The Spine

Degenerative disease

 

Degenerative disease of the spine is a major health problem in the Western World. It is of considerable economical importance, since back pain and sciatica caused by degenerative disease lead to numerous consultations and prevent the patient from working for long periods. Furthermore, the patients are often middle-aged and therefore belong to one of the most important groups of workers. The diagnostic work is often difficult because in many patients there is a poor correlation between radiological findings and clinical symptoms. Thus, it is not unusual that a patient has advanced degenerative changes with large osteophytic spurs in the spine without symptoms, while others have severe symptoms with rather discrete radiological findings. The discrepancy between clinical symptoms and findings is especially pronounced in the cervical region. Another complicating element is the combination of psychosocial factors and low back pain. Therefore, it is of extreme importance that the radiologist liaises closely with the clinicians, preferably during dedicated clinico-radiological conferences. There are a number of different modalities and techniques that can be used for diagnosis and the choice between these is based on several factors, such as availability, cost and experience of the radiologist with the method. Furthermore, the situation is also changing due to the rapid spread, technical development and improved film quality of MRI, which certainly will influence working practice in the future.

Modalities

Plain films
Although plain films do not give information about the soft tissue, each evaluation should start with this modality which gives a good overview and shows anomalies, which are important for determining the level at which surgery should be undertaken. Plain films also show the degree of spondylosis, scoliosis and different dislocations. Such images guide the choice of modality for further soft tissue evaluation.

Myelography
Myelography has been used for many decades for evaluation of low-back pain and most radiologists are familiar with the method. The advantages of the method are that it gives a good overview, shows intradural morphology with a very high spatial resolution and is not as sensitive for patient motion as MRI. Furthermore, it has the capability of showing spinal block and provocation can easily be performed. This is of special importance in patients with spinal stenosis. In patients with intradural cyst formation and inflammatory adhesions, myelography is usually superior to CT and also to MR in the majority of cases. Myelography is of special value in patients with spinal stenosis, and even more when the spinal stenosis is combined with scoliosis. In patients with cervical root symptoms, in whom MRI has failed to give an explanation of the patient's symptoms, myelography and CT-myelography are also valuable. The disadvantages of myelography are that the method is invasive and that the area beyond the root-sleeve is not visualized. When there is a free choice of modality in well-equipped centres, myelography is nowadays seldom used routinely and is used only in selected patients.

Computed tomography (CT)
Today almost every major hospital in the Western World is equipped with a CT -scanner and most radiologists have considerable experience in the diagnosis of lumbar disease with this modality. CT is very valuable for the evaluation of degenerative disease in the spine for many reasons: the method is non-invasive, it is quick, it provides excellent visualization of bone in the axial projection and it shows the root-canals and paraspinal area. It is usually easy to make a diagnosis of disc herniation, as well as to detect bony elements narrowing the spinal canal, recesses and/or root-canals. CT is therefore especially useful for the diagnosis of the different causes of spinal stenosis. The draw-backs are that for practical reasons only a limited number of disk levels are routinely covered at most institutions, and only direct axial views can be obtained. Projections other than the axial view can be obtained by using thin slices and reformatting. However, if a large area is to be visualized, there will be significant irradiation of the patient. Another, and probably more important drawback is that the content of the dural sac is not visualized, unless contrast has been injected into the subarachnoid space, and therefore intradural pathology, such as a tumor, might be missed.

Magnetic resonance imaging (MRI)
MRI is the latest modality for morphological evaluation of the lumbar spine. The method has been available already for more than ten years, but the images obtained before the introduction of surface coils were of poor quality. There is a dramatic technical development still going on in this field, and the quality is steadily improving. Furthermore, MRI is now rapidly becoming more available in general hospitals, and the modality must therefore be evaluated against others in the diagnostic work-up in patients with degenerative disease of the spine. The advantages of the method are that it is non-invasive, it gives a good overview from the sacrum to the conus, all slice orientations can be obtained, and the content of the dural sac, root-canals and paraspinal area are visualized (Fig. 14). MRI also provides good information about the bone marrow. Cortical bone is not as well visualized as with CT, but with improving quality this difference has been diminished. Another drawback of the method is that

