Книги по МРТ КТ на английском языке / MRI for Orthopaedic Surgeons Khanna ed 2010

Intravertebral herniation refers to a herniated disc in the craniocaudal direction through a defect in the vertebral body end plate. Herniations can also be described as contained or uncontained. Contained herniations are displaced disc material that is retained by the outer annulus. Uncontained herniations are not retained by the outer annulus. Each abnormal presentation of disc pathology has specific features that can be detected with MRI (see below).

■ Degenerative Conditions

Along with cervical degenerative disorders, lumbar degenerative disc disease and the associated stenosis are the most common indications for MRI of the spine. Most patients present with low back pain, lower extremity pain, or symptoms of neurogenic claudication. They usually have had at least 6 weeks of unsuccessful nonoperative management and often have already been evaluated with conventional radiography. The purpose of MRI in this situation is most frequently to evaluate for the presence or absence of spinal stenosis, disc herniation, and degenerative disc disease.

Lumbar spine degeneration typically includes a constellation of changes, such as degenerative disc disease, arthritic and hypertrophic changes involving the facet joints, and hy-

pertrophy of the ligamentum flavum (see Chapter 10 for a discussion of inflammatory arthropathies, including ankylosing spondylitis). It is important to note that patients exhibiting MRI changes may not necessarily be symptomatic.30 A study of 33 asymptomatic, elite tennis players showed that 15.2% had a normal MRI evaluation and 84.8% had abnormalities: 27.3% had pars lesions and 39.4% showed evidence of disc desiccation and bulging.30 The high incidence of abnormal lumbar spine MRI studies was described by Boden et al.31 in 67 asymptomatic patients. A follow-up study of those 67 patients concluded that the MRI findings were not predictive of the development or duration of low back pain.32

Disc and End Plates

Degenerative Disc Disease

As noted above, Fardon and Milette25 have suggested the use of the terms normal, spondylosis deformans, and intervertebral osteochondrosis to describe the degenerative lumbar disc (Fig. 11.11). The specific changes seen on MRI correlate with the pathogenesis of degenerative disc disease, which results from the spectrum of changes that occur in the various parts of the vertebrodiscal complex. The nucleus pulposus becomes increasingly hypointense on T2-weighted images because of desiccation. An alternative finding is the inter-

vertebral disc vacuum phenomenon secondary to a collection of intradiscal nitrogen, which manifests as a linear area of signal void on T1-weighted and T2-weighted sequences. Gradient-echo sequences may show this particular finding even better than do T1-weighted and T2-weighted images.33 Early signs of disc degeneration on MRI include infolding of the anterior annulus and a hypointense central region often seen before any loss of disc signal intensity, which may be associated with reproduction of pain at discography.33 Advanced degeneration may present with a linear hyperintensity parallel to the end plate, which is thought to represent separation of the nucleus pulposus from the hyaline cartilage end plate.33

Pfirrmann et al34 introduced a grading system for lumbar degenerative disc disease based on MRI findings on sagittal T2-weighted images. This classification system, which describes five grades of progressively increasing degenerative disc disease, is complex, which may be the reason it is not commonly used by most clinicians. To summarize, the grading system describes the lumbar disc degenerative process as a continuum that progresses from a normal disc, to loss of the normal disc signal on T2-weighted images, to increasing

loss of disc height, to degenerative end-plate changes and sclerosis.

Modic et al.26,29 described signal changes within the vertebral body bone marrow and end plate adjacent to degenerating discs. The first finding in the sequence of changes is fibrovascular ingrowth that results in diminished signal intensity on T1-weighted images and a corresponding increase in signal intensity on T2-weighted images (type 1) (Fig. 11.15). The more chronic, type 2 changes involve a change from hematopoietic (red) to fatty (yellow) marrow, leading to relatively increased signal on T1-weighted images and slightly diminished signal intensity on T2-weighted images (Fig. 11.16). Type 3 changes consist of decreased signal intensity on T1-weighted and T2-weighted sequences and are associated with subchondral sclerosis on radiographs27,33 (Fig. 11.17). Among the three types of degenerative end-plate changes, type 1 changes have been found to have the greatest correlation with the presence of discogenic back pain.28,35,36

In addition to an assessment of the type of lumbar degenerative disc disease using the methods described above, one should also describe the degree of lumbar disc degeneration by noting the amount of disc space height loss (Fig. 11.18).

Fig. 11.16 Modic type 2 (fatty) changes. Sagittal T1-weighted (A) and fat-suppressed T2-weighted (B) images showing the typical pattern (arrows on each) of increased signal intensity on the T1-weighted

image and decreased signal on the T2-weighted image at the L5-S1 level that is seen with Modic type 2 end-plate changes. Note that degenerative changes and stenosis are also seen at other levels.

