Overview of CNS Imaging

Overview of CNS Imaging

  • Nuclear Medicine Applications:

    • Assessment of surgery or radiation therapy efficacy.
    • Evaluation of tumor involvement, lesion growth, and residual lesions post-therapy.
    • Use of CT/MRI for anatomical correlation (e.g., PET/CT or SPECT/CT is useful).

  • Central Nervous System (CNS):

    • Composed of the brain and spinal cord.
    • Transfers information between the body and the external environment.

Neuron Structure

  • Three Main Parts of Neurons:

    1. Cell Body (Soma): Contains the nucleus.
    2. Dendrites: Conduct nerve impulses toward the cell body.
    3. Axons: Conduct impulses away from the cell body to other neurons or tissues.

  • Synapse: Interface between neurons for communication.

  • Primary CNS Cell Types:

    1. Neurons: Principal functional unit.
    2. Glial Cells: Support functions of neurons (e.g., astrocytes, oligodendrocytes).
    3. Cells of Blood Vessels and Meninges.

Neuronal Function and Communication

  • Approximately 10^11 neurons in the brain; no new neurons formed in adulthood.
  • Neuronal Communication:
    • Transmit action potentials via gap junctions or neurotransmitters.
    • Neurotransmitters include amines (e.g., norepinephrine, dopamine, serotonin) and peptides.
    • Released into the synaptic cleft and interact with neuroreceptors.
    • Effects may be excitatory, inhibitory, or modulatory:
    • Example: Acetylcholine contracts skeletal muscle but relaxes cardiac muscle.

Anatomy of the Brain

  • Divisions of the Brain:
    1. Cerebrum:
    • Gray matter; two hemispheres connected by the corpus callosum.
    • Subdivided into four lobes:
      • Frontal Lobes: Higher mental functions (planning, judgment).
      • Temporal Lobes: Hearing, language, memory.
      • Parietal Lobes: Sensory functions.
      • Occipital Lobes: Vision.
    1. Cerebellum: Memory and skeletal muscle movement.
    2. Diencephalon:
    • Thalamus: Pain and temperature sensation.
    • Hypothalamus: Body temperature, water balance, pituitary function, hunger.
    1. Brain Stem: Medulla, pons, midbrain—controls cardiac and respiratory functions.

Cerebrospinal Fluid (CSF) and Blood-Brain Barrier (BBB)

  • Cavities in the Brain: Called ventricles; filled with cerebrospinal fluid (CSF).

  • Functions of CSF:

    • Shielding the brain from shocks.
    • Transporting nutrients and removing waste.

  • Estimated total volume of CSF in adults: 125-150 ml.

  • Blood Supply:

    • Brain requires 20% of the body's oxygen supply.
    • Glucose is the main energy source; areas performing mental functions exhibit increased glucose use.

  • Blood-Brain Barrier (BBB):

    • Protects the brain from potentially toxic substances.
    • Substances cross via active transport or depend on lipophilicity.
    • BBB damage from trauma can allow restricted substances to enter the brain.

Neuroimaging Techniques

  • Traditional Brain Scanning (Prior to CT/MRI):

    • Used for diagnosis of brain tumors and strokes via nuclear medicine techniques.
    • Radiopharmaceuticals: Tc-99m agents to visualize neural activity.

  • Modern Imaging Approaches:

    • Planar Scanning: Non-3D view. Useful for immediate blood flow assessments.
    • SPECT or PET: 3D imaging reflecting metabolic activity, cerebral blood flow, and neurotransmitter dynamics.

  • Cancer Imaging: PET brain scans can reveal tumor aggressiveness based on glucose metabolism (e.g., Alzheimer’s disease, epilepsy).

Alzheimer’s Disease and Tau Imaging

  • Alzheimer's Disease:

    • Dementia characterized by cognitive decline, memory loss, and personality changes.
    • FDG PET detects low metabolism areas in the brain, indicating neuronal degeneration.

  • New Radiopharmaceuticals:

    • Beta Amyloid Plaque Agents: Three FDA-approved: Florbetapir, Flutemetamol, Florbetaben.
    • Detect amyloid deposition but cannot determine the severity of the disease based on uptake alone.

  • TAUVID (flortaucipir F 18): Evaluates tau neurofibrillary tangles density.

Other Radiopharmaceuticals and Their Uses

  • Fluorodopa F 18: Evaluates dopaminergic nerve terminals in Parkinson’s patients. Administer 5 mCi IV and wait approximately 80 minutes for imaging.

  • DaTscan (I-123 ioflupane): Visualizes dopamine transporters; dosage typically 5 mCi.

  • Radionuclide Cisternograms: Used for diagnosing hydrocephalus; involves injecting 500 µCi In-111 DTPA subarachnoid.

    • Normal Imaging: Ascending activity in CSF spaces within expected time frames.
    • Abnormal Imaging: Persistent activity indicates potential hydrocephalus or shunt failure.

Summary of Procedures and Techniques

  • Correlate imaging results with anatomical studies (CT/MRI) for more accurate diagnosis.
  • Imaging performed with patients in supine position to maintain orientation.
  • Head should be secured to prevent movement during imaging for accurate results.