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                      The History and Evolution of PET

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The PET scanner is a new kind of medical instrument which is radically
different from the tools which the physicians had to make images of the
brain in a non-invasive way, that is, without having to open the skull in
order to peer inside or to actually take samples of brain tissue.

And why PET is different ?

The first non-invasive technique to get images from the brain was a big
revolution in itself. It was the radiography, invented by a German physicist
named Wilhelm Röntgen, in 1896, and it used x-rays, an invisible
eletromagnetic radiation discovered also by him. To Röntgens astonishement,
x-rays were able to penetrate the body, as it were transparent, and to
produce a negative photograph of the body's interior, showing with startling
detail the bones, cavities and other anatomical structures on its path.
Röntgen could see immediately the medical value of his discovery, and the
first radiograph ever made was from his wife's hand.

However, x-rays can shown only the anatomical structures, and nothing else.
The function of these structures could be inferred from anatomical changes,
but only when they happened. Enlargement, movement and flow of substances
could be observed in some selected organs (for example, the heart or the
intestines, or, by using some liquids which are opaque to the x-rays, named
contrasts), but not much more. Since the brain does not move, radiographys
is of little value to study function, particularly normal function.

The immense value of PET for studying normal brain function was highlighted
by several path-breaking investigations. The UCLA researcher Michael E.
Phelps, for instance, was one of the first to show in striking detail how
different parts of the brain are activated when performing mental tasks such
as hearing, reading, talking, thinking, etc.

                      The first PET scanners had a small number of radiation
                      sensors to build the image, and they could do only a
slice at a time. The slices were also very thick. Thus, the images obtained
with the PET had a low quality and definition. It was impossible to get the
finer details of localization of function in the brain, so their clinical
usefulness was quite limited, as compared with modern models.

                                    Thisfigureshows the evolution of image
                                    quality from the first PET scanner,
available in 1975, to the latest and most sophisticated model, ECAT Exact
HR+. You can easily see how the finer detais of internal brain structures
can be visualized much better in the latest model. This improvement has been
achieved with a larger number and better radiation sensors, better computer
programs and the possibility of getting several slices at the same time
(using many rings of sensors). All models of PET machines depicted here were
produced by the Siemens biomedical equipment manufacturer, which has
installed the majority of PET scanners in the world.

                        Modern
                        PET
scanners are very expensive and sophisticated pieces of equipment. They are
also much easier to install and to operate, and have many new capabilities
which clinicians use with advantage to perform many feats of brain imaging,
such as a higher speed in obtaining results. For example, as shown here,
they can be used to produce movies of parts of the body

Since PETs are so expensive (actually, it's
rather the whole installation, including the
cyclotron to produce the radiopharmaceuticals which adds to the cost), there
are only about 150 installations around the world, which are highly
concentrated on the USA (particularly in the East and West coasts), Europe
(particularly in the Anglo-Saxon countries and France) and in Japan. In the
Southern hemisphere, only Australia and Argentina have a very small number
of PET facilities.

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Image credits: The Crump Institute for Biological Imaging, Department of
Pharmacology, University of California at Los Angeles. CTI and The History
of Medicine CD-ROM,

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From: The PET Scan: A New Window Into the Brain
By: Renato M.E. Sabbatini, PhD
In: Brain & Mind Magazine, March 1997.