•Normal

•Bulge (Fig. 11.19)

•Protrusion (Fig. 11.20)

•Extrusion (Fig. 11.21)

The status of the annulus provides insight into the status of a herniated disc. A disc protrusion is a herniation with an intact annulus, confined by the posterior longitudinal ligament. Extrusions occur when the nuclear material breaches the outer annular fibers. If a herniated disc becomes detached from the parent disc, it is termed sequestrated. The sequestrated fragment can migrate superiorly, inferiorly, or

occasionally, posterior to the thecal sac. Intradural disc herniation is very rare.33

It often is di cult to di erentiate protrusion from extrusion. Several MRI signs may be used to aid this di erentiation, including the following:

•If the AP diameter of the herniated disc is >50% of the spinal canal diameter, then an extrusion is present in >90% of cases.33,41

•Examination of the base of the disc shows that a protrusion usually has a broad base against the parent disc, broader than any other part of the hernia; an

Fig. 11.21 Lumbar disc extrusion. Midline sagittal (A) and parasagittal (B) T2-weighted images showing a large disc extrusion at the L4-L5 level and distal migration of the disc fragment (arrow on each) to behind the L5 vertebral body in a patient with transitional lumbosacral anatomy. Note the advanced degenerative disc disease at

extrusion has a base that is more narrow than the extruded material.

•Protrusions and extrusions can also be distinguished by their outlines. Protrusions are limited by the outer annular fibers and tend to have a smooth outline; in contrast, extrusions have a poorly defined outer margin.33,42

When reviewing an MRI study that shows lumbar disc displacement, the clinician or radiologist should use the appropriate term to describe the morphology of the disc

this level. (C) An axial T2-weighted image at the level of the L4-L5 disc showing what appears to be a central disc bulge (arrow). (D) However, an axial T2-weighted image at the L5 vertebral body level shows the disc extrusion (arrow).

(bulge, protrusion, extrusion, sequestration) and should also describe several additional key characteristics of the disc pathology, including the following (see Figs. 11.19 through 11.21 for examples of such descriptions):

•Level of the disc pathology

•Precise location relative to the disc space

•Size and degree of neural compression

With regard to location of the disc pathology, the axial and sagittal T2-weighted images should be carefully evaluated

to determine the location of the disc protrusion or other pathology. The following terms should be used to describe the location of the protrusion (primarily based on the appearance of the axial T2-weighted image) (Figs. 11.22 and 11.23):

•Central (Fig. 11.24)

•Posterolateral or lateral recess (Fig. 11.25)

•Foraminal (Fig. 11.26)

•Far lateral (Fig. 11.27)

Of all lumbar disc herniations, 90% are central or paracentral (5% are foraminal and 5% are far lateral).43 It should be noted that a typical posterolateral disc protrusion compresses the traversing nerve root, whereas a far lateral disc

Fig. 11.23 Schematic representation of the anatomic “zones” identified on axial images. (From Wiltse LL, Berger PE, McCulloch JA. A system for reporting the size and location of lesions of the spine. Spine 1997;22:1534–1537. Reprinted with permission.)

protrusion compresses the exiting nerve root. Thus, for example, a posterolateral disc protrusion at the L4-L5 level will likely produce an L5 radiculopathy, whereas a far lateral disc protrusion at the same level will likely produce an L4 radiculopathy.

Thoracic disc herniations are rare, comprising only 1% to 2% of all disc herniations.44 When they do occur, they are seen most often in the lower thoracic spine, likely the result of the increased mobility and load in this region. They can also be seen in association with Scheuermann’s disease. Sagittal T2weighted images show thoracic disc herniations, and axial T2-weighted images allow for additional characterization of the size, location, and morphology of the lesion (Fig. 11.29).

Schmorl’s Nodes

Schmorl’s nodes represent herniations of the intervertebral disc through weak areas in the adjacent vertebral end plates

and into the vertebral body.45 They are found most commonly in the thoracic and lumbar spine and occur in approximately 10% of the population, with no dependency on age or gender.46 Scheuermann’s kyphosis is one of several processes

that is associated with Schmorl’s nodes and premature disc degeneration. Patients may be asymptomatic or have nonspecific pain that may not be directly related to the presence of the Schmorl’s node. When symptoms are the result of

Fig. 11.26 Foraminal disc protrusion. (A) A sagittal T2-weighted image showing disc bulges at the L3-L4, L4-L5, and L5-S1 levels. (B) A parasagittal T2-weighted image at the level of the neural foramen shows disc material (arrow) within the left L4-L5 foramen with re-

sultant compression of the exiting nerve root. Note the patency of the neural foramen at the L3-L4 and L5-S1 levels (arrowheads). (C) An axial T2-weighted image at the L4-L5 level showing a left-side disc protrusion (arrow) in the foraminal zone.

Fig. 11.27 Far lateral disc protrusion. Sagittal T2-weighted images obtained in the midline (A), a few millimeters lateral to midline (B), at the level of the pedicle (C), and lateral to the pedicle and foramen (D)

the Schmorl’s node(s), the patient may present with axial back pain. MRI allows for the optimal detection of Schmorl’s nodes (Fig. 11.29); they appear as extensions of disc material (with direct continuity with the disc) into the vertebral body, surrounded by a rim of low signal intensity second-

show a right-side far lateral disc herniation that is seen primarily on D (arrow on each) at the L4-L5 level. (E) The axial T2-weighted image confirms that the disc protrusion is in the far lateral zone (arrow).

ary to reactive sclerosis.33 Cases in which the Schmorl’s node is associated with increased T2-weighted signal in the adjacent bone marrow are more commonly associated with back pain and may represent an acute or subacute Schmorl’s node.43,47