6A. Neuroimaging

Page created on June 3, 2021. Last updated on March 20, 2023 at 10:41

We can separate neuroimaging into two types:

• Structural neuroimaging – gives diagnosis of gross intracranial disease, like tumour or injuries
• Functional neuroimaging – used to examine certain functions

Computer tomography (CT)

CT is usually used for neurological emergencies, especially to rule out haemorrhage as a cause of stroke. It’s also used for traumatic injuries of the brain and spine.

The advantages of CT include:

• Fast (few minutes)
• Cheap
• No contraindications except pregnancy
• Patient cooperation not as difficult as with MRI, as the scan is faster
• Claustrophobia is not a problem as the machine is more of a donut than a cylinder

The disadvantages are that it exposes the patient to radiation and that it’s not especially good for soft tissue imaging and precise imaging of the CNS.

On native CT, blood is shown as a hyperdense lesion. Ischaemic lesions are hypodense but only visible after 24 hours.

CT with contrast can be used for cancer, as cancers show enhancement by contrast. CT angiography can be used to find occluded vessels and examine collateral blood supply. CT perfusion can differentiate salvageable ischaemic brain (penumbra) from infarcted brain in ischaemic stroke.

Magnetic resonance imaging (MR)

MR is widely used in neurology for non-acute conditions, like:

• Soft tissue imaging
• Demyelinating disorders
• Infectious CNS disorders
• Inflammatory CNS disorders
• Neurodegenerative disorders
• General imaging of brain, spinal cord, peripheral nerves, cranial nerves
• Epilepsy
• Muscle atrophy, fatty degeneration of muscle

Advantages:

• No radiation
• Good for soft tissue imaging

Disadvantages:

• Patient cooperation is required, as they must lie still for some time – anxiolytic or anaesthetic may be required
  • The patient must lie in a closed space, which can be problematic for claustrophobics
• Slow (20-30 minutes)
• Expensive
• Contraindicated for some metals and pacemakers

There are many MR modalities which each have individual purposes:

• T1: good for anatomical structures (brain, spine, muscles)
• T2: grey and white matter lesions
• Contrast-enhanced T1: shows BBB damage
• FLAIR: more sensitive for white matter lesion
• DWI: very sensitive for recent lesions (ischaemia, oedema, inflammation)
• SWI: visualization of deposition of metals
• Tractography: visualizes main tracts before neurosurgery or in ALS
• MR angiography: same as CT angiography, but doesn’t require contrast
• MR spectroscopy: measure metabolites like NAA (marker of neuronal loss) or lactate (marker of metabolic disease)
• Functional MRI: localizes certain functional areas (speech centre, motor centre) before neurosurgery

Ultrasound (US)

In neurology ultrasound is usually used to examine the extracranial (carotid) vessels for carotid stenosis. This is useful in patients with high risk for stroke, or in people who have had TIA or stroke, to look for atherosclerosis in the carotids. Transcranial doppler can be used to examine the intracranial vessels, usually for vasospasm in SAH or embolism in ischaemic stroke.

Advantages:

• Fast
• Cheap
• No radiation

Disadvantages:

• Result depends on operator experience

Digital subtraction angiography (DSA)

DSA uses fluoroscopy and interventional radiology for visualizing blood vessels. Interventional radiological procedures, like stenting, thrombectomy, and aneurysm coiling, can be performed during the same procedure.

Single photon emission computed tomography (SPECT)

SPECT measures the cerebral blood flow, which can be used in the diagnosis of epilepsy, Alzheimer, or Parkinson.

Positron emission tomography

PET measures metabolic activity, which can be used in the diagnosis of epilepsy, Alzheimer, or tumours.