Relevant MRI anatomy - dorsolumbar spine

The posterior spinal arteries arise either from the intracranial vertebral arteries or from the posterior inferior cerebellar arteries and run posterolaterally along the cord.

From both the anterior and posterior spinal arteries, anastomosing vessels arise that are arteriolar in size and encircle the spinal cord, forming a pial plexus that has been named the vasa corona.

The C1/C2 articulation

Sagittal images clearly demonstrate the C1/C2 articulation. Immediately behind the odontoid, an area of decreased signal intensity represents the transverse ligament. The transverse ligament arches across the ring of the atlas and retains the odontoid process in contact w i th the anterior arch of the atlas. A small fasciculus extends upward and another extends downwards from the transverse ligament as it crosses the odontoid. The upper fasciculus attaches to the basilar portion of the occipital bone and the lower fasciculus is fixed to the posterior surface of C2. This entire ligament is named the cruciate ligament of the atlas. Axial and coronal images through the top of the odontoid shows a portion of the alar ligaments. The alar ligaments attach to the top of the odontoid and insert on the medial aspect of the occipital condyles. The transverse ligament is present posterior to the odontoid process, attaching to the ring of C1. Anterior and posterior synovial joints make up the median atlantoaxial joint. The former is between the anterior arch of C1 and the odontoid, and the latter is between the odontoid and the transverse ligament. On either side of the odontoid are the lateral atlantoaxial joints, which are gliding joints. The rotation of the head occurs at the atlantoaxial joints.

RELEVANT MRI ANATOMY - THE DORSOLUMBAR

SPINE (Fig. 3. 21)

Vertebral bodies

The signal intensity derived from the vertebral body in the spine is dependent on the quantity of yellow marrow relative to haemopoietic red marrow. In adulthood, yellow marrow predominates and results in signal hyperintensity throughout the vertebral body on both T1 -weighted and fast spin echo T2-weighted scans. The presence of small quantities of red marrow produces some signal heterogeneity and signal suppression. Accompanying degenerative disc disease, MRI allows the identification of marrow changes adjacent to the endplate originally described by Modic. Following loss of disc hydration, morphology and function, the forces of weight bearing are directly translated to the endplate. The applied impaction forces produce endplate subarticular trabecular microfractures. Associated haemorrhage and oedema, manifest as Modic type 1 endplate marrow change, produce signal hypointensity on T1- and hyperintensity on T2-weighted scans. Chronically applied forces produce reactive fatty infiltration of the endplate, manifest as signal hyperintensity on T1-weighted and signal isointensity or hyperintensity on T2-weighted scans: Modic type 2 change. Ultimately, endplate fatty change becomes replaced by fibrosis and is manifest as signal hypointensity on T1- and T2-weighted scans: so-called Modic type 3 change.

Focal fatty deposits frequently appear within the vertebral body during ageing and show signal characteristics of epidural fat.

The basivertebral venous trunk is well seen on MRI in midsagittal sections and can be identified as a horizontal band along the posterior margin of the vertebral body midway between the superior and inferior endplates.

Lumbar facet joints The lumbar articular facets are curvilinear, w i th an anterior component that is oriented toward the coronal plane and a posterior component oriented toward the sagittal plane.

When a lumbar facet joint is viewed on an axial plane, the anterior facet belongs to the caudad vertebra and the posterior facet to the cephalad vertebra. The facet joints are true synovial joints, w i th articular surfaces covered by hyaline cartilage 2-4 mm thick. The capsule of the facet joint is a continuation of the ligamentum flavum. Chronic joint effusion may lead to the development of a synovial cyst, which may extend to the epidural space.

The lateral recess

The anterolateral portion of the spinal canal is called the lateral recess. It is bounded anteriorly by the posterior surface of the intervertebral disc and vertebral body, laterally by the pedicle and posteriorly by the superior articular process. The descending nerve root lies in the lateral recess, which is otherwise described as the radicular canal. The lateral recess is usually more than 3 mm in anteroposterior diameter, narrowing being produced by hypertrophic change in the superior facet of the vertebral body below and by posterior bulging of the vertebral body and endplate. It is important to recognize that there are a significant number of individuals w i th lateral recess narrowing w i t hout symptoms of nerve entrapment. In most cases in these patients the nerve root is located medially w i t h in the spinal canal.

