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

Fig. 11.29 Thoracic disc protrusion and stenosis. Sagittal (A) and axial (B) T2-weighted images showing moderate-severe stenosis at the T10-T11 level secondary to a moderate-sized central disc protru-

sion (arrows on each) and underlying degenerative stenosis. Note the multilevel degenerative disc disease at other levels and the Schmorl’s nodes (A, arrowheads).

Facets

Facet Arthropathy

Although it is now accepted that the facet joints may be a cause of pain in the degenerated spine, it is di cult to associate them with a particular clinical syndrome.48–50 One of the earliest MRI findings of facet arthropathy is seen as fluid-like intraarticular signal intensity on sagittal or axial T2-weighted images (Fig. 11.30). Renfrew and Heitho 22 described a practical and simple way to assess facet arthropathy:

•Mild: mild undulation of the margins with small (1- to 3-mm) osteophytes, minimal subchondral sclerosis,

mild narrowing of articular cartilage, and <25% increase in facet joint transverse dimension (Fig. 11.31A)

•Moderate: more pronounced changes, osteophytes up to 3 to 5 mm, and 25% to 50% increase in facet transverse dimension (Fig. 11.31B)

•Severe: additional progression of disease with near complete loss of cartilage, osteophytes >5 mm, and joint width >50% of expected transverse dimension (Fig. 11.31C)

Facet joint hypertrophy may cause canal, subarticular recess, or foraminal stenosis and neural compromise. E u- sions may also be seen within facet joints, reflecting synovi-

tis from osteoarthritis or a synovial proliferative process in an inflammatory arthritis. Finally, asymmetric facet disease may predispose to degenerative disc disease and eventual scoliosis.51–53

Synovial Cyst

A synovial cyst, which originates most commonly from lumbar facet joints, also may cause neural compression and may appear on a sagittal T2-weighted image as a hyperintense

cyst with a hypointense rim (Fig. 11.32). T2-weighted MR images in the axial plane show the degree of lateral recess stenosis. Neural foramen stenosis may arise from a reduction in the height of the neural foramen because of degenerative narrowing of the intervertebral disc space, facet-joint hypertrophy and osteophyte formation, or posterolateral encroachment from the disc in the form of bulges, protrusions, and extrusions. This compression is evaluated best on far lateral parasagittal T1-weighted and T2-weighted images that visualize the neural foramina in cross section.54 On

Fig. 11.32 Lumbar synovial cyst. Sagittal T2-weighted (A), T1weighted (B), and postgadolinium fat-suppressed T1-weighted (C) images showing a large L4-L5 lesion compatible with a facet joint cyst when correlated with the axial T2-weighted image (D). Note the intense peripheral enhancement of the lesion on C, the postgadolin-

occasion, synovial cysts may contain air or may calcify, in which case they may not have the typical bright fluid signal on T2-weighted images and may appear gray or dark on all sequences.

ium T1-weighted image. (D) The axial T2-weighted image shows that the cyst (arrowhead) likely originates from the right L4-L5 facet joint and that the thecal sac (between arrows) is severely compressed and shifted toward the left.

Lumbar Spinal Stenosis

The term spinal stenosis describes the compression of the neural elements in the spinal canal, lateral recesses, or neural

Fig. 11.33 Degenerative upon congenital lumbar stenosis. (A) A sagittal T2-weighted image showing multilevel degenerative disc disease at L3-L4, L4-L5, and L5-S1 with evidence of stenosis from disc bulges at these levels (arrowheads), ligamentum flavum hypertrophy (arrows), and a generalized narrow appearance of the spinal canal

foramina. The evaluation of patients with known or suspected lumbar spinal stenosis is one of the primary indications for MRI of the lumbar spine. Patients with lumbar stenosis typically present with combinations of radicular leg pain or weakness, neurogenic claudication, and low back pain. After nonoperative management fails for such patients and conventional radiographs have been obtained, MRI can be considered. The MR images should be evaluated to determine the degree (i.e., mild to severe), level (i.e., L1 to S1), and type (e.g., degenerative, congenital) of lumbar spinal stenosis.

