What is the difference between PET and MRI images?
Until recently PET and MRI imaging measured different
things. MRI is generally a slow process, it takes about 15 mins to
complete and thus the image is only able to reveal still structures -
no movement can be displayed. PET can however display the areas of
the brain which are activated at a certain time and those that are
inactivated and so actual behaviour can be observed. MRI however has a
new improved piece of apparatus called Echoplanar which forms a brain
image in a tenth of a second and therefore blood can be seen as it
flows through the brain. It is believed it may in fact take over from
PET as it is a non-invasive technique, it is not radioactive so it
can be used repeatedly, and it has a higher spatial resolution than
PET.
They work in quite different ways. PET imaging involves the
injection of a tracer substance labelled with a positron emitting
radionuclide for example Flurodeoxyglucose (FDG). The cells use the
glucose in this for nutrition, and thus the active cells absorb more
FDG. The FDG then decays emitting a positron which eventually
produces a photon, and it is this that is detected by the scanner.
MRI works differently. Atoms with an odd atomic weight like
hydrogen rotate. Under normal conditions the atoms axis of rotation
points in a random direction but by introducing an outside magnetic
field the atoms can be aligned. Creating a radio - frequency field
makes the atoms move, and turning them off relaxes them and causes
electromagnetic energy to be released. This energy is measured and
the image of the brain is produced.
There were differences and therefore advantages to using PET,
such as its being able to reveal which part of the brain react to
certain stimuli, for example in reading. However with the
improvements in MRI it can now do this too. Thus as the new
technology becomes more widespread, PET's use may decline.
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