The neural foramen This is actually a fibro-osseous canal, directly continuous w i th the lateral recess. It extends superoinferiorly from the pedicle above to the pedicle below, and from the posterior margin of the vertebral body anteriorly to the superior and inferior facets posteriorly. It has a wider upper portion that contains the exiting nerve roots, the dorsal root ganglion and their accompanying arteries and veins surrounded by

102 ANATOMY FOR DIAGNOSTIC IMAGING

1. Posterior epidural space 2. Interspinous ligament 3. Subcutaneous fat 4. Latissimus dorsi muscle attachments 5. Spinous process of L4 6. Anterior epidural fat pad 7. Presacral fat space 8. Intervertebral disc (central nucleus pulposus) 9. Basivertebral vein 10. L2 vertebral body 11. Conus at L1 12. Cauda equina nerve roots 13. Posterior longitudinal ligament

Sequential axial images through L5 to S1: (c) at the level of the inferior end plate of L5; (d) at the level of the L5/S1 disc; (e) at the level of the superior end plate of S1; (f) at the level of the mid-S1 vertebral body.

1. L5 vertebral body 2. Common iliac veins 3. Psoas muscle 4. L5 nerve root (exited at foramen above the L5/S1 disc space) 5. S1 nerve root in the lateral recess G. Thecal sac 7. Posterior epidural space

8. Erector spinae muscle 9. Lamina of L5 10. Inferior articular facet of L5 11. Superior articular facet of S1 12. L5/S1 intervertebral disc 13. Spinous process of L5 14. Exit foramen at L5/S1 15. Central disc bulge to anterior epidural fat 16. Pedicle of S1 17. L5/S1 facet joint 18. Transverse process of S1 (sacral ala) 19. S1 vertebral body 20. Presacral plexus 2 1. Anterior epidural space

fat. The sinuvertebral or recurrent nerve originates partly from the spinal nerve in the neuroforamen and re-enters the spinal canal. It supplies the posterior portion of the annulus, the posterior longitudinal ligament and the periosteum of the vertebral body.

The posterior longitudinal ligament This is a thin narrow band of fibrous tissue that extends on the posterior surface of the vertebral body. In the lumbar and lower thoracic regions the ligament becomes broader at each level and adheres firmly to the posterior surface of the annulus fibrosus and the adjacent vertebral cortex, but not to the midportion of the vertebral body, where epidural veins and some fat lie between the two. The ligamentum flavum This is a fibroelastic structure that covers the bony gaps between the laminae in the posterior wall of the spinal canal. There is a right and a left ligament at each level. Anterolaterally, the ligament continues as the capsule of the facet joint.

The lumbar intervertebral disc

There are five lumbar intervertebral discs, which are the largest in the body. Owing to the lumbar lordotic curve, the lower lumbar discs may be more or less wedge-shaped.

Similar to the cervical spine, each disc is composed of a cartilaginous endplate, an annulus fibrosus and a nucleus

pulposus. The cartilaginous endplate has a peripheral part which is fibrocartilaginous and covers the apophyseal ring, and a central part which is hyaline cartilage and covers the vertebral endplate. The annulus fibrosus is composed of two sets of fibres, fibrocartilaginous and collagenous, arranged in a circular and concentric fashion. The fibrocartilaginous fibres (type 2 collagen) arise from the peripheral portion of the cartilaginous endplate and merge imperceptibly w i th the nucleus pulposus. The collagenous fibres (type 2 collagen) are short and form the peripheral part of the annulus, inserting into the ring apophysis and crossing over the edge of the endplate to insert into the vertebral cortex via Sharpey's fibres. These collagenous fibres also insert into the anterior and posterior longitudinal ligaments. The annulus is stronger anteriorly, being firmly attached to the anterior longitudinal ligament, than posteriorly where the attachment to the posterior longitudinal ligament is loose. The posterior borders of the upper three discs are flat, in contrast to the lower two vertebral discs that are flat. The vascular supply to the disc decreases in maturity. In infants and during childhood, a network of vessels both supplies the outer annulus and penetrates deep into the disc substance. In adulthood, deep penetrating vessels are not identified, vessels supplying the outer annulus alone. Supply to the annulus is thought to account for frequently identified enhancement of the outer annulus in up to 60% of cases. Enhancement, reflecting vascularity, appears to increase in the annulus of degenerative discs, most likely reflecting local inflammatory changes and response to prostaglandins. During maturity, the water content of the nucleus pulposus reduces from 95% to between 70 and 75% at 70 years of age.

