THE SPINAL COLUMN AND ITS CONTENTS
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 with 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, with 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, with 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 with lateral recess narrowing without symptoms of nerve entrapment. In most cases in these patients the nerve root is located medially within the spinal canal. The neural foramen This is actually a fibro-osseous canal, directly continuous with 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
101
