Neuroimaging

Medical Imaging

  • Different imaging methods use different technologies, providing different types of images.
  • Some techniques are best for visualizing anatomical structures, while others are best for analyzing functions.
  • Neuroimaging is viewed in one of three planes:
    • Sagittal
    • Coronal
    • Horizontal

Structural vs. Functional Medical Imaging

  • Structural Imaging:
    • X-rays
    • Computed Tomography (CT)
    • Magnetic Resonance Imaging (MRI)
    • Diffusion Tensor Imaging (DTI)
    • Cerebral angiography
  • Functional Imaging:
    • Functional Magnetic Resonance Imaging (fMRI)
    • Positron Emission Tomography (PET)
    • Electroencephalography (EEG)
    • Magnetoencephalography (MEG)

1. X-Rays

  • X-rays penetrate the body to create a 2D image.
  • Dense structures absorb X-rays and appear whiter.
  • Hollow/soft structures do not absorb X-rays and appear darker.
  • X-rays are a type of radiation.
  • Used for:
    • Bone fractures
    • Infections (e.g., pneumonia)
    • Joints (e.g., arthritis)

2. Computed Tomography (CT)

  • Multiple X-rays are rotated around the patient to measure relative tissue density.
  • A computer reconstructs the data to create detailed 3D images.
  • Tissue density appearance:
    • More dense tissue appears WHITE (e.g., metal, bone).
    • Less dense tissue appears BLACK (e.g., CSF, air).
    • Tissue with high water content appears DARK GRAY (e.g., fat, white matter).
    • Tissue with high protein content appears LIGHT GRAY (e.g., gray matter).
  • Preferred choice in emergencies for a quick image.
  • Depicts fractures and bone abnormalities, as well as acute intracranial hemorrhages.
  • Not good for fine tissue detail.
  • Involves radiation exposure.
  • Hemorrhages appear HYPERDENSE.

3. Magnetic Resonance Imaging (MRI)

  • Magnetic fields cause hydrogen protons in tissue to align.
  • Radiofrequency wave is then pulsed to the tissue, changing proton alignment.
  • Protons return to their original position, generating an electrical signal.
  • The speed at which the proton returns to position depends on the density and mobility of the tissue, creating contrast between various tissues.
  • This creates an extremely detailed image presented in slices.
  • Good for:
    • Soft tissue disease (including the nervous system) – tumors, MS, inflammation
  • Not ideal for:
    • Bone or acute hemorrhage imaging
    • Patients with metal implants
    • People who experience claustrophobia
  • Narrow, takes longer, and is loud.
  • Expensive.

MRI Scan Types

  • T1 and T2 weighted images
  • Fluid-attenuated inversion recovery (FLAIR)
  • Diffusion-tensor imaging (DTI) and diffusion-weighted imaging (DWI)
  • Contrast MRI

3a. Diffusion Tensor Imaging (DTI)

  • Measures how water diffuses around axon bundles.
  • Useful for assessment of white matter/tracts/pathways.
  • Example: Corticospinal tract (CST) – for movement.
  • Can show the effect of stroke on the CST, indicating fewer fibers on the side of the stroke.

4. Cerebral Angiography

  • 3D reconstruction of blood vessels.
  • Radio-opaque contrast agent injected into an artery and imaged using sequential X-rays, CT, or MRI scans.
  • Useful for screening for:
    • Narrowing (stenosis)
    • Dilation (aneurysm)
    • Abnormal connections (malformations)

Functional Imaging

  • Shows how the typical & atypical brain is working to assist in neuro-rehabilitation.
  • Shows brain regions used during thought, speech, movement, and sensation.
  • Helps assess effects of stroke & degenerative disease (e.g. Alzheimer's) on brain function.
  • Monitors the growth of brain tumors.
  • A task is performed during the imaging process; determines brain activity before & during task.

1. Functional Magnetic Resonance Imaging (fMRI)

  • The fMRI detects changes in blood flow and oxygenation that occur in response to neural activity while performing a task.
  • Safe and non-invasive.

2. Positron Emission Tomography (PET)

  • Injection of radioactive isotopes.
  • Detectors measure gamma rays as they travel in cerebral blood vessels.
  • Highlights metabolically active cellular regions.
  • Not used frequently because it is invasive, expensive, and involves radiation.

3. Electroencephalography (EEG)

  • Detects and records electrical activity in the brain.
  • Non-invasive electrodes (sensors) placed on the scalp, while an electronic device (electroencephalogram) records activity.
  • Can help diagnose abnormal brain electrical activity, e.g., epilepsy, sleep disorders.

4. Magnetoencephalography (MEG)

  • Non-invasive imaging test that measures magnetic fields produced by electrical currents in the brain.
  • Identifies functional areas of the brain (e.g., sensory, motor, language, memory activities) AND the precise location of abnormal activities (e.g., epilepsy, brain tumors).