Relevant MRI anatomy - cervical spine

Spinal angiography This is a lengthy procedure involving injection of contrast into the vertebral arteries, the deep cervical arteries, several intercostal and lumbar arteries and the sacral branch of the internal iliac artery to identify the radiculomedullary arteries, which are variable in number and level of origin. No single injection w i ll opacify the entire anterior spinal artery. The main feeding vessel in spinal angiomas may arise distant from the lesion and may be missed if a thorough examination is not performed.

The arteria radicularis magna (artery of Adamkiewicz) usually arises on the left side between T8 and T12. When opacified it is seen to turn cranially from its origin towards the midline of the spinal cord and to bifurcate into a small ascending and a large descending contribution to the anterior spinal artery. This angiographic feature is called the 'hairpin appearance'.

Angiographically directed embolization of angiomatous vessels is sometimes possible.

Bronchial angiography In the catheterization of intercostal arteries and their branches, such as for bronchial angiography and in particular for bronchial embolization, there is a risk of damage to the blood supply of the cord by embolization to the radiculomedullary branches.

Magnetic resonance imaging (see Fig. 3. 12) MRI is useful for imaging the spinal cord - it is noninvasive and the entire cord can be seen in a few sagittal images. The spinal cord is well seen against the CSF in either T1- or T2-weighted images. The enlargements of the cord in the cervical and lower thoracic areas are visualized, and the cauda can be seen close to the posterior aspect of the subarachnoid space beyond the lower limit of the cord. Differentiation between grey and white matter within the cord is not always possible. A thin line of low-T1, high-T2 signal intensity seen down the centre of the normal cord on sagittal sections is not the central canal but represents a truncation artefact.

Nerve roots are visible, especially where they are outlined by fat in the intervertebral foramen. Epidural fat is invariably present posteriorly and, to a lesser extent anteriorly. The amount of epidural fat in the nerve roots increases from their cranial to their caudal ends. In contrastenhanced MRI, bright epidural veins contrast with negligible signal from nerve roots. The spinal nerves of the cauda equina can be seen outlined by the CSF, usually lying posteriorly in the thecal space, except for those passing anteriorly from the exit foramina.

RELEVANT MRI ANATOMY - CERVICAL SPINE

(Fig- 3. 20)

Vertebral body In order to maintain the axial load imposed by weight bearing, the vertebral body is composed of both a thick outer cortical shell of compact bone and an inner supporting

Fig. 3. 20 MRI of the cervical spine, (a) Midline sagittal T1-weighted and (b) fat-suppressed inversion recovery images.

1. Obliquus capitis superior 2. Rectus capitis posterior minor 3. Semispinalis capitis muscle 4. Ligamentum nuchae 5. Interspinous ligament 6. Spinous process of C7 7. Spinal cord 8. C4 vertebral body

9. Posterior longitudinal ligament 10. Odontoid peg 11. Anterior arch of C1 12. Medulla oblongata 13. Vertebral body haemangioma 14. Ligamentum flavum 15. Basivertebral vein 16. Anterior longitudinal ligament

(c) Axial T2-weighted image in the midcervical spine at the level of the true vocal cords or fourth cervical vertebral body.

1. Thyroid cartilage and vocal cords 2. Vertebral body and longus colli muscles 3. Ventral nerve root 4. Sternocleidomastoid muscle 5. Scalenus medius 6. Vertebral artery in the foramen transversarium

7. Levator scapulae 8. Semispinalis cervicis 9. Semispinalis capitis 10. Trapezius 11. Splenius capitis 12. Splenius capitis 13. Facet joint 14. Carotid sheath 15. Posterior nerve root

network of vertically oriented trabeculae or cancellous bone. The posterior element, having less of a role in weight bearing and functioning primarily to stabilize and prevent subluxation, is composed of compact cortical bone alone. In adulthood up to 50% of the vertebral body is cancellous bone relative to the femoral neck, where the cancellous bone only constitutes 30% by volume. Such a relationship explains why changes of osteoporosis occur earlier in the vertebral body than in the femoral neck, and explains why screening for osteoporosis is aimed primarily at the vertebral body.

In youth and early adulthood, the vertebral body contains primarily vascularized red marrow and is metaboli¬ cally active. Such a marrow pattern explains the distribution of bloodborne metastases to the vertebral body and explains why changes of osteoporosis tend to affect the vertebral body rather than the posterior element and peripheral appendicular sites.

Intervertebral disc anatomy These joints are amphiarthrodial, w i th only slightly movable articulations connected by fibrocartilage. Although only a slight amount of motion is possible at each joint, the spine has considerable motion because of the number of joints present. The cervical discs are thicker anteriorly than posteriorly and this shape contributes to the lordosis of the cervical spine.

The major components of the intervertebral disc are the nucleus pulposus, the annulus fibrosus and the cartilaginous endplates. The nucleus pulposus, located centrally in the intervertebral disc, is composed of fibrocartilage that is predominantly type 2 collagen and proteoglycans, which include hyaluronic acid and sulphated glycosamino¬ glycans. The disc absorbs and retains water because of the negative charge of the proteoglycans. In adults, the nucleus pulposus is hyperintense on T2-weighted scans and has indistinct boundaries w i th the annulus fibrosus. The annulus fibrosus, which is a highly ordered laminated structure, is divided into an outer and an inner ring. The outer ring inserts on to the ring apophyses of the adjacent vertebrae and the adjacent cartilaginous endplates. The outer ring is made of type 1 collagen and is hypointense on MRI; the inner ring is made of type 2 collagen w i th more proteoglycan and f l u i d, and is hyperintense on MRI, indistinguishable from the nucleus pulposus. Hyaline cartilage makes up the cranial and caudal aspects of the disc and attaches to the osseous endplates by numerous collagenous fibres. In the vertebral end plate are numerous pores through which the vascular channels extend. Diffusion of gadolinium into the disc has been shown through these channels.

In youth, the water content of the nucleus pulposus is as high as 80%, although this decreases w i th age. On MRI the relative signal intensity of the disc varies w i th water content. In health, a collagen band is often noted to traverse the equator of the disc and is termed the intranuclear cleft. Neural anatomy There are eight cervical nerves and seven cervical bodies. The first cervical nerve exits between the occipital bone and the C1 vertebral body. The eighth cervical nerve exits the spinal canal through the C7-T1 intervertebral foramen. The exiting nerve roots are clearly identified on sagittal oblique images. Each spinal nerve is composed of a dorsal and a ventral root, the dorsal root contributing sensory fibres, the ventral root contributing motor fibres. The dorsal and ventral roots merge to form the spinal ganglion, from where the spinal nerve arises. The spinal ganglion usually lies inferiorly within the exit foramen in the concavity of the superior articular process of the subjacent vertebra.

The spinal cord On T2-weighted axial images in the spinal cord, the central grey matter can be faintly identified. The grey matter is composed of the dorsal, ventral and intermediolateral horns and an intermediate zone. The dorsal horn contains cell clusters involved in sensory function and receives axons from the dorsal nerve root. The ventral horn contains multipolar motor neurons that give rise to the ventral motor root. Within surrounding white matter there are three discrete tracts which allow information transit either to the brain or to the extremities. The anterior and lateral funiculi contain ascending and descending tracts; the posterior funiculus contains ascending fibres alone.

Arterial anatomy The vertebral artery The vertebral artery is composed of four segments. The first extends from its origin at the subclavian artery and usually enters the transverse foramen of C6. In about 5% of the population the artery has an anomalous origin from the aortic arch. The second segment is vertically oriented within the transverse foramina of C6-C1 The third segment begins at the transverse foramen of C1 and extends posteriorly and horizontally on the posterior arch of C1. The fourth segment pierces the dura mater and enters the intracranial cavity.

The blood supply to the spinal cord consists of a single anterior spinal artery and paired posterior spinal arteries. These arteries cannot be seen on MR images of the spinal cord. The anterior spinal artery is formed from branches of the vertebral arteries that originate before the vertebral arteries form the basilar artery. These vessels pass inferiorly and merge to form the anterior spinal artery in the anterior median fissure. The artery extends caudally to the filum terminale. The anterior spinal artery may receive branches in the cervical region from radicular branches of the vertebral arteries, branches from the thyrocervical or costocervical trunk in the lower cervical region, and from the ascending pharyngeal arteries in the upper cervical region.