Chapter 14: Myelography & Other Central Nervous System Imaging

Detailed Chapter Overview

Chapter 14 explores myelography, an invasive and highly specialized imaging procedure of the central nervous system (CNS). Although the prevalence of myelography has decreased with the advancement of Magnetic Resonance Imaging (MRI), it remains an indispensable diagnostic tool for specific clinical scenarios, particularly when MRI is contraindicated or when dynamic imaging is required to assess spinal stability. This chapter provides a definitive guide to the intricate anatomy of the spinal cord and its protective meningeal layers, which is foundational to understanding the procedure. A core focus is on the subarachnoid space—the target for contrast injection—and the circulation of cerebrospinal fluid (CSF). The chapter meticulously details the indications for myelography, such as identifying spinal cord compression from herniated discs, tumors, or bone fragments. It provides a comprehensive discussion of the nonionic, water-soluble contrast media used, patient preparation, contraindications, and crucial post-procedural care. The procedural steps for both lumbar and cervical punctures are outlined with an emphasis on sterile technique and patient safety. The chapter explains the dynamic use of table angulation (tilting) under fluoroscopy to control the flow of contrast to the specific region of interest. Furthermore, it details the subsequent imaging protocols, which almost always include a post-myelogram CT scan to provide detailed cross-sectional images, combining the benefits of both modalities. For every aspect of the procedure, rigorous protocols and evaluation criteria are established, ensuring the radiographer can play their critical role in this complex diagnostic examination.

In-Depth Study Guide

Anatomy of the Central Nervous System (CNS)

The CNS is composed of the brain and the spinal cord, which are enclosed and protected by the skull and vertebral column, respectively.

1. The Spinal Cord

• Structure: A slender, elongated structure extending from the medulla oblongata of the brainstem down to the lower border of the first lumbar vertebra (L1).
• Conus Medullaris: The tapered, pointed distal end of the spinal cord, located around the L1-L2 level.
• Cauda Equina: Below the conus medullaris, the spinal nerves continue down through the vertebral canal in a bundle that resembles a horse's tail, hence the name "cauda equina".
• Filum Terminale: A delicate fibrous strand that extends from the tip of the conus medullaris and attaches the cord to the upper coccygeal segment.

2. The Meninges

The brain and spinal cord are enclosed in three protective membranes, or meninges.

• Pia Mater: The innermost layer, a delicate and highly vascular membrane that adheres directly to the surface of the brain and spinal cord.
• Arachnoid Mater: The middle layer, a delicate, web-like membrane. It is separated from the pia mater by the subarachnoid space.
• Dura Mater: The outermost and toughest layer, forming a strong, fibrous protective sheath. The space between the arachnoid and dura mater is the subdural space. The space external to the dura is the epidural space.

3. Cerebrospinal Fluid (CSF) and Subarachnoid Space

• Subarachnoid Space: This is the crucial space located between the arachnoid and pia mater. It is filled with **cerebrospinal fluid (CSF)**, which bathes and cushions the brain and spinal cord.
• Myelography Target: The subarachnoid space is the specific anatomical location where contrast medium is injected during a myelogram. The contrast mixes with the CSF, allowing for radiographic visualization of the spinal cord and nerve roots.

Myelography: The Procedure

Myelography is the radiographic study of the spinal cord and its nerve root branches that employs a contrast medium.

1. Clinical Indications and Contraindications

• Indications: Primarily used to demonstrate extrinsic spinal cord compression caused by a herniated disk, bone fragments, or tumors, as well as spinal cord swelling resulting from traumatic injury. It is often performed when MRI is contraindicated (e.g., patient has a pacemaker or metallic implants).
• Contraindications:
  • Blood in the CSF (indicates irritation within the subarachnoid space).
  • Arachnoiditis (inflammation of the arachnoid membrane).
  • Increased intracranial pressure.
  • Recent lumbar puncture (within the last 2 weeks).

2. Contrast Media and Patient Care

• Contrast Type: Nonionic, water-soluble, iodinated contrast medium is used exclusively. It provides good visualization and is absorbed by the body.
• Patient Preparation: The patient should be well-hydrated to reduce the risk of post-procedural headache. Pre-medication (a sedative or muscle relaxant) is often administered.
• Post-Procedure Care: The patient's head and shoulders should be kept elevated 30 to 45 degrees for several hours after the procedure to prevent the contrast medium from flowing into the cranial ventricles, which can cause severe headaches or seizures. Bed rest with increased fluid intake is recommended for 8-24 hours.

3. Puncture Sites and Needle Placement

The injection is performed under fluoroscopic guidance by a radiologist or neuroradiologist using strict sterile technique.

• Lumbar Puncture: The most common site. The patient is placed in a lateral or prone position. The needle is inserted into the subarachnoid space at the **L3-L4 level**. This site is well below the end of the spinal cord (conus medullaris), minimizing the risk of injuring it.
• Cervical Puncture (C1-C2): Less common and performed only by experienced physicians. The patient is erect or prone. The needle is inserted into the subarachnoid space between the C1 and C2 vertebrae. This approach is used when there is a complete blockage in the lumbar or thoracic spine preventing the upward flow of contrast.

Imaging Protocols

Conventional Myelography (Fluoroscopy)

• Contrast Flow: After the contrast is injected, the patient is tilted in Trendelenburg and Fowler positions under fluoroscopy. This uses gravity to control the flow of the contrast column to the specific area of the spinal cord being examined (cervical, thoracic, or lumbar).
• Radiographs: Horizontal beam lateral and various oblique projections are taken to visualize the nerve roots and any areas of compression. For the cervical region, a "Swimmer's" lateral is often necessary to see the C7-T1 area.

CT Myelography

• The Modern Standard: Today, conventional myelography is almost always followed by a CT scan of the spine while the contrast is still in the subarachnoid space.
• Advantages: CT myelography provides significantly more detail than conventional radiographs. It creates detailed cross-sectional images that can demonstrate the size, shape, and position of the spinal cord and nerve roots with great precision. It is particularly excellent for visualizing subtle disk herniations or bone spurs compressing the neural elements.
• Procedure: After the fluoroscopic portion is complete, the patient is moved to the CT scanner. Thin-slice axial images are acquired through the region of interest, which can then be reconstructed into sagittal and coronal planes for a full three-dimensional evaluation.