The authors’ suggested method for the assessment of lumbar spinal stenosis begins with a systematic evaluation of the midsagittal T2-weighted images. These images show the conus medullaris in the patient without scoliosis. The clinician or radiologist should carefully trace the posterior margin of the vertebral bodies and intervening discs to ensure that there is no e acement of the CSF space. Next, the dorsal margin of the thecal sac should be evaluated on these images to evaluate for focal hypertrophy of the ligamentum

relative to the AP diameter of the vertebral bodies. (B) An axial T2weighted image at the L4-L5 level shows minimal to moderate stenosis secondary to underlying congenital stenosis with superimposed degenerative changes, including ligamentum flavum hypertrophy (arrowheads) and facet arthropathy (arrows).

flavum. This procedure should be repeated on the parasagittal T2-weighted images in each direction (left and right from center) to evaluate for lateral recess and foraminal stenosis. After the sagittal T2-weighted images have been evaluated, the axial T2-weighted images are sequentially evaluated from the sacrum toward the upper lumbar spine. Specifically, CSF should be seen ventral to the cauda equina, which is often displaced posteriorly within the spinal canal, given that most studies are obtained with the patient in a supine position. The lateral recess and foraminal region should be evaluated bilaterally at each level to rule out stenosis secondary to disc, facet, or ligamentum flavum pathology.

Spinal stenosis may involve the neural foramina, lateral recesses, or central canal of the lumbosacral spine and is usually developmental or acquired in nature. Developmental spinal stenosis, which constitutes approximately 15% of all cases of spinal stenosis, is hereditary-idiopathic or associated with disorders of skeletal growth.54 MRI of the hereditary form shows minor hypoplasia of the posterior osseous

Midline posterior vertebral ridging

Exiting nerve root

Exiting nerve root

Normal nerve root

B

Neuroforamina

Thecal sac compression

arch of the vertebrae, short pedicles, and narrowing of the cross-sectional area of the central spinal canal (Fig. 11.33). Sagittal images may show progressive narrowing of the AP dimension of the spine in the caudal direction, indicating developmental spinal stenosis.

Acquired central spinal canal stenosis may arise from hypertrophic or degenerative changes of the intervertebral discs, facet joints, or ligamentum flavum (Fig. 11.34). On MRI, central canal stenosis is characterized by compression of the thecal sac, best seen on sagittal and axial T2-weighted images. Fat-suppressed T2-weighted and STIR images provide a “myelographic e ect,” in which the CSF is seen as bright signal anterior and posterior to the neural elements on sagittal and axial images. E acement, discontinuity, or displacement of this CSF space is seen in patients with focal and concentric spinal stenosis (Fig. 11.35).

There are several objective measures of lumbar spinal stenosis.55,56 Hamanishi et al.55 found that a cross-sectional area of <100 mm2 at more than two of three lumbar intervertebral levels was highly associated with the presence of intermittent neurogenic claudication. Speciale et al.56 evaluated observer variability in assessing lumbar spinal stenosis on MRI in relation to cross-sectional spinal canal area and found only a fair level of agreement among the observers; however, they found that the ability of the various readers to predict the degree of central stenosis was high.

Although such formal measurements of lumbar stenosis on MRI are well known, most clinicians and radiologists tend to grade the degree of spinal stenosis as mild, moderate, or severe. The authors use the following terms and definitions:

•Mild: stenosis in which the canal begins to assume a triangular shape, the thecal sac is not compressed, and

Fig. 11.35 Lumbar stenosis. (A) A sagittal T2-weighted image showing multilevel stenosis in the lumbar spine. (B) An axial T2-weighted image at the L4-L5 level shows moderate-severe stenosis secondary

there is only minimal (<2 mm) thickening of the ligamentum flavum. The AP canal diameter is >75% of expected normal without nerve root crowding.

•Moderate: findings similar to those of mild stenosis but with compression and minimal flattening and deformity of the thecal sac. The AP canal diameter is between 50% and 75% of expected normal.

•Severe: advanced stenosis with very pronounced flattening and deformity of the thecal sac that is obvious on both sagittal and axial T2-weighted images. The ligamentum flavum is often thickened to >4 mm. The AP canal diameter is <50% of expected normal.

It should be noted that in some cases, canal narrowing can be downgraded if there is ample CSF surrounding the neural structures and upgraded if the surrounding CSF is scant. Similar terminology can be applied to grading stenosis in the subarticular recesses or foramina.

In addition to evaluating the degree of central and canal stenosis, the lateral recess, foraminal, and extraforaminal zones should also be specifically assessed (Fig. 11.23). Lateral recess and foraminal stenosis are most often the result of a combination of pathology: facet arthropathy, ligamentum flavum hypertrophy, and disc bulge or protrusion. Specifically, hypertrophy of the superior articular process from the caudal level often leads to the development of foraminal stenosis. In

to contributions from a central disc bulge (arrow), ligamentum flavum hypertrophy (arrowheads), and facet arthropathy (asterisks).

addition, degenerative disc disease with the associated loss of disc height and subsequent loss of foraminal height and volume can lead to the development or exacerbation of foraminal stenosis from other degenerative pathologies. Many clinicians and radiologists evaluate for the presence of foraminal stenosis in the axial plane. However, parasagittal images are also quite useful in confirming the presence of foraminal stenosis (Figs. 11.26 and 11.27). The normal foramen has an ovoid configuration on parasagittal images (see Chapter 2) where the superior aspect of the foramen contains the exiting nerve root and the inferior aspect of the foramen shows high signal intensity on both T1-weighted (from perineural fat) and T2-weighted (from CSF within the nerve root sleeve) images. On parasagittal images, patients with foraminal stenosis have progressive narrowing of the foramen, with resultant compression of the nerve root.

Cauda Equina Syndrome

Cauda equina syndrome is typically characterized by unilateral or bilateral sciatica, perianal or saddle anesthesia, bowel and bladder incontinence, and sensory and motor deficits in the lower extremities.57 Often, it is caused by a spaceoccupying mass compressing against the cauda equina and/ or conus terminale. There can be numerous etiologies, in-

Fig. 11.36 Artist’s sketch of the Meyerding classification, which is used to quantify the degree of spondylolisthesis. Grade 1 is 0% to 25% slip, grade 2 is 26% to 50% slip, grade 3 is 51% to 75% slip, and grade 4 is 76% to 99% slip. A = width of the superior end plate of S1, a = distance between the posterior edge of the inferior end plate of L5 and the posterior edge of the superior end plate of S1. (From Cavalier R, Herman MJ, Cheung EV, Pizzutillo PD. Spondylolysis and spondylolisthesis in children and adolescents. I. Diagnosis, natural history, and nonsurgical management. J Am Acad Orthop Surg 2006;14:417–424. This reprinted illustration was modified with permission from Herman MJ, Pizzutillo PD, Cavalier R. Spondylolysis and spondylolisthesis in the child and adolescent athlete. Orthop Clin North Am 2003;34:461–467. Reprinted with permission.)

cluding disc herniation, severe stenosis, trauma, tumor, or infection.58,59

MRI is the preferred imaging modality for the evaluation of the patient with suspected cauda equina syndrome. MRI allows visualization of space-occupying lesions within the spinal canal as well as identification of compression of neural structures. Lumbar myelography with CT of the lumbar spine is indicated in patients who are unable to undergo MRI. Given that the treatment of cauda equina syndrome is

urgent decompression, one of these imaging studies should be obtained without delay.

Spondylolisthesis

Spondylolisthesis is defined as anterior displacement of a vertebral body relative to the one caudal to it. Retrolisthesis is seen when the superior vertebral body is displaced posterior to the one caudal to it. Wiltse et al.60 classified lumbar spondylolisthesis on the basis of etiology: dysplastic, isthmic, degenerative, traumatic, iatrogenic, or pathologic. Meyerding61 described the various degrees of forward slippage (from grade 1 to grade 4) based on a division of the superior surface of the lower vertebra into quarters (Fig. 11.36). According to this system, a complete slip of L5 on S1 is termed spondyloptosis. Each manifestation of spondylolisthesis has specific associated MRI findings.

The system of Wiltse et al.60 details the features of spondylolisthesis as follows:

•Dysplastic spondylolisthesis: may present with degeneration and pseudobulging of the lumbosacral disc, with potential compression of the cauda equina between the neural arch of L4 and the superoposterior aspect of the sacrum (for a slip at L5). A parasagittal T1-weighted SE image may show severe compression of the exiting L5 nerve root.

•Isthmic spondylolisthesis: sagittal T2-weighted images often show obvious spondylolisthesis at the L5S1 level (Fig. 11.37). Parasagittal images at the level of the pedicle may show compression of the exiting L5 nerve root between the bulging L5-S1 disc and the undersurface of the L5 pedicle, and reduction of foraminal height. The parasagittal images should also be scrutinized for the presence of a pars intraarticularis defect or reparative granulation tissue in that region; CT imaging may help confirm the presence of the pars defect.

•Degenerative spondylolisthesis: seen most commonly at the L4-L5 level. MRI can be used to evaluate narrowing of the central canal, lateral recesses, and neural foramina, and compression of the cauda equina and exiting nerve roots. Facet joint cysts are not uncommon in the presence of degenerative spondylisthesis. Sagittal and axial T2-weighted images delineate these entities clearly (Fig. 11.38).

•Traumatic spondylolisthesis: MRI shows the associated soft-tissue injury, which may include rupture of the intervertebral disc and posterior ligamentous complex, as seen with bilateral facet dislocation (Fig. 11.9).

•Pathologic spondylolisthesis: MRI shows very focal changes at the level of the pars intraarticularis based on the specific pathology involved.

•Iatrogenic spondylolisthesis: may occur after laminectomy, facetectomy, and extensive resection of the

Fig. 11.37 Isthmic spondylolisthesis. (A) A lateral radiograph showing bilateral pars intraarticularis defects (arrow) at the L5-S1 level with Meyerding grade 2 spondylolisthesis. (B) A sagittal T2-weighted MR image obtained on a closed system with the same patient in a supine position showing grade 1 spondylolisthesis. (C) A sagittal T2weighted MR image obtained on an open MRI system with the patient

facet joint and neural arch without fusion. MRI shows changes directly correlated to the specific areas altered during surgery.

Recently, the focus in spondylolisthesis has moved beyond the slippage of the vertebral bodies to include, among other issues, its etiologic factors and spinopelvic alignment. This change in focus has led to the development of more comprehensive classification systems that may be better at pre-

in a standing position shows that the spondylolisthesis progresses to grade 2. (D) A sagittal T2-weighted image obtained on an open MRI system with the patient in a flexed position shows that the grade 2 spondylolisthesis progresses compared with images in the neutral

(C) and supine (B) positions.

dicting progression of the disease, especially in younger individuals.62–64

Scoliosis

Scoliosis is a lateral curvature of the vertebral column in the coronal plane involving lateral and rotational vectors, and it may be associated with spinal cord or other neuronal abnormalities that are best visualized with MRI before op-

Fig. 11.38 Degenerative spondylolisthesis. (A) A sagittal T2-weighted image showing Meyerding grade 1 spondylolisthesis at the L4-L5 level with severe stenosis and evidence of a high-intensity zone at

erative intervention. The most common indication for MRI in patients with scoliosis is degenerative scoliosis. In this scenario, MRI is obtained to evaluate for the presence, degree, and levels of stenosis. Because of the unique challenge of obtaining contiguous visualization of the spinal canal content in the scoliotic spine, specific protocols should be followed to obtain the best views. Redla et al.65 described the use of sagittal T1-weighted SE and T2-weighted FSE sequences, beginning from above the foramen magnum and including the brainstem down to the sacrum. However, for typical thoracolumbar scoliosis, MR images are obtained of the thoracic and lumbar spine only. When the curve is severe, sagittal sequences are obtained parallel to the two major portions of the curve and are planned from the coronal plane. Axial T1-weighted images are obtained through the apices of the curve, providing a second view of the cord. A coronal T1-weighted sequence also is obtained, especially to assess the vertebral bodies for congenital anomalies. In patients with degenerative stenosis, the sagittal and axial T2-weighted images should be evaluated in correlation with each other to determine the degree and type of stenosis at each level (Fig. 11.39). This information helps determine the levels for decompression in a patient who is being considered for surgical intervention.

Aside from the evaluation of the patient with lumbar degenerative scoliosis and suspected stenosis, the indications for MRI in patients with scoliosis have been the subject of debate (see Chapter 13). Although some studies have shown

the posterior annulus of the L4-L5 disc. (B) An axial T2-weighted image showing severe stenosis from a central disc bulge, ligamentum flavum hypertrophy, and facet arthropathy.

that routine MRI is not indicated for patients with adolescent idiopathic scoliosis who have a typical right thoracic curve and who are neurologically intact,66–68 others have found a high incidence of spinal cord abnormalities (17.6% to 26%65,69) in patients with infantile and juvenile forms of scoliosis. Those studies stressed the importance of MRI in children younger than 11 years old. Spinal cord abnormality is suggested by several physical examination findings, including a left thoracic curve, absent abdominal reflexes, lower limb neurologic deficits, and cutaneous stigmata of occult spinal dysraphism.65

Specific MRI findings for abnormalities seen with scoliosis secondary to an underlying neurologic abnormality include the following:

•Tethered cord: thickened filum terminale and spinal lipoma seen on sagittal T1-weighted SE and T2weighted FSE images

•Syringohydromyelia: dissection of CSF through the cord substance best seen on T2-weighted images with increased signal within the cord, sometimes associated with sacculation

•Diastematomyelia: a midline sagittal cleft of the spinal cord most commonly involving the lower thoracic and lumbar region, seen as two hemicords on MRI, with each having a single dorsal and ventral horn and a septum seen from the dorsal aspect of the vertebral body and extending into the cleft between the cords