Where Grey Matter meets Dark Matter

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Episode 2 - 22 December 2008

Anti-matter medical scans sound like something straight out of sci-fi, rather than straight out of your local hospital, but Positron Emission Tomography (PET) scans are a tried and tested technique in medical imaging. It works like this: radioactive isotopes of oxygen, carbon or flourine are produced in a particle accelerator, then shuttled off to a chemistry lab post-haste to be incorporated into a a tracer molecule. This tracer is then injected into the patient who then sits down and waits for their body to shift the tracer around to the interesting spots (well if they're already patient they don't mind waiting). The isotopes decay, producing anti-matter (positrons) which collide with electrons and generate gamma rays. These gamma rays are what radiographers detect with their fancy machines. There are lots of references on the internet, but I got my info from good old paper:

  • Turner, Robert & Jones, Terry (1975), 'Techniques for imaging neuroscience', British Medical Bulletin, 65, pp. 3--20.

Disgusting slime balls are roaming free on the ocean floor. While some of us run in fear, paleontologists are rubbing their little hands together with glee. Gromia sphaerica is a huge (up to 3cm wide) single-celled organism, filled with bacteria in a symbiotic relationship. This ball rolls around on the sea floor leaving clearly identifiable tracks. The discovery that single-celled organisms can leave such obvious traces has profound consequences for our understanding of evolution. It had been thought that only multicellular critters could leave tracks like this, and so it was assumed that multi-cellular life originated quite early, despite a glaring dearth of fossils from this time period. These slime balls could fill the gap.
Popculture: In the 1930s a poor kid suffered a head injury from falling off his bike. During the years following this crash, he developed crippling seizures, possibly due to the accident. Doctors decided to try an experimental treatment, which involved removing a significant portion of his brain. The seizures stopped, but from that point on H.M. (the initials by which he came to be known) could no longer form new memories, a condition called Anterograde Amnesia. This condition was the basis for the films 50 First Dates, Memento and Clean Slate. We look at the life and psychology of H.M.
The end of show quiz was enjoyed by all.

Oops. Mistakes we shouldn't have made but did:

In the quiz, the "correct" answer that Anthony gave for the ordering of geological time periods was incorrect. Here is the actually correct ordering, from longest to shortest: eon, era, period, epoch. He has been suitably upbraided.

In answer to Anita's question about the dangers of gamma rays, Anthony mentioned that yes they can be dangerous since they are ionising radiation. However, this is not the whole story. There are three ways in which gamma rays interact with the molecules in our body: Compton scattering, pair-production, nuclear interactions (which can change the atomic number), and the photoelectric effect. This might be worse than confusing, but we'll go into these on a future podcast.

When talking about the size of the piece that was removed from H.M.'s brain, Anita mentioned that it was the size of a fist - this came from the article at http://www.brainconnection.com/topics/?main=fa/hm-memory4. Other sources give it as the size of two fingers. We're not sure how big it was.


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