Imagine that it’s a cold winter’s morning. You turn the ignition
key on your car and all you get is a depressing clunk. The battery is flat.
Of course, if you open the car bonnet you don’t see a squashed battery.
A flat battery means that it can’t provide the power to start the engine,
it has stopped working.
When you go to the hospital and have an X-ray, it shows what your body looks
like underneath your skin. But what it doesn’t reveal is how well your
body is working. Like the car battery, your body relies on chemistry to work,
so what we need is a way of imaging the body’s chemistry.
We can now do this with a technique invented by physicists and known as Positron
Emission Tomography, or PET for short. For example, to detect cancer we give
the patient an injection of labelled sugar. Tissues need sugar to grow and
cancerous tissues grow faster than normal. So if PET shows that some parts
of the body are using sugar more quickly than expected then cancer might be
present. PET shows your body’s biochemistry and gives the doctors a better
chance of finding out what is wrong.
To do PET imaging requires the skills of many different scientists; physicists,
chemists, engineers, radiologists. So far Aberdeen is the only place in Scotland
that can do PET. But the NHS in Scotland has now decided to set up more PET
centres so that it is available to all patients who need it. That is just one
example of how physics helps medicine. So don't be puzzled if your doctor says
you need a PET, he may just think that your body's battery is flat.
Positron Emission Tomography (PET)
Our body relies on chemistry to work. So disease can sometimes be diagnosed
by looking for changes in the body’s chemistry. This may be done by taking
a blood sample and then analysing it in a laboratory. But it would be more
useful if we could take images showing the body’s biochemistry. That
way we would also see where in the body things were going wrong. This is what
PET imaging does.
A very small amount of a chemical is injected into the patient. The most common
one is glucose since sugar is one of the body’s major fuels. An image
showing where in the body the sugar is being used tells us how effectively
different tissues are working. In particular we use this for looking for cancer
as cancer cells often work much harder than normal cells and so use a lot more
sugar.
To give us our “sugar image” we attach a very small amount of
radioactive material to the sugar, in this case fluorine. Fluorine gives off
positrons. These are electrons but they have a positive charge. As a positron
travels through tissue it quickly meets a normal negatively charged electron
and changes into two gamma rays. These are like light but have a lot more energy.
So, unlike normal light, the gamma rays can pass through several centimetres
of tissue and escape from the body. By detecting these pairs of gamma rays
as they come out of the patient we can build up a 3D PET image showing how
our body is using the sugar.
With many hundreds of different chemicals that can be used, PET is providing
the doctor with a unique insight into the biochemistry of disease. Over one
hundred years ago the discovery of X-rays gave us images of what the body looked
like beneath the skin, now PET gives us biochemical imaging.
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