Central Nervous System: Cerebrum

Central Nervous System (CNS)

• The CNS divides into the brain and spinal cord
• The brain occupies the cranial cavity
• The brain consists of gray matter (cortex - outer portion) and white matter (inner portion)
• The brain divides into the cerebrum, cerebellum, and brain stem
• The midbrain connects the cerebrum to the pons and cerebellum

Cerebrum

• The cerebrum, also known as the forebrain, forms the largest part of the brain
• Its surface has shallow sulci and deeper grooves (fissures)
• These divide it into four lobes, each named for its location
• The cerebrum is divided into right and left hemispheres by the longitudinal sulcus (interhemispheric fissure)
• The corpus callosum is a large band of nerve fibers between the right and left hemispheres
• Located midline inferior to the longitudinal sulcus
• Each hemisphere has a fluid-filled cavity called the lateral ventricle
• The midbrain is a stem-like portion connecting the cerebrum to the pons and cerebellum

Cerebellum

• The hindbrain's largest part
• It is separated from the cerebrum by a deep transverse cleft
• The hemispheres of the cerebellum connect, via a median constricted area called the vermis
• The surface of the cerebellum has numerous transverse sulci
• This gives it a cauliflower-like appearance
• The diencephalon (interbrain) contains the thalamus and hypothalamus
• The thalamus' oval-shaped gray matter masses surround the third ventricle
• The pituitary gland, the master endocrine gland, lies inferior to the diencephalon
• The pituitary gland resides in the hypophyseal fossa of the sella turcica
• The folia consists of tissue between the curved sulci
• The pons forms the upper part of the hindbrain and is the commissure (bridge)
• It lies in between the cerebrum, cerebellum, and medulla oblongata
• The medulla oblongata extends between the pons and spinal cord
• It forms the hindbrain's lower portion
• All fiber tracts between the brain and spinal cord pass through the medulla

Spinal Cord

• The spinal cord is a slender, elongated structure
• It consists of an inner gray, cellular substance, with a butterfly shape on the transverse section and outer white, fibrous substance
• It extends from the brain at the medulla oblongata (foramen magnum) to between L1 and L2
• The conus medullaris is the spinal cord's pointed extremity
• The filum terminale is a delicate, fibrous strand that extends from the terminal tip
• It attaches the cord to the upper coccygeal segment
• The adult spinal cord is 18 to 20" long, connected to 31 pairs of spinal nerves
• The nerves transmit through the intervertebral and sacral foramina
• The cauda equina is spinal nerves resembling a horse's tail
• It lies below the spinal cord's termination, extending inferiorly through the vertebral canal
• The spinal cord and nerves transmit and receive sensory, motor, and reflex messages to and from the brain

Meninges

• The meninges consist of 3 continuous protective membranes enclosing brain & spinal cord
• The pia mater is the inner sheath, highly vascular, closely adhered to underlying brain and cord structure
• The arachnoid mater is delicate, central, with a subarachnoid space, wide, lying between arachnoid and pia mater
• Subarachnoid cisterns are areas of subarachnoid space that are increased in width
• The cisterna magna is the widest of subarachnoid cisterns
• It sits in the lower posterior fossa between the base of the cerebellum and the dorsal surface of the medulla oblongata
• The dura mater is the outermost sheath, strong and fibrous
• It's 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
• It consists of 2 layers:
  • Outer layer lines cranial bones, made of periosteum to their inner surface.
  • Inner layer protects the brain, supports the blood vessels
• Dura mater consists of 4 partitions:
  • Falx cerebri runs through longitudinal tissues, supporting and separating cerebral hemispheres
  • Falx cerebelli separates the cerebellar hemispheres
  • Tentorium cerebelli, a tent-shaped fold separating cerebrum and cerebellum
  • Dural sac located in lower portion, encloses cauda equina

Ventricular System

• Consists of 4 irregular, fluid-containing cavities (ventricles) that communicate through connecting channels
• Has 2 right and left lateral ventricles (upper identical cavities) sitting on each side of the medial superior part (MSP) of the corresponding hemisphere of the cerebrum
• Each consists of a body (central portion) prolonged into 3 horn-like portions that give the ventricle a U shape
  • Anterior, posterior (occipital), and inferior (temporal) horns
• The Interventricular foramen (Foramen of Monro) connects lateral ventricles to the third ventricle
• The third ventricle is a slit-like cavity, quadrilateral in shape
• It lies in the MSP just inferior to bodies of lateral ventricles
• The cerebral aqueduct (aqueduct of Sylvius) connects third and fourth ventricles
• Fourth ventricle is diamond-shaped
• Located in the area of the hindbrain
• Anterior to the cerebellum, posterior to the pons and upper portion of the medulla oblongata
• A distal, pointed end continues with the central canal of the medulla oblongata
• The Median and lateral apertures (foramina of Magendie and Luschka) are where CSF exits the fourth ventricle

CNS Imaging Procedures

• Radiographic examination
• Myelography
• Computed tomography (CT)
• Magnetic resonance imaging (MRI)
• Vascular and interventional procedures

