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

Fig. 2.58 A sagittal proton-density fat-suppressed image of the ankle showing the Achilles tendon and plantar aponeurosis. Flexor and extensor tendons crossing the ankle are also seen.

and metatarsals. Precise identification of the central osseous structures is facilitated by concurrent correlation with the axial and coronal images. The first MTP joint structures are visualized particularly well in the sagittal plane. Although coronal and axial images are better suited for identification of the ligamentous complexes of the ankle, the plantar plate and ligament structures of the MTP joints can be identified in the sagittal plane.

The Achilles tendon, easily identified as the most posterior tendon inserting onto the posterior calcaneal tuberosity, can be inspected for thickening, intratendinous signal heterogeneity, partial tearing, or complete rupture with retraction (Fig. 2.58). Immediately deep to this structure is the retrocalcaneal bursa, which can show increased signal intensity on T2-weighted images, suggestive of retrocalcaneal bursitis. The posterior tibial tendon is the first structure posterior to the medial malleolus. The course of this tendon can be appreciated as it curves around the medial malleolus to insert onto the tuberosity of the navicular bone. The next two medial tendons are the flexor digito-

rum longus and FHL (Fig. 2.59). The flexor digitorum longus tendon is located just posterior to the posterior tibialis tendon and medial to the posterior neurovascular bundle. The FHL, the deepest muscle in the posterior compartment, lies against the tibia. Lateral to the FHL are the peroneal tendons, which course posterior to the lateral malleolus and then toward the plantar midfoot. The anterior tibialis, extensor hallucis longus, and extensor digitorum longus tendons can be visualized anteriorly. In particular, the anterior tibialis tendon can be identified on sagittal imaging and should be evaluated for tenosynovitis, partial tear, or rupture.

■ General Spine Anatomy

Intervertebral Discs

The normal discovertebral complex has three components:

•Cartilaginous end plate

•Annulus fibrosus

•Nucleus pulposus

Overall, the intervertebral discs show intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images. The outer annulus appears hypointense on T2-weighted images, whereas the inner annulus, which is composed of fibrocartilage and a high proportion of type II collagen, is indistinguishable from the hyperintense nucleus pulposus. Compared with normal vertebral marrow, the nucleus pulposus, which is composed of a proteoglycan matrix and type II collagen, appears hyperintense on T2-weighted images and hypointense on T1-weighted images.14 As the disc degenerates and as patients age, the signal intensity of the nucleus pulposus decreases on T2-weighted images and becomes dark on all pulse sequences with advanced degenerative changes. Additional pulse sequences, such as fat-suppressed T2-weighted images, are useful for accentuating fluid and edema, which may help in delineating spine pathology. Special attention should be given to the posterior aspect of the disc on both sagittal and axial T2-weighted images to evaluate for disc protrusion, extrusion, sequestration, or other pathology that may be contributing to central canal or neural foraminal stenosis.

The end plate is a flat, osseous disc with a slightly elevated rim secondary to the attached ring apophysis, which produces a central depression in the end plate that is occupied by hyaline cartilage.14 The annulus is composed of type I collagen fibers (Sharpey’s fibers) that are attached to the ring apophysis periosteum. The annulus also blends with the anterior and posterior longitudinal ligaments. T1-weighted images provide optimal evaluation of anatomy, fracture lines, and other osseous detail. T1-weighted images also clearly

A

Spinous process

C2

C7

T1

D

Spinous process

Lamina

B

C

E

delineate the relationships of the vertebral bodies, intervertebral discs, central canal, and posterior elements.

Vertebral Bodies

Normal lordosis should be seen in the cervical and lumbar spine, and normal kyphosis should be present in the thoracic

spine. The superior and inferior end plates should be intact and parallel to one another in the absence of vertebral compression fractures, other fractures, or infection. The subaxial cervical vertebral bodies all have a similar appearance (Fig. 2.60), whereas the atlas (C1) has no vertebral body and the axis (C2) has an odontoid process. Each of the lumbar vertebral bodies also has a similar appearance (Fig. 2.61), as do the

A

Spinous

process

Lamina

C

Vertebral body

Superior articular process

Mamillary process

B

Transverse Inferior process articular process

Superior articular process

Vertebral body

Transverse process

Spinous process

thoracic vertebral bodies (Fig. 2.62). The anterior and posterior longitudinal ligaments, seen along the vertebral bodies, have low signal intensity. The entry site of the basivertebral veins is seen at the midportion of the posterior vertebral bodies. The spinal and radicular arteries course around the cord’s anterior, posterior, and lateral aspects within the vertebral canal (Fig. 2.63). Fat-suppression techniques can be used to nullify marrow signal and increase the sensitivity in the evaluation of neoplastic and infiltrative processes.

