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ScienceWise - Spring 2012

A positron approach

Article Illustration
A PET scanner
Article Illustration
The twin gamma rays emitted when positrons annihilate with electrons form the basis of PET scan technology

Antimatter in medicine

Compared to a century ago, doctors have a bewildering range of diagnostic and imaging techniques at their disposal one of which is the Positron Emission Tomography or PET scan.

In preparation for the scan the patient is injected with a sugar solution in which some of the hydrogen atoms have been replaced with radioactive fluorine 18 (18F). This substitution is possible because fluorine and hydrogen both have a single electron in their outer shells giving them similar chemical properties.  Once inside the body this radioactive sugar is taken up by cells. Highly active cells such as growing cancers absorb far more sugar than normal cells around them.

Because 18F has a half-life of only a couple of hours, it soon decays inside the body emitting positrons (anti electrons). Because these positrons are antimatter, they rapidly annihilate when they meet electrons in the body. When this happens the resulting energy release is in the form of a pair of gamma rays emitted in almost exactly opposite directions. A ring of special detectors record these gamma rays. By calculating their directions of flight and time of arrival it’s then possible to build up a three dimensional picture of the body.

One the principal advantages of PET, is that it’s a functional imaging technique enabling doctors to not only see structures within the body, but also which structures are the most active. However there are limitations too.
One of the problems is that the radiation dose received by the patient is roughly equivalent to their annual yearly limit. Another is that the positrons travel a little way from the fluorine atom that emits them before they decay, resulting in a significant loss of resolution.

The latter problem is due to the energy with which the positrons are emitted. Although the decay of 18F produces comparatively low energy positrons, due to the relatively low density of human tissue, these still travel several mm before annihilating.

The best approach to fixing both these problems is to go back to the underlying physics and that’s exactly what scientists like Dr James Sullivan from the Centre for Antimatter-Matter Studies are doing.

“One of the main problems in designing better PET scanners is that we still have quite a limited understanding of how positrons behave when they interact with matter. A lot of what we currently know about safe radiation doses is based on electron data yet positrons can behave quite differently.” Dr Sullivan says, “We have to put a lot more physics into this and build a better understanding from the bottom up.”

The first step in doing this was to look at the way in which positrons scatter from very simple atoms such as hydrogen and helium using the Australian Positron Beamline Facility at the Australian National University in Canberra. “This is very much a problem in fundamental quantum mechanics,” Dr Sullivan explains, “The sensible approach is to begin with a system with as few variables as possible then build to more complex molecules, which might be useful in a more applied context.”

One such molecule is pyrimidine from which the DNA bases cytosine, thymine and uracil are constructed. “There’s been some quite encouraging results reported recently that suggest that the data collected from pyrimidine does indeed have direct relevance to the interaction of radiation with DNA, so we believe that we’re on the right track here.” Dr Sullivan says. “The long term goal of this work is to use this improved knowledge of the physics of antimatter to better understand the underlying process of PET scans - ultimately working towards better PET scanners than we have today.”

The scientists are hoping that eventually this work will result in an increase in the resolution of PET scans, enabling them to see very small secondary tumours much earlier which should offer a better outcome for patients. A better understanding of how positrons interact with DNA will also lead to better defined radiation dose limits which should in turn, help reduce risk to patents.

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