PET in Medical Physics
Positron emission tomography (PET) is a nuclear medicine, functional imaging technique that produces a three-dimensional image of functional processes in the body using beta+ emitting isotopes.
The isotope decays emitting a positron (which is a positive electron, also called a beta+ particle, and is a particle of antimatter). The positron can only travel about 1 mm before losing its energy and slowing down. When it slows down enough, it will meet a negative electron from a nearby atom, and they will ‘annihilate’, leaving no particles. Their energy is converted into two gamma rays which travel in opposite directions so that momentum is conserved.
The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule. Three-dimensional images of tracer concentration within the body are then constructed by computer analysis. In modern PET-CT scanners, three dimensional imaging is often accomplished with the aid of a CT X-ray scan performed on the patient during the same session, in the same machine.
A PET scanner has a ring of detectors so that both gamma rays are seen, and is connected to a computer which can work out where the gamma rays came from and produces an image.
Computer software enables us to look at the kidney from all directions.
Not all hospitals have PET scanners as they need large, expensive machines called cyclotrons nearby to produce the positron-emitting isotopes by bombarding atoms with accelerated protons. (radionuclides). The isotopes have a shorter half-life than the gamma emitters used in traditional nuclear medicine (e.g. Carbon-11, which has a half-life of 20.5mins and 2 mins for Oxygen-15). The patient is injected with a drug and the scanner measures where the drug went. If we choose a drug which is found in cancer, then we can work out where the cancer is.
PET imaging is often used to detect tumours. As cancers are growing quickly they need a large supply of energy, which they get from glucose. A chemical called fluoxyglucose can be labelled with positron emitting fluorine-15, which then collects in the tumour and shows up as a bright spot in the PET scan (like in the rib in the picture below).
Below is a picture from a PET scan from somebody who’s been injected with a drug which flows round the blood stream and collects in the cancer. This allows the medical team to work out where the cancer is. The heart and kidneys show up bright, because that’s where there’s a lot of blood, and the very bright region is a tumour.
Some PET scanners now have a CT scanner next to them so both types of scan can be done at the same time. This can easily be done as both types of scanner are shaped. This image is a combined PET/CT image. The excellent contrast from the PET scan, in which the brain and bladder show up as bright red, is combined with the anatomical detail) good spatial resolution) from the CT shown in grey).
The information from the PET scan (in colour in the picture above) can be superimposed on an x-ray CT image (grey/blue on slide). In this way, doctors get the benefit of high contrast from the PET scan and good spatial resolution from the CT image.
Making a PET scan
PET imaging carried out by injecting patient with a tracer that produces gamma rays (indirectly).
Gamma rays detected using ring of detectors around patient. Signal from detectors used by computer to build a functional image of organs such as the brain.
After being emitted positron slows down (after travelling about 1 mm) and interacts with an electron inside patient’s body. Annihilation of electron and positron produces two gamma rays.
PET offers detailed imaging because:
Gamma rays that do not arrive in pairs are ignored
Computer works out position of source by “drawing lines” between gamma rays that arrive at the same time (within nanoseconds of each other).
Gamma rays produced must travel in opposite directions to conserve momentum (both electron and positron have negligible momentum before annihilation)
With thanks to the Institute of Physics for producing most of the resources on PET medical physics that I have used in this report.