A few days ago I learned a parent of a close friend will be having a PET scan in the near future. There are several non-invasive methods of looking into a living body each with its own niche. Most were developed by physicists using instrumentation and techniques developed for studying physics - a nifty side effect that has impacted millions of people.
Positron Emission Tomography is right out of particle physics with a bit of biochemistry mixed in for good measure. You need a cyclotron, gamma ray detectors and computational techniques that were mostly well developed by the 60s and further refined since then. Tomography is the reconstruction of images from image sections that were created by some form of penetrating wave. CT scans, MRI, fMRI, and PET are commonly used in medicine and biology, but the technique is broadly applicable elsewhere. Underground water, pockets of oil and natural gas, even the interiors of pyramids.
Positron emission is the sexy part...
There are times when you want to map out regions of the body that happen to have very high metabolic rates - active cancer sites for example. You want the cancer stand out from surrounding tissue. Bonus points for being able to place it in three dimensions and measure metabolic rates in as much detail as possible. To do this you fool part of the metabolic process.
A tracer called fluorodeoxyglucose does the trick (I never remember what it stands for and had to look it up - I just remember it as [18F]FDG). The name gives a hint. It is a glucose-like molecule that your body will happily substitute for garden variety glucose. An oxygen atom has been replaced by a radioactive isotope of fluorine with nine neutrons and nine protons in its nucleus - fluorine 18 or 18F. About 98% of the time it undergoes a positive beta decay to oxygen 18 with eight protons and ten neutrons. Effectively a proton has been converted into a neutron lowering the atom's atomic number by one and turning it into oxygen, with a positron and an electron neutrino leaving the nucleus to make the physics work out. Directly detecting the neutrino is very difficult, but fortunately the positron provides the useful signal.
A positron is a bit of antimatter - an anti-electron. It has a positive charge in contrast to the electron's negative charge but both have the same rest mass - 511 keV (thousand electron volts).1 Since opposite charges attract and it is moving out into a world of matter its life is numbers. After a very short trip - usually less than a millimeter, it runs into an electron and the two of them annihilate becoming two packets of pure energy. These packets, or gamma ray photons, each have an energy of 511 keV and leave in opposite directions at the speed of light. Some will be stopped by flesh and bone, but mostly they just keep cruising.
To catch and detect the gammas a tube shape detector surrounds the person. It is composed of segments of a scintillator material - usually an inorganic crystal - that glows when a gamma ray strikes it. The brightness of the glow is proportional to the gamma ray's energy and a photodetector makes note of the time and the energy deposited.
Remember that I said the gamma rays left in opposite directions? That was in the rest frame of the positron. The positron was moving relative to the patient so the gamma rays will not be seen as 180° apart, but nearly so. The scintillator array has segments lighting up randomly in position and energy. When two segments light up at the 511 keV annihilation signature and a straight line running through the person's body connects them, the detector's logic says - 'aha! I know how to draw a line roughly through the point in the body where the annihilation took place. Correlated events are saved to memory - all uncorrelated scintillator flashes are ignored.
The process continues and thousands of events are noted allowing a 3d image can be constructed. Higher intensity corresponds to a higher metabolism. For a good image you need a lot of 18F - a dose of roughly 14 milli sievert or about twice a chest CT scan or 100 chest X-Rays.2 The 18F has a half life of about 1.8 hours. 24 hours after it is injected it has gone through a bit over 13 half lives - only (1/2)13 of it remains about about 1/8,192 of the original amount. Its radiation level is very low at this point and it is safe for normal handling.3 But that presents a problem - at such low dosages it is useless as a radioactive tracer. Even with overnight shipping you would have to make dangerously large and expensive amounts of it to be useful. You need an on site cyclotron.
A cyclotron is a simple particle accelerator. Old cathode-ray tubes are simple linear accelerators (much more fun to use for that purpose than for watching TV). The got up to about 25 keV ... not enough to be very useful in nuclear medicine. More powerful accelerators tend to be long. A neat trick found in the thirties was to make them circular - the cyclotron accelerates a charged particle, usually a proton, up to a few million electron volts in its simple form. The CERN accelerator uses a similar principal refined over the decades. Most medical cyclotrons are used to make 18F for on-site FDG. A few million dollars of kit allows you accelerate a non-trivial number of protons (tens of milliamps)to about 99.5% the speed of light and direct them to a target of 18O enriched water (a different sort of heavy water). The clock is now ticking and you have to do some rather fast chemistry to make the FDG .. this isn't exactly easy, but is done every day at a few hundred sites around the world.
So if you or a loved one needs the technique you have instrumentation physics people at Brookhaven National Lab and several universities around the world and the budget for fundamental science to thank...
1 The positron and electron have some other differences, but they're not factors in nuclear medicine. Physicists measure mass-energy in a variety of units - an electron volt is very small. A gram is about 5.6 * 1032 eV. Technically it is energy change when you move an electron across a potential difference of one volt.
2 Not to worry - while that sounds a lot, the body can handle a few a year. The danger from radiation is much lower than exploratory surgery!
3 Some of it remains in the body, much of it is excreted.
Thanksgiving is coming -- what more can I say? If you're not an American it still makes sense to have a holiday devoted to family and friends and a bonding meal. A warning though... a few years ago I counted the pre-meal goodies before guests and again just before the meal. The average person had about 1,200 calories of nuts, chocolate, and other treats before dinner!