14: Myelography and Other Central Nervous System Imaging

The cerebrum is the largest part of the brain and is referred to as the forebrain. Its surface is convoluted by shallow sulci and deeper grooves (fissures) that divide it into lobes and lobules. There are four lobes of the cerebrum, each named for its location within the cranium: the frontal lobe, parietal lobe, occipital lobe, and temporal lobe. The stemlike portion that connects the cerebrum to the pons and cerebellum is termed the midbrain. The cerebellum, pons, and medulla oblongata make up the hindbrain.

A deep cleft, called the longitudinal sulcus (interhemispheric fissure), separates the cerebrum into right and left hemispheres, which are closely connected by bands of nerve fibers, or commissures. The largest commissure between the cerebral hemispheres is the corpus callosum. The corpus callosum is a midline structure inferior to the longitudinal sulcus. Each cerebral hemisphere contains a fluid-filled cavity called a lateral ventricle. At the diencephalon, or second portion of the brain, the pair of oval-shaped gray matter masses of the thalamus surround the third ventricle. Inferior to the diencephalon is the pituitary gland, the master endocrine gland of the body. The pituitary gland resides in the hypophyseal fossa of the sella turcica.

The cerebellum, the largest part of the hindbrain, is separated from the cerebrum by a deep transverse cleft. The hemispheres of the cerebellum are connected by a median constricted area called the vermis. The surface of the cerebellum contains numerous transverse sulci that account for its cauliflower-like appearance. The tissues between the curved sulci are called folia. The pons, which forms the upper part of the hindbrain, is the commissure, or bridge, between the cerebrum, cerebellum, and medulla oblongata. The medulla oblongata, which extends between the pons and spinal cord, forms the lower portion of the hindbrain. All the fiber tracts between the brain and spinal cord pass through the medulla.

In an adult, the spinal cord is 18 to 20 inches (46 to 50 cm) long and is connected to 31 pairs of spinal nerves. Each pair of spinal nerves arises from two roots (dorsal and ventral) at the sides of the spinal cord. The nerves are transmitted through the intervertebral and sacral foramina. Spinal nerves below the termination of the spinal cord extend inferiorly through the vertebral canal. These nerves resemble a horse’s tail and are referred to as the cauda equina. The spinal cord and nerves work together to transmit and receive sensory, motor, and reflex messages to and from the brain.

The outermost sheath, called the dura mater (Latin, meaning “tough mother”), forms the strong fibrous covering of the brain and spinal cord. The dura is separated from the arachnoid by the subdural space and from the vertebral periosteum by the epidural space. These spaces do not communicate with the ventricular system. The dura mater is composed of two layers throughout its cranial portion. The outer layer lines the cranial bones, serving as periosteum to their inner surface. The inner layer protects the brain and supports the blood vessels. The inner layer also has four partitions that provide support and protection for the various parts of the brain. One of these partitions, the falx cerebri, runs through the longitudinal fissure and provides support for the cerebral hemispheres. The tentorium cerebelli is a tent-shaped fold of dura that separates the cerebrum and cerebellum. Changes in the normal positions of these structures often indicate pathology. The dura mater extends below the spinal cord (to the level of the second sacral segment) to enclose the spinal nerves, which are prolonged inferiorly from the cord to their respective exits. The lower portion of the dura mater is called the dural sac. The dural sac encloses the cauda equina.

Each lateral ventricle consists of a central portion called the body of the cavity. The body is prolonged anteriorly, posteriorly, and inferiorly into horn-like portions that give the ventricle an approximate U shape. The prolonged portions are known as the anterior, posterior, or occipital, and inferior or temporal horns. Each lateral ventricle is connected to the third ventricle by a channel called the interventricular foramen, or foramen of Monro, through which it communicates directly with the third ventricle and indirectly with the opposite lateral ventricle.

The fourth ventricle is diamond-shaped and located in the area of the hindbrain. The fourth ventricle is anterior to the cerebellum and posterior to the pons and the upper portion of the medulla oblongata. The distal, pointed end of the fourth ventricle is continuous with the central canal of the medulla oblongata. CSF exits the fourth ventricle into the subarachnoid space via the median aperture (foramen of Magendie) and the lateral apertures (foramen of Luschka). The central canal continues inferiorly to the conus medullaris and functions to carry CSF and nutrients throughout the spinal cord (Fig. 14.7).