Intranuclear cleft

In adults a septum-like structure can be seen to transect the middle of the nucleus pulposus in the horizontal plane. On T2-weighted images of the normal lumbar discs there is often a linear area of reduced signal traversing the centre of the nucleus from anterior to posterior on sagittal views the so-called intranuclear cleft. This is thought to represent a remnant of the collagenous skeleton of the disc, being a zone of more prominent fibrous tissue than the surrounding nucleus.

The dural sac

The dural sac extends inferiorly to behind the midbody of the sacrum. In most adults it tapers progressively for 2-4 cm prior to its lowest limit and becomes more posteriorly placed behind L5/S1 than behind L4/5. As a result, the epidural space is more capacious immediately behind the L5 vertebral body than behind the L4 vertebral body. The epidural space In the lumbar spine this space contains the epidural veins, fat and nerve roots. The diameter of the anterior epidural space increases-progressively towards the sacrum, being a few millimetres behind the L5/S1 body. The anterior epidural space is divided by a midline fibrous septum attached to the posterior margin of the vertebral body and the posterior longitudinal ligament. At the level of the disc, the posterior longitudinal ligament attaches directly to the outer annulus and the septum does not exist. The posterior epidural space is most extensive at the L3/4, L4/5 interspaces, and for this reason is most frequently chosen as the site for epidural injections. The caudal extent of the spinal cord is the conus medullaris, from which extends the filum terminale internum. The filum terminale internum extends within the dural sac to its inferior point behind S2. It continues inferiorly below this point as the filum terminale externum to insert on the first coccygeal segment.

Spinal nerves There are five pairs of lumbar spinal nerves. Each nerve is formed by the union of a ventral (anterior) and a dorsal (posterior) nerve root. The nerve roots in turn are formed by the union of multiple rootlets that emerge from the anterolateral and posterolateral surfaces of the spinal cord. The dorsal root has an oval enlargement called the dorsal root ganglion, which lies in the neural foramen. The lumbar spinal nerve is formed just after the dorsal root ganglion and extends for only a small distance, when it divides into the ventral and dorsal rami of the spinal nerve.

Somatic and autonomic nerve supply to the lumbar intervertebral disc

The anatomic basis for the origin and mediation of clinical signs and symptoms relating to discogenic dysfunction is complex but relates primarily to influences from somatic innervation and from the neural rami arising from the paravertebral autonomic neural plexus.

Discogenic pain arises partly from fibres that arise from the recurrent meningeal nerve of Luschka, supplying the posterior longitudinal ligament, the meninges, the blood vessels, the posterior extent of the outermost fibres of the annulus fibrosus, a portion of the periosteum of the vertebral bodies and the underlying bone. In addition, a variable small branch from the ventral ramus of the somatic spinal nerve root may directly innervate the posterolateral aspect of the vertebral body.

Posterior spinal elements, including the facet joints, receive their innervation primarily from the dorsal rami of the spinal nerves.

In addition to somatic innervation, sympathetic fibres arising from the paravertebral autonomic plexus extend posteriorly and form the bulk of the recurrent meningeal nerve to supply the bulk of the disc periphery. Pain referred to the lumbosacral zones of Head is thought to reflect autonomic nerve impulses derived from the segmental embryologic expression of the somatic tissues.

Other procedures Before the advent of CT and MRI, the following procedures were sometimes used for evaluation of the vertebral canal and its contents:

• Phlebography: opacification of the epidural veins by injection of the lumbar veins via a lumbar approach; • Epidurography: injection into the epidural space (between dura and the vertebral bone) either via a

lumbar route or by injection from below into the sacral canal; and • Endomyelography: injection of contrast into the spinal cord under general anaesthesia, usually for differentiation of cystic swelling and central canal expansion.

Therapeutic interventions

• Epidural injection - guided injection of steroid to the posterior epidural space following contrast epidurography; • Epidural nerve block - selective injection of steroid and local anaesthetic to the epidural fat within the epidural space surrounding the exiting nerve root; and • Facet joint block - injection of steroid and local anaesthetic to the facet joint following contrast confirmation or arthrography.