CSF

Like other fluids, CSF has low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. On sagittal and axial images, CSF is seen surrounding the spinal cord in the cervical, thoracic, and lumbar spine and around the cauda equina in the lower lumbar spine. Thus, T2-weighted images provide a myelographic appearance that allows for the detection of spinal stenosis.

Spinal Cord

Sagittal T2-weighted images provide a myelographic e ect that allows for the evaluation of spinal cord morphology and the presence of extrinsic compression. The cord should have homogeneous signal intensity in the absence of intrinsic pathology. On axial T2-weighted images, the central gray matter can be faintly identified. Specialized sequences obtained with high magnetic field magnets allow for detailed evaluation of central gray matter (Fig. 2.64). The spinal cord should terminate at or above the L1-L2 level in adults.

Ligaments

Normal ligaments should have low signal intensity on all pulse sequences. The transverse ligament can be seen posterior to the odontoid process (Fig. 2.65) and is best visualized on axial images. The ligamentum flavum connects the lamina of adjacent vertebrae and is seen as a hypointense band posterior to the dura. The anterior longitudinal ligament is broad, thick, and adherent to the anterior vertebral bodies. The posterior longitudinal ligament is thin, attached to the posterior vertebral cortex via the midline septum,

Branch to

Aorta vertebrae and dura

Segmental a.

Anterior spinal a.

Dura

Posterior spinal aa.

Paravertebral ganglion

Anterior radicular a.

Intercostal a.

Spinal a.

Posterior intercostal a.

Posterior radicular a.

Branches to posterior

spinal muscles

and defines the posterior margin of the anterior epidural space at the midline (Fig. 2.66). The anterior epidural space is not apparent at the disc level because the posterior longitudinal ligament fuses to the discs. Visualization of these structures may be limited with MRI because of partial volume averaging with the adjacent vertebral body.15

Roots and Foramina

The dorsal and ventral nerve roots can be identified within the neural foramina on the parasagittal images and the axial images, and the exiting nerve root is located just posterior to the vertebral artery in the cervical spine (Fig. 2.67). Additional neural structures, including the sympathetic chain, are located in and around the cervical vertebral bodies (Fig. 2.68). Note that the C2 nerve root exits the spine above the C2 pedicle and runs parallel to the C1-C2 facet joint (Fig. 2.69). The nerve roots have intermediate signal intensity and are surrounded by high signal intensity fat on T1-weighted images and by high signal intensity CSF on T2weighted images.

■ Cervical Spine

Sagittal Images

The T1-weighted and T2-weighted sagittal images should be reviewed first to evaluate the spinal anatomy (Fig. 2.70). The midsagittal image from the cervicomedullary to cervicothoracic junctions is a good anatomic screen of the cervical vertebral bodies, intervertebral discs, spinal cord, thecal sac, and posterior elements. To find the midsagittal image, the sagittal series should be reviewed for the image that best shows the entire cervical spinal cord and the odontoid process, basion, opisthion, midbrain, and fourth ventricle.

Sequential evaluation of the sagittal series away from the midsagittal image allows for assessment of facet joints and neural foramina (Fig. 2.71). The foramina have an oblique orientation to the sagittal plane, which can cause distortion and lead to artifactual narrowing. T1-weighted images are somewhat limited because they a ord poor di erentiation of the vertebral body, the annular-posterior longitudinal ligament complex, and the CSF in the adjacent thecal sac16; the posterior longitudinal ligament, a disc herniation, an osseous

spur, and CSF all appear as a low-intensity signal on T1weighted sagittal images.16 The CSF–extradural interface is well defined on T2-weighted images, and a myelographic appearance is produced on T2-weighted sagittal images because of the CSF’s high signal. The intervertebral discs show intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted and gradient-echo images.

The cervical vertebral body has bright signal intensity on T1-weighted images because of the presence of normal fatty

bone marrow. Normal cervical spine lordosis should be observed. The AP diameter of the canal tapers from the first to third cervical levels and then is relatively constant. Parasagittal images show the short cervical pedicles. The anterior and posterior longitudinal ligaments are depicted as low signal intensity linear structures along the vertebral bodies.15,17,18 Often, a small cortical defect is seen in the midportion of the posterior vertebral body; this defect represents a vascular channel and is a normal finding.