Re: PET and MRI Imaging

From: HARNAD Stevan (
Date: Mon Jun 03 1996 - 16:14:47 BST

> From: "Stent, Hannah Mary" <>
> Date: Thu, 23 May 1996 09:53:27 GMT
> Until recently PET and MRI imaging measured different
> things.

Spell them out the first time you mention them.

> 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.

For an A, you need to discuss the kinds of task used and what PET and
MRI can reveal.

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