Radiographic Examination

• This starts with noninvasive imaging procedures
• Radiographs of the cerebral and visceral cranium and the vertebral column
• Radiographs are obtained to detect bone injury, subluxation, or dislocation of the vertebral column and to determine the extent and stability of the bone injury
• CT is often employed first in a trauma because of its speed & ability to demonstrate both soft tissue and bony anatomy
• A traumatized patient with possible CNS involvement requires a cross-table lateral C spine radiograph
• This will rule out fracture or misalignment of the C spine
• Radiographs of the spine have to be obtained before myelography

Myelography

• A radiologic examination of the CNS structures within the spine
• Requires contrast introduction into the subarachnoid space via intrathecal injection by spinal puncture
• Puncture is made at L2-L3 or L3-L4 interspace or into the thecal sac, via a lateral C1–C2 sternal puncture
• Usually performed as an outpatient procedure, with patients recovering for about 4 to 8 hours before being released
• Often MRI is used instead
• Myelography is still used for patients with contraindications for MRI, such as those with pacemakers and metal fusion rods:
• Shows extrinsic spinal cord compression caused by herniated disks, bone fragments, tumors, spinal cord swelling, narrowing of the subarachnoid space, and CSF leaks
• Contrast is generally water-soluble, nonionic, iodinated:
  • Provides a good visualization of nerve roots
  • Allows for good enhancement for follow-up CT of the spine
  • Body readily absorbs these agents
• Use of ionic contrast may cause severe and fatal neurotoxic reactions

Myelography Preparation

• The room should be prepared by the radiologic technologist (RT) before patient arrival
• Table and equipment have to be cleaned
• Footboard and shoulder supports are attached
• Radiographic equipment is checked
• Image intensifier locked to prevent contact with sterile field or spinal needle
• An informed consent form is completed and signed by the patient and physician
• Premedication may be needed
• A patient should be well hydrated
• Note all procedural details, including any table movement & sensations
• Scout images made:
  • A cross-table lateral lumbar spine, prone
  • The Prone position is usually selected for spinal puncture
  • Lateral with spine flexed may also be used
• A local anesthetic is given at the puncture site
• A spinal needle is inserted
• CSF is usually withdrawn and sent to laboratory (in a healthy system: clear; unhealthy: yellow)
• Contrast injected and needle is removed
• The table angle and gravity are used to move contrast under fluoroscopy
• Spot images are taken as needed
• Where contrast moves into cervical area, the head needs to be positioned in acute extension
• This prevents contrast from entering the ventricular system
  • acute extension compresses cisterna magna which is the only position to prevent contrast from entering ventricles
• After the procedure monitoring is required;
  • the head and shoulders should be elevated 30 to 45 degrees
  • bed rest for several hours
  • fluid encouraged
  • puncture site checked before release

Lumbar puncture

• It involves a similar setup to myelography, but without contrast administration steps
• Spinal needle is inserted, and CSF is withdrawn for laboratory analysis, or medication given intrathecally
• It is performed for diagnosis to detect infections (meningitis), demyelinating diseases (multiple sclerosis), bleeding (subarachnoid hemorrhage) & increased intracranial pressure
• It's performed to inject medications
• May be performed for treatment purposes

Vertebral Augmentation

• Comprises several percutaneous techniques aimed at stabilizing weakened vertebral bodies
• Vertebroplasty, kyphoplasty, and mesh-container-plasty are interventional radiology procedures
• These treat spinal osteoporotic compression fractures and other pathologies
• Used in cases of severe pain that does not improve over many weeks of conservative treatment.
• Percutaneous vertebroplasty:
  • consists of injecting a radiopaque bone cement into a painful compression fracture under fluoroscopic guidance
  • is performed in a special procedures suite or the operating room with the patient sedated but awake.
• 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
  • Helps restore the spine to a more normal curvature and reduce hunchback deformities.
• Percutaneous mesh-container-plasty:
  • This utilizes a bone expansion brace to cut the bone tissues
  • After the brace is withdrawn, a mesh container is advanced into the cavity, then the cement is injected into the container
  • This technique restores vertebral body height and strengthens bony trabeculae.

Other Neuroradiographic Procedures

• Include provocative diskography and interventional pain management

Provocative diskography

• It's performed under fluoroscopic guidance to determine whether the disc is the source of a person’s chronic back pain
• A small quantity of a water-soluble, nonionic iodinated media is injected into the center of the disk
• It's used in the investigation of internal disk lesions, such as rupture of the nucleus pulposus, which cannot be shown by other imaging procedures
• CT is usually performed after diskography to look for clefts or tears
• MRI & CT have largely replaced diskography

Interventional pain management

• A treatment for chronic back pain that does not respond to conservative treatment
• Fluoroscopy, CT, and ultrasonography are used to confirm needle placement
• Corticosteroids & local anesthetics are used to reduce inflammation and improve symptoms
• Performed at all levels of the spine, including facet injections, nerve root blocks, and epidural steroid injections
• Various needle types can be used, but needles with a stylet are most common to prevent tissue from being trapped in the lumen