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.

 
 
  An Australian Academy of Science project, supported by the Science and
   Technology Awareness Program of the Commonwealth Department of
     Industry, Science and Resources and the Australian Foundation for
                                  Science.

   This topic is sponsored by the Howard Florey Institute of Experimental
                          Physiology and Medicine.
 

       © Australian Academy of Science | Email: aas@science.org.au