a	Figure 14. MRI and CT of patient with a huge central disc herniation at the L5-SI level. a) T1-weighted image shows a disc herniation and also a normal conus ending at the L1-L2-level. b) T2-weighted image shows the border of the disc better and degeneration of the L5-SI disk. Other discs have an essentially normal nucleus pulposus. c) Axial T1-weighted image shows central position of the disc herniation. d) Corresponding CT-examination. In a case like this any modality would show the disc herniation but the advantage of MRI compared to CT is that it also shows other levels and the content of the dural sac.
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it is rather time-consuming, but this is changing due to technical development allowing high resolution examinations with modem scanners in a few minutes. Several investigations have shown that MRI provides equal or better information than myelography or CT in the diagnosis of degenerative disease in the lumbar spine. With modem MR-scanners this will probably shift further in favour of MRI. A standard evaluation of the lumbar spine should include T1-weighted sagittal images covering the area from the conus down to the sacrum, from neural foramen to neural foramen. Axial views should always be obtained at the levels of interest. This is especially important in the evaluation of the lateral part of the spinal canal which is difficult to evaluate on sagittal views. T2-weighted images with standard spin-echo-technique are time-consuming and usually not required, but can be helpful when the disk is difficult to delineate and when a more thorough evaluation of the disk content is necessary, as for instance in a suspected infection. Using newer techniques, fast spin echo T2weighted images can be obtained in a much shorter time, and this technique might be the standard for evaluation in the near future. The specificity of MRI is high since not only pathology in the spinal canal is disclosed, but also pathology in adjacent organs, such as the aorta, which might mimic disease originating in the spine (Fig. 15). MRI is therefore superior to other methods, especially in cases with unclear symptoms. However, in patients with a typical history and location of the symptoms to one level and one side, CT is usually sufficient for diagnosis. If all modalities are available the following strategy for evaluation of patients with low-back pain and sciatica is suggested: Routine patient: 1) plain films, 2) MRI. Patient with advanced spinal stenosis or severe scoliosis: 1) plain films, 2) myelography, 3) CT at selected levels, after myelography.

Discography
Discography is necessary in the evaluation of patients intended for chemonucleolysis and might be helpful in the evaluation before percutaneous discectomy. The method has also been used for the evaluation of patients presenting diagnostic difficulty and in selecting the proper level in cases with multiple pathology. However, there are different opinions on the value of the method in this respect. Some authors have tried to correlate the induced pain with the therapeutical result. In a recent large study, the value of the method in this respect was found to be low. 

a	Figure 15. An aneurysm of the aorta which has ruptured into the psoas muscle found in a patient sent for routine MRI of the spine with suspicion of disc herniation. a) Sagittal view shows the neck of the aneurysm b + c) Complementary coronal view shows the full extent of the aneurysm.
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Diagnostic lumbar nerve-rootblock

Another method for evaluation of patients with normal findings on MR, CT or myelography, or multiple pathological levels, is diagnostic nerveroot block. In this method, a small amount of carbocain is injected in dose relation to the nerve under investigation. The neurological state before and after nerve-block is carefully recorded. The method is easy to perform and has been shown to be of value in the determination of the level of pathology and in the evaluation of patients with uncertain findings.

Pathology

Lumbar spine

Anatomical considerations
The spinal card usually ends at the L1-L2-level and puncture of the subarachnoid space should not be performed above the L2-L3-level. The nerve-roots have a more cranio-caudal course in the lower lumbar spine than in the upper, and the root-sleeves of the lower lumbar nerves are larger and longer than those in the upper lumbar spine. Common normal variants are root-sleeve cysts and conjoined nerve-roots. This can cause diagnostic problems using CT or MRI, because they can be misinterpreted as disk herniation (Fig. 16). The epidural space is thickest at the L5-S1 level and tissue which contains numerous vessels is normally seen as a symmetrical soft tissue mass in dose relation to the L5 - S1 disk. It is important not to misinterpret this as a disk herniation. On MRI and CT images the dural sac is surrounded by fat, which is of great help for diagnosis. Also the nerve-roots in the root-canals are surrounded by fat in the normal patient. An important area to identify is the lateral recess where the nerve-root is turning in the lateral direction and then passes under the pedicle into the root-canal. The ligamentum flavum and the intervertebral joints can usually easily be identified on both CT and MRI.

Disc herniation
A bulging disc is a common finding and is often seen in combination with reduction of the height of a disc. In bulging discs there is a general expansion of the disc beyond the margins of the adjacent vertebral endplates. Bulging discs, not necessarily related to symptoms, are very common in the middle-aged population and there is a considerable risk that

