PET Scans, Visualized by Lavoisier

Brain imaging. It was anticipated as far back as the 18^th century by Lavoisier, writes Stanislas Dehaene in Reading in the Brain! Lavoisier noticed that an organ uses more energy when it is at work than when it is at rest. Shouldn’t the same principle apply to the brain, wrote Lavoisier:

“One can evaluate… how many pounds of weight correspond to the efforts of a man who recites a speech… One might even assess the mechanical content in the work of a philosopher as he thinks.”

It would take another 200 years before this “simple idea” could be put into practice. Via what we call a PET scan, which stands for Positron Emission Tomography. It involves the injection of a small amount of radioactive water, where the normal Oxygen¹⁶ has been replaced by its isotope Oxygen¹⁵. This water spreads over the entire body quickly via the blood.

 

In the brain in particular, radioactivity accumulates in regions where blood flow is the fastest, which are the same regions where brain activity is most intense. Oxygen¹⁵ spontaneously turns into the stable Oxygen¹⁶ by emitting a positron - an antimatter particle. This is the Positron Emission part of the PET scan.

 

Remember what happens matter and antimatter collide? They annihilate each other and produce energy. When this emitted positron hits an electron, well, they annihilate each other and release two high-energy photons.

“The scanner detects these particles of light as they escape from the scalp in opposite directions.”

The signal from these scanners is fed to a computer which then “reconstructs the location where the disintegration initially occurred”. Thus a 3D picture is produced slice by slice. This is the Tomography part of the PET scan, from the Greek words tomos (“slice”) and graphein (“drawing”).

 

Put it all together and we have the PET scan. When done on a brain, it can tell which parts of the brain are using the most energy, just as Lavoisier had imagined.