Getting our heads around the brain
Box 3 | Brain imaging
One of the oldest technologies for monitoring brain activity is an electroencephalograph (EEG). This records the electrical activity of the brain using signals produced by electrodes taped to the head.
Other, more sophisticated, imaging techniques were spawned by the computer revolution of the 1970s. In computerised axial tomography (CAT scans), the brain is X-rayed from a variety of angles. A computer combines the results of the different X-rays to produce a cross-sectional image.
In positron emission tomography (PET), a radioisotope that emits positrons (similar to electrons, but with a positive charge) is injected into the bloodstream. A scanner can then detect the location of the radioisotope in the body.
This technique can be used to determine regions of brain activity. Sugar, an energy source for cells, is 'labelled' with a radioisotope and injected into the blood stream. A PET scan will show those regions of the brain containing more radioisotopes – these will be the ones that used more sugar because they were more active.
Magnetic resonance imaging (MRI) is a technology that is often used to diagnose damage to tissue, including brain tissue. This technique forms images by detecting protons which respond to a magnetic field. MRI helps detect active areas of the brain by identifying the location of oxygen-rich blood. There are several advantages to MRI, including that it requires no injection of material into the body and no radioactive substances are used.
Diagnosing diseased brains
Not all imaging techniques pick up all neurological diseases, so the technology used for the search has to match the target.
- Alzheimer's disease, in which memory is affected by the degeneration
of neurons in the temporal lobes of the cerebral cortex, shows
up in PET scans but not in MRI.
- Brain tumours show up on MRI, but PET scans are needed to
determine whether they are malignant.
- Epilepsy, which affects 1 per cent of the population, is caused
when a large collection of neurons 'fire' at the same time causing
a seizure. During seizures, severe epileptics experience a range
of symptoms including involuntary movements, hallucinations, and
emotional changes. They might also have feelings of fear, anger,
paranoia and deja vu. Both PET scans and MRI can often detect
epilepsy by registering reductions in activity in the parts of
the brain that are affected by seizures.
- Parkinson's disease, the symptoms of which include involuntary
tremors and rigidity, is caused by dysfunction of neurons in the middle part of the brain, and is diagnosed from PET scans.
- MRI is used to diagnose stroke. A stroke occurs when a blood vessel supplying a given part of the brain becomes blocked. The functioning of that portion of the brain affected is impaired: muscles controlled by that region, for instance, may no longer function. Depending on the region affected, a stroke may be fatal.
Boxes
Box 1. The human nervous system
Box 2. Functions of the left and right sides of the brain
Box 4. Neurotransmitters and drugs
Related sites
Brain imaging (University of Washington, USA)
What are some diagnostic tests used to diagnose neurological disorders? (National Institute of Neurological Disorders and Stroke, USA)
Tests and Procedures (Mayo Clinic, USA)
Brain tumor basics (American Brain Tumor Association)
Brain imaging (ABC radio's Health Report, 15 June 1998)
Page updated June 2010.