BioEd Online

PET scanning could become cheaper and more widespread, thanks to a new bench-top way to produce rare radioactive atoms¹. Current methods of making radioisotopes render the medical-imaging technique cumbersome and expensive.

Positron-emission tomography (PET) scans of tumours and organs rely on the radioactive decay of isotopes such as carbon-11 and fluorine-18. Isotopes are versions of chemical elements that differ in the number of neutrons in each atom. Carbon's most common isotope, carbon-12, for example, has six neutrons per atom, whereas carbon-11 has five.

The isotopes used for PET decay by emitting positrons, which are like positively charged electrons, made from antimatter; normal electrons have a negative charge. When positrons collide with electrons, they annihilate each other in a flash of gamma rays.

By detecting these gamma rays, a PET scanner works out where in the body the decaying isotopes are. So patients swallow substances containing carbon-11 or fluorine-18, which are then tracked as they move through the body's organs.

The problem is that these isotopes decay quickly - within minutes or hours. So they have to be made at the same place and time as the scan, by particle accelerators that fire beams of protons at other materials. "Due to the size, cost and shielding required for such installations, PET is limited to only a few facilities," explains Victor Malka of the École Polytechnique in Palaiseau.

Recently scientists have found that proton beams can be produced by blasting a solid target with a very bright laser beam. The beam converts some of the material into a plasma - a gas of electrically charged particles, including protons.

Malka's team now show that a set-up like this can produce carbon-11 and fluorine-18 in quantities close to those needed for PET scanning. The researchers use a relatively cheap titanium-sapphire laser that generates a rapid-fire string of light pulses.

They calculate that firing the laser at some aluminium foil produces a beam of fast-moving protons that, aimed at a suitable target material for half an hour, is nearly intense enough to generate the quantity of radio-isotopes needed for a single patient's dose in PET scanning.

To boost the proton beam to the level for making a full dose, the team hopes to increase the current firing rate of the laser pulses about 100-fold.