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

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•Neurofibromatosis: a congenital condition that may show dural ectasia, pseudomeningocele, and neurofibromas on MRI

•Pars defects: a common occurrence in patients with developmental scoliosis; may occur in up to 6.2% of patients with idiopathic scoliosis70

Epidural Lipomatosis

Epidural lipomatosis is a condition in which there is excessive deposition of fat in the epidural space that, in turn, leads to spinal stenosis and neural compression. The syn-

Fig. 11.39 Scoliosis. (A) A coronal T1-weighted image showing advanced thoracolumbar degenerative scoliosis that is primarily left convex with the apex of the curve at the L2 level. (B) An axial T2-weighted image at the L2 level showing rotation of the vertebral body but no substantial stenosis. (C) An axial T2-weighted image at the L3-L4 level showing rotation of the vertebral body level and moderate-severe stenosis secondary to degenerative changes.

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drome is primarily associated with excessive glucocorticoid levels, which may be exogenous or endogenous but also may be spontaneous or idiopathic.71–73 In the lumbar spine, epidural fat surrounding and compressing the thecal sac is the key finding. In the thoracic spine, the presence of >6 mm of fat posterior to the cord is diagnostic. The characteristic feature of lumbar epidural lipomatosis is the presence of epidural tissue that follows the signal characteristics of subcutaneous fat on all pulse sequences, including fat-suppressed sequences (Fig. 11.40). The primary di erential consideration includes an intraspinal lipoma, which is typically focal and is often located in the anterior

thoracic spine.74 Other epidural abnormalities tend to have a low T1-weighted signal and can be excluded.

the imaging modality of choice for the detection of this process, with a sensitivity of >82% and a specificity of 53% to 94%.77 Infection involving the vertebral body occurs through one of three primary routes78:

■ Infectious Conditions

Vertebral Osteomyelitis

Cases of vertebral osteomyelitis comprise between 2% and 4% of all skeletal infections.75,76 MRI is usually regarded as

•Hematogenous (most common)

•Direct inoculation through surgery or penetrating trauma

•Contiguous spread from an adjacent soft-tissue infection

Hematogenous seeding occurs through nutrient arterioles of the vertebral bodies or by retrograde spread through the paravertebral venous plexus of Batson.78–80 Infection then spreads from the vertebral body and marrow to the contiguous intervertebral disc and adjacent vertebral body, often sparing the central portion of the vertebral bodies.78,80

The general MRI signal changes in patients with osteomyelitis include the following:

•Decreased signal intensity of the intervertebral disc and adjacent vertebral bodies, with a discernible margin between the two on T1-weighted images

•Increased signal intensity of vertebral bodies adjacent to the involved disc on T2-weighted images

•An abnormal configuration and increased signal intensity of the intervertebral disc with loss of the nuclear cleft on T2-weighted images76,81 (Fig. 11.41)

Gadolinium enhancement may be seen in adjacent vertebral bodies. However, gadolinium also may cause edematous marrow to blend in with the normal fatty marrow.82 Combining fat-suppressed T1-weighted images with gadolinium contrast enhancement eliminates this problem.81 STIR images may also be used for the MRI evaluation of osteomyelitis because they suppress the high signal intensity from fat and provide increased contrast.83 STIR image are especially useful when combined with the anatomic detail from T1weighted sequences.

The pattern of vertebral body involvement in vertebral osteomyelitis should be di erentiated from that in patients with spinal tumors (see Chapter 12). Patients with vertebral osteomyelitis tend to have the epicenter of the pathologic change at the disc space, and those with tumors tend to have the epicenter at the vertebral body. In other words, infectious processes are based at and cross the disc space, whereas neoplastic processes are typically based in the vertebral body and do not cross the disc space.

Discitis

Disc infection often causes edema, which leads to hyperintensity of the disc and the end plate on T2-weighted images; it also causes loss of definition of the vertebral end plates, inflammatory changes in the adjacent vertebral marrow, and gadolinium enhancement within the disc.33,84,85 In addition, it is not uncommon to have an associated paraspinal inflammatory mass.

Spondylodiscitis, which essentially is discitis with vertebral osteomyelitis, may be seen after lumbar disc surgery. It also may be seen after discography or myelography.86 Postoperative spondylodiscitis is believed to occur in 0.1% to 3% of patients.86 Intraoperative contamination usually is the most common mechanism for infection. The most common infecting organisms are Staphylococcus epidermidis and

Staphylococcus aureus. MR images often show Modic type 1 changes at the level of the operated disc (vertebral end plate with decreased signal on T1-weighted images and increased signal intensity on T2-weighted images) and enhancement of the disc when contrast is used (Fig. 11.42). No enhancing tissue should be seen outside the intervertebral space. Normal vertebral bone marrow has low signal on T1-weighted images and high signal intensity on T2-weighted and con- trast-enhanced images.86 If a rim of soft tissue around the a ected intervertebral space is noted to enhance, concern about septic spondylodiscitis arises, which can be ruled out with disc biopsy.

