast.

Injections of anesthesia, steroid, or both, can also be performed into degenerated or painful facet and/or sacroiliac joints. As mentioned earlier, synovial cysts resulting from degeneration of the facet joints can be aspirated and injected under fluoroscopy or CT guidance. For longer relief of facet related pain, the median nerves supplying sensation to the facet joints can be selectively ablated using specialized radiofrequency probes in an imaging guided procedure referred to as a median branch rhizotomy.

Many recent advances have been made in the minimally invasive treatment of vertebral compression fractures. In vertebroplasty, needles are percutaneously placed into the affected vertebral bodies using a transpedicular approach and radioopaque cement injected under fluoroscopic guidance. This is an outpatient procedure requiring approximately 4-6 hours in the hospital and resulting in almost instant relief of pain. If restoration of vertebral body height is required, kyphoplasty can be performed. This procedure is performed in the operating room and a balloon catheter is inserted into the vertebral body which is inflated before the cement is delivered.