a	Figure 16. a) CT shows right-sided disc herniation with compression of the L5-root. b) Myelography shows disc herniation with compressed L5-root and root-sleeve. c) CT at L5-S1 level shows increased soft tissue in front of dural sac on left side which could be misinterpreted for a disk herniation. d) Myelography shows a big root-sleeve and conjoined nerveroots (arrow).
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bulging discs are misinterpreted as disc herniations. In disk herniations, which affect the L4-L5 or L5-S1 discs in 90 %, there is a focal extension of different size of the disk outside the vertebral body. In international literature this is called "focal disc bulge" or "protrusion" . These terms are used interchangeably and usually represent different degrees of the same condition, protrusion being the larger disc herniation. Common to both is that the nucleus material remains within the confines of the outermost fibres of the anulus fibrosus which is focally weakened. In "prolapse" or "extruded  disk" the nucleus material has penetrated the annulus fibrosus but is contained in front of the posterior longitudinal ligament. With the present imaging modalities it is often impossible to differentiate between these types of disc herniation. In sequestration or free fragment the disc material is no longer contiguous with the intervertebral disk and usually it has penetrated the posterior longitudinal ligament (Fig. 17). This is important for the choice of surgical technique. Most disk herniations are found in the posterolateral direction (Fig. 18), because the posterior longitudinal ligament is weakest in this part. Disk herniations are divided into central, posterolateral, lateral (foraminal) (Fig. 19), and lateral (extraforaminal). The latter group is also called "far lateral" disk herniation. Sequesters or free fragment can migrate both in the upward and downward direction and can sometimes be found at considerable 

Figure 17. Schematic illustration of a bulging disc and various types of disc herniation. The thick dark grey line represents the posterior longitudinal ligament. a) Normal b) Bulging disk c) Focal bulge or protrusion. The nucleus material remains within the outermost fibres of the annulus fibrosus. d) Prolapse or extrusion. The nucleus material has penetrated the annulus fibrosus but is contained in front of the posterior longitudinal ligament. e) Sequester or free fragment.

Figure 18. CT examination showing posterolateral disk herniation at L4-L5-level on the right side causing compression of the L5-root.

a	Figure 19. CT and MRl of patient with lateral (foraminal) disc herniation at L2-L3-level. a) CT Disc herniation is seen in right root canal, compressing the L2-nerve. Note normal low attenuation fat on left side. b) T1-weighted sagittal MRl shows disc herniation in root-canal, replacing normal fat. Compare with the level below. c) Axial T1-weighted view, showing disk herniation in right root-canal and high signal fat in left. Note intermediate signal from cauda equina in posterior dural sac and low signal in CSF-containing anterior dural sac.
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distance from the disc of origin. Intradural disc herniations are very rare. A firm attachment between the dura and posterior longitudinal ligament due to inflammatory adhesion is thought to play an important role in intradural disc herniation. A sudden increase in disc pressure can then push the disc material through the dura into an intradural position.

In the radiological report it is important not only to describe the disc herniation but also its influence on nerve-roots and the dural sac. The general rule is that the common posterolateral disc herniation compresses the nerve-root which exits in the neural foramen below the intervertebral disc, i.e. a L4-L5 disc herniation will compress the L5-root. In lateral disc herniation, the root in the root-canal will be compressed and in this situation a L4-L5 disc herniation will compress the L4-root. It is not uncommon for disc herniation to disappear on conservative treatment (Fig. 20). Imaging following surgery should be interpreted with caution since there is a poor correlation between findings on imaging and clinical outcome. All of the three modalities, myelography, CT and MRI, can be used for the diagnosis of a disc herniation with the advantages and disadvantages previously described. On CT, disc herniation will have a high attenuation (Fig. 14 d, 16 a, 18, 19 a) relative to the dural sac, and on T1-weighted MRI, the signal intensity will be increased in relation to the subarachnoid space (Fig. 14 a + c, 19 a + c, 20 a), and on T2-weighted images decreased (Fig. 14 b).

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Figure 20. T1-weighted axial MR-image, showing huge right-sided posterolateral disc herniation and compression of the S1-nerve (a). Following conservative treatment, the disk herniation has almost disappeared six months later (b). Note that epidural fat is now seen in front of the dural sac on left side, but there is still some compression of the epidural fat on right side.

A special problem is the differentiation between recurrent disc herniation and postoperative scar. Scar tissue is more vascularized than disk material, and therefore it is possible to differentiate these two tissues by using intravenous contrast media. This can be done by using CT, but is more efficient using contrast enhanced MRI with gadolinium-containing compounds. Using this technique, contrast enhancement will usually be seen early after injection (approximately 5 minutes) in scar tissue but not in disc herniations (Fig. 21). However, in disc herniations there is not uncommonly a rim enhancement caused by surrounding granulation tissue. The differentiation between hemia and scar is very important since reoperation on scar tissue is associated with poor results, while reoperation of disc herniation often leads to cure.

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Figure 21. T1-weighted MRl before contrast (a) shows tissue adjacent to the L4-L5