Epidural Abscess

An epidural abscess is a purulent epidural collection of material without involvement of the vertebral body or the disc space. Such collections are usually located anteriorly in the spinal canal and originate from the posterior aspect of the vertebral body and disc space. If the abscess originates from hematogenous sources, then it may be associated with a positive blood culture. MRI is very sensitive in the detection of these abscesses.82 MRI also is useful in visualizing phlegmon and epidural abscesses, which usually are isointense or hypointense compared with the spinal cord on T1-weighted images and which usually have high signal intensity on T2-weighted images83,86 (Fig. 11.43). The di erentiation of epidural abscess and CSF may be di cult, necessitating gadolinium enhancement for better visualization. The high signal intensity of the enhancing mass can be distinguished easily from the lower signal intensity of the CSF and spine on T1-weighted images.86 Sometimes fat suppression is necessary to di erentiate an abscess from epidural fat.86 Contrast enhancement also can aid in di erentiating between epidural phlegmon and an abscess; dense homogeneous enhancement of the mass suggests phlegmon, whereas peripheral or ring enhancement of the mass suggests an abscess. Paraspinal abscesses, also well visualized with MRI, usually have low signal intensity on T1-weighted images and are commonly associated with swelling of the psoas on T1-weighted images and increased signal on T2-weighted images. Gadoliniumenhanced images provide additional abscess delineation.86

Tuberculosis

Tuberculosis has reappeared in the developed world because of the emergence of AIDS; 5% of all cases of tuberculosis affect the musculoskeletal system.87

Tuberculosis usually results from hematogenous seeding. The rich vascular supply of the vertebral bodies makes the spine susceptible to infection. A vertebral body receives its blood supply inferiorly from the ascending branch of the posterior spinal artery and superiorly from the descending

Fig. 11.42 Discitis. Sagittal T2-weighted (A), T1-weighted (B), and postgadolinium T1-weighted (C) images show the typical findings of discitis at the L2-L3 level (arrows on A and C). Note the increase in

branch of this artery. These two arteries anastomose and create a network of vessels in the anterior epidural space. The network then leads to three or four arteries that enter the vertebral body through the nutrient foramen. Children are at an increased risk for discitis because they still have an arterial anastomosis between the vertebral end plate and the disc.87 The intervertebral disc becomes less vascularized in adolescence. Arteries end within the vertebral bodies, which result in increased rates of infection within the bodies.87

Tuberculous osteomyelitis usually involves the ventral trabecular bone marrow adjacent to the intervertebral disc, and it spreads via the anterior longitudinal ligament to adjacent vertebral bodies.86 MRI signs of tuberculosis infection are hypointense signal on T1-weighted images and hyperintense signal on T2-weighted images seen in the subchondral tissue. There also may be a hyperintense signal within the disc on T2-weighted images (Fig. 11.44). As the disease progresses, it may lead to collapse of vertebral bodies, with an associated epidural abscess. Continuous destruction of the

signal at the disc space on the T2-weighted image, the decrease in signal on the T1-weighted image, and the postgadolinium enhancement of the small epidural component (C, arrow).

anterior cortices of single or contiguous vertebral bodies can lead to kyphotic deformity. Tuberculosis may also directly involve the spinal cord, a condition termed tuberculosis myelitis; it is usually seen in individuals younger than 30 years old. MRI with gadolinium is the best imaging modality for this phenomenon.87

It is not uncommon to see paraspinal involvement with abscess formation. Gadolinium enhancement usually is seen only in the periphery of abscesses, unlike granulation tissue, which enhances throughout. The observation of disc-space sparing and enhancement of granulation tissue is highly suggestive of tuberculosis.77

■ Postoperative MRI Findings

Previously, MRI had been considered to have limited use in evaluating the instrumented spine because of the resultant artifacts that were commonly seen. However, the introduction of titanium pedicle screws and specialized pulse se-

Fig. 11.43 Epidural abscess. (A) A sagittal T2-weighted image showing a ventral epidural collection (arrow) posterior to the L5 vertebral body in a patient with infectious symptoms and findings. (B) A sagittal T1-weighted image shows the same collection. (C) A sagittal post-

quences has improved the MRI visualization of the central spinal contents88–92 (see also Chapter 16). It is important to note that MRI may not always be the best imaging modality for the evaluation of the postoperative lumbar spine. Conventional radiographs are typically the starting point

gadolinium T1-weighted image shows peripheral enhancement. (D) An axial postgadolinium T1-weighted image shows the ventral epidural collection (arrow), again with peripheral enhancement, which is producing moderate-severe stenosis.

for the study of patients with an instrumented lumbar fusion, given that they allow for the evaluation of overall spinal alignment, position of instrumentation, and evidence of fusion. CT may be best for more precise determination of the presence or absence of fusion, infection, loosening, and

postoperative fluid collections. The addition of myelography to CT imaging provides excellent determination of the presence and degree of spinal stenosis, or its absence. MRI may be best for evaluating for the presence or absence of infection, postoperative fluid collection (such as hematoma or CSF), recurrent disc herniation, and residual or recurrent stenosis.

Surgical procedures in the spine are typically performed with the following goals in mind:

Fig.11.44 Tuberculosisofthespine.Sagittal T2-weighted (A) and T1-weighted (B) images showing destruction and partial collapse of the L5 vertebral body (between arrows on A) with relative preservation of the intervertebral discs. (C) An axial T2-weighted image at the L5 level showing signal change within the anterior aspect of the left psoas muscle (arrow) compatible with an abscess and also heterogeneous signal within the vertebral body (arrowheads).

•Decompression of a stenotic spinal canal or neural foramen

•Removal of herniated disc material

•Stabilization and fusion of motion segments, for existing instability (such as spondylolisthesis, scoliosis, or posttraumatic injury) or after iatrogenic instability (such as with facetectomy or multilevel laminectomies)

•Excision of tumor or infection

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(Fig. 11.48) and posterior spinal fusion. Although CT is excellent for visualizing osseous detail and the precise location of spinal instrumentation relative to the spinal canal and neural foramen, MRI is superior for visualizing postoperative fluid collections such as an epidural hematoma and the size, location, and degree of compression of the thecal sac it produces.

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Pseudomeningocele

A pseudomeningocele typically occurs in a patient who sustains a durotomy during an open surgical procedure or in a patient who undergoes resection of an intradural lesion. In both of these situations, there may be an incomplete closure of the dural opening, with resultant leakage of CSF into the