For a traumatized patient with possible CNS involvement, a cross-table lateral cervical spine radiograph may be obtained to rule out fracture or misalignment of the cervical spine. Approximately two-thirds of significant pathologic conditions affecting the spine can be detected on this initial image. Care must be taken to image the entire cervical spine adequately, including the C7–T1 articulation. Employing the Swimmer’s technique (see Chapter 9) may be necessary to show this anatomic region radiographically.

After the cross-table lateral radiograph has been checked and cleared by a physician or advanced practitioner, the following cervical spine projections should be obtained: AP, bilateral AP oblique (trauma technique may be necessary), and AP to show the dens. A vertebral arch, or pillar image, of the cervical spine may provide additional information about the posterior portions of the cervical vertebrae (see Chapter 9). An upright lateral cervical spine radiograph may also be requested to show alignment of the vertebrae better and to assess the normal lordotic curvature of the spine.

Radiographs of the spine should always be obtained before myelography. Routine images of the vertebral column are helpful in assessing narrowed disk spaces because of degeneration of the disk, osteoarthritis, postoperative changes in the spine, and other pathologies of the vertebral column. Because the contrast agents used in myelography may obscure some anomalies, noncontrast spinal images complement the myelographic examination and often provide additional information.

Routine skull images may be obtained when the possibility of a skull fracture exists. In trauma patients, a cross-table lateral or upright lateral skull radiograph may be obtained to show air-fluid levels in the sphenoid sinus. In many instances, these air-fluid levels may be the initial indication of a basilar skull fracture. A noncontrast head CT is indicated in head trauma patients who experience a loss of consciousness or other neurologic symptoms. In addition, skull images are helpful in diagnosing reactive bone formation and general alterations in the skull resulting from various pathologic conditions, including Paget disease, fibrous dysplasia, hemangiomas, and changes in the sella turcica.

Most myelograms are performed on an outpatient basis, with patients recovering for approximately 4 to 8 hours after the procedure before being released to return home. In many parts of the United States, magnetic resonance imaging (MRI) (see Chapter 26) has largely replaced myelography. Myelography continues to be the preferred examination method for assessing disk disease in patients with contraindications to MRI, such as pacemakers or metallic posterior spinal fusion rods.

Myelography is employed to show extrinsic spinal cord compression caused by a herniated disk, bone fragments, or tumors and spinal cord swelling resulting from traumatic injury. These encroachments appear radiographically as a deformity in the subarachnoid space or an obstruction of the passage of the column of contrast medium within the subarachnoid space. Myelography is also useful for identifying a narrowing of the subarachnoid space by evaluating the dynamic flow patterns of the CSF. Myelography may also demonstrate CSF leak and be used for surgical planning.

Percutaneous vertebroplasty is defined as the injection of a radiopaque bone cement (e.g., polymethyl methacrylate) into a painful compression fracture under fluoroscopic guidance. This procedure is typically performed in the special procedures suite or the operating room with the patient sedated but awake. A specialized trocar needle is advanced into the fractured vertebral body under fluoroscopy (Fig. 14.17). Intraosseous venography using nonionic contrast media is performed to confirm needle placement. When the physician is satisfied with the needle placement, the cement is injected (Fig. 14.18). The cement stabilizes fracture fragments and leads to reduction in pain. Postprocedural imaging includes AP and lateral projections of the spine to confirm cement position (Fig. 14.19). A CT scan may also be performed.

Percutaneous kyphoplasty differs from vertebroplasty in that a balloon catheter is used to expand the compressed vertebral body to near its original height before injection of the bone cement. Inflation of the balloon creates a pocket for the placement of the cement. Kyphoplasty can help restore the spine to a more normal curvature and reduce hunchback deformities.

Percutaneous mesh-container-plasty utilizes a bone expansion brace to cut the bone tissues. After the brace is withdrawn, a mesh container is advanced into the cavity and the cement is injected into the container. This technique restores vertebral body height and strengthens bony trabeculae.

The success of these procedures is measured by significant reduction of pain, reduced disability, and improved quality of life reported by the patient. With proper patient selection and technique, success rates of 80% to 90% have been reported. Vertebral augmentation procedures have risks of serious complications, however. Major complications occur in less than 1% of patients treated for compression fractures. The most common complication is leakage of the cement before it hardens. Pulmonary embolism and death, although rare, have been reported. Patients should be encouraged to discuss risks, benefits, and alternatives with their physicians. Technologists who perform these procedures need to be properly educated and ensure that